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PROFESSOR: Today we are
going to discuss color vision

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and adaptation.

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About 2/3 of it's going
to be color vision and one

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third on adaptation.

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Now, I'm going to have several
demonstrations on the screen

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here for you.

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And I would like to forewarn
on you that for some reason,

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they still haven't fixed the
light bulb in this projector.

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And this, you see this is a
bluish color to it, speaking

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of color vision?

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And actually, it's
supposed to be gray.

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But there's some loss of
balance there in this bulb.

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And they've promised to
replace it some 10 days ago

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and it still hasn't happened.

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So once we come to
the demonstrations,

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they're not going to be perfect.

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But they will also be
available on the internet,

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as well on Stellar.

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So you can look at them
there and maybe you'll

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get a better picture of
them there than here.

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But we'll do the best we can.

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So anyway, let me
first give you a list

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of things we are
going to discuss

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and the questions
we're going to pose.

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First of all, you're going to
ask, what are the basic facts

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and laws of color vision?

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Now, one of the nice
things about color vision

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is that their number of laws--
it's a very basic phenomenon.

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And its in many ways
very close to physics.

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The second one is, what are the
major theories of color vision

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that we are going to discuss?

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00:02:01,150 --> 00:02:03,680
And then we are
going to examine how

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00:02:03,680 --> 00:02:06,370
color is processed in
the retina geniculate.

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Then we're going to
move on and examine

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what happens in the cortex.

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And then we are
going to discuss what

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is the nature of
colorblindness, which

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I think will be of
interest to most of you

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because colorblindness is not
that uncommon, unfortunately,

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among humans.

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And then we're going to
look at how adaptation

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is achieved in
the visual system.

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That's when we switch
from color to adaptation.

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And then we are going to ask the
question, what are afterimages?

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How are they produced?

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And what are its effects?

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So let's begin, then,
with color vision.

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And the first thing I would
like to say about this

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is that as so often happens in
the course of history, often

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there had been great
misconceptions about color.

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And one of the
great misconceptions

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was that people thought that
white light is the pure light.

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And that was
exemplified in the fact

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that before the 20th century,
for example, most nuns

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were required to
wear a white outfit.

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They were asked to
wear the white outfit

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because it meant
that they were pure.

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So what happened
then in the 1600s,

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one of the greatest geniuses
of our time came along.

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That was Newton.

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And at that time-- this is just
an interesting coincidence--

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it happened that the art of
making chandeliers has emerged.

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And the chandeliers
in those cases

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consisted of little pieces
of glass cut in various ways.

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And people noticed, and
so especially did Newton,

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that when you looked
at these chandeliers,

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you saw all sorts
of colors there.

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And so Newton said, my god.

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How can that be?

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What's going on?

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And so he began
to analyze color,

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which also came
about then for him

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because that's when they also,
in addition to the chandeliers,

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they came up with prisms.

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And so what Sir Isaac
Newton did was--

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let me skip this for a minute
and I'll come back to it.

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What he did was that he put
a little opening in a screen

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and let the light come
through from the sun.

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White light.

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And then he put that beam
of light through a prism.

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And he discovered
that he got an image

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much like what you see when
you have these chandeliers.

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Namely, that you get all
sorts of colors projecting out

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of the prism.

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And then he performed yet
another little experiment.

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He added another
prism, same kind

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of prism, where the light
was separated from the red.

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And then there was no
further separation.

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That was a remarkable
discovery on his part.

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And he became interested not
only in the physics of it,

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but he also became
very interested in how

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we organize our
color perceptions.

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And he was the first
person to come up

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with what I will talk a lot
about, the so-called color

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circle.

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So this is what Sir Isaac
Newton then came up with.

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And he established that we have
a huge range of frequencies.

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00:06:04,390 --> 00:06:07,190
And a very narrow
section of it right here

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is the one that falls
into the visible range.

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00:06:09,780 --> 00:06:12,870
And if you break that up
like this and enlarge it,

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00:06:12,870 --> 00:06:15,400
much like the
colors of a rainbow,

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you see all of these colors.

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00:06:17,840 --> 00:06:20,600
And so the conclusion
to which he came

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is something that
was quite remarkable.

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He was at that age
just 29 years old.

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And that was done-- to go
back here-- at the age of 29

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in 1672.

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And so he concluded at
that time that white light

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is a mixture of all the colors.

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It's white because
it's an equal mixture

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of the different
wavelengths here.

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So rather than being
pure, white light

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is a conglomeration of
all the little wavelengths

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00:06:55,240 --> 00:07:01,545
in the visible range
that the eye can process.

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So that was so stunning
for him at the time

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00:07:08,060 --> 00:07:12,790
that he delayed publication
of it for more than 30 years.

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And then when he
published this, this

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was extensively debated
even 30 years later.

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And one of the people
who debated it a lot

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was a famous German
poet called Goethe.

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You probably all of
you know who he is.

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And he said, Newton
is a charlatan.

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He just made this up.

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It can't possibly be so.

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White light is pure.

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And so what he did, he took
a prism just like Newton did.

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But instead of reflecting the
light, he looked into the prism

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towards the light and he
didn't see any colors.

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And he said, Newton
is all full of junk.

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And so he asked his associates
at the time, Schopenhauer,

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00:08:00,200 --> 00:08:05,280
who is also a very
famous philosopher,

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00:08:05,280 --> 00:08:08,710
and said, why don't you
do some experiments?

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00:08:08,710 --> 00:08:12,560
Let us prove that
Newton is all wrong.

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00:08:12,560 --> 00:08:15,560
And so Schopenhauer did the
experiment right and said,

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00:08:15,560 --> 00:08:18,110
oh my god, Newton is right.

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What am I going to do?

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00:08:19,240 --> 00:08:22,950
How can I tell Goethe,
my boss, that he's wrong

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00:08:22,950 --> 00:08:25,140
and Newton is right?

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00:08:25,140 --> 00:08:30,460
So that became quite a thorn
in his hide at the time.

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But of course
eventually, we all came

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to recognize indeed that
this is the situation,

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and that white light is
a mixture of all colors.

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So let me now go
back a minute to some

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of the basic facts, which I
will elaborate on as we proceed.

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First of all, when
we talk about color,

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there are all kinds
of systematic things

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that we are going to discuss.

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And you're going
to become educated

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about the processing
of color as a result

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00:08:58,950 --> 00:09:02,270
because we know a lot of very
important basic facts about it.

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It's solid science.

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00:09:04,490 --> 00:09:06,940
So when we talk about
color, we typically

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make a distinction between hue,
brightness, and saturation.

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Hue means what is the color.

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Is it red, green, and blue?

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Brightness is how intense
the impression is.

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And saturation is that every
color can be kind of washed out

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or it can be very sharp.

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Can you see a bright red?

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Or you can see a
really washed-out red

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00:09:28,890 --> 00:09:30,723
that's barely different
from the background?

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This is very important.

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We have to make a
clear distinction

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between the psychological
and physiological attributes,

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or the physical, actually,
attributes of color.

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So what do we mean by that?

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When we talk about color,
that's an impression we have.

