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The first thing I
would like to tell you,

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which I did not mention the
last time-- that in the reading

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assignment, the syllabus that
we handed out-- and by the way,

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does everybody have one?

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If you don't, we have
some extras here.

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You all have one?

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Good.

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Anyway, as you can
see on the first page,

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there is a section called
preparatory reading.

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So I would recommend that
if you can each time read

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this recommended reading,
which is available, of course,

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on the internet.

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And that will make it easier
for you to follow the lecture,

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because this course is both
in vision and audition,

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and relies on imparting
a great many facts

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about the workings
of these two systems.

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And it's easier to be able to
memorize these facts by doing

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the preparatory
reading, if you can.

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00:01:30,020 --> 00:01:32,770
All right, so today, then,
what we are going to do

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is initially start to
talk about the layout

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of the visual system.

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And then we're going to talk
about the retina for much

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of the course.

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At the very end, we'll
talk a little bit

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about the lateral
geniculate nucleus.

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In each case, I
will try to tell you

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00:01:49,870 --> 00:01:52,680
about how some people
thought about things,

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00:01:52,680 --> 00:01:57,620
and what the progression
is of the discoveries that

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have been made about
the visual system.

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00:01:59,810 --> 00:02:02,380
It is certainly an
area of research

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which has been
incredibly successful,

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00:02:05,220 --> 00:02:08,630
and has resulted in
quite a number of Nobel

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00:02:08,630 --> 00:02:12,040
Prizes for the discoveries
that individuals

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had made in uncovering the
workings of the system.

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00:02:16,140 --> 00:02:21,340
Furthermore, many of the
uncoverings that investigators

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had done came about as
incredible surprises,

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because so many of the
findings were quite unexpected.

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00:02:30,460 --> 00:02:38,000
And I will try to point these
things out during the course.

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00:02:38,000 --> 00:02:40,390
And my section, as
I mentioned before,

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will consist of 12 lectures.

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And then that will be
followed on October 23rd

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00:02:47,820 --> 00:02:49,540
by a midterm exam.

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00:02:49,540 --> 00:02:52,010
And again, to
reiterate, that's going

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00:02:52,010 --> 00:02:56,190
to consist of a series of
multiple-choice questions.

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And then following
that, we'll move on

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00:02:59,730 --> 00:03:05,540
to talk about the
auditory system

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that Chris Brown is going
to be presenting to you.

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All right, so the first
thing I want to do,

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then, is to talk about the basic
wiring of the visual system,

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which in itself has yielded a
number of unexpected surprises,

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and also raised some
interesting questions.

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So what I want to
start with first of all

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is that when you look
at some animals that

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have sideways-looking eyes, like
the rabbit and most amphibians,

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the two eyes look out in
two different directions--

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to the left and to the right.

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And that enables them to
see a very large portion

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of the visual field.

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And when that is the case,
as in this picture shown here

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for the rabbit, what
you find is that there's

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only a small area which
is seen by both eyes.

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00:04:01,880 --> 00:04:07,630
But then when we look at higher
mammals, and particularly

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primates and humans,
what you find

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is that there was a huge change
in the course of evolution,

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00:04:13,950 --> 00:04:16,490
bringing the eyes to the front.

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00:04:16,490 --> 00:04:22,670
Now that change is very
interesting, and we can ask,

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why on earth did that happen?

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00:04:24,660 --> 00:04:27,450
Because it meant
that, obviously,

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a human cannot see
behind themselves,

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where these rabbits can
see a much larger portion

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of the visual field.

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And so you sacrificed
ability to see all the way

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around the world for being
able to see with both eyes

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to the front.

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And so the question is,
why did this happen?

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Now we are going to discuss
this in some detail,

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because one of the prime
reasons-- I believe

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at least-- that
this has happened

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is to be able to process
information about depth better

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00:05:01,060 --> 00:05:04,670
than it can be done by
rabbits and other animals.

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00:05:04,670 --> 00:05:06,950
And so when we talk
about depth perception,

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00:05:06,950 --> 00:05:10,650
we are going to deal with
this issue in more detail.

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00:05:10,650 --> 00:05:16,040
Now if you look at an animal
like a rabbit, what you find--

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and also animals which are,
as I've mentioned already,

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that you also have in
fish and amphibians,

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00:05:27,940 --> 00:05:30,090
you have these
sideways looking eyes.

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00:05:30,090 --> 00:05:32,650
And now we can ask
the question, how

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00:05:32,650 --> 00:05:36,250
do these two eyes
make their connections

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00:05:36,250 --> 00:05:40,070
through the ganglion cells to
the central nervous system?

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00:05:40,070 --> 00:05:43,450
And that, in fact, was
quite an issue at one time,

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00:05:43,450 --> 00:05:46,817
believe it or not, before
anatomical techniques

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00:05:46,817 --> 00:05:47,900
became more sophisticated.

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And the reason it
became an issue is

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because it was discovered
predominantly, actually,

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by Cajal, who I've mentioned
had received the Nobel Prize

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for the beautiful work he
had done on not only vision,

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00:06:04,750 --> 00:06:09,820
but in general about the
nervous system, in 1906

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00:06:09,820 --> 00:06:13,350
with Golgi, he
discovered, as did

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00:06:13,350 --> 00:06:17,630
several other investigators,
that the optic nerve crosses

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00:06:17,630 --> 00:06:20,190
over at the so-called chiasm.

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And so the input
from the left eye

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projects into the right
half of the brain,

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00:06:25,440 --> 00:06:28,110
and from the right half to
the left half of the brain.

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00:06:28,110 --> 00:06:32,350
And that became a huge issue--
why on earth would this happen?

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00:06:32,350 --> 00:06:36,210
And we are going to deal
with this repeatedly.

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It's a very interesting issue.

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00:06:39,700 --> 00:06:43,380
And even more
interesting is the fact

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that when the eyes move to
the front, what happened

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is that this kind of connection
became much more complicated.

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But before I do that, let me
just say a few more words here.

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First of all, that in
fish and amphibians,

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00:06:59,330 --> 00:07:02,790
you have a huge structure
called the optic tectum,

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00:07:02,790 --> 00:07:08,100
which is the primary visual
processing center in the brain.

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00:07:10,360 --> 00:07:13,930
And there is also the
lateral geniculate nucleus

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of the thalamus
in these animals.

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But that's quite small, and
not very well developed.

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And there is a cortex
in these animals,

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but it is, again, a
primitive area compared

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to primates, for example.

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And the lateral geniculate
nucleus projects to the cortex.

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So the left cortex
in these animals

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gets input from the right
eye, and the obverse

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is the case for
the right cortex.

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And so consequently,
this cortex is

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this part of the visual
field, and this cortex

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is this part of
the visual field.

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It's almost like an inversion.

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Now we come to what happened
as a result of the eyes

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moving to the front.

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00:07:55,350 --> 00:07:59,200
When the eyes move to the front
in the course of evolution,

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that necessitated a major
rewiring in the visual system.

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00:08:04,760 --> 00:08:08,330
So that's what we are going to
look at next, because that's

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00:08:08,330 --> 00:08:09,090
us.

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00:08:09,090 --> 00:08:12,630
All right, so what
we have here then,

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we have the two eyes
looking to the front.

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00:08:14,900 --> 00:08:17,440
And imagine that these two
eyes are looking at something.

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00:08:17,440 --> 00:08:18,790
There's a dot here.

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00:08:18,790 --> 00:08:21,220
And both eyes are
looking at the same dot,

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00:08:21,220 --> 00:08:23,570
so that you only
see a single dot.

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00:08:23,570 --> 00:08:24,260
OK.

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So now what happens is
really something surprising.

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What happens is that the--
let me interrupt for a second

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and tell you that imagine that
you cut your eye-- I mean,

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00:08:37,309 --> 00:08:40,570
don't do it-- but imagine that
you cut your eye vertically

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in half, each eye, OK?

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00:08:43,100 --> 00:08:46,510
And that means that each
eye has, if you will,

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a nasal half and a
temporal half, all right?

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00:08:50,200 --> 00:08:51,930
So here we have the two eyes.

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Here's the left eye.

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And what we have is a nasal
half, projects across,

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and the same thing is
true for the right eye--

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00:09:02,790 --> 00:09:04,380
projects across.

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00:09:04,380 --> 00:09:06,430
But now if you look at
the temporal hemiretinae--

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00:09:06,430 --> 00:09:10,660
hemiretinae, remember
that's the word for it--

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00:09:10,660 --> 00:09:12,780
something very
interesting happens.

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The temporal hemiretinae
don't cross over,

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00:09:16,170 --> 00:09:18,570
but they remain
on the same side.

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00:09:18,570 --> 00:09:23,960
So we have a color code here
that the red parts project

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00:09:23,960 --> 00:09:29,250
across, and the black parts also
project across from one eye,

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00:09:29,250 --> 00:09:32,230
but stay ipsilateral
for the other eye.

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00:09:32,230 --> 00:09:35,270
So you say, my god,
why did this happen?

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00:09:35,270 --> 00:09:37,780
What on earth is going on?

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00:09:37,780 --> 00:09:44,330
And then if you study this, you
realize that this is actually

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one of the few
truly logical things

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that you encounter when
you study the brain.

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Many of the things
you study in the brain

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00:09:51,600 --> 00:09:54,460
don't seem to be too
logical, because in the force

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00:09:54,460 --> 00:09:56,820
of evolution, you
have to change things

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00:09:56,820 --> 00:10:00,510
in very peculiar, subtle
ways to make things work.

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00:10:00,510 --> 00:10:03,980
You couldn't just redo the
whole system from scratch.

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00:10:03,980 --> 00:10:05,820
But this is certainly
a logical one,

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and I'll explain it to
you in just a minute.

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00:10:08,660 --> 00:10:14,170
OK, so now what we do
next is that again, we

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00:10:14,170 --> 00:10:16,500
have the two lateral
geniculate nuclei,

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but now these are
in the thalamus.

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00:10:19,030 --> 00:10:23,000
These two nuclei have
grown tremendously in size,

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and became much more important
in visual processing,

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00:10:26,370 --> 00:10:29,160
as did, then, the cortex.

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00:10:30,394 --> 00:10:32,560
This process, by the way,
is called encephalization.

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So in primitive animals, they
have a very small cortex.

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00:10:37,290 --> 00:10:39,300
And then the course of
evolution, the cortex

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00:10:39,300 --> 00:10:40,990
grew, and grew,
and grew, and grew,

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00:10:40,990 --> 00:10:43,500
and became a more
and more important

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00:10:43,500 --> 00:10:48,720
structure in being able to
analyze just about anything,

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00:10:48,720 --> 00:10:51,090
and certainly analyze vision.

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00:10:51,090 --> 00:10:55,360
Now here we have a cortical
area in the posterior part

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00:10:55,360 --> 00:10:58,640
of the brain, as I pointed
out to you the last time.

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00:10:58,640 --> 00:11:00,330
And what you have
is a whole series

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00:11:00,330 --> 00:11:02,400
of visual areas in the cortex.

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00:11:03,690 --> 00:11:05,640
The most central
one, I guess, would

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00:11:05,640 --> 00:11:10,830
be area V1-- the so-called
primary visual cortex to which

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00:11:10,830 --> 00:11:14,040
the lateral geniculate
projects most profusely.

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00:11:14,040 --> 00:11:18,150
Then we have a whole bunch
of other visual areas,

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00:11:18,150 --> 00:11:21,650
as we'll discuss in
more detail later on.

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00:11:21,650 --> 00:11:26,510
And then what happens is that
we have several other cortical

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00:11:26,510 --> 00:11:27,060
areas.

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00:11:27,060 --> 00:11:28,520
The cortex-- I
think you all of you

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00:11:28,520 --> 00:11:33,400
know that-- is divided
into four lobes, OK?

206
00:11:33,400 --> 00:11:36,320
The frontal, temporal,
parietal, and the rear one

207
00:11:36,320 --> 00:11:38,010
is called the occipital lobes.

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00:11:38,010 --> 00:11:42,140
All right, now the visual
areas that you see in the back

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00:11:42,140 --> 00:11:44,790
here make extensive
interconnections

210
00:11:44,790 --> 00:11:50,060
and connections to these
other lobes in the brain

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00:11:50,060 --> 00:11:53,380
to be able to analyze
the visual scene.

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00:11:53,380 --> 00:11:56,820
Now in addition,
what we have still--

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00:11:56,820 --> 00:11:58,570
as we had in more
primitive animals--

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00:11:58,570 --> 00:12:01,320
we have a superior colliculus--
actually two of them,

215
00:12:01,320 --> 00:12:04,970
one on each side-- and then
we have another set of areas

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00:12:04,970 --> 00:12:06,890
called the terminal
nuclei, the nucleus

217
00:12:06,890 --> 00:12:09,330
of the optic tract,
about which we will just

218
00:12:09,330 --> 00:12:13,780
talk a little bit,
because they are

219
00:12:13,780 --> 00:12:16,260
specializing in certain
visual functions,

220
00:12:16,260 --> 00:12:20,870
but not nearly as compelling
and interesting as the work that

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00:12:20,870 --> 00:12:24,010
has been done in
the cortical areas

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00:12:24,010 --> 00:12:25,900
and in the superior colliculi.

223
00:12:25,900 --> 00:12:27,440
So this is the arrangement.

224
00:12:27,440 --> 00:12:29,660
But now we want to
understand, why do you

225
00:12:29,660 --> 00:12:32,660
have this strange
connection here?

226
00:12:32,660 --> 00:12:37,450
So to understand that, what
we are going to talk about

227
00:12:37,450 --> 00:12:41,960
is the so-called horopter
or the Vieth-Muller circle.

228
00:12:41,960 --> 00:12:42,850
Now what is that?

229
00:12:42,850 --> 00:12:45,970
Some clever guy came up
with this observation.

230
00:12:45,970 --> 00:12:49,540
They made a circle that
goes through the nodal point

231
00:12:49,540 --> 00:12:50,590
of the eye.

232
00:12:50,590 --> 00:12:54,890
And its diameter depends
on where you're fixating.

233
00:12:54,890 --> 00:13:01,700
So fixating here, in
this case, the red area,

234
00:13:01,700 --> 00:13:04,850
which is your left
visual hemifield,

235
00:13:04,850 --> 00:13:08,630
impinges on the nasal
retina of the left eye

236
00:13:08,630 --> 00:13:10,565
and the temporal retina
of the other eye.