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That's our own personal
psychological experience.

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But the scientific way to look
at it is to call it wavelength.

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00:10:03,680 --> 00:10:06,850
The same thing is true for
luminance and brightness.

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00:10:06,850 --> 00:10:10,065
Now before I go onto this,
let me back up for a minute.

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Here, let me say one
more thing about this

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00:10:15,320 --> 00:10:17,387
because it's an interesting
way to remember it

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00:10:17,387 --> 00:10:19,220
and also it's relevant
to what you are going

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to hear in the second
half of this course, which

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is going to be on audition.

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So here's a classic question
people have often been asked,

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especially when you were still
in grammar school or maybe even

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00:10:29,370 --> 00:10:30,530
in high school.

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00:10:30,530 --> 00:10:33,300
It was, when a tree
falls in the forest,

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and there's nobody around,
does it make a sound?

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00:10:37,510 --> 00:10:40,870
And so people debate this, and
debate this, and debate this.

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00:10:40,870 --> 00:10:43,680
Well, from my point
of view, there's

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00:10:43,680 --> 00:10:45,580
no question there at all.

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00:10:45,580 --> 00:10:47,425
When a tree falls in
the forest and there

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00:10:47,425 --> 00:10:49,470
are all these cracks
and everything,

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00:10:49,470 --> 00:10:53,850
there is no sound because sound
is a psychological attribute

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that we hear and interpret.

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00:10:56,190 --> 00:10:58,240
There is, of course,
the production

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of wavelengths as a
result of the fall.

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00:11:01,390 --> 00:11:03,280
So that's up in the air there.

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But you need a human being to
turn that various frequencies

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into what we call sound.

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00:11:13,030 --> 00:11:17,555
So now the next thing I would
like to briefly approach--

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00:11:17,555 --> 00:11:20,080
I'm going to talk about it in
much more detail in a minute--

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00:11:20,080 --> 00:11:22,734
is that it's been
established by now,

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00:11:22,734 --> 00:11:24,400
but initially there
was quite a debate--

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I'll come back to it-- that we
have three major kinds of cone

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receptors in humans
and in many primates.

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00:11:33,500 --> 00:11:36,485
And actually, in some
animals and some birds,

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00:11:36,485 --> 00:11:38,440
there are actually four of them.

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00:11:38,440 --> 00:11:43,105
And these three are the short,
middle, and long wavelength

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00:11:43,105 --> 00:11:43,605
cones.

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00:11:45,950 --> 00:11:49,810
Of course, we can call those--
for short, we mean blue,

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00:11:49,810 --> 00:11:53,090
for medium, we mean green,
and for long, we mean red.

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00:11:53,090 --> 00:11:54,245
And then we have the rods.

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00:11:55,960 --> 00:11:58,530
And these numbers here--
this one is misspelled.

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00:11:59,791 --> 00:12:00,290
bb?

216
00:12:00,290 --> 00:12:01,340
I don't know what bb is.

217
00:12:01,340 --> 00:12:07,010
At any rate, these are
nanometers at which they peak.

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00:12:07,010 --> 00:12:08,890
And we'll come back
to that in a minute.

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00:12:08,890 --> 00:12:13,080
But before I go on with that,
let's imagine for a minute--

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00:12:13,080 --> 00:12:14,220
and we can skip this.

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00:12:16,280 --> 00:12:25,890
Suppose that you are the
emperor of the universe

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00:12:25,890 --> 00:12:34,240
since the beginnings of time
1,500 million years ago.

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00:12:34,240 --> 00:12:40,770
And you decided that you're
going to create animals.

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00:12:40,770 --> 00:12:42,710
And once you've
created animals, you

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have decided that they're
going to have to see things.

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00:12:45,310 --> 00:12:49,310
And so they have to have
an eye with which to see.

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00:12:49,310 --> 00:12:54,290
And then you have to decide,
well, if that's the case,

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00:12:54,290 --> 00:12:55,290
color is very important.

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00:12:55,290 --> 00:12:56,956
You looked around in
the world and said,

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00:12:56,956 --> 00:12:58,630
oh, all these beautiful colors.

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00:12:58,630 --> 00:13:01,850
How are we going to have
these animals and these humans

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00:13:01,850 --> 00:13:03,480
see all the colors?

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00:13:03,480 --> 00:13:07,736
And then you said, well,
there are hundreds of colors.

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00:13:07,736 --> 00:13:08,860
So what are we going to do?

235
00:13:08,860 --> 00:13:10,870
Are we going to
create the receptor

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00:13:10,870 --> 00:13:13,890
for every one of these colors
and put them in the eye?

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00:13:13,890 --> 00:13:16,700
And you said, oh dear, that's
a problem because then we

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00:13:16,700 --> 00:13:18,870
would need a gigantic eye.

239
00:13:18,870 --> 00:13:21,015
And so the question
became, of course,

240
00:13:21,015 --> 00:13:23,260
how else would you
come around this?

241
00:13:24,610 --> 00:13:28,210
And so here the idea, that
here you have sensitivity,

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00:13:28,210 --> 00:13:30,170
and then here you
have wavelength.

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00:13:30,170 --> 00:13:33,190
And so the idea was that
you could create hundreds

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00:13:33,190 --> 00:13:38,270
of very sharply tuned
photoreceptors like that

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00:13:38,270 --> 00:13:39,975
so you could get all the colors.

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00:13:39,975 --> 00:13:47,590
Well, that did not seem to be a
very good way of doing things.

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00:13:47,590 --> 00:13:53,200
And so getting more back
to the present time--

248
00:13:53,200 --> 00:13:56,490
still a long ways off from
the present, actually-- people

249
00:13:56,490 --> 00:13:59,520
began to hypothesize about,
they say, well, what can

250
00:13:59,520 --> 00:14:03,090
we do to minimize the
number of receptors

251
00:14:03,090 --> 00:14:05,340
with different
sensitivities color

252
00:14:05,340 --> 00:14:07,450
and still be able to see well?

253
00:14:07,450 --> 00:14:10,960
And so one theory that came
up before they knew anything

254
00:14:10,960 --> 00:14:16,520
about the eye and the
three cone photoreceptors,

255
00:14:16,520 --> 00:14:20,260
Young and Helmholtz
came up with the idea

256
00:14:20,260 --> 00:14:23,220
that if you had just
three types of cones

257
00:14:23,220 --> 00:14:26,560
that are broadly
tuned, they could

258
00:14:26,560 --> 00:14:31,040
take care of most of the
ability to see various colors.

259
00:14:31,040 --> 00:14:33,470
And so that became
a very interesting,

260
00:14:33,470 --> 00:14:35,680
very, very powerful theory.

261
00:14:35,680 --> 00:14:38,460
And actually, speaking
of theories and models,

262
00:14:38,460 --> 00:14:42,190
it still is in my mind
probably the greatest model

263
00:14:42,190 --> 00:14:44,500
or theory that has
ever been developed

264
00:14:44,500 --> 00:14:47,640
about how the brain works
because it subsequently

265
00:14:47,640 --> 00:14:51,010
did turn out that
indeed, there were three

266
00:14:51,010 --> 00:14:54,820
of these receptors that are
broadly tuned that can provide

267
00:14:54,820 --> 00:14:56,620
all that information for you.

268
00:14:56,620 --> 00:14:59,710
And that actually, as you
will see, became a huge issue.

269
00:15:02,411 --> 00:15:02,910
Sorry.

270
00:15:02,910 --> 00:15:07,990
Let me go back for
a minute to this.

271
00:15:07,990 --> 00:15:10,040
And let me just reiterate.

272
00:15:10,040 --> 00:15:17,130
These are the nanometers for
the three types of cones.

273
00:15:17,130 --> 00:15:21,430
And at the end here, we have
the nanometers for the rods.

274
00:15:21,430 --> 00:15:26,170
Then as a result of
analyzing all this stuff,

275
00:15:26,170 --> 00:15:29,240
people have come up with
all sorts of rules and laws.

276
00:15:29,240 --> 00:15:32,870
And that's what is one of the
nice things about color vision.

277
00:15:32,870 --> 00:15:37,460
And one of the rules is called--
actually, laws in this case--

278
00:15:37,460 --> 00:15:39,490
is called Grassmann's laws.

279
00:15:40,660 --> 00:15:43,900
And he said every color
has a complimentary which,

280
00:15:43,900 --> 00:15:46,730
when mixed properly,
yields gray.

281
00:15:46,730 --> 00:15:50,310
And I will explain this to
you in just a few minutes.

282
00:15:50,310 --> 00:15:53,290
And the other is that
non-complementary colors

283
00:15:53,290 --> 00:15:54,123
yield intermediates.

284
00:15:55,250 --> 00:15:58,880
Just keep that in your head for
me until I fully explain it.

285
00:15:58,880 --> 00:16:02,580
And the other law, which I'm not
going to talk too much about,

286
00:16:02,580 --> 00:16:05,390
is that the luminance of a
mixture of differently colored

287
00:16:05,390 --> 00:16:08,340
lights is equal to the
sum of the luminance

288
00:16:08,340 --> 00:16:09,960
of its components.

289
00:16:09,960 --> 00:16:12,855
So these are very, very
basic rules-- laws.

290
00:16:14,170 --> 00:16:16,920
So here we go
again, and move on.

291
00:16:18,310 --> 00:16:22,530
Here is what is called the
CIE chromaticity diagram.

292
00:16:22,530 --> 00:16:26,140
And it was initially
devised in 1931.

293
00:16:26,140 --> 00:16:29,250
And let me explain to
you how this came about.

294
00:16:29,250 --> 00:16:31,320
This became an
international undertaking.

295
00:16:32,560 --> 00:16:37,580
And it came about
because it was highly

296
00:16:37,580 --> 00:16:40,100
desirable to be
able to communicate

297
00:16:40,100 --> 00:16:43,500
throughout the world
your particular color

298
00:16:43,500 --> 00:16:45,640
desire or experience.

299
00:16:45,640 --> 00:16:52,040
So for example, if you had a
particular hat you had bought,

300
00:16:52,040 --> 00:16:54,380
say a blue hat of some
sort, and then said

301
00:16:54,380 --> 00:16:57,680
now I would like to get
a dress that fits it.

302
00:16:57,680 --> 00:16:59,420
How can I do that?

303
00:16:59,420 --> 00:17:02,230
Well, what you can
do now as a result

304
00:17:02,230 --> 00:17:06,859
of this chromaticity diagram,
there's a scale here.

305
00:17:06,859 --> 00:17:11,220
You can see the vertical values
and the horizontal values

306
00:17:11,220 --> 00:17:15,270
going from 0 to close to 100.

307
00:17:15,270 --> 00:17:19,290
And then, if you can
specify a particular color,

308
00:17:19,290 --> 00:17:21,589
say you want this
color here, you simply

309
00:17:21,589 --> 00:17:24,405
can state what that color
is by giving it the number.

310
00:17:25,670 --> 00:17:29,780
And then you can send that
number to China or to,

311
00:17:29,780 --> 00:17:32,520
I don't know, to South
Africa or something,

312
00:17:32,520 --> 00:17:34,610
to a particular
company, and say I

313
00:17:34,610 --> 00:17:38,480
want to have a dress like
that with that color.

314
00:17:38,480 --> 00:17:40,800
And then because this
is international,

315
00:17:40,800 --> 00:17:42,640
they were able to
produce that color

316
00:17:42,640 --> 00:17:45,064
as you specified
on this diagram.

317
00:17:45,064 --> 00:17:46,730
So that was a very
powerful undertaking.

318
00:17:47,830 --> 00:17:52,070
And this arrangement is such
that actually, the colors

319
00:17:52,070 --> 00:17:55,620
go from the center outward,
become more saturated.

320
00:17:55,620 --> 00:17:58,240
Think, for example, we're
going to here to here.

321
00:17:58,240 --> 00:18:03,620
And the center of this,
which is about 333 333

322
00:18:03,620 --> 00:18:07,320
on the chromaticity
diagram, is white,

323
00:18:07,320 --> 00:18:09,480
which is not that obvious
here, partly because

324
00:18:09,480 --> 00:18:11,520
of the colors of the background.

325
00:18:11,520 --> 00:18:17,490
So that is the famous
1931 chromaticity diagram.

326
00:18:17,490 --> 00:18:21,940
And what one can do with
this is to superimpose

327
00:18:21,940 --> 00:18:27,340
on this some rules
about the human or, say,

328
00:18:27,340 --> 00:18:29,897
the primate color
vision abilities.

329
00:18:29,897 --> 00:18:32,230
And the person who came up
with this, as I've mentioned,

330
00:18:32,230 --> 00:18:33,130
was Newton.

331
00:18:33,130 --> 00:18:37,030
He came up with the so-called
famous color circle.

332
00:18:37,030 --> 00:18:40,810
Now, the color circle
is described here.

333
00:18:40,810 --> 00:18:43,210
And I'll show it to you
in just a minute head-on.

334
00:18:44,850 --> 00:18:48,380
This is green, this is red,
this is yellow, this is blue.

335
00:18:48,380 --> 00:18:50,370
And of course, the
question is, why do we

336
00:18:50,370 --> 00:18:52,390
have them set up
in this fashion?

337
00:18:52,390 --> 00:18:54,860
And we'll explain
that in just a minute.

338
00:18:54,860 --> 00:18:59,620
So here is the color circle, the
two-dimensional color circle,

339
00:18:59,620 --> 00:19:02,600
where things are pretty
much equiluminant across it.

340
00:19:03,790 --> 00:19:06,330
Now, if you go from
the center out,

341
00:19:06,330 --> 00:19:08,930
you increase saturation,
as I had already said.

342
00:19:08,930 --> 00:19:11,405
And as you go around,
you change hue.

343
00:19:13,440 --> 00:19:15,345
So those are the
basic attributes.

344
00:19:16,370 --> 00:19:21,150
Now to anticipate
the issues here,

345
00:19:21,150 --> 00:19:23,910
let me tell you this-- this
is yellow, this is blue,

346
00:19:23,910 --> 00:19:26,130
this is green, and this is red.