237
00:13:10,565 --> 00:13:14,710
So if you have an
object here, that

238
00:13:14,710 --> 00:13:20,070
hits corresponding retinal
elements in the two eyes.

239
00:13:20,070 --> 00:13:21,870
And that is a basic rule.

240
00:13:21,870 --> 00:13:26,500
Anywhere along this
horopter, a spot

241
00:13:26,500 --> 00:13:30,970
will activate corresponding
regions in the two eyes.

242
00:13:30,970 --> 00:13:35,020
And then what happens is that
those corresponding points

243
00:13:35,020 --> 00:13:39,340
go to the same location in the
lateral geniculate nucleus,

244
00:13:39,340 --> 00:13:42,830
and in the superior colliculus,
and in the visual cortex.

245
00:13:42,830 --> 00:13:45,210
So the logic of
this arrangement is

246
00:13:45,210 --> 00:13:50,120
to be able to have the
two eyes have input

247
00:13:50,120 --> 00:13:53,080
to corresponding
points in the brain.

248
00:13:53,080 --> 00:13:58,010
That way, you can see
something uniform and clear,

249
00:13:58,010 --> 00:13:59,660
rather than seeing double.

250
00:13:59,660 --> 00:14:01,500
So that is, then,
the arrangement

251
00:14:01,500 --> 00:14:02,655
that you have in primates.

252
00:14:03,760 --> 00:14:07,210
Now just to anticipate a
little bit-- whenever images

253
00:14:07,210 --> 00:14:09,470
fall inside or
outside the circle,

254
00:14:09,470 --> 00:14:11,832
they do fall on
non-correspondent points,

255
00:14:11,832 --> 00:14:13,290
and that will have
something to do.

256
00:14:13,290 --> 00:14:16,880
And I keep you just curious
about that for the time being,

257
00:14:16,880 --> 00:14:19,260
about how we process depth.

258
00:14:20,430 --> 00:14:24,710
And that is a mechanism
that is involved,

259
00:14:24,710 --> 00:14:26,160
so it's called stereopsis.

260
00:14:26,160 --> 00:14:28,660
I've mentioned that very
briefly the last time.

261
00:14:28,660 --> 00:14:34,700
OK, so that then summarises
the basic connections

262
00:14:34,700 --> 00:14:35,800
of the system.

263
00:14:35,800 --> 00:14:37,780
And then what we
are going to do next

264
00:14:37,780 --> 00:14:39,420
is we're going to
go to the retina,

265
00:14:39,420 --> 00:14:43,770
and look at in more detail
about how the retina is made.

266
00:14:43,770 --> 00:14:46,380
And then we will go
in successive steps

267
00:14:46,380 --> 00:14:49,490
to higher areas, sometimes
going back and forth,

268
00:14:49,490 --> 00:14:53,510
to try to understand visual
processing in general.

269
00:14:53,510 --> 00:14:57,350
All right, so now let's
begin by looking at the eye.

270
00:14:58,995 --> 00:15:04,380
Now I don't expect you to
know all these names here,

271
00:15:04,380 --> 00:15:07,640
but you should remember, of
course, the lens, the cornea.

272
00:15:07,640 --> 00:15:08,980
You know that already.

273
00:15:08,980 --> 00:15:10,434
You know the iris, right?

274
00:15:10,434 --> 00:15:11,100
What's the iris?

275
00:15:12,214 --> 00:15:13,450
AUDIENCE: [INAUDIBLE]

276
00:15:13,450 --> 00:15:17,340
PROFESSOR: OK, now
what do we base

277
00:15:17,340 --> 00:15:22,580
our views on the color of
the eye when you say somebody

278
00:15:22,580 --> 00:15:25,920
has blue eyes, or
somebody has brown eyes?

279
00:15:27,030 --> 00:15:29,720
It's based on the fact
of what the coloring is

280
00:15:29,720 --> 00:15:32,330
of the iris, all right?

281
00:15:32,330 --> 00:15:33,530
Now, what's the iris?

282
00:15:33,530 --> 00:15:39,210
The iris can get
bigger and smaller

283
00:15:39,210 --> 00:15:44,860
to control the size of
the opening into the lens.

284
00:15:44,860 --> 00:15:46,050
All right?

285
00:15:46,050 --> 00:15:50,780
It's much like what you
have in cameras, where

286
00:15:50,780 --> 00:15:53,700
they have the so-called
f-stop, right?

287
00:15:53,700 --> 00:15:57,360
So if you have a
large number, say f/16

288
00:15:57,360 --> 00:16:01,910
on a camera, that is set
to 16 if you do it manually

289
00:16:01,910 --> 00:16:03,480
when it's bright out there.

290
00:16:03,480 --> 00:16:07,020
But when it's dim out there,
like we have in this camera

291
00:16:07,020 --> 00:16:09,580
here, we have the lens
wide open because it's

292
00:16:09,580 --> 00:16:11,130
pretty dark in here.

293
00:16:11,130 --> 00:16:17,830
Then you have a low f-stop,
like an f/4 or even lower,

294
00:16:17,830 --> 00:16:22,540
to allow more light to come in--
more photons to enter the eye.

295
00:16:22,540 --> 00:16:25,590
Now next is-- here's the lens.

296
00:16:25,590 --> 00:16:30,747
And the fascinating thing about
the lens is that in the eye,

297
00:16:30,747 --> 00:16:35,380
the lens is totally different
from the way it is in a camera.

298
00:16:37,390 --> 00:16:40,740
Now, maybe by now
many of you don't even

299
00:16:40,740 --> 00:16:44,750
know the details about a
camera, but one of the details,

300
00:16:44,750 --> 00:16:53,680
of course, is that to
bring an image into focus,

301
00:16:53,680 --> 00:16:58,510
you have to adjust the distance
of the lens from the film

302
00:16:58,510 --> 00:17:02,080
onto which the
image projects, OK?

303
00:17:02,080 --> 00:17:05,520
So the closer an image, the
further out the lens has to go.

304
00:17:06,619 --> 00:17:10,829
Now, if that were the case
in animals and humans,

305
00:17:10,829 --> 00:17:12,607
you would have a
super-bulging eye,

306
00:17:12,607 --> 00:17:14,190
so when you look at
something closely,

307
00:17:14,190 --> 00:17:15,525
the eye would go out like that.

308
00:17:15,525 --> 00:17:17,500
And that would be
really disturbing.

309
00:17:17,500 --> 00:17:23,869
So that is not the case,
because what happened instead

310
00:17:23,869 --> 00:17:28,780
is something very clever that
you don't see in cameras, which

311
00:17:28,780 --> 00:17:33,520
is that you have this lens here,
which is-- at least in younger

312
00:17:33,520 --> 00:17:41,140
people-- is an organ that
can vary in thickness.

313
00:17:42,730 --> 00:17:47,700
So you have here some
muscles, the ciliary muscles,

314
00:17:47,700 --> 00:17:53,680
which are just the
thickness of this lens,

315
00:17:53,680 --> 00:17:58,470
to allow it to focus
properly on the retina.

316
00:17:58,470 --> 00:18:01,710
Now here, the
retina goes around,

317
00:18:01,710 --> 00:18:03,750
is against the inner
wall of the eye.

318
00:18:03,750 --> 00:18:05,000
You all know that, of course.

319
00:18:06,110 --> 00:18:10,860
And to explain how this
is done, then-- very,

320
00:18:10,860 --> 00:18:12,680
very crudely-- here we have it.

321
00:18:12,680 --> 00:18:14,630
If you have an
object really near,

322
00:18:14,630 --> 00:18:17,570
you want the lens to be
thick to properly focus

323
00:18:17,570 --> 00:18:19,950
the image on the retina.

324
00:18:19,950 --> 00:18:23,220
And if the image is
far, you have to thin

325
00:18:23,220 --> 00:18:29,060
the lens to get the proper
focus on the retina.

326
00:18:30,410 --> 00:18:33,862
So that's how the
lens works, at least

327
00:18:33,862 --> 00:18:35,070
when you're reasonably young.

328
00:18:35,070 --> 00:18:38,120
But then when you get
older, what happens

329
00:18:38,120 --> 00:18:43,210
is that the lens
becomes progressively

330
00:18:43,210 --> 00:18:44,150
stiffer and stiffer.

331
00:18:46,400 --> 00:18:50,440
And because of that, you have
to start wearing glasses.

332
00:18:50,440 --> 00:18:53,860
Now, in the olden
days, they used

333
00:18:53,860 --> 00:18:57,890
to have lenses which came
in two parts-- bifocals.

334
00:18:58,892 --> 00:19:00,350
So if you looked
in the upper part,

335
00:19:00,350 --> 00:19:01,558
you could look at a distance.

336
00:19:01,558 --> 00:19:04,320
If you looked in the lower
part, you could look close.

337
00:19:04,320 --> 00:19:07,200
Nowadays, they have
graduated lenses,

338
00:19:07,200 --> 00:19:10,580
so if you look in the upper
part, you can look far.

339
00:19:10,580 --> 00:19:12,200
And if you want to
look [INAUDIBLE]

340
00:19:12,200 --> 00:19:15,370
look down below, and then
you can see that in focus.

341
00:19:17,160 --> 00:19:20,000
So once you get
older, these lenses

342
00:19:20,000 --> 00:19:22,960
don't adjust as much--
still a little bit.

343
00:19:22,960 --> 00:19:28,890
But the proof that this is
not that important, thank god,

344
00:19:28,890 --> 00:19:34,380
now that we have these graded
lenses and we have bifocals--

345
00:19:34,380 --> 00:19:38,040
is that a lot of
people in old age,

346
00:19:38,040 --> 00:19:43,040
who develop-- and sometimes
not even in old age-- in India,

347
00:19:43,040 --> 00:19:46,205
for example, there are a lot of
young kids who have cataracts.

348
00:19:47,810 --> 00:19:48,970
Now what are cataracts?

349
00:19:48,970 --> 00:19:54,380
Cataracts interfere with
the processing of vision

350
00:19:54,380 --> 00:19:58,270
through the lens system,
and things become unclear,

351
00:19:58,270 --> 00:20:03,980
because all kinds of blockage
occurs in the lens itself.

352
00:20:03,980 --> 00:20:06,650
So how is that corrected?

353
00:20:06,650 --> 00:20:09,590
The way that's corrected
nowadays, is that you have

354
00:20:09,590 --> 00:20:11,860
what is called cataract surgery.

355
00:20:16,310 --> 00:20:19,420
So what they do,
then, is they actually

356
00:20:19,420 --> 00:20:23,440
literally remove the
lens from the eye,

357
00:20:23,440 --> 00:20:28,760
and they put in a fixed lens
made out of plastic or glass.

358
00:20:30,560 --> 00:20:34,240
And then, obviously, since
there's no adjustment,

359
00:20:34,240 --> 00:20:38,610
you do have to wear glasses
to look at things far away

360
00:20:38,610 --> 00:20:40,210
and look at things close.

361
00:20:40,210 --> 00:20:44,720
And most of the time, those
would be graduated lenses.

362
00:20:44,720 --> 00:20:48,420
And so people who
have cataract surgery

363
00:20:48,420 --> 00:20:52,510
can see extremely well,
certainly a lot better

364
00:20:52,510 --> 00:20:56,825
than having to try to deal with
lenses that have cataracts.

365
00:20:57,870 --> 00:20:58,460
All right.

366
00:20:58,460 --> 00:21:00,970
Now there's one other point
I want to make about the eye

367
00:21:00,970 --> 00:21:05,020
here before we move
on-- maybe two points.

368
00:21:05,020 --> 00:21:09,440
One is about this
part of the eye--

369
00:21:09,440 --> 00:21:14,130
that you have here your
so-called iris, OK?

370
00:21:14,130 --> 00:21:17,610
And what do you call what's
in the center of the iris?

371
00:21:17,610 --> 00:21:18,540
AUDIENCE: Pupil.

372
00:21:18,540 --> 00:21:20,170
PROFESSOR: Pupil, very good.

373
00:21:20,170 --> 00:21:23,300
Now what is the
color of the pupil?

374
00:21:26,240 --> 00:21:29,240
In most people,
it's black, right?

375
00:21:29,240 --> 00:21:30,500
Why is it black?

376
00:21:30,500 --> 00:21:32,661
How come the pupil is black?

377
00:21:32,661 --> 00:21:33,660
Any thoughts about that?

378
00:21:36,260 --> 00:21:37,870
OK, well, let me tell you.

379
00:21:37,870 --> 00:21:41,400
And I think what I'll do
is actually I'll move on.

380
00:21:42,480 --> 00:21:46,280
First I'll leave this unanswered
for just a few minutes.

381
00:21:47,430 --> 00:21:50,410
This I've shown
you before, which

382
00:21:50,410 --> 00:21:53,480
is another amazing
thing about the eye--

383
00:21:53,480 --> 00:21:57,980
namely that in the
fovea, you have something

384
00:21:57,980 --> 00:22:05,910
like 200,000 cones per
square millimeter-- 200,000.

385
00:22:05,910 --> 00:22:10,990
As you go five degrees out,
it reduces tenfold to 20,000.

386
00:22:10,990 --> 00:22:15,170
And then you go 10 degrees,
it reduces by half.

387
00:22:15,170 --> 00:22:18,070
So because of this
very high density--

388
00:22:18,070 --> 00:22:22,120
I've mentioned this
before-- in the fovea,

389
00:22:22,120 --> 00:22:24,510
you have high acuity.

390
00:22:24,510 --> 00:22:26,120
And that is one of
the prime reasons

391
00:22:26,120 --> 00:22:29,260
you have to move your eye around
so that you can see things

392
00:22:29,260 --> 00:22:30,070
in fine detail.

393
00:22:31,320 --> 00:22:36,430
All right, so now let's
take another step here,

394
00:22:36,430 --> 00:22:38,425
and look at the retina itself.

395
00:22:40,330 --> 00:22:42,800
And let me first of
all point out to you,

396
00:22:42,800 --> 00:22:47,732
I have reversed the image
to have it right-side up.

397
00:22:47,732 --> 00:22:49,565
The light actually comes
in from the bottom,

398
00:22:49,565 --> 00:22:52,500
and goes through all
these cells until it

399
00:22:52,500 --> 00:22:53,500
hits the photoreceptors.

400
00:22:54,780 --> 00:22:56,320
Here are the photoreceptors.