347
00:19:26,130 --> 00:19:28,765
These are called
the cardinal axes.

348
00:19:32,570 --> 00:19:34,820
As long as a line goes
through the center,

349
00:19:34,820 --> 00:19:36,350
that's a cardinal axis.

350
00:19:36,350 --> 00:19:38,170
These are the ones
which are best known,

351
00:19:38,170 --> 00:19:39,420
the red/green and blue/yellow.

352
00:19:41,110 --> 00:19:43,750
And the fascinating
fact about this

353
00:19:43,750 --> 00:19:49,140
is that this explains a number
of very interesting facts

354
00:19:49,140 --> 00:19:51,640
about our ability to see colors.

355
00:19:51,640 --> 00:19:56,480
So let me tell you this--
if you mix yellow and blue,

356
00:19:56,480 --> 00:19:59,690
as we have talked about
the law, if you mix them

357
00:19:59,690 --> 00:20:05,530
in equal luminances, you get
what's in the center here.

358
00:20:05,530 --> 00:20:06,500
You get white.

359
00:20:07,550 --> 00:20:11,140
Furthermore, and
because of this,

360
00:20:11,140 --> 00:20:14,270
there is no such
experience in our existence

361
00:20:14,270 --> 00:20:17,040
that's called yellowish blue.

362
00:20:17,040 --> 00:20:20,360
And there's no such
existence in our minds,

363
00:20:20,360 --> 00:20:23,030
as far as color's concerned,
that's reddish green.

364
00:20:24,310 --> 00:20:27,190
On the other hand, if you
don't go across the center,

365
00:20:27,190 --> 00:20:30,330
but you say you had yellow
here and you have green here,

366
00:20:30,330 --> 00:20:33,230
there is yellowish green.

367
00:20:33,230 --> 00:20:35,130
There is yellowish red.

368
00:20:35,130 --> 00:20:36,910
There is bluish red.

369
00:20:36,910 --> 00:20:38,270
And there's bluish green.

370
00:20:39,380 --> 00:20:43,580
So we can process those and
see those in-between colors,

371
00:20:43,580 --> 00:20:47,450
but we cannot do that
along the cardinal axes.

372
00:20:47,450 --> 00:20:53,240
So this incredible
color circle essentially

373
00:20:53,240 --> 00:20:57,100
explains the very essence
of how we can see color.

374
00:20:57,100 --> 00:20:59,980
Now, there's one more
factor here, is that we also

375
00:20:59,980 --> 00:21:04,130
have to take into
account luminance values.

376
00:21:04,130 --> 00:21:09,510
And so people turn this color
circle for some purposes

377
00:21:09,510 --> 00:21:12,550
into a three-dimensional
entity that's shown here.

378
00:21:12,550 --> 00:21:14,410
Here's a color circle.

379
00:21:14,410 --> 00:21:18,200
And the third dimension
going up and down here,

380
00:21:18,200 --> 00:21:22,240
as you go from white to black,
if you will, in the center.

381
00:21:22,240 --> 00:21:25,650
So here, things are brighter
and here, things are darker.

382
00:21:25,650 --> 00:21:28,220
So that is sort of
a complete, then,

383
00:21:28,220 --> 00:21:31,760
arrangement for your
color impressions.

384
00:21:31,760 --> 00:21:33,530
But what we are
going to do, we are

385
00:21:33,530 --> 00:21:36,770
going to concentrate on
the color circle itself

386
00:21:36,770 --> 00:21:38,610
as we move along.

387
00:21:38,610 --> 00:21:44,070
So now let's next turn
to the outgrowth of this,

388
00:21:44,070 --> 00:21:46,870
starting with Newton's color
circle, which has been somewhat

389
00:21:46,870 --> 00:21:51,830
modified in the manner
that I had just shown you.

390
00:21:51,830 --> 00:21:57,280
And as a result of all this,
a number of competing theories

391
00:21:57,280 --> 00:21:58,150
have emerged.

392
00:21:58,150 --> 00:21:59,960
And I'm going to talk
about two of them.

393
00:21:59,960 --> 00:22:02,970
The first one is the famous
Young-Helmholtz theory.

394
00:22:04,220 --> 00:22:07,580
Young initially, and he
collaborated with him

395
00:22:07,580 --> 00:22:10,030
many years later,
with Helmholtz,

396
00:22:10,030 --> 00:22:13,400
came up with the idea that
you could experience colors

397
00:22:13,400 --> 00:22:19,600
by just having three types of
cones that are broadly tuned.

398
00:22:19,600 --> 00:22:22,900
And so he said there
are three types

399
00:22:22,900 --> 00:22:25,390
of broadly tuned
color receptors.

400
00:22:25,390 --> 00:22:27,030
The color experience
is the product

401
00:22:27,030 --> 00:22:29,635
of the relative
degree of activation.

402
00:22:30,700 --> 00:22:33,050
Now, that's a fantastic theory.

403
00:22:33,050 --> 00:22:35,420
But there's a big
problem with it.

404
00:22:35,420 --> 00:22:37,830
The big problem
with that theory is

405
00:22:37,830 --> 00:22:41,410
that he doesn't explain
Grassmann's laws.

406
00:22:41,410 --> 00:22:43,780
Remember what
Grassmann's law is?

407
00:22:43,780 --> 00:22:47,690
That if you mix things along the
cardinal axes, you get white.

408
00:22:47,690 --> 00:22:49,150
And you only get
other colors when

409
00:22:49,150 --> 00:22:52,775
you mix them not along the axes.

410
00:22:53,900 --> 00:22:56,350
So that became a problem.

411
00:22:56,350 --> 00:23:03,410
And because of that, another
famous person, Herring,

412
00:23:03,410 --> 00:23:05,910
came up with an
alternate theory.

413
00:23:05,910 --> 00:23:09,820
He came up with a theory which
said that color opponency is

414
00:23:09,820 --> 00:23:12,500
based on the observation
that red and green, as well

415
00:23:12,500 --> 00:23:17,040
as blue and yellow, are mutually
exclusive, just as I had said.

416
00:23:17,040 --> 00:23:18,630
The nervous system
probably treats

417
00:23:18,630 --> 00:23:22,390
red/green and blue/yellow
as antagonistic pairs,

418
00:23:22,390 --> 00:23:24,260
with a third pair
being black and white.

419
00:23:24,260 --> 00:23:26,260
That's where the third
dimension comes in.

420
00:23:26,260 --> 00:23:29,230
So therefore he argued
that we need something

421
00:23:29,230 --> 00:23:34,380
like color opponency to be
able to see colors right.

422
00:23:34,380 --> 00:23:36,010
Now, the interesting
thing about this

423
00:23:36,010 --> 00:23:38,440
is that he became
very famous coming up

424
00:23:38,440 --> 00:23:40,230
with this incredible theory.

425
00:23:40,230 --> 00:23:43,310
But then if you go
back in history,

426
00:23:43,310 --> 00:23:49,250
you find that Leonardo da
Vinci had this same idea

427
00:23:49,250 --> 00:23:51,700
many, many, many
years before that.

428
00:23:51,700 --> 00:23:53,910
And this is from
his autobiography,

429
00:23:53,910 --> 00:23:55,810
with a very poor translation.

430
00:23:55,810 --> 00:23:58,560
It says, "Of different
colors equally perfect,

431
00:23:58,560 --> 00:24:01,130
that will appear
most excellent, which

432
00:24:01,130 --> 00:24:03,940
is seen near its
direct contrary,

433
00:24:03,940 --> 00:24:06,730
blue near yellow,
green near red,

434
00:24:06,730 --> 00:24:11,270
because each color is seen,
when opposed to its contrary,

435
00:24:11,270 --> 00:24:14,330
than any other similar to it."

436
00:24:14,330 --> 00:24:17,105
It's not written in English
really, but you get the idea.

437
00:24:18,160 --> 00:24:20,770
So we have a major two theories.

438
00:24:20,770 --> 00:24:24,880
And then numerous
experiments subsequently

439
00:24:24,880 --> 00:24:26,680
emerge, especially
when it became

440
00:24:26,680 --> 00:24:30,220
possible to record the
neural activity of cells

441
00:24:30,220 --> 00:24:36,120
to determine to what degree
these theories are correct.

442
00:24:36,120 --> 00:24:39,820
And of course, the first
correct aspect of both theories

443
00:24:39,820 --> 00:24:44,480
was that indeed, we have
three types of cones

444
00:24:44,480 --> 00:24:46,337
that are selected to
red, green, and blue,

445
00:24:46,337 --> 00:24:47,670
and that they are broadly tuned.

446
00:24:54,860 --> 00:24:59,470
So to now understand
better how it really

447
00:24:59,470 --> 00:25:01,960
happens in the nervous
system, let's take a look

448
00:25:01,960 --> 00:25:04,690
at the basic physiology
of color processing.

449
00:25:06,100 --> 00:25:08,670
I showed you this
slide once before.

450
00:25:08,670 --> 00:25:12,240
I pointed out to
you that contrary

451
00:25:12,240 --> 00:25:16,470
to red and green cones, blue
cones are much less numerous.

452
00:25:16,470 --> 00:25:19,900
Only one out of eight are blue.

453
00:25:19,900 --> 00:25:22,400
And furthermore,
if you look at this

454
00:25:22,400 --> 00:25:25,300
in the retinal surfaces
in the fovea area,

455
00:25:25,300 --> 00:25:29,280
there appear to be very few
blue ones in the fovea itself.

456
00:25:30,480 --> 00:25:32,635
So the blue cones
are less numerous.

457
00:25:34,820 --> 00:25:38,860
And consequently,
it became a puzzle

458
00:25:38,860 --> 00:25:43,040
of how do they contribute
to color vision.

459
00:25:43,040 --> 00:25:48,750
Now, to exemplify this further,
let me show you a slide here.

460
00:25:50,180 --> 00:25:52,250
Here what we vary is
a spatial frequency.

461
00:25:53,350 --> 00:25:55,550
And what you can see
is-- I think most of you

462
00:25:55,550 --> 00:25:57,480
probably don't
see anything here.

463
00:25:57,480 --> 00:26:00,310
But most of you
probably still see this.

464
00:26:00,310 --> 00:26:02,640
This is the same spatial
frequency as this.

465
00:26:02,640 --> 00:26:06,000
But this activates
the blue cones mostly.

466
00:26:06,000 --> 00:26:09,000
And this activates
your red cones.

467
00:26:10,070 --> 00:26:13,205
And you can see that your
acuity is much, much lower

468
00:26:13,205 --> 00:26:15,080
when you only have your
blue cones available.

469
00:26:16,270 --> 00:26:20,200
And that's because only
one out of eight blue cones

470
00:26:20,200 --> 00:26:22,540
exist in the retina.

471
00:26:22,540 --> 00:26:24,435
So there's this very,
very clear distinction.

472
00:26:25,570 --> 00:26:30,306
So now let's talk about
the photoreceptors.

473
00:26:32,290 --> 00:26:38,690
Here we have an
absorption spectrum,

474
00:26:38,690 --> 00:26:42,320
or I should say a series
of absorption spectra,

475
00:26:42,320 --> 00:26:45,810
for the four kinds
of photoreceptors.

476
00:26:45,810 --> 00:26:47,945
And the fourth one is your rods.

477
00:26:48,980 --> 00:26:51,020
So what you see here--
and each of them

478
00:26:51,020 --> 00:26:52,625
are fairly broadly tuned.

479
00:26:55,113 --> 00:26:56,113
Here we have nanometers.

480
00:26:57,910 --> 00:27:00,540
And I'm sure all of
you know this already,

481
00:27:00,540 --> 00:27:03,396
that 1 nanometer is a
billionth of a meter.

482
00:27:03,396 --> 00:27:05,270
So we're talking about
incredibly, incredibly

483
00:27:05,270 --> 00:27:07,050
high frequencies.

484
00:27:09,070 --> 00:27:12,390
So that the important
thing to remember here

485
00:27:12,390 --> 00:27:17,170
is that each of these cones
is fairly broadly tuned.

486
00:27:17,170 --> 00:27:20,520
And so consequently, any light
that comes into the eye tends

487
00:27:20,520 --> 00:27:25,710
to activate all of the cones
unless they're at the very

488
00:27:25,710 --> 00:27:27,140
extremes. .

489
00:27:27,140 --> 00:27:32,220
And so indeed, as Young
and Helmholtz had proposed,

490
00:27:32,220 --> 00:27:34,920
somehow we have to derive
our color experience

491
00:27:34,920 --> 00:27:36,840
from the relative
amount of activity

492
00:27:36,840 --> 00:27:42,660
from these different cone types.

493
00:27:42,660 --> 00:27:45,912
But then as I've
mentioned to you,

494
00:27:45,912 --> 00:27:52,430
Herring felt that that was not
sufficient to explain our color

495
00:27:52,430 --> 00:27:53,380
abilities.

496
00:27:53,380 --> 00:28:00,010
And so what he did then is
to move on and had people,

497
00:28:00,010 --> 00:28:04,700
especially much more recently,
examine more closely what

498
00:28:04,700 --> 00:28:11,030
the center-surround organization
is of the different midget

499
00:28:11,030 --> 00:28:12,850
cells in the retina.

500
00:28:12,850 --> 00:28:15,640
And if you remember,
initially I told you

501
00:28:15,640 --> 00:28:19,200
that the prime theory was
that the center comes up,

502
00:28:19,200 --> 00:28:22,560
is comprised in the central
retina of just a single cone

503
00:28:22,560 --> 00:28:25,160
and the surround of its
color opponent cone.

504
00:28:26,520 --> 00:28:30,770
But then, when people began to
study this very carefully using

505
00:28:30,770 --> 00:28:35,110
a combination of
recordings and anatomy,

506
00:28:35,110 --> 00:28:37,310
they found that actually
the surround is not

507
00:28:37,310 --> 00:28:40,660
specific to one
type of cone input.

508
00:28:40,660 --> 00:28:41,940
It's mixed.