401
00:22:56,320 --> 00:22:59,680
And at the closest
point to eye, what we

402
00:22:59,680 --> 00:23:03,300
have is called the
pigment epithelium.

403
00:23:05,529 --> 00:23:07,070
OK, so what's the
pigment epithelium?

404
00:23:08,380 --> 00:23:10,850
The pigment epithelium
is a single layer

405
00:23:10,850 --> 00:23:16,210
of cells that are
pigmented to such a degree

406
00:23:16,210 --> 00:23:19,180
that when photons
come into the eye

407
00:23:19,180 --> 00:23:23,990
and get to the
pigment epithelium,

408
00:23:23,990 --> 00:23:28,530
they get absorbed just
like a black surface,

409
00:23:28,530 --> 00:23:33,050
that you see as a black
surface, absorbs the photons

410
00:23:33,050 --> 00:23:34,200
and doesn't reflect them.

411
00:23:35,370 --> 00:23:38,875
So because of that, there's
no reflection back out.

412
00:23:40,960 --> 00:23:48,480
And that explains why
the pupil looks black,

413
00:23:48,480 --> 00:23:51,860
because the light that
comes into the eye

414
00:23:51,860 --> 00:23:53,880
doesn't reflect back out.

415
00:23:55,050 --> 00:23:57,080
Now there's a good
reason for that,

416
00:23:57,080 --> 00:24:01,060
because if it were to reflect
back out, it would scatter,

417
00:24:01,060 --> 00:24:06,090
and it would activate many
more photoreceptors receptors

418
00:24:06,090 --> 00:24:09,750
than it should, and because of
that, you get blurred vision.

419
00:24:09,750 --> 00:24:10,940
Now how do we know this?

420
00:24:13,710 --> 00:24:20,280
Can you think of any
one species of animal

421
00:24:20,280 --> 00:24:24,708
where the pigment
epithelium is not black?

422
00:24:24,708 --> 00:24:25,458
AUDIENCE: In cats.

423
00:24:27,930 --> 00:24:29,940
PROFESSOR: OK, in cat, OK.

424
00:24:29,940 --> 00:24:33,260
So there's some
animals, you see mostly

425
00:24:33,260 --> 00:24:39,060
in animals that specialize
in nocturnal vision, that

426
00:24:39,060 --> 00:24:43,860
have a pigment epithelium
which actually has purposefully

427
00:24:43,860 --> 00:24:46,180
reflecting molecules in it.

428
00:24:47,230 --> 00:24:49,590
For them, this is called,
actually, the tapetum.

429
00:24:51,310 --> 00:24:57,070
And the purpose of that
is to improve the ability

430
00:24:57,070 --> 00:25:01,080
to see so that the photons
activate the photoreceptors not

431
00:25:01,080 --> 00:25:05,170
only in one direction, but
also when they bounce back.

432
00:25:05,170 --> 00:25:08,630
Now the tapetum is arranged in
such a way, in most animals,

433
00:25:08,630 --> 00:25:10,910
that it doesn't freely
scatter the light,

434
00:25:10,910 --> 00:25:14,180
but scatters in a fairly
directional manner.

435
00:25:14,180 --> 00:25:16,860
And because of that,
you can see pretty well.

436
00:25:16,860 --> 00:25:21,450
Now I'm sure that many
of you have noticed

437
00:25:21,450 --> 00:25:28,120
that when you drive at
night, in a the deserted area

438
00:25:28,120 --> 00:25:32,750
in the United States-- which
are decreasing, of course--

439
00:25:32,750 --> 00:25:39,070
you can see sometimes
deer on the highway,

440
00:25:39,070 --> 00:25:41,190
or rabbits for that matter.

441
00:25:41,190 --> 00:25:43,870
And when you see them,
it looks like they

442
00:25:43,870 --> 00:25:48,970
have two flashlights
projecting at you.

443
00:25:48,970 --> 00:25:49,720
It's almost scary.

444
00:25:50,740 --> 00:25:56,420
But it also helps you by being
able to avoid hitting them.

445
00:25:56,420 --> 00:25:59,530
Now that's because they have
that reflecting tapetum,

446
00:25:59,530 --> 00:26:00,030
all right?

447
00:26:01,080 --> 00:26:02,500
Let's look at a
different example.

448
00:26:03,670 --> 00:26:08,930
What other human can you
think of who does not

449
00:26:08,930 --> 00:26:14,030
have a black pigment
epithelium or for that matter,

450
00:26:14,030 --> 00:26:15,100
some animals as well.

451
00:26:15,100 --> 00:26:15,600
Yes?

452
00:26:15,600 --> 00:26:16,180
AUDIENCE: Albino.

453
00:26:16,180 --> 00:26:17,570
PROFESSOR: Very good-- albinos.

454
00:26:18,620 --> 00:26:19,880
Now what's albinos?

455
00:26:19,880 --> 00:26:23,580
Albinos are
individuals, or animals,

456
00:26:23,580 --> 00:26:30,720
who lack pigment in their
skin, and, of course,

457
00:26:30,720 --> 00:26:32,560
in their retinae.

458
00:26:32,560 --> 00:26:39,770
So if you look at
the eye of an albino,

459
00:26:39,770 --> 00:26:42,390
actually their
pupil is not black.

460
00:26:42,390 --> 00:26:44,354
Their pupil is--

461
00:26:44,354 --> 00:26:45,410
AUDIENCE: Red.

462
00:26:45,410 --> 00:26:46,960
PROFESSOR: Reddish, very good.

463
00:26:46,960 --> 00:26:48,500
Why is it reddish?

464
00:26:48,500 --> 00:26:59,280
Because you have thousands
of-- in the retina,

465
00:26:59,280 --> 00:27:03,240
especially mostly on the
outside here as well,

466
00:27:03,240 --> 00:27:05,125
you have blood vessels.

467
00:27:06,535 --> 00:27:07,910
And the blood
vessels, since they

468
00:27:07,910 --> 00:27:12,030
carry blood, when light
reflects from them,

469
00:27:12,030 --> 00:27:14,400
they will have a
reddish color to them,

470
00:27:14,400 --> 00:27:17,980
and thereby endow, if you will,
the pupil with a reddish color.

471
00:27:17,980 --> 00:27:23,100
The pupil with a reddish color.

472
00:27:23,100 --> 00:27:27,625
Now, the proof that the
main function of the pigment

473
00:27:27,625 --> 00:27:31,530
epithelium is to absorb the
incoming light, to prevent

474
00:27:31,530 --> 00:27:35,740
scattering, is in
the fact that albinos

475
00:27:35,740 --> 00:27:38,810
that lack this pigment
epithelium have

476
00:27:38,810 --> 00:27:39,740
very poor vision.

477
00:27:40,940 --> 00:27:45,490
They often have vision to
20 over 200 or something,

478
00:27:45,490 --> 00:27:51,120
because the light scatters
from the epithelium which

479
00:27:51,120 --> 00:27:53,840
is no longer pigmented
in these people,

480
00:27:53,840 --> 00:27:58,370
and activates many
photoreceptors,

481
00:27:58,370 --> 00:28:02,415
thereby reducing the ability to
see things clearly and sharply.

482
00:28:03,920 --> 00:28:08,090
All right, so now let's go
through this retina here.

483
00:28:08,090 --> 00:28:11,375
First of all, we have
the rods and the cones.

484
00:28:13,610 --> 00:28:18,370
Now this brings me to a very
interesting story-- at least

485
00:28:18,370 --> 00:28:19,720
for me an interesting story.

486
00:28:22,050 --> 00:28:27,590
Back in the 19th century,
there was a person

487
00:28:27,590 --> 00:28:36,260
whose name was Max Schultze.

488
00:28:36,260 --> 00:28:38,880
I'm trying to remember
when he lived.

489
00:28:38,880 --> 00:28:45,390
He lived from 1825 to 1874.

490
00:28:45,390 --> 00:28:49,355
He developed some new
anatomical procedures.

491
00:28:50,770 --> 00:28:57,020
And as I mentioned before,
whenever new techniques emerge

492
00:28:57,020 --> 00:29:00,000
or are invented, it's
likely that you're

493
00:29:00,000 --> 00:29:02,170
going to make some
interesting new discoveries.

494
00:29:03,370 --> 00:29:11,280
And so he was the first person,
by then looking at the retina

495
00:29:11,280 --> 00:29:13,300
and looking at the
photoreceptors,

496
00:29:13,300 --> 00:29:16,740
that he said there are
two different kinds

497
00:29:16,740 --> 00:29:23,470
of photoreceptors--
the rods and the cones.

498
00:29:26,100 --> 00:29:29,550
The rods are sort of, like
the name implies, rods.

499
00:29:29,550 --> 00:29:32,770
And the cones are sort
of, as the name implies,

500
00:29:32,770 --> 00:29:33,750
conish looking.

501
00:29:35,080 --> 00:29:38,640
So he looked at that and looked
at that, and said why on earth

502
00:29:38,640 --> 00:29:43,450
would we have in most
animals rods and cones?

503
00:29:43,450 --> 00:29:44,890
What is their function?

504
00:29:44,890 --> 00:29:46,930
This was a totally
new discovery.

505
00:29:46,930 --> 00:29:50,360
When he first published
this, nobody believed him.

506
00:29:50,360 --> 00:29:52,380
They said, oh, they
just vary randomly,

507
00:29:52,380 --> 00:29:56,807
and some are more rod-like
and some are more cone-like,

508
00:29:56,807 --> 00:29:58,390
but it's just the
same photoreceptors.

509
00:30:00,050 --> 00:30:03,220
So he, of course,
did not accept that.

510
00:30:03,220 --> 00:30:05,960
He saw, by being very
careful about it,

511
00:30:05,960 --> 00:30:08,942
that it was definitely
two different classes

512
00:30:08,942 --> 00:30:10,275
just like it is in this picture.

513
00:30:12,160 --> 00:30:14,540
And so he began to
ask the question, why

514
00:30:14,540 --> 00:30:16,860
on earth do we have this?

515
00:30:16,860 --> 00:30:20,710
So imagine yourself in
the 19th century coming up

516
00:30:20,710 --> 00:30:23,180
with this incredible
discovery, and trying

517
00:30:23,180 --> 00:30:24,810
to figure out why we have them.

518
00:30:26,400 --> 00:30:28,840
Now if you're a
really good scientist,

519
00:30:28,840 --> 00:30:31,210
then you start to
think analytically.

520
00:30:31,210 --> 00:30:32,590
And so what did he do?

521
00:30:33,890 --> 00:30:35,970
I'm so impressed to this day.

522
00:30:35,970 --> 00:30:39,800
He said, well, let me take
a closer look at the retina

523
00:30:39,800 --> 00:30:43,110
and see how the
rods and the cones

524
00:30:43,110 --> 00:30:46,950
are distributed on
the retinal surface.

525
00:30:46,950 --> 00:30:53,190
And he discovered, to go back to
the previous slide, that there

526
00:30:53,190 --> 00:30:56,990
is a small region
here, the fovea,

527
00:30:56,990 --> 00:30:59,480
where there are only cones.

528
00:30:59,480 --> 00:31:00,439
There are no rods.

529
00:31:00,439 --> 00:31:02,730
And I'll show you a better
picture of that in a minute.

530
00:31:03,790 --> 00:31:08,180
And he said, my god,
if that's the case,

531
00:31:08,180 --> 00:31:10,690
there must be something
different about the way

532
00:31:10,690 --> 00:31:14,720
we see in the fovea than
we can see in areas where

533
00:31:14,720 --> 00:31:17,550
you have lots of rods.

534
00:31:17,550 --> 00:31:19,370
So what did he discover?

535
00:31:19,370 --> 00:31:26,885
He discovered that in the fovea
we don't see too well at night.

536
00:31:28,090 --> 00:31:32,340
OK, on the basis of
that, he concluded

537
00:31:32,340 --> 00:31:35,280
that the rods are
for night vision.

538
00:31:36,830 --> 00:31:38,600
Now this was
absolutely incredible.

539
00:31:40,290 --> 00:31:43,410
There's only one sad
part in this story, which

540
00:31:43,410 --> 00:31:51,890
is that this discovery was
made in the early 19th century,

541
00:31:51,890 --> 00:31:56,570
but before the Nobel
Prize was initiated,

542
00:31:56,570 --> 00:32:00,260
because had the Nobel Prize
been initiated before you made

543
00:32:00,260 --> 00:32:02,520
this discovery,
there's no question

544
00:32:02,520 --> 00:32:04,700
that he would have received
the Nobel Prize for it.

545
00:32:06,320 --> 00:32:09,500
It's such an incredible
magnitude of discovery.

546
00:32:09,500 --> 00:32:11,620
All right, so let's
go back here then,

547
00:32:11,620 --> 00:32:14,060
and I pointed out the rods
and the cones and the pigment

548
00:32:14,060 --> 00:32:14,560
epithelium.

549
00:32:15,590 --> 00:32:17,910
Now let's go through
here and I'll

550
00:32:17,910 --> 00:32:20,130
tell you about the
rest of the elements.

551
00:32:20,130 --> 00:32:24,860
And then this time, and also
in several subsequent sessions,

552
00:32:24,860 --> 00:32:28,830
we'll talk about some of these
elements in much more detail,

553
00:32:28,830 --> 00:32:32,780
especially about the
retinal ganglion cells.

554
00:32:32,780 --> 00:32:37,420
So anyway, when you come
here to the next stage here,

555
00:32:37,420 --> 00:32:41,300
we have the so-called horizontal
cells and bipolar cells.

556
00:32:41,300 --> 00:32:45,350
It's been established that
the photoreceptors connect

557
00:32:45,350 --> 00:32:47,070
with both of these.

558
00:32:47,070 --> 00:32:51,790
And then the bipolar cells
connect into the inner portion

559
00:32:51,790 --> 00:32:54,260
of the retina, just as
I'll show you in a minute.

560
00:32:54,260 --> 00:32:58,480
Now this region here
is called the OPL,

561
00:32:58,480 --> 00:33:00,870
and it's easy to remember--
outer plexiform layer.

562
00:33:02,270 --> 00:33:04,825
Now then, when you get into
the deeper parts of the retina,

563
00:33:04,825 --> 00:33:09,400
you have the so-called amacrine
cells and ganglion cells,

564
00:33:09,400 --> 00:33:13,300
and they reside in the
inner plexiform layer.