509
00:28:41,940 --> 00:28:45,050
And so then people
began to model that.

510
00:28:45,050 --> 00:28:47,760
And they found that
this arrangement

511
00:28:47,760 --> 00:28:50,220
is almost as good
as this arrangement.

512
00:28:50,220 --> 00:28:52,050
And this is the truth, actually.

513
00:28:52,050 --> 00:28:53,310
That's how it is.

514
00:28:53,310 --> 00:28:57,840
And so then, just to remind
you, the parasol cells

515
00:28:57,840 --> 00:29:00,560
have a mixture of these
inputs, both in the center

516
00:29:00,560 --> 00:29:01,870
and the surround.

517
00:29:01,870 --> 00:29:06,380
And as we had discussed, the
parasol system cannot tell you

518
00:29:06,380 --> 00:29:06,945
about colors.

519
00:29:08,230 --> 00:29:09,980
This I mentioned to you before.

520
00:29:09,980 --> 00:29:12,510
The midget system gives
a more sustained response

521
00:29:12,510 --> 00:29:14,510
than the parasol system.

522
00:29:14,510 --> 00:29:17,250
So now I also showed
you this diagram.

523
00:29:17,250 --> 00:29:23,490
And we established that the
green and the red cones each

524
00:29:23,490 --> 00:29:25,940
give rise to an
on and off system

525
00:29:25,940 --> 00:29:28,090
at the level of
the bipolar cells,

526
00:29:28,090 --> 00:29:32,810
and then give rise to the
on and off ganglion cells.

527
00:29:32,810 --> 00:29:35,200
The red and green on
and off ganglion cells.

528
00:29:35,200 --> 00:29:39,220
Now, the blue system
is more complicated

529
00:29:39,220 --> 00:29:46,510
because if you would have had
four different kinds of cones,

530
00:29:46,510 --> 00:29:49,210
meaning a blue one
and a yellow one,

531
00:29:49,210 --> 00:29:51,540
then probably would have
had a similar arrangement.

532
00:29:51,540 --> 00:29:58,090
But nature somehow had failed to
create a yellow cone because it

533
00:29:58,090 --> 00:30:00,640
felt it was not necessary--
because if you have

534
00:30:00,640 --> 00:30:07,204
an equal mix of red and
green, you get yellow,

535
00:30:07,204 --> 00:30:08,995
because remember,
they're not on opposites.

536
00:30:14,140 --> 00:30:14,900
Sorry.

537
00:30:14,900 --> 00:30:16,580
They're opposite.

538
00:30:16,580 --> 00:30:25,630
But if you mix them, you can
create an impression of yellow

539
00:30:25,630 --> 00:30:29,710
because they're not along
the exact axes for the colors

540
00:30:29,710 --> 00:30:30,740
themselves.

541
00:30:30,740 --> 00:30:33,060
So the argument was
therefore that you

542
00:30:33,060 --> 00:30:42,400
must have blue on and
blue/yellow on ganglion cells.

543
00:30:42,400 --> 00:30:44,450
But whether this
is really the case

544
00:30:44,450 --> 00:30:46,434
is still to some degree debated.

545
00:30:46,434 --> 00:30:47,975
People have done a
lot of recordings.

546
00:30:49,750 --> 00:30:52,450
And much of this
was done not only

547
00:30:52,450 --> 00:30:55,282
in the retina, but also your
lateral geniculate nucleus.

548
00:30:55,282 --> 00:30:56,740
And so the question
was, let's just

549
00:30:56,740 --> 00:31:02,600
find out what is the
color tuning of cells

550
00:31:02,600 --> 00:31:05,910
in the lateral geniculate
nucleus to understand this.

551
00:31:05,910 --> 00:31:10,440
And to do this, when we went
back to the color circle

552
00:31:10,440 --> 00:31:14,690
and presented stimuli
along the color circle

553
00:31:14,690 --> 00:31:17,240
through the receptive
fields of these cells

554
00:31:17,240 --> 00:31:18,735
to see how they responded.

555
00:31:20,070 --> 00:31:24,950
And here's an example of
a so-called blue ON cell.

556
00:31:24,950 --> 00:31:29,285
It shows here you [INAUDIBLE]
the cell is tuned.

557
00:31:30,680 --> 00:31:34,020
Sharply tuned,
mostly to 90 degrees.

558
00:31:34,020 --> 00:31:35,420
The yellow cell is the opposite.

559
00:31:36,650 --> 00:31:41,090
And green OFF cell here
and a green ON cell here.

560
00:31:41,090 --> 00:31:45,640
So what happens then is when you
take a lot of sample of these,

561
00:31:45,640 --> 00:31:48,610
huge sample of
them, what you find

562
00:31:48,610 --> 00:31:51,770
is that in the lateral
geniculate nucleus,

563
00:31:51,770 --> 00:31:56,150
you don't get any cells
that are at the diagonals.

564
00:31:56,150 --> 00:31:59,910
All the cells fall into
these major four categories

565
00:31:59,910 --> 00:32:01,365
along your cardinal axes.

566
00:32:04,480 --> 00:32:06,680
And then if you take
a big huge sample,

567
00:32:06,680 --> 00:32:09,380
you come up with the
following summary--

568
00:32:09,380 --> 00:32:11,790
that you have red ON
cells, red OFF cells.

569
00:32:11,790 --> 00:32:14,390
You have green ON cells and
you have green OFF cells.

570
00:32:14,390 --> 00:32:16,420
And at least some people
claim that you only

571
00:32:16,420 --> 00:32:18,250
have blue ON and Yellow ONs.

572
00:32:18,250 --> 00:32:20,410
You don't have blue
OFFs and yellow OFFs.

573
00:32:20,410 --> 00:32:21,725
That is still under debate.

574
00:32:23,360 --> 00:32:25,870
But certainly, if you
do extensive recordings,

575
00:32:25,870 --> 00:32:29,100
if you find any OFF
blue and yellow OFFs,

576
00:32:29,100 --> 00:32:31,290
they're extremely rare
if they exist at all.

577
00:32:33,900 --> 00:32:40,450
So now to try to gain
yet further understanding

578
00:32:40,450 --> 00:32:51,720
of the role various areas
play in color vision,

579
00:32:51,720 --> 00:32:54,040
is that one can make lesions.

580
00:32:54,040 --> 00:32:56,300
And I've already
told you what happens

581
00:32:56,300 --> 00:32:59,850
when you make either
parvocellular or magnocellular

582
00:32:59,850 --> 00:33:00,550
lesion.

583
00:33:00,550 --> 00:33:05,650
That blocks the parasol
or the midget systems.

584
00:33:05,650 --> 00:33:09,290
And that the midget system is
essential for color vision.

585
00:33:09,290 --> 00:33:11,360
And then I showed you this.

586
00:33:11,360 --> 00:33:14,650
Here is a geniculate.

587
00:33:14,650 --> 00:33:18,930
If you take out this area,
you block the midget system.

588
00:33:18,930 --> 00:33:23,550
And if you take out this area,
you block the parasol system.

589
00:33:23,550 --> 00:33:25,970
So if you do that--
here is an overall view

590
00:33:25,970 --> 00:33:27,330
of the monkey brain.