565
00:33:13,300 --> 00:33:21,370
Now it is the ganglion cells
that project out of the eye

566
00:33:21,370 --> 00:33:26,290
and then project,
as we had shown you,

567
00:33:26,290 --> 00:33:32,583
to various regions in the
brain, most notably that we're

568
00:33:32,583 --> 00:33:35,890
going to talk about today the
lateral geniculate nucleus.

569
00:33:39,610 --> 00:33:51,440
Now let me also point out at
this stage to you some numbers.

570
00:33:51,440 --> 00:33:55,720
Here is an enlarged, highly
simplified view of rods.

571
00:33:56,810 --> 00:34:02,490
Now, the rods are constructed
in an incredibly complex way.

572
00:34:02,490 --> 00:34:06,000
They have these individual
so-called disks.

573
00:34:06,000 --> 00:34:09,160
And within each of these
disks-- there are about 1,000

574
00:34:09,160 --> 00:34:12,330
of them-- I only have
25, here, I think, or 24.

575
00:34:12,330 --> 00:34:17,050
There are about 1,000 of
these in each rod, OK?

576
00:34:17,050 --> 00:34:21,050
And within each of
these disks, you

577
00:34:21,050 --> 00:34:24,410
have about 10,000
rhodopsin molecules.

578
00:34:27,300 --> 00:34:30,230
There are several kinds
of opsins, so-called.

579
00:34:30,230 --> 00:34:33,040
Rhodopsins are the
ones you see in rods,

580
00:34:33,040 --> 00:34:35,105
in there are different
kinds of opsins in cones.

581
00:34:36,929 --> 00:34:45,070
And these are the photosensitive
molecules in your receptors,

582
00:34:45,070 --> 00:34:49,420
so that when a photon
comes in and hits

583
00:34:49,420 --> 00:34:54,280
one of these molecules,
they change shape.

584
00:34:54,280 --> 00:34:56,469
And the simplest way
of thinking about this

585
00:34:56,469 --> 00:35:00,350
is to say that these molecules,
these rhodopsin molecules,

586
00:35:00,350 --> 00:35:02,095
come in two different shapes.

587
00:35:03,330 --> 00:35:08,530
And these two different
shapes are what we call,

588
00:35:08,530 --> 00:35:14,930
very simply, those
that are open and those

589
00:35:14,930 --> 00:35:17,070
are the sort of
closed, if you're

590
00:35:17,070 --> 00:35:18,900
describing the molecular shape.

591
00:35:18,900 --> 00:35:21,740
And the easiest way to think
of them as that they're

592
00:35:21,740 --> 00:35:23,840
bleached or unbleached.

593
00:35:23,840 --> 00:35:27,720
And so there are millions
of these molecules,

594
00:35:27,720 --> 00:35:33,710
and what is that, 10
million in each rod.

595
00:35:33,710 --> 00:35:37,320
And you know how many rods
there are in the average eye?

596
00:35:37,320 --> 00:35:44,840
About 120 million, and about
five or six million cones.

597
00:35:44,840 --> 00:35:46,850
And you're talking about
unbelievable numbers.

598
00:35:46,850 --> 00:35:49,060
It's just hard to comprehend.

599
00:35:49,060 --> 00:35:51,860
But at any rate,
the molecules here,

600
00:35:51,860 --> 00:35:57,670
then, are two different states--
bleached and unbleached.

601
00:35:57,670 --> 00:36:01,230
What that means is
when a molecule shifts

602
00:36:01,230 --> 00:36:03,050
from being unbleached
to bleached

603
00:36:03,050 --> 00:36:06,310
as a result of a
photon hitting it,

604
00:36:06,310 --> 00:36:13,960
that changes the overall--
I'm talking about, of course,

605
00:36:13,960 --> 00:36:18,970
hundreds of molecules,
typically-- the overall degree

606
00:36:18,970 --> 00:36:22,100
of de- and hyper-polarization
of the cell.

607
00:36:22,100 --> 00:36:25,150
And that will then determine
whether the cell is going

608
00:36:25,150 --> 00:36:27,220
to communicate to the
next elements below,

609
00:36:27,220 --> 00:36:30,220
and I'll describe that in much
more detail in just a minute.

610
00:36:31,370 --> 00:36:35,620
But it gives you an idea of how
unbelievably complex this is.

611
00:36:35,620 --> 00:36:37,580
Yet another
complexity is that you

612
00:36:37,580 --> 00:36:44,520
have this huge number of rods,
but if you think about it,

613
00:36:44,520 --> 00:36:51,820
will these rods be there
in each in these rods

614
00:36:51,820 --> 00:36:55,940
with each of these disks
persist for a lifetime?

615
00:36:58,180 --> 00:37:00,970
So the average lifetime
is 80 or 90 years.

616
00:37:00,970 --> 00:37:06,200
Well these same guys exist
in the eye all the time?

617
00:37:06,200 --> 00:37:07,720
Well, the answer is no.

618
00:37:07,720 --> 00:37:11,270
What happens is there is yet
another process, amazingly,

619
00:37:11,270 --> 00:37:21,810
that replaces in steps these
disks in the rods, usually

620
00:37:21,810 --> 00:37:24,050
a few disks every few days.

621
00:37:24,050 --> 00:37:26,670
So it's a constant,
dynamic process

622
00:37:26,670 --> 00:37:35,350
keeping your rods, if not your
whole body, young, so to speak.

623
00:37:35,350 --> 00:37:37,350
All right, so anyway,
this gives you

624
00:37:37,350 --> 00:37:39,765
sort of a crude sense of the
complexity of this system.

625
00:37:41,450 --> 00:37:44,940
But now we are going
to shift over, and talk

626
00:37:44,940 --> 00:37:49,605
about the retinal
ganglion cells.

627
00:37:51,160 --> 00:37:53,430
And then we'll come
back, and we talk

628
00:37:53,430 --> 00:37:58,330
in a bit more detail about the
preretinal ganglionic elements.

629
00:37:58,330 --> 00:38:02,780
Here is a set of
pictures created

630
00:38:02,780 --> 00:38:07,060
by a wonderful scientist
called [? Poyak ?], showing you

631
00:38:07,060 --> 00:38:10,820
different classes of
types, if you will,

632
00:38:10,820 --> 00:38:12,890
of retinal ganglion cells.

633
00:38:12,890 --> 00:38:16,690
The top row here
have been named,

634
00:38:16,690 --> 00:38:19,990
because of their small size
and the small dendritic arbors,

635
00:38:19,990 --> 00:38:22,500
have been named midget cells.

636
00:38:24,460 --> 00:38:29,660
Whereas these ones below--
some of them, at least--

637
00:38:29,660 --> 00:38:32,770
are much, much bigger and have
much larger dendritic arbors,

638
00:38:32,770 --> 00:38:36,180
as you can see here, have
been called parasol cells.

639
00:38:36,180 --> 00:38:38,290
And why they've been
called parasol cells

640
00:38:38,290 --> 00:38:40,560
I'll explain to you
in just a minute.

641
00:38:40,560 --> 00:38:47,280
Now this set of pictures was
created by the Golgi stain

642
00:38:47,280 --> 00:38:53,430
that I mentioned to you before,
that was invented by Golgi,

643
00:38:53,430 --> 00:38:56,670
and extensively
studied by Cajal.

644
00:38:57,710 --> 00:38:59,350
And for this
incredible work they

645
00:38:59,350 --> 00:39:01,950
had done, a lot of
it on the retina,

646
00:39:01,950 --> 00:39:06,560
they received the
Nobel Prize in 1906.

647
00:39:06,560 --> 00:39:10,720
So what this disclosed,
then-- an initial view--

648
00:39:10,720 --> 00:39:14,450
was that there are different
classes of retinal ganglion

649
00:39:14,450 --> 00:39:15,160
cells.

650
00:39:15,160 --> 00:39:16,600
And so therefore,
the big question

651
00:39:16,600 --> 00:39:19,660
came up, why do we have
different classes of cells

652
00:39:19,660 --> 00:39:21,630
like that, retinal
ganglion cells?

653
00:39:21,630 --> 00:39:23,320
What do they do?

654
00:39:23,320 --> 00:39:25,490
Well, initially
some people again--

655
00:39:25,490 --> 00:39:26,940
which is good-- I
think one should

656
00:39:26,940 --> 00:39:29,000
be resistant to
change and new ideas

657
00:39:29,000 --> 00:39:31,160
until they are
really proven well.

658
00:39:35,440 --> 00:39:38,560
Some people argue that
[INAUDIBLE] just continue,

659
00:39:38,560 --> 00:39:42,800
but then it became evident
that these are indeed

660
00:39:42,800 --> 00:39:44,370
different classes.

661
00:39:44,370 --> 00:39:48,080
And I will present some
evidence to prove that.

662
00:39:48,080 --> 00:39:49,790
So there are actually
quite a number

663
00:39:49,790 --> 00:39:51,980
of different classes of
retinal ganglion cells

664
00:39:51,980 --> 00:39:53,750
that perform different jobs.

665
00:39:53,750 --> 00:39:56,290
We're going to try to understand
what are the different jobs

666
00:39:56,290 --> 00:39:57,550
that they perform.

667
00:39:57,550 --> 00:39:59,590
And we're going to
concentrate mostly

668
00:39:59,590 --> 00:40:04,370
on these midget
and parasol cells,

669
00:40:04,370 --> 00:40:08,440
because they are by far the
most numerous in the retina,

670
00:40:08,440 --> 00:40:13,130
and make a major, major
contribution to overall vision.

671
00:40:13,130 --> 00:40:17,970
All right, so now let's
look at the physiology

672
00:40:17,970 --> 00:40:19,344
of retinal ganglion cells.

673
00:40:19,344 --> 00:40:21,760
Now that we know we have these
different kinds of ganglion

674
00:40:21,760 --> 00:40:26,010
cells, how do we find
out what they do?

675
00:40:26,010 --> 00:40:30,820
Well what was recognized
in the early portions

676
00:40:30,820 --> 00:40:38,450
of the 20th century was that to
find out what these cells do,

677
00:40:38,450 --> 00:40:42,460
one needs to study
their activity.

678
00:40:42,460 --> 00:40:44,780
And to study their
activity, what

679
00:40:44,780 --> 00:40:49,600
was developed after
many different trials,

680
00:40:49,600 --> 00:40:53,890
is to either record
from individual axons,

681
00:40:53,890 --> 00:40:58,210
or to use a microelectrode that
you could put into the eye,

682
00:40:58,210 --> 00:41:02,680
and record from individual
cells, and see what they do.

683
00:41:02,680 --> 00:41:06,990
Well, the first person who
did this was Keffer Hartline.

684
00:41:08,060 --> 00:41:09,770
And for the beautiful
work he had done,

685
00:41:09,770 --> 00:41:13,660
he received the
Nobel Prize in 1967.

686
00:41:13,660 --> 00:41:17,550
Now what he did-- he was
a tremendous surgeon.

687
00:41:17,550 --> 00:41:20,250
And so what he did-- he did
this in primitive animals

688
00:41:20,250 --> 00:41:25,790
like frogs-- he would
take the optic nerve,

689
00:41:25,790 --> 00:41:30,920
and he would dissect one
fiber from the optic nerve,

690
00:41:30,920 --> 00:41:34,230
hook it up, and record
electrically from it.

691
00:41:34,230 --> 00:41:37,580
And then what he would
do, he would shine a light

692
00:41:37,580 --> 00:41:44,300
into the eye, move it around
until this cell generated

693
00:41:44,300 --> 00:41:45,840
action potentials.

694
00:41:45,840 --> 00:41:47,500
That was just a
very small region,

695
00:41:47,500 --> 00:41:50,120
because as I've
mentioned, each cell

696
00:41:50,120 --> 00:41:52,100
in the retina--
each ganglion cell--

697
00:41:52,100 --> 00:41:55,600
only sees a teeny, teeny, little
portion of the visual field.

698
00:41:56,930 --> 00:41:59,410
So that's how he
did his experiments.

699
00:41:59,410 --> 00:42:02,340
And he made some really
remarkable discoveries

700
00:42:02,340 --> 00:42:05,720
which further then
were elaborated

701
00:42:05,720 --> 00:42:07,530
by using slightly
different methods.

702
00:42:07,530 --> 00:42:09,280
A slightly different
method, first of all,

703
00:42:09,280 --> 00:42:13,050
was to put a microelectrode
into the eye.

704
00:42:13,050 --> 00:42:14,930
And I've mentioned
that to you before.

705
00:42:14,930 --> 00:42:20,620
The initial microelectrode
was a very fine, tube of glass

706
00:42:20,620 --> 00:42:23,680
which was heated and then
pulled until the tip was only

707
00:42:23,680 --> 00:42:26,370
about a micron in
diameter, which

708
00:42:26,370 --> 00:42:31,230
enabled you to record from
individual cells in the retina,

709
00:42:31,230 --> 00:42:33,270
or for that matter,
anywhere in the brain.

710
00:42:34,370 --> 00:42:36,960
Now another change in
the methods that emerged

711
00:42:36,960 --> 00:42:43,910
is that instead of shining a
light directly into the eye,

712
00:42:43,910 --> 00:42:49,170
people use reflected
light, because in the world

713
00:42:49,170 --> 00:42:52,150
most of what we see
is reflected light.

714
00:42:52,150 --> 00:42:54,680
Of course, nowadays this
has changed a little bit,

715
00:42:54,680 --> 00:42:58,620
because we have
computers and we have

716
00:42:58,620 --> 00:43:02,920
TV, which were set up in
such a way as to mimic

717
00:43:02,920 --> 00:43:05,450
reflected light, if you will.

718
00:43:05,450 --> 00:43:08,410
But in the natural
world for the most part,

719
00:43:08,410 --> 00:43:11,170
we see reflected light.

720
00:43:11,170 --> 00:43:15,240
All right, so when this was
done-- this initial work that

721
00:43:15,240 --> 00:43:19,190
was done by Keffer Hartline
for which he received the Nobel

722
00:43:19,190 --> 00:43:25,470
Prize-- that he discovered
three major classes of cells

723
00:43:25,470 --> 00:43:28,730
based on that recording
of this, which he called,

724
00:43:28,730 --> 00:43:31,290
first of all, on
cells, secondly off

725
00:43:31,290 --> 00:43:34,240
cells, and then
thirdly on-off cells.

726
00:43:35,400 --> 00:43:37,300
Now the on cells
were those which,

727
00:43:37,300 --> 00:43:41,990
when you shown light into their
so-called receptive field area,

728
00:43:41,990 --> 00:43:44,000
they would discharge vigorously.

729
00:43:44,000 --> 00:43:48,680
You turn the light on here, and
you can see the on cell fires.

730
00:43:49,760 --> 00:43:53,000
An off cell, on the other
hand, fired when you turned off

731
00:43:53,000 --> 00:43:57,090
the light using this
particular method.

732
00:43:57,090 --> 00:43:59,610
And on-off cells
were those that fired

733
00:43:59,610 --> 00:44:02,590
both at the onset and the
termination of the light.

734
00:44:02,590 --> 00:44:04,750
And that's why they were
called, and are still

735
00:44:04,750 --> 00:44:06,940
called to this day,
on and off cells.

736
00:44:08,860 --> 00:44:11,340
So that was an
incredible discovery,

737
00:44:11,340 --> 00:44:16,140
and has created
a major evolution

738
00:44:16,140 --> 00:44:19,650
in the study of
the visual system.

739
00:44:20,770 --> 00:44:27,750
All right, so now doing this
kind of work in more detail,

740
00:44:27,750 --> 00:44:30,360
using especially
mostly reflected

741
00:44:30,360 --> 00:44:36,430
light, several other major
discoveries were made.

742
00:44:36,430 --> 00:44:42,030
A fellow at Harvard
in his later years,

743
00:44:42,030 --> 00:44:46,040
originally at Johns Hopkins, did
experiments carefully studying

744
00:44:46,040 --> 00:44:50,460
the receptive field
structure of these cells.

745
00:44:50,460 --> 00:44:53,410
He would record, in this
case with microelectrodes,

746
00:44:53,410 --> 00:44:57,240
from a single cell--
retinal ganglion cell--

747
00:44:57,240 --> 00:45:00,430
and then see how it responded
when he fiddled around

748
00:45:00,430 --> 00:45:03,360
with the light in
the receptive field.

749
00:45:03,360 --> 00:45:05,760
So imagine, then,
that the eye is fixed.

750
00:45:05,760 --> 00:45:06,990
It's looking out.

751
00:45:06,990 --> 00:45:10,390
You're putting an electrode,
in this case, into the eye

752
00:45:10,390 --> 00:45:12,660
or wherever you put
it, and then you

753
00:45:12,660 --> 00:45:15,590
can use lights, maybe
like a projector,

754
00:45:15,590 --> 00:45:19,210
move it around to find out
where the receptive field is.

755
00:45:19,210 --> 00:45:22,750
And then you can present
a small spot of light,

756
00:45:22,750 --> 00:45:25,290
make it a big spot, make
it different colors,

757
00:45:25,290 --> 00:45:26,740
and so on, and so on.

758
00:45:26,740 --> 00:45:30,380
So when this was done,
a remarkable discovery

759
00:45:30,380 --> 00:45:38,140
was made by Kuffler, namely that
the so-called on and off cells

760
00:45:38,140 --> 00:45:42,360
were not just responding to the
onset and termination of light.

761
00:45:42,360 --> 00:45:46,600
Instead, what they did was
they responded vigorously

762
00:45:46,600 --> 00:45:50,380
to a small spot in the center of
the so-called receptive field,

763
00:45:50,380 --> 00:45:53,090
but if you used a
large spot like that,

764
00:45:53,090 --> 00:45:54,790
there was much
less of a response.

765
00:45:56,970 --> 00:45:59,400
And that was true also
for the off cells,

766
00:45:59,400 --> 00:46:01,130
and for the on-off cells.

767
00:46:01,130 --> 00:46:03,240
So the surround you
can think of as being

768
00:46:03,240 --> 00:46:06,330
inhibitory with
respect to the center.

769
00:46:06,330 --> 00:46:11,000
And so the big question
arose, why on earth

770
00:46:11,000 --> 00:46:15,020
did this complex
arrangement evolve

771
00:46:15,020 --> 00:46:19,270
of having cells that have not
only an excitatory center,

772
00:46:19,270 --> 00:46:21,155
but also have an
inhibitory surround?

773
00:46:22,420 --> 00:46:25,070
And now I'm going to let you
think about this for a minute,

774
00:46:25,070 --> 00:46:28,320
and eventually I
will tell you as

775
00:46:28,320 --> 00:46:29,905
to why we have
this organization.

776
00:46:31,120 --> 00:46:33,690
Because we're going to
devote a whole session

777
00:46:33,690 --> 00:46:38,790
to the on and off systems,
and also a complete session

778
00:46:38,790 --> 00:46:40,690
to the midget and parasol cells.

779
00:46:40,690 --> 00:46:43,630
And that's when we are going
to discuss this in detail.

780
00:46:43,630 --> 00:46:49,220
Now, the important thing as
you learn about these things

781
00:46:49,220 --> 00:46:52,150
is to be an active thinker.

782
00:46:52,150 --> 00:46:54,757
And if something comes up, you
say, well, why did this happen?

783
00:46:54,757 --> 00:46:55,590
Why did that happen?

784
00:46:55,590 --> 00:46:56,140
Why is this?

785
00:46:56,140 --> 00:46:56,910
Why is that?

786
00:46:56,910 --> 00:46:58,860
When you actively
think about that,

787
00:46:58,860 --> 00:47:00,650
then it becomes interesting.

788
00:47:00,650 --> 00:47:03,430
And also, once the
answers come, it

789
00:47:03,430 --> 00:47:07,670
becomes insightful and
exciting to understand it.

790
00:47:07,670 --> 00:47:14,750
OK, so now a series
of investigators

791
00:47:14,750 --> 00:47:17,370
did experiments in
which they-- this sounds

792
00:47:17,370 --> 00:47:19,630
like a fairly simple
experiment-- in which they

793
00:47:19,630 --> 00:47:24,930
did labeling of just the
cell bodies in the retina,

794
00:47:24,930 --> 00:47:26,840
and did what is
called a whole mounts.

795
00:47:26,840 --> 00:47:27,980
What's a whole mount?

796
00:47:27,980 --> 00:47:29,110
You take the retina.

797
00:47:29,110 --> 00:47:30,200
You flatten it out.

798
00:47:30,200 --> 00:47:35,030
Then you look at this whole
layout through a microscope.

799
00:47:35,030 --> 00:47:37,650
And if you label
these cells-- and this

800
00:47:37,650 --> 00:47:41,080
was a so-called Nissl stain
that stains the cell bodies,

801
00:47:41,080 --> 00:47:47,870
but doesn't stain the processes
like the dendrite of the axons.

802
00:47:47,870 --> 00:47:51,440
What they found was-- they
did this quantitatively--

803
00:47:51,440 --> 00:47:56,600
they could distinguish three
very distinct different classes

804
00:47:56,600 --> 00:47:57,675
of cells.

805
00:47:57,675 --> 00:48:00,020
It turned out later that
there were more than three,

806
00:48:00,020 --> 00:48:02,830
but in this case, just
doing it crudely like that,

807
00:48:02,830 --> 00:48:04,760
you can see those
big cells-- that one,

808
00:48:04,760 --> 00:48:08,570
that one, that one, that one,
and so on-- maybe one out

809
00:48:08,570 --> 00:48:12,000
of seven or eight in this
sample-- are these huge cells.

810
00:48:12,000 --> 00:48:15,050
And then you have
some smaller cells.

811
00:48:15,050 --> 00:48:17,690
And then you also
some tiny cells.

812
00:48:17,690 --> 00:48:19,570
And if you did this
quantitatively,

813
00:48:19,570 --> 00:48:25,840
they argued that they formed
distinct separate populations.

814
00:48:25,840 --> 00:48:27,570
They were not a continuum.

815
00:48:27,570 --> 00:48:29,840
All right, so then
that, of course,

816
00:48:29,840 --> 00:48:33,780
is a basic requirement to
prove that indeed, these

817
00:48:33,780 --> 00:48:35,810
are different
classes of cells that

818
00:48:35,810 --> 00:48:38,590
probably perform different jobs.

819
00:48:38,590 --> 00:48:42,480
So what can you do to
prove this further?

820
00:48:42,480 --> 00:48:46,680
Well, subsequent to this
kind of work, people

821
00:48:46,680 --> 00:48:53,100
began to look at another
interesting question about how

822
00:48:53,100 --> 00:48:54,700
these retinal
ganglion cells send

823
00:48:54,700 --> 00:48:56,630
a signal to the
central nervous system.

824
00:48:57,760 --> 00:48:59,890
And it's been discovered,
not just in the retina,

825
00:48:59,890 --> 00:49:06,830
but in many other parts of
the human and animal body,

826
00:49:06,830 --> 00:49:16,140
that the rapidity with which
an axon can send its action

827
00:49:16,140 --> 00:49:19,830
potentials down from one
location to the next--

828
00:49:19,830 --> 00:49:27,300
from the cell body, say,
to the axon terminals--

829
00:49:27,300 --> 00:49:30,650
very much depends on
the size of the cell

830
00:49:30,650 --> 00:49:31,910
and the size of the axon.

831
00:49:33,020 --> 00:49:38,160
So if you have a small
cell and a very thin axon,

832
00:49:38,160 --> 00:49:40,900
the conduction is
slower than if you

833
00:49:40,900 --> 00:49:44,240
have a big cell and a big axon.

834
00:49:44,240 --> 00:49:47,620
So if that's the
case, people argued

835
00:49:47,620 --> 00:49:52,540
that if you were to
take the optic nerve

836
00:49:52,540 --> 00:49:56,330
and stimulate it at one
side and a little bit

837
00:49:56,330 --> 00:50:00,240
down, maybe back
towards the retina,

838
00:50:00,240 --> 00:50:04,580
record and see what kind
of overall general--

839
00:50:04,580 --> 00:50:08,430
not action potentials, but
overall activation takes place.

840
00:50:08,430 --> 00:50:13,190
And when that was done, a very
amazing discovery was made.

841
00:50:13,190 --> 00:50:18,950
In this case, you can see
we have two major dips here.

842
00:50:18,950 --> 00:50:20,680
This is time, OK.

843
00:50:20,680 --> 00:50:22,740
Here is electrical stimulation.

844
00:50:22,740 --> 00:50:25,470
Here's the activation
of hundreds and hundreds

845
00:50:25,470 --> 00:50:26,460
of fibers.

846
00:50:26,460 --> 00:50:29,130
And it shows that they
formed distinct groups--

847
00:50:29,130 --> 00:50:32,840
a very rapidly conducting group,
a slower conducting group,

848
00:50:32,840 --> 00:50:35,350
and a very slow,
more diffuse group.

849
00:50:36,470 --> 00:50:41,460
So then it was subsequent--
I'll talk about that later-- it

850
00:50:41,460 --> 00:50:44,230
was subsequently
established that these very

851
00:50:44,230 --> 00:50:51,720
rapidly conducting axons come
from the so-called parasol

852
00:50:51,720 --> 00:50:52,660
cells.

853
00:50:52,660 --> 00:50:56,330
The medium conducting ones
come from the midget cells,

854
00:50:56,330 --> 00:50:57,570
all right?

855
00:50:57,570 --> 00:51:00,660
I'll tell you exactly
how that was established,

856
00:51:00,660 --> 00:51:03,500
but now that has
become a solid fact.

857
00:51:03,500 --> 00:51:07,750
OK, so now we go back to the
anatomy of these cell types,

858
00:51:07,750 --> 00:51:13,890
if you look at them at
equivalent distances,

859
00:51:13,890 --> 00:51:19,490
over eccentricity, one,
three, and 5.7 degrees,

860
00:51:19,490 --> 00:51:23,560
here are examples
of the midget cell

861
00:51:23,560 --> 00:51:26,270
dendritic orbits
[INAUDIBLE] looking straight

862
00:51:26,270 --> 00:51:28,960
down at the cell.

863
00:51:28,960 --> 00:51:34,300
You can see these are very
constricted dendritic orbits.

864
00:51:34,300 --> 00:51:36,930
Now by contrast,
the parasol cells

865
00:51:36,930 --> 00:51:41,322
have much, much larger-- three
times larger in diameter--

866
00:51:41,322 --> 00:51:42,030
dendritic orbits.

867
00:51:43,320 --> 00:51:45,670
And now I can tell
you-- which I deferred

868
00:51:45,670 --> 00:51:48,320
before-- is that if
you look at these,

869
00:51:48,320 --> 00:51:50,510
the reason they're called
parasol cells is what?

870
00:51:51,770 --> 00:51:54,810
Because it looks
like an umbrella.

871
00:51:55,960 --> 00:51:57,000
OK.

872
00:51:57,000 --> 00:52:01,440
So this looks like an umbrella,
and of course that's parasol.

873
00:52:01,440 --> 00:52:04,310
So that's why they are
called parasol cells.

874
00:52:04,310 --> 00:52:06,242
And these guys, much
less interesting,

875
00:52:06,242 --> 00:52:08,200
are called midget cells
because they're midget.

876
00:52:08,200 --> 00:52:09,870
They're small.

877
00:52:09,870 --> 00:52:13,310
All right, so that
then established--

878
00:52:13,310 --> 00:52:15,910
this work was done
just 20 years ago--

879
00:52:15,910 --> 00:52:19,690
it was established that you
have indeed these two very, very

880
00:52:19,690 --> 00:52:22,380
distinct populations of cells.

881
00:52:22,380 --> 00:52:25,560
So now the big
question came up, why

882
00:52:25,560 --> 00:52:27,360
do we have on and off cells?

883
00:52:27,360 --> 00:52:29,760
Why do we have midget
and parasol cells?

884
00:52:29,760 --> 00:52:31,460
And as I say, we
are going to discuss

885
00:52:31,460 --> 00:52:33,180
this in considerable detail.

886
00:52:33,180 --> 00:52:36,360
But let me just give you
a crude introduction.

887
00:52:36,360 --> 00:52:38,540
If you look at
the parasol cell--

888
00:52:38,540 --> 00:52:41,380
obviously, because of the
huge dendritic orbit--

889
00:52:41,380 --> 00:52:46,070
they have large, relatively
speaking, much larger receptive

890
00:52:46,070 --> 00:52:49,250
fields-- three times
bigger than midget cells.

891
00:52:49,250 --> 00:52:54,370
The midget cells are so
specific that each cell

892
00:52:54,370 --> 00:52:58,570
in central retina gets input for
the excitatory center from just

893
00:52:58,570 --> 00:53:00,870
one single cone.

894
00:53:00,870 --> 00:53:01,870
Now what does that mean?