591
00:33:27,330 --> 00:33:29,290
Here's area V1 Here's the V2.

592
00:33:29,290 --> 00:33:31,990
And here's area V4, which
I've mentioned to you

593
00:33:31,990 --> 00:33:37,210
before had been believed to
play a central role in color.

594
00:33:37,210 --> 00:33:39,310
So then it becomes
important to see just

595
00:33:39,310 --> 00:33:42,616
what happens when you make
lesions in these areas.

596
00:33:42,616 --> 00:33:44,240
In this case, you
can make a lesion V4.

597
00:33:45,400 --> 00:33:48,500
And then we can examine
color discrimination.

598
00:33:49,540 --> 00:33:51,040
And I told you in
a monkey, what you

599
00:33:51,040 --> 00:33:53,810
do is you use an oddity task.

600
00:33:53,810 --> 00:33:57,720
After fixation, you can
present the odd stimulus either

601
00:33:57,720 --> 00:33:59,510
in the intact parts
of the visual field

602
00:33:59,510 --> 00:34:02,425
or those that had been lesions.

603
00:34:04,850 --> 00:34:06,780
This is a high contrast.

604
00:34:06,780 --> 00:34:08,370
And if you do that
experiment-- I

605
00:34:08,370 --> 00:34:11,050
showed you part of
this data before.

606
00:34:11,050 --> 00:34:17,429
I showed you that after
lesion of the parvocellular

607
00:34:17,429 --> 00:34:21,110
geniculate, which blocks
the midget system,

608
00:34:21,110 --> 00:34:24,159
you totally lose the
ability to discriminate

609
00:34:24,159 --> 00:34:26,980
even these high colors,
red, green, and blue.

610
00:34:29,489 --> 00:34:32,679
But no deficit
arises when you make

611
00:34:32,679 --> 00:34:35,850
a lesion in the magnocellular
portion of the geniculate.

612
00:34:35,850 --> 00:34:37,639
And then the big
question became what

613
00:34:37,639 --> 00:34:44,719
happens in V4, since
that had for decades been

614
00:34:44,719 --> 00:34:50,040
declared to be a color area.

615
00:34:50,040 --> 00:34:54,320
And what was surprising about
that is that after V4 lesion,

616
00:34:54,320 --> 00:34:57,660
there was only a
small deficit in color

617
00:34:57,660 --> 00:35:00,430
for this high-contrast stimuli.

618
00:35:00,430 --> 00:35:04,450
So therefore people have to
go on and do a more careful

619
00:35:04,450 --> 00:35:06,740
detailed study to
see what happens

620
00:35:06,740 --> 00:35:10,300
if you use less
saturated colors.

621
00:35:10,300 --> 00:35:11,990
Remember that the
degree of saturation

622
00:35:11,990 --> 00:35:15,520
is one of the important
factors for analyzing color.

623
00:35:16,850 --> 00:35:20,320
And here we have an example
of a very low saturation.

624
00:35:20,320 --> 00:35:22,700
And here we have a
somewhat high saturation.

625
00:35:22,700 --> 00:35:25,100
And you can vary
that systematically

626
00:35:25,100 --> 00:35:28,160
and see what happens
after V4 lesion

627
00:35:28,160 --> 00:35:30,300
and after other
kinds of lesions.

628
00:35:30,300 --> 00:35:33,230
And here's an example of
what happens in a V4 lesion.

629
00:35:33,230 --> 00:35:35,590
And this is what
happens in an MT lesion.

630
00:35:35,590 --> 00:35:38,990
It shows no deficit at
all with an MT lesion,

631
00:35:38,990 --> 00:35:42,530
indicating that MT does
not play a crucial role

632
00:35:42,530 --> 00:35:44,100
in analyzing color.

633
00:35:44,100 --> 00:35:47,410
We do get this
significant deficit here,

634
00:35:47,410 --> 00:35:48,510
but it's a small one.

635
00:35:49,800 --> 00:35:54,250
So even at low
saturations, the monkey

636
00:35:54,250 --> 00:35:56,670
still can do colors
reasonably well.

637
00:35:56,670 --> 00:36:00,550
So it's not like V4
is the color area.

638
00:36:00,550 --> 00:36:02,570
There apparently
are several areas

639
00:36:02,570 --> 00:36:04,760
in the brain that
can process color.

640
00:36:04,760 --> 00:36:09,370
And V4 does that in
addition to performing

641
00:36:09,370 --> 00:36:13,980
several other analyses
that we will discuss

642
00:36:13,980 --> 00:36:16,330
in a couple of
sessions from now.

643
00:36:16,330 --> 00:36:21,700
So now we're going to turn
to another very interesting

644
00:36:21,700 --> 00:36:24,525
phenomenon, which is what
is called isoluminance.

645
00:36:25,840 --> 00:36:27,460
What is isoluminance?

646
00:36:27,460 --> 00:36:32,310
Isoluminance is the
presentation of different colors

647
00:36:32,310 --> 00:36:37,940
that have the same
illumination level.

648
00:36:39,480 --> 00:36:43,040
So I'm going to give you
a few examples of this.

649
00:36:44,859 --> 00:36:46,400
If you look at this--
how many of you

650
00:36:46,400 --> 00:36:48,460
can read those words there?

651
00:36:48,460 --> 00:36:49,860
Pretty tough, isn't it?

652
00:36:52,580 --> 00:36:55,120
This can be set up in such
a way that it's even worse.

653
00:36:55,120 --> 00:37:00,220
But because of the light bulb in
there, it's not really perfect.

654
00:37:00,220 --> 00:37:01,820
But just to give
you a sense of it

655
00:37:01,820 --> 00:37:04,020
obviously goes to a
lot easier to read.

656
00:37:05,510 --> 00:37:08,450
So that's because this
is close to isoluminance,

657
00:37:08,450 --> 00:37:13,720
at which our ability to see
objects is much impeded.

658
00:37:13,720 --> 00:37:15,800
Not eliminated, but impeded.

659
00:37:15,800 --> 00:37:18,550
So what can you do to do
this kind of experiment

660
00:37:18,550 --> 00:37:19,680
systematically?

661
00:37:19,680 --> 00:37:24,560
What you can do here is you
can use various capabilities.

662
00:37:24,560 --> 00:37:29,270
Stereopsis, motion parallax--
just motion-- and texture.

663
00:37:29,270 --> 00:37:34,460
And if you do that, and you vary
the red/green luminance ratio,

664
00:37:34,460 --> 00:37:39,100
you can see that there's a
dramatic drop off at around

665
00:37:39,100 --> 00:37:42,430
very close to a luminous
ratio of one and one.

666
00:37:43,870 --> 00:37:47,330
So indeed, our ability
to process information

667
00:37:47,330 --> 00:37:50,180
in the absence of
luminance information

668
00:37:50,180 --> 00:37:51,385
is greatly compromised.

669
00:37:53,440 --> 00:37:55,780
So then the question
came up-- I mean,

670
00:37:55,780 --> 00:38:01,560
we're talking about here such
things as motion perception,

671
00:38:01,560 --> 00:38:04,020
texture perception,
and stereopsis.