895
00:53:03,390 --> 00:53:06,070
What I didn't emphasize
before, but I'm sure

896
00:53:06,070 --> 00:53:10,380
you all know, that we have
three major classes of cones,

897
00:53:10,380 --> 00:53:14,570
which some people call
red, green, and blue.

898
00:53:14,570 --> 00:53:18,050
And then when you say that, the
real aficionados say, come on,

899
00:53:18,050 --> 00:53:19,860
you can't say that.

900
00:53:19,860 --> 00:53:23,530
You're supposed to say short,
medium, and long wavelength

901
00:53:23,530 --> 00:53:24,810
sensitive neurons.

902
00:53:24,810 --> 00:53:26,960
But we'll just call them
red, green, and blue.

903
00:53:26,960 --> 00:53:30,230
Now the reason, then,
is that the center

904
00:53:30,230 --> 00:53:34,890
of one class of midget cells
gets input only from red cones,

905
00:53:34,890 --> 00:53:37,740
another from green cones,
and some from blue cones.

906
00:53:37,740 --> 00:53:39,900
I'll show that in more
detail in a minute.

907
00:53:39,900 --> 00:53:41,815
So we have these three subtypes.

908
00:53:42,980 --> 00:53:46,530
And so therefore, you can
say that the midget cells,

909
00:53:46,530 --> 00:53:48,400
because they're so
teeny, they should

910
00:53:48,400 --> 00:53:50,590
be able to see fine detail.

911
00:53:50,590 --> 00:53:53,560
And because of this
wiring, they should

912
00:53:53,560 --> 00:53:55,780
be able to tell you about color.

913
00:53:55,780 --> 00:53:58,720
By contrast, if you look
at the parasol cells,

914
00:53:58,720 --> 00:54:01,610
they get a mix of inputs,
both in the center

915
00:54:01,610 --> 00:54:03,960
and their surround,
so it's unlikely

916
00:54:03,960 --> 00:54:06,070
that they can tell
you about color.

917
00:54:06,070 --> 00:54:08,960
And we'll examine that's
in much more detail soon.

918
00:54:08,960 --> 00:54:11,130
All right, so now the
other interesting thing

919
00:54:11,130 --> 00:54:14,040
about this system is
it makes you speculate

920
00:54:14,040 --> 00:54:15,860
as to why we have
them-- and we'll

921
00:54:15,860 --> 00:54:19,220
elaborate on that-- is that the
midget system, when you turn

922
00:54:19,220 --> 00:54:21,910
on the light, when you shine
a small spot in the center

923
00:54:21,910 --> 00:54:25,520
receptive field, gives a
fairly sustained response

924
00:54:25,520 --> 00:54:29,400
for the brief time that was
maybe a second of two, OK?

925
00:54:29,400 --> 00:54:34,260
But the parasol system gives
you only a burst, like that,

926
00:54:34,260 --> 00:54:35,800
instead of a sustained response.

927
00:54:36,850 --> 00:54:38,190
So think about that.

928
00:54:38,190 --> 00:54:40,340
Say why would we
have two systems

929
00:54:40,340 --> 00:54:43,120
which are not only different
in size, but also different

930
00:54:43,120 --> 00:54:44,695
in the profile of
their responses?

931
00:54:46,190 --> 00:54:48,810
And then, once you have
this question in your mind,

932
00:54:48,810 --> 00:54:52,130
then you'll be eager to find
out when we talk about that time

933
00:54:52,130 --> 00:54:53,610
after next.

934
00:54:53,610 --> 00:54:56,240
OK, so now let's go
back to the beginning

935
00:54:56,240 --> 00:54:58,760
here, and talk about the
photoreceptors in just a bit

936
00:54:58,760 --> 00:55:04,750
more detail, so you can
get a better picture of it.

937
00:55:04,750 --> 00:55:06,820
Here, as I mentioned to
you before, in the fovea,

938
00:55:06,820 --> 00:55:12,143
you have a very, very tightly
packed cone photoreceptors.

939
00:55:13,260 --> 00:55:15,825
They're almost hexagonally
shaped rather than round,

940
00:55:15,825 --> 00:55:18,100
they're so tightly packed.

941
00:55:18,100 --> 00:55:20,560
But then when you
go five degrees out,

942
00:55:20,560 --> 00:55:24,050
you first of all see that the
cones got to be much larger.

943
00:55:25,220 --> 00:55:29,840
And now you have an intermixture
of rods there that you can see.

944
00:55:31,750 --> 00:55:35,930
| in this region, you can
process both information

945
00:55:35,930 --> 00:55:38,950
at low illumination levels
and high illumination levels,

946
00:55:38,950 --> 00:55:46,020
whereas here as Keffer
Hartline had proven,

947
00:55:46,020 --> 00:55:52,075
you can only see under fairly
bright, normal illumination

948
00:55:52,075 --> 00:55:52,575
conditions.

949
00:55:56,980 --> 00:56:01,084
OK, so now if you go even
further out in the retina--

950
00:56:01,084 --> 00:56:03,000
this is a beautiful,
three-dimensional looking

951
00:56:03,000 --> 00:56:06,365
picture-- it shows the rods
and the cones in the periphery.

952
00:56:06,365 --> 00:56:09,800
There are many, many
more rods per unit area,

953
00:56:09,800 --> 00:56:12,340
and the cones have
gotten even bigger.

954
00:56:13,480 --> 00:56:18,700
And so obviously, the ability
for the cones to process fine

955
00:56:18,700 --> 00:56:23,990
information has been
degraded as a result.

956
00:56:23,990 --> 00:56:26,530
Now again, as I've
mentioned to you before,

957
00:56:26,530 --> 00:56:32,460
we have in humans about 120
million rods and five million

958
00:56:32,460 --> 00:56:34,390
cones in each retina.

959
00:56:35,490 --> 00:56:38,610
OK, so now if you look at
the overall distribution--

960
00:56:38,610 --> 00:56:40,410
this is not too
important-- you can

961
00:56:40,410 --> 00:56:43,990
see that as you go
away from the fovea,

962
00:56:43,990 --> 00:56:49,080
there's a very rapid decline
in the number of cones.

963
00:56:49,080 --> 00:56:50,940
This is the hash line.

964
00:56:50,940 --> 00:56:54,510
And then there are
no rods in the fovea,

965
00:56:54,510 --> 00:56:57,550
but they increase rapidly,
and then they fall off.

966
00:56:57,550 --> 00:57:00,320
So that's sort of the
overall distribution.

967
00:57:00,320 --> 00:57:03,420
And therefore, you can see
fine detail in the center,

968
00:57:03,420 --> 00:57:08,500
and you're very sensitive
for night vision

969
00:57:08,500 --> 00:57:14,940
not in the far periphery, but
sort of in the mid section

970
00:57:14,940 --> 00:57:21,850
here, anywhere between 20 and
40 degrees from the fovea.

971
00:57:21,850 --> 00:57:26,210
Let me remind you, because I'm
sure you know all this already,

972
00:57:26,210 --> 00:57:27,940
a few facts, OK?

973
00:57:32,960 --> 00:57:34,960
I will just write down a
couple of these things.

974
00:57:34,960 --> 00:57:42,447
First of all, what is
degree of visual angle?

975
00:57:42,447 --> 00:57:43,405
That's a good question.

976
00:57:44,640 --> 00:57:47,050
What does it mean on
the retinal surface?

977
00:57:47,050 --> 00:57:50,590
Just let me give you an
interesting mnemonic.

978
00:57:50,590 --> 00:57:53,640
If you stick out
your arm like that,

979
00:57:53,640 --> 00:57:56,150
and you look at
your thumbnail, that

980
00:57:56,150 --> 00:58:00,605
imprints one degree of visual
angle on the retinal surface.

981
00:58:01,971 --> 00:58:02,470
OK?

982
00:58:03,850 --> 00:58:08,760
Now we are going to talk a lot
about very small measurements.

983
00:58:08,760 --> 00:58:11,590
And so when we talk
about small measurement,

984
00:58:11,590 --> 00:58:14,400
I mentioned already that
the tip of a microelectrode

985
00:58:14,400 --> 00:58:15,900
is about one micron.

986
00:58:15,900 --> 00:58:19,615
And what is a
micron, anybody know?

987
00:58:19,615 --> 00:58:21,100
Yes.

988
00:58:21,100 --> 00:58:23,080
AUDIENCE: 10 to the
negative 6 [INAUDIBLE].

989
00:58:24,570 --> 00:58:31,240
PROFESSOR: OK, one micron is
one-thousandth of a millimeter,

990
00:58:31,240 --> 00:58:31,740
OK?

991
00:58:34,650 --> 00:58:38,000
Now, talking about that in terms
of millimeters, which you're

992
00:58:38,000 --> 00:58:41,450
going to do a lot-- that's the
greatest convention-- you need

993
00:58:41,450 --> 00:58:46,760
to also know what is the
relationship between inches

994
00:58:46,760 --> 00:58:50,190
and millimeters, centimeters?

995
00:58:50,190 --> 00:58:58,390
OK, so if you take one inch--
you all know what an inch is,

996
00:58:58,390 --> 00:58:59,656
I guess.

997
00:58:59,656 --> 00:59:01,530
Roughly, if you look at
your thumbnail again,

998
00:59:01,530 --> 00:59:03,650
it's maybe a little
bit under an inch.

999
00:59:05,000 --> 00:59:17,180
Now 1 inch equals
2.54 millimeters.

1000
00:59:17,180 --> 00:59:20,240
So that's a very interesting
mnemonic to remember.

1001
00:59:20,240 --> 00:59:23,570
So whenever you look
at various things

1002
00:59:23,570 --> 00:59:27,140
and try to understand
size, you may

1003
00:59:27,140 --> 00:59:32,170
need to make that conversion,
especially if you have always

1004
00:59:32,170 --> 00:59:34,720
thought about things
in terms of inches.

1005
00:59:34,720 --> 00:59:35,400
Yes.

1006
00:59:35,400 --> 00:59:37,170
AUDIENCE: I think you
meant to write centimeters.

1007
00:59:37,170 --> 00:59:37,770
PROFESSOR: What?

1008
00:59:37,770 --> 00:59:39,618
AUDIENCE: Did you mean
to write centimeters?

1009
00:59:42,390 --> 00:59:49,995
PROFESSOR: You're right, 2.5
centimeters, 254 millimeters.

1010
00:59:51,020 --> 00:59:52,210
You're right, OK.

1011
00:59:52,210 --> 00:59:53,530
Sorry about that.

1012
00:59:53,530 --> 00:59:54,030
Very good.

1013
00:59:54,030 --> 00:59:55,410
Thank you for pointing that out.

1014
00:59:56,450 --> 00:59:59,850
OK, so now what
we're going to do

1015
00:59:59,850 --> 01:00:02,930
next is we're going
to move on, and I'll

1016
01:00:02,930 --> 01:00:06,190
tell you another clever
experiment that had been done.

1017
01:00:08,050 --> 01:00:10,480
These people here,
McCrane et al.,

1018
01:00:10,480 --> 01:00:14,790
developed a method which enabled
them to selectively label

1019
01:00:14,790 --> 01:00:16,270
the blue cells in the retina.

1020
01:00:17,500 --> 01:00:19,650
And this shows a
picture of that.

1021
01:00:19,650 --> 01:00:23,550
All the rest of them
are red and green cells.

1022
01:00:23,550 --> 01:00:38,700
So only one out of eight
cones is in the foveal area,

1023
01:00:38,700 --> 01:00:41,370
near foveal area, I should
say-- this is both the fovea

1024
01:00:41,370 --> 01:00:45,580
and the perifovea is black.

1025
01:00:45,580 --> 01:00:48,390
The rest of them are
red and green cones, OK?

1026
01:00:49,980 --> 01:00:54,030
Now let's become
specific about the facts

1027
01:00:54,030 --> 01:00:58,760
here, and tell you first of all
that one degree-- roughly one

1028
01:00:58,760 --> 01:01:02,820
degree, which I pointed
out is like a thumb,

1029
01:01:02,820 --> 01:01:07,400
comprises 200 microns
on the retinal surface.

1030
01:01:08,980 --> 01:01:15,005
The intercone distance in the
fovea is only 2.4 microns, OK?

1031
01:01:17,180 --> 01:01:23,470
Whereas when you-- per
square millimeter I already

1032
01:01:23,470 --> 01:01:28,810
mentioned this-- you
have 200,000 cones.

1033
01:01:28,810 --> 01:01:31,460
You go out five degrees,
it reduces tenfold.

1034
01:01:32,510 --> 01:01:35,230
And then-- this I already
pointed out to you

1035
01:01:35,230 --> 01:01:36,060
several times.

1036
01:01:36,060 --> 01:01:37,980
That's a good thing to remember.

1037
01:01:37,980 --> 01:01:39,680
And one thing I haven't
mentioned before

1038
01:01:39,680 --> 01:01:43,680
is that if you look
at 12 font letter, I,

1039
01:01:43,680 --> 01:01:47,020
and you look at it
at 23 centimeters,

1040
01:01:47,020 --> 01:01:48,820
that activates about 80 cones.

1041
01:01:48,820 --> 01:01:50,870
So that gives you
sort of a handle

1042
01:01:50,870 --> 01:01:54,480
on what is the nature
of activation there.

1043
01:01:54,480 --> 01:01:56,220
And I already said
that several times--

1044
01:01:56,220 --> 01:02:01,960
namely that each rod has 1,000
disks, and each disk in it

1045
01:02:01,960 --> 01:02:05,790
has 10,000 molecules.

1046
01:02:05,790 --> 01:02:11,360
OK, and as I've just mentioned,
one out of eight cones is blue.

1047
01:02:11,360 --> 01:02:13,540
The others are fairly
equal in number,

1048
01:02:13,540 --> 01:02:16,200
but varies from
person to person.

1049
01:02:16,200 --> 01:02:24,170
OK, so now we are
going to tell you

1050
01:02:24,170 --> 01:02:27,480
about yet another experiment.

1051
01:02:27,480 --> 01:02:30,470
And this is an incredibly
clever experiment

1052
01:02:30,470 --> 01:02:33,555
that was carried out
by Ursula Drager.

1053
01:02:35,092 --> 01:02:37,300
At that time, when she did
this beautiful experiment,

1054
01:02:37,300 --> 01:02:40,910
she did this at Harvard, at
the Harvard Medical School.

1055
01:02:40,910 --> 01:02:47,430
And what she did was this-- she
took the words Fiat Lux, put it

1056
01:02:47,430 --> 01:02:52,730
on a screen, and
exposed it that too

1057
01:02:52,730 --> 01:02:57,720
dark to the animal-- the
animal being a mouse, OK?

1058
01:02:57,720 --> 01:03:01,550
And then her procedure
enabled her to label,

1059
01:03:01,550 --> 01:03:06,080
using a monoclonal antibody
label, those cells that

1060
01:03:06,080 --> 01:03:08,290
had been activated by Fiat Lux.

1061
01:03:09,490 --> 01:03:15,270
OK, so this is, then, the actual
retina, flat-mount retina,

1062
01:03:15,270 --> 01:03:17,175
showing you what
had been activated.

1063
01:03:18,510 --> 01:03:20,950
Now that's
remarkable, beautiful,

1064
01:03:20,950 --> 01:03:24,230
and she came, and
gave a talk here

1065
01:03:24,230 --> 01:03:26,810
at MIT maybe about 15 years ago.

1066
01:03:28,660 --> 01:03:33,580
And when she presented this
picture, one of the students,

1067
01:03:33,580 --> 01:03:38,060
it wasn't any of you, I
guess, raised her hand

1068
01:03:38,060 --> 01:03:44,940
and was very upset-- said, but,
but, but-- and so Ursula said,

1069
01:03:44,940 --> 01:03:46,030
yes?

1070
01:03:46,030 --> 01:03:52,160
So what do you think
this listener asked?

1071
01:03:53,890 --> 01:03:56,450
Well, this was a
very active thinker.

1072
01:03:56,450 --> 01:04:00,712
This person said, but, but,
but, was this image right-side

1073
01:04:00,712 --> 01:04:01,795
up on the retinal surface?

1074
01:04:02,990 --> 01:04:04,730
That was the question.

1075
01:04:04,730 --> 01:04:08,700
And Ursula Drager said,
no, it was upside down.

1076
01:04:08,700 --> 01:04:12,980
I just rotated it 180
degrees so you can see it.

1077
01:04:12,980 --> 01:04:20,990
All right, well,
interestingly enough,

1078
01:04:20,990 --> 01:04:23,340
there were times when
this became an issue.

1079
01:04:23,340 --> 01:04:27,160
First of all, before
people knew about lenses

1080
01:04:27,160 --> 01:04:36,210
and that lenses turn images
upside down, and secondly,

1081
01:04:36,210 --> 01:04:39,990
because many people
didn't believe

1082
01:04:39,990 --> 01:04:42,226
that the images were
upside down in the retina.

1083
01:04:42,226 --> 01:04:44,225
I mean, if images are
upside down in the retina,

1084
01:04:44,225 --> 01:04:47,140
they're upside
down on your brain,

1085
01:04:47,140 --> 01:04:48,830
and yet the world
is right-side up.

1086
01:04:50,880 --> 01:04:52,210
What's going on?

1087
01:04:52,210 --> 01:04:59,980
So I would say about
eight or ten years ago, I

1088
01:04:59,980 --> 01:05:09,400
was asked to review a paper
for a high visibility journal--

1089
01:05:09,400 --> 01:05:15,770
I can't name the journal-- in
which the investigator very,

1090
01:05:15,770 --> 01:05:22,980
very vociferously proclaimed
that we have all been wrong,

1091
01:05:22,980 --> 01:05:25,140
and that the images are
right-side up on the retina

1092
01:05:25,140 --> 01:05:26,620
after all.

1093
01:05:26,620 --> 01:05:29,040
So what was his proof?

1094
01:05:29,040 --> 01:05:35,640
His proof was that he took an
ox eye, a slaughtered ox's eye,

1095
01:05:35,640 --> 01:05:38,350
and cut out a little piece
in the back of the eye,

1096
01:05:38,350 --> 01:05:41,780
and put a translucent
piece of paper over it,

1097
01:05:41,780 --> 01:05:44,870
and then presented a
stimulus out there.

1098
01:05:44,870 --> 01:05:47,740
And when he looked at
the back of the eye,

1099
01:05:47,740 --> 01:05:49,620
at the translucent
part, the image

1100
01:05:49,620 --> 01:05:53,630
was right-side up-- a little
bit blurred but right-side up.

1101
01:05:53,630 --> 01:05:57,421
And so he felt that he proved
that images are right-side

1102
01:05:57,421 --> 01:05:58,545
up on the retina after all.

1103
01:06:00,000 --> 01:06:01,140
Well, I was flabbergasted.

1104
01:06:02,220 --> 01:06:05,560
And it took me quite
a while to figure out

1105
01:06:05,560 --> 01:06:08,280
why this guy got what he
got, because he'd obviously

1106
01:06:08,280 --> 01:06:09,670
got what he got.

1107
01:06:09,670 --> 01:06:12,740
So can you guys think
of any reason for this?

1108
01:06:12,740 --> 01:06:14,806
How come he got what he got?

1109
01:06:14,806 --> 01:06:16,780
AUDIENCE: Wait, what did he do?

1110
01:06:16,780 --> 01:06:21,130
PROFESSOR: He took the
eye, OK, and in the back,

1111
01:06:21,130 --> 01:06:25,182
he cut out an opening and
put a transparency over it,

1112
01:06:25,182 --> 01:06:26,640
so that anything
came in there, you

1113
01:06:26,640 --> 01:06:31,070
could see what would
have been normally

1114
01:06:31,070 --> 01:06:33,610
on the retinal surface.

1115
01:06:33,610 --> 01:06:36,010
And it was a clever experiment--
no question about that.

1116
01:06:36,852 --> 01:06:38,435
So what was wrong
with the experiment?

1117
01:06:39,562 --> 01:06:41,110
Well, I'll tell you.

1118
01:06:41,110 --> 01:06:43,760
What was wrong with
the experiment was

1119
01:06:43,760 --> 01:06:46,040
back to what I told
you before, about

1120
01:06:46,040 --> 01:06:50,480
the way the lens works, right?

1121
01:06:50,480 --> 01:06:53,940
The lens works so that when
you look at something close,

1122
01:06:53,940 --> 01:06:58,550
the muscles of the lens are
such that it makes the lens fat.

1123
01:06:58,550 --> 01:07:09,200
And when the muscles relax,
it makes the lens thin, OK?

1124
01:07:09,200 --> 01:07:15,450
So that means-- just to
be graphic about it--

1125
01:07:15,450 --> 01:07:23,310
that if you have a thick lens,
you have a short focal length.

1126
01:07:23,310 --> 01:07:27,530
And if you have a
thin lens, you have

1127
01:07:27,530 --> 01:07:31,430
a much longer focal
length, like that, OK?

1128
01:07:31,430 --> 01:07:39,985
So now if you look at
the back of the eye

1129
01:07:39,985 --> 01:07:43,700
here, where the photoreceptors
are, in this case,

1130
01:07:43,700 --> 01:07:45,410
it looks like it
would be upside down.

1131
01:07:45,410 --> 01:07:47,400
But if they were in
the same distance here,

1132
01:07:47,400 --> 01:07:50,310
it would be blurred and it
would remain right-side up,

1133
01:07:50,310 --> 01:07:53,540
just like it is with a
magnifying glass, OK?

1134
01:07:53,540 --> 01:07:56,710
So what this guy
didn't know is the rule

1135
01:07:56,710 --> 01:08:03,430
of how the muscle works in
the eye to control the lens.

1136
01:08:03,430 --> 01:08:08,760
And when the animal was
killed, it no longer

1137
01:08:08,760 --> 01:08:12,440
had the lens at the
proper focal depth.

1138
01:08:14,080 --> 01:08:16,763
And therefore, the
image was right-side up.

1139
01:08:16,763 --> 01:08:18,179
So therefore, guess
what happened?

1140
01:08:20,000 --> 01:08:23,609
I wrote back to this
high visibility journal

1141
01:08:23,609 --> 01:08:27,200
saying that this guy is
wrong for this reason.

1142
01:08:28,420 --> 01:08:32,979
OK, so that was certainly
an interesting experience,

1143
01:08:32,979 --> 01:08:38,819
and that's why I managed to
learn a little bit about how

1144
01:08:38,819 --> 01:08:44,580
the lens works in the eye, which
is a remarkable way it works,

1145
01:08:44,580 --> 01:08:46,529
since it's so totally
different from what

1146
01:08:46,529 --> 01:08:49,840
we are familiar with
when it comes to cameras.

1147
01:08:49,840 --> 01:08:58,450
OK, so now let's go to the
next step, one down from your

1148
01:08:58,450 --> 01:09:00,939
from photoreceptors.

1149
01:09:00,939 --> 01:09:03,880
And we come to the
so-called bipolar cells.

1150
01:09:03,880 --> 01:09:06,779
Here's a series of pictures,
the so-called [? cat ?]

1151
01:09:06,779 --> 01:09:08,290
bipolar cells.

1152
01:09:08,290 --> 01:09:11,830
And what was discovered is
that the bipolar cells here

1153
01:09:11,830 --> 01:09:13,963
in the outer
plexiform layer, they

1154
01:09:13,963 --> 01:09:15,296
connect with the photoreceptors.

1155
01:09:16,425 --> 01:09:17,800
And in the inner
plexiform layer,

1156
01:09:17,800 --> 01:09:23,590
their axonal projections are
such that in the upper layer

1157
01:09:23,590 --> 01:09:33,040
here, which is actually layer A,
the cells there are OFF cells.

1158
01:09:33,040 --> 01:09:35,939
And the ones that
project below this here

1159
01:09:35,939 --> 01:09:37,710
are the so-called ON cells.

1160
01:09:37,710 --> 01:09:40,859
So there's a separation of
the so-called ON and OFF

1161
01:09:40,859 --> 01:09:44,990
cells at the bipolar cell level
in the manner in which they

1162
01:09:44,990 --> 01:09:45,960
connect.

1163
01:09:45,960 --> 01:09:50,069
And so therefore, those ganglion
cells that connect with these

1164
01:09:50,069 --> 01:09:51,479
are OFF ganglion cells.

1165
01:09:51,479 --> 01:09:55,860
And those that connect with
these are ON ganglion cells.

1166
01:09:57,070 --> 01:10:00,690
All right, so that's
basically the bipolar cells.

1167
01:10:00,690 --> 01:10:03,020
And then we come to
the horizontal cells.

1168
01:10:03,020 --> 01:10:06,746
Here's a beautiful
example of a Procion

1169
01:10:06,746 --> 01:10:12,580
yellow-labeled horizontal cell.

1170
01:10:12,580 --> 01:10:15,810
And if that is then
analyzed in detail,

1171
01:10:15,810 --> 01:10:17,295
it looks something like this.

1172
01:10:17,295 --> 01:10:24,410
This particular cell
has 15 connections

1173
01:10:24,410 --> 01:10:26,403
with the photoreceptors.

1174
01:10:27,630 --> 01:10:29,930
And these cells-- I might
as well anticipate--

1175
01:10:29,930 --> 01:10:32,460
we'll talk about that
in more detail later--

1176
01:10:32,460 --> 01:10:35,580
they play a central
role in providing

1177
01:10:35,580 --> 01:10:38,700
the surround inhibition
that I described to you

1178
01:10:38,700 --> 01:10:41,030
for those retinal
ganglion cells.

1179
01:10:41,030 --> 01:10:43,920
Then if we proceed to
the amacrine cells--

1180
01:10:43,920 --> 01:10:45,970
and again, things
are so complicated--

1181
01:10:45,970 --> 01:10:50,950
the numerous, numerous amacrine
cells, different types--

1182
01:10:50,950 --> 01:10:52,650
and they have been
studied extensively--

1183
01:10:52,650 --> 01:10:53,645
they are more than 20.

1184
01:10:55,140 --> 01:10:59,090
And here's a clever experiment
I want to point out to you.

1185
01:10:59,090 --> 01:11:03,510
What you can do here, if you
look at the called cholinergic

1186
01:11:03,510 --> 01:11:07,890
amacrine cells-- that's one
class-- you can selectively

1187
01:11:07,890 --> 01:11:09,950
label them, because
they're cholinergic.

1188
01:11:09,950 --> 01:11:11,460
And here's an example.

1189
01:11:11,460 --> 01:11:16,310
All those lighter blue ones
are your cholinergic cells.

1190
01:11:16,310 --> 01:11:18,060
And then what you can
do, you can put this

1191
01:11:18,060 --> 01:11:19,620
under a microscope.

1192
01:11:19,620 --> 01:11:22,160
And then you can
lower a microelectrode

1193
01:11:22,160 --> 01:11:25,037
into one of these cells,
just like I've done here.

1194
01:11:25,037 --> 01:11:26,620
And if you look at
this closely-- this

1195
01:11:26,620 --> 01:11:28,550
is not the best
of pictures-- this

1196
01:11:28,550 --> 01:11:31,130
is a Procion
yellow-labeled amacrine

1197
01:11:31,130 --> 01:11:35,270
cell-- you can see it
has all these processes.

1198
01:11:35,270 --> 01:11:37,910
So then if you look at
this on the microscope

1199
01:11:37,910 --> 01:11:42,790
under the high quality,
you can draw these out.

1200
01:11:42,790 --> 01:11:45,490
And this is what
they look like, OK?

1201
01:11:45,490 --> 01:11:48,320
So this is a cholinergic cell.

1202
01:11:48,320 --> 01:11:49,620
This is another type.

1203
01:11:49,620 --> 01:11:54,040
It's just called A2, which
has a lot to do with rods.

1204
01:11:54,040 --> 01:11:56,765
And then yet another class
called open [? minergic ?].

1205
01:11:56,765 --> 01:11:58,640
I'm not going to go
into detail about it.

1206
01:11:58,640 --> 01:12:01,960
But you can see that each of
these now-- scientific methods

1207
01:12:01,960 --> 01:12:06,550
enable you to specify what
these amacrine cells are like.

1208
01:12:06,550 --> 01:12:08,500
And on the basis of
that, people have come up

1209
01:12:08,500 --> 01:12:12,190
with ideas what the
amacrine system does.

1210
01:12:12,190 --> 01:12:17,810
OK, now we come to the crux of
things in the last few minutes.

1211
01:12:17,810 --> 01:12:21,070
We're going to talk about
the electrical responses

1212
01:12:21,070 --> 01:12:22,770
in the retina.

1213
01:12:22,770 --> 01:12:27,100
Now this is going to be,
again, fairly complicated.

1214
01:12:27,100 --> 01:12:32,490
This work-- again remarkable
work which, in my view,

1215
01:12:32,490 --> 01:12:35,630
merits a Nobel Prize,
but it has not yet

1216
01:12:35,630 --> 01:12:40,515
been awarded to John
Darling at Harvard.

1217
01:12:44,080 --> 01:12:47,326
And let me just introduce
this by telling you

1218
01:12:47,326 --> 01:12:48,825
how they went about
this experiment.

1219
01:12:51,060 --> 01:12:53,360
They realized that the
cells in the retina

1220
01:12:53,360 --> 01:12:55,230
are very, very small.

1221
01:12:55,230 --> 01:12:58,200
And so it's very difficult
to record from them

1222
01:12:58,200 --> 01:12:59,520
intercellularly.

1223
01:12:59,520 --> 01:13:01,930
You have to do that
if you want to be

1224
01:13:01,930 --> 01:13:04,550
able to label them anatomically.

1225
01:13:04,550 --> 01:13:09,080
And so they looked around
for a species of animal

1226
01:13:09,080 --> 01:13:12,700
that had large
cells in the retina.

1227
01:13:12,700 --> 01:13:14,750
And they discovered
after a lot of search

1228
01:13:14,750 --> 01:13:20,180
that the so-called mudpuppy--
Necturus maculosus--

1229
01:13:20,180 --> 01:13:25,810
is an animal that has unusually
large retinal ganglion cells.

1230
01:13:25,810 --> 01:13:29,200
Then they developed a
technique of removing the eye

1231
01:13:29,200 --> 01:13:33,230
from this animal, and
putting it into a dish,

1232
01:13:33,230 --> 01:13:35,560
because that makes
it real stable.

1233
01:13:35,560 --> 01:13:39,340
And then they were able to
put an electrode in there,

1234
01:13:39,340 --> 01:13:43,920
define what the cells did
functionally by shining light

1235
01:13:43,920 --> 01:13:46,420
on them and looking
at the activity,

1236
01:13:46,420 --> 01:13:50,160
and then they were
able to label the cell

1237
01:13:50,160 --> 01:13:52,340
by injecting a
labeling substance.

1238
01:13:52,340 --> 01:13:54,160
So that's what they did.

1239
01:13:54,160 --> 01:13:57,980
And so here is a description
of that arrangement.

1240
01:13:57,980 --> 01:14:01,520
Here you have an
inverted mudpuppy retina,

1241
01:14:01,520 --> 01:14:04,190
and here you have a
DC-recording electrode.

1242
01:14:04,190 --> 01:14:06,720
And what you do then, you
look at it in an oscilloscope.

1243
01:14:06,720 --> 01:14:13,690
And at this point, the cell is
entered by the DC recording.

1244
01:14:13,690 --> 01:14:16,050
And then once it's
inside, cells inside

1245
01:14:16,050 --> 01:14:20,730
are negatively charged with
respect to the surround,

1246
01:14:20,730 --> 01:14:22,765
usually up to about
70 millivolts.

1247
01:14:23,830 --> 01:14:26,320
In this case, I have
50 millivolts labeled.

1248
01:14:26,320 --> 01:14:27,900
And so once you're
inside the cell,

1249
01:14:27,900 --> 01:14:30,830
you see a sudden drop
to a minus level.

1250
01:14:30,830 --> 01:14:34,250
And then if the cell
discharges an action potential,

1251
01:14:34,250 --> 01:14:38,540
then you see that, of
course, in the oscilloscope.

1252
01:14:38,540 --> 01:14:41,370
And then you can study this
cell in all kinds of detail

1253
01:14:41,370 --> 01:14:44,080
as to how it responds,
what it responds and so

1254
01:14:44,080 --> 01:14:47,490
on, to determine what
its characteristics are.

1255
01:14:47,490 --> 01:14:50,150
So then when they
did that, they made

1256
01:14:50,150 --> 01:14:52,100
the following
central discoveries--

1257
01:14:52,100 --> 01:14:55,350
they discovered, first
of all, that receptors

1258
01:14:55,350 --> 01:14:58,310
all hyperpolarize to light.

1259
01:14:58,310 --> 01:15:03,520
Now this is the opposite of
what you would have thought,

1260
01:15:03,520 --> 01:15:06,762
because the principle
of the way cells

1261
01:15:06,762 --> 01:15:08,470
operate-- and I'm sure
you know all this,

1262
01:15:08,470 --> 01:15:10,750
but I will also describe
it in a bit more detail

1263
01:15:10,750 --> 01:15:17,230
in a minute-- cells
when they hyperpolarize,

1264
01:15:17,230 --> 01:15:24,180
they are less likely to cause
a release in neurotransmitters.

1265
01:15:24,180 --> 01:15:29,750
If they depolarize, they
will increase the likelihood

1266
01:15:29,750 --> 01:15:34,390
of neurotransmitters
being released.

1267
01:15:34,390 --> 01:15:36,390
And by the way, the
neurotransmitter

1268
01:15:36,390 --> 01:15:39,620
for these receptors
is glutamate.

1269
01:15:39,620 --> 01:15:45,850
And also, which I
should emphasize here,

1270
01:15:45,850 --> 01:15:49,490
is that the receptors never
give action potentials.

1271
01:15:49,490 --> 01:15:52,170
They only give
graded potentials.

1272
01:15:52,170 --> 01:15:54,080
That's also true for
the horizontal cells

1273
01:15:54,080 --> 01:15:56,250
and the bipolar cells.

1274
01:15:56,250 --> 01:16:02,090
They found that all of receptors
hyperpolarize to light, meaning

1275
01:16:02,090 --> 01:16:05,820
that they activate subsequent
elements in the retina

1276
01:16:05,820 --> 01:16:08,070
when it gets darker
out there, rather than

1277
01:16:08,070 --> 01:16:10,725
when it gets lighter, which
is the opposite of what we all

1278
01:16:10,725 --> 01:16:11,475
would've expected.

1279
01:16:14,570 --> 01:16:16,490
Same thing for the
horizontal cells.

1280
01:16:16,490 --> 01:16:18,930
But then when you
come to bipolar cells,

1281
01:16:18,930 --> 01:16:22,420
suddenly you find that there
are two different classes-- one

1282
01:16:22,420 --> 01:16:26,350
class which mimics
what the receptors do,

1283
01:16:26,350 --> 01:16:27,975
the other class that
does the opposite.

1284
01:16:29,050 --> 01:16:31,850
And so the bipolar cells
of these two types--

1285
01:16:31,850 --> 01:16:36,910
this type has sign
conserving synapses,

1286
01:16:36,910 --> 01:16:40,015
and this type has sign
inverting synapses.

1287
01:16:43,550 --> 01:16:47,360
Now amacrine cells
come in various types,

1288
01:16:47,360 --> 01:16:49,070
and some give action potentials.

1289
01:16:49,070 --> 01:16:51,820
And ganglion cells all
give action potentials.

1290
01:16:51,820 --> 01:16:54,580
So to go through this
again, all receptors

1291
01:16:54,580 --> 01:16:56,000
hyperpolarize to light.

1292
01:16:56,000 --> 01:17:00,480
Horizontal cells
hyperpolarize to light.

1293
01:17:00,480 --> 01:17:04,210
About half of bipolar cells
hyperpolarize and half

1294
01:17:04,210 --> 01:17:04,710
depolarize.

1295
01:17:05,940 --> 01:17:07,680
Now this is
accomplished-- by the way,

1296
01:17:07,680 --> 01:17:09,940
I'll go into that in
more detail later--

1297
01:17:09,940 --> 01:17:16,740
by virtue of having different
kinds of neurotransmitter

1298
01:17:16,740 --> 01:17:17,750
receptor sites.

1299
01:17:19,539 --> 01:17:21,330
And I'll talk about
that in a lot of detail

1300
01:17:21,330 --> 01:17:24,054
when we talk about
the ON and OFF cells.

1301
01:17:24,054 --> 01:17:25,720
OK, and then we have
the amacrine cells.

1302
01:17:25,720 --> 01:17:27,594
Some hyperpolarize, some
depolarize, and some

1303
01:17:27,594 --> 01:17:28,840
give action potentials.

1304
01:17:28,840 --> 01:17:32,600
And all ganglion cells
give action potentials.

1305
01:17:32,600 --> 01:17:36,600
So that is a major, major
discovery, and not only major,

1306
01:17:36,600 --> 01:17:40,930
but totally unexpected from
what one would have thought.

1307
01:17:40,930 --> 01:17:43,687
All right, so now
let me just explain

1308
01:17:43,687 --> 01:17:44,770
this in a bit more detail.

1309
01:17:46,450 --> 01:17:52,770
If you go inside a cell, then
if this cell is activated,

1310
01:17:52,770 --> 01:17:57,190
it can be either
get-- what you see

1311
01:17:57,190 --> 01:17:59,780
is an excitatory post
synaptic potential, which

1312
01:17:59,780 --> 01:18:02,350
goes from minus
towards plus, or you

1313
01:18:02,350 --> 01:18:07,080
can get an IPSP, an inhibitory
post synaptic potential.

1314
01:18:07,080 --> 01:18:10,210
And this discovery
was made by Eccles,

1315
01:18:10,210 --> 01:18:13,160
which is a remarkable discovery
for which he received the Nobel

1316
01:18:13,160 --> 01:18:15,965
Prize some years back.

1317
01:18:17,170 --> 01:18:21,650
All right, so now if we look at
those cells which give action

1318
01:18:21,650 --> 01:18:24,820
potentials, what happens is
when you get an EPSP-- that's

1319
01:18:24,820 --> 01:18:28,490
why it's called excitatory
post synaptic potential-- then

1320
01:18:28,490 --> 01:18:31,940
you increase the likelihood
that an action potential

1321
01:18:31,940 --> 01:18:36,010
will be generated that
will send its signal down

1322
01:18:36,010 --> 01:18:41,950
the axon to the next
location of the cells.

1323
01:18:41,950 --> 01:18:43,720
All right, so that's
the basic process.

1324
01:18:44,910 --> 01:18:46,950
So again to reiterate,
photoreceptors

1325
01:18:46,950 --> 01:18:48,960
hyperpolarize the light.

1326
01:18:48,960 --> 01:18:51,200
Therefore, glutamate
is released when

1327
01:18:51,200 --> 01:18:55,050
there's a decrease in
illumination-- the opposite

1328
01:18:55,050 --> 01:18:56,720
of what anyone
would have expected.

1329
01:18:58,330 --> 01:19:02,680
All right, so that
I think brings us

1330
01:19:02,680 --> 01:19:04,300
to what I want to cover today.

1331
01:19:04,300 --> 01:19:06,320
I'll wait until
next time to talk

1332
01:19:06,320 --> 01:19:09,390
about the lateral geniculate
nucleus, which is very brief.

1333
01:19:09,390 --> 01:19:12,640
But let me go through the
four receptor basics again.

1334
01:19:12,640 --> 01:19:16,460
All photoreceptors
hyperpolarize the light.

1335
01:19:16,460 --> 01:19:18,310
Depolarization of
the photoreceptor

1336
01:19:18,310 --> 01:19:19,470
releases glutamate.

1337
01:19:20,770 --> 01:19:24,640
Photon absorption by the
photo pigment results

1338
01:19:24,640 --> 01:19:27,870
in isomerization of the
chromophore from 11 cis

1339
01:19:27,870 --> 01:19:29,010
to 11 trans.

1340
01:19:29,010 --> 01:19:32,160
This is what I told you
to think about in a very

1341
01:19:32,160 --> 01:19:40,520
simple fashion-- to say that
the photoreceptor molecules--

1342
01:19:40,520 --> 01:19:44,940
rhodopsin in this case-- is
either bleached or unbleached,

1343
01:19:44,940 --> 01:19:45,980
OK?

1344
01:19:45,980 --> 01:19:49,020
So once it's bleached, further
light has no effect on it.

1345
01:19:49,020 --> 01:19:53,060
And at any point, given whatever
the level of illumination is,

1346
01:19:53,060 --> 01:19:56,600
a certain percentage of
these rhodopsin molecules

1347
01:19:56,600 --> 01:19:59,250
are bleached and a certain
percentage is unbleached.

1348
01:19:59,250 --> 01:20:01,970
The darker it is, the
more are unbleached.

1349
01:20:03,100 --> 01:20:06,080
All right, then we
have these two classes

1350
01:20:06,080 --> 01:20:08,150
that we talked about--
the ON and the OFF cells

1351
01:20:08,150 --> 01:20:12,340
that we will discuss in some
detail-- the synaptic junction

1352
01:20:12,340 --> 01:20:15,250
of OFF bipolars is sign
conserving, as I just said,

1353
01:20:15,250 --> 01:20:17,710
and that of ON bipolars
is sign inverting.

1354
01:20:18,830 --> 01:20:22,740
Now the ON bipolar receptor
is-- I should mention that now,

1355
01:20:22,740 --> 01:20:26,660
and I will deal with it in
detail-- is called the mGluR6.

1356
01:20:29,050 --> 01:20:31,623
It's activation leads
to closing of channels

1357
01:20:31,623 --> 01:20:32,706
causing hyperpolarization.

1358
01:20:34,240 --> 01:20:36,863
that's the basic layout,
then, of the photoreceptors.

1359
01:20:37,950 --> 01:20:42,100
And I will now
stop at this point.

1360
01:20:42,100 --> 01:20:43,980
And next time, we'll
talk a little bit

1361
01:20:43,980 --> 01:20:47,720
about the lateral
geniculate nucleus.

1362
01:20:47,720 --> 01:20:50,770
And then we're going to move
on to talk about the cortex.

1363
01:20:50,770 --> 01:20:54,500
then if you look at
you assignment sheet,

1364
01:20:54,500 --> 01:20:58,060
you can see here that
on September 16th,

1365
01:20:58,060 --> 01:21:00,400
we're going to talk about
the ON and OFF channels.

1366
01:21:00,400 --> 01:21:03,520
And then we'll deal in more
detail with these rather

1367
01:21:03,520 --> 01:21:06,550
complex things which are
pretty hard to remember,

1368
01:21:06,550 --> 01:21:11,140
about hyperpolarization,
depolarization, glutamate,

1369
01:21:11,140 --> 01:21:13,320
glutamate receptor
sites, and so on.

1370
01:21:13,320 --> 01:21:16,920
OK, thank you.