672
00:38:04,020 --> 00:38:05,570
And as we had
discussed before, we

673
00:38:05,570 --> 00:38:09,590
have already established that
texture and stereopsis are

674
00:38:09,590 --> 00:38:13,260
processed to a large extent
by the midget system.

675
00:38:13,260 --> 00:38:15,380
And that motion
to a large extent

676
00:38:15,380 --> 00:38:17,320
is processed by
the parasol system.

677
00:38:19,130 --> 00:38:22,940
But all three of these
types of capabilities

678
00:38:22,940 --> 00:38:25,460
are compromised isoluminance

679
00:38:25,460 --> 00:38:31,550
So a series of experiments
had been carried out

680
00:38:31,550 --> 00:38:37,590
in which people argued that
when you present stimuli

681
00:38:37,590 --> 00:38:48,730
at isoluminance, you render
the parasol system unresponsive

682
00:38:48,730 --> 00:38:51,360
because it gets equal input
in the center-surround,

683
00:38:51,360 --> 00:38:53,610
some red and green
cones and blue cones.

684
00:38:55,790 --> 00:38:59,240
So therefore, they
concluded, that if there's

685
00:38:59,240 --> 00:39:05,370
a deficit in
performance like this,

686
00:39:05,370 --> 00:39:10,280
that must reflect the fact
that the parasol system plays

687
00:39:10,280 --> 00:39:12,510
an important role
in the analysis.

688
00:39:12,510 --> 00:39:14,860
But as I told you
before, stereopsis

689
00:39:14,860 --> 00:39:17,200
is processed predominantly
by the midget system

690
00:39:17,200 --> 00:39:18,830
and so is texture.

691
00:39:18,830 --> 00:39:20,890
So that raised quite a problem.

692
00:39:20,890 --> 00:39:24,680
And so people began to run
experiments in which they

693
00:39:24,680 --> 00:39:31,360
recorded from the parasol system
to see what happens actually

694
00:39:31,360 --> 00:39:35,670
to unit responses when
you are at isoluminance.

695
00:39:35,670 --> 00:39:37,450
And that resulted
in quite a surprise.

696
00:39:39,320 --> 00:39:43,710
So here is an example
of a magnocellular cell,

697
00:39:43,710 --> 00:39:46,313
meaning a cell that gets
input from the parasol system

698
00:39:46,313 --> 00:39:47,740
of the retina.

699
00:39:47,740 --> 00:39:51,460
And you alternate between red
and green here repeatedly,

700
00:39:51,460 --> 00:39:55,460
collect the data, and you
vary the red/green ratio.

701
00:39:56,990 --> 00:40:01,190
And you can see here that
throughout the whole thing,

702
00:40:01,190 --> 00:40:02,895
the cell continues to respond.

703
00:40:05,080 --> 00:40:07,410
So that became quite a puzzle.

704
00:40:07,410 --> 00:40:10,790
We were not able to
silence the magnocellular

705
00:40:10,790 --> 00:40:12,530
system at isoluminance.

706
00:40:15,280 --> 00:40:19,830
So the question is, how come?

707
00:40:19,830 --> 00:40:23,120
Well, the answer is,
as you had seen before,

708
00:40:23,120 --> 00:40:26,270
that the parasol system
is extremely sensitive.

709
00:40:26,270 --> 00:40:29,000
It gets input even in the
center of the receptive

710
00:40:29,000 --> 00:40:32,175
field of each cell
for many of-- I

711
00:40:32,175 --> 00:40:36,490
should maybe say
several-- different cones.

712
00:40:38,360 --> 00:40:40,880
And because of that,
there's much more

713
00:40:40,880 --> 00:40:44,800
information and excitation
coming into those cells

714
00:40:44,800 --> 00:40:47,180
than into those that
are in the midget system

715
00:40:47,180 --> 00:40:48,630
that most of which
only get input

716
00:40:48,630 --> 00:40:51,050
from a single cone
for the center.

717
00:40:51,050 --> 00:40:54,980
So that being the case
then, we can move on

718
00:40:54,980 --> 00:40:57,150
and ask a question
of what about if you

719
00:40:57,150 --> 00:41:00,055
do the same kind of
experiment in area MT.

720
00:41:00,055 --> 00:41:01,110
Now, who remembers?

721
00:41:01,110 --> 00:41:04,120
Area MT gets most of its
input from which system?

722
00:41:06,090 --> 00:41:06,810
Good.

723
00:41:06,810 --> 00:41:11,550
Mostly from the parasol system
going through the magnocellular

724
00:41:11,550 --> 00:41:12,090
layers.

725
00:41:12,090 --> 00:41:15,040
So now you can
record in the area MT

726
00:41:15,040 --> 00:41:17,620
and do the same kind of
experiment I just described

727
00:41:17,620 --> 00:41:21,580
to you the retina in the
lateral geniculate nucleus.

728
00:41:21,580 --> 00:41:24,970
And so what you can do
here is you have a monkey.

729
00:41:24,970 --> 00:41:26,230
The monkey fixates.

730
00:41:26,230 --> 00:41:31,730
And you move a bar of light
across the receptor field

731
00:41:31,730 --> 00:41:34,870
back and forth which is
isoluminant-- red/green

732
00:41:34,870 --> 00:41:36,305
in this case.

733
00:41:36,305 --> 00:41:39,330
And then you see how
well the cell responds.

734
00:41:39,330 --> 00:41:42,030
And then for comparison
what you can do

735
00:41:42,030 --> 00:41:45,680
is you can use a luminance
grading, bright and dark

736
00:41:45,680 --> 00:41:49,590
in the background,
to which we know

737
00:41:49,590 --> 00:41:57,600
that the parasol cells and
the cells in the area MT

738
00:41:57,600 --> 00:41:59,270
respond vigorously.

739
00:41:59,270 --> 00:42:03,950
So now we are going to
compare what the difference is

740
00:42:03,950 --> 00:42:07,550
between these two conditions,
meaning luminance grading as

741
00:42:07,550 --> 00:42:11,400
opposed to an isoluminant
color grading.

742
00:42:11,400 --> 00:42:14,540
So if you do that experiment,
you're in for surprise.

743
00:42:14,540 --> 00:42:17,710
Here's an example of one
cell in which there's

744
00:42:17,710 --> 00:42:19,850
a much more vigorous
response to luminance.

745
00:42:19,850 --> 00:42:22,545
Here are the various
luminance contrasts.

746
00:42:24,290 --> 00:42:26,670
And here is a chrominance one.

747
00:42:26,670 --> 00:42:28,790
And the cell doesn't
respond as well here,

748
00:42:28,790 --> 00:42:31,180
but it still responds
reasonably well.

749
00:42:31,180 --> 00:42:34,550
Then you take another cell
and you get the opposite.

750
00:42:34,550 --> 00:42:37,620
And this particular
cell, still in area MT,

751
00:42:37,620 --> 00:42:39,930
responds a bit more
vigorously to chrominance

752
00:42:39,930 --> 00:42:41,280
than to luminance.

753
00:42:41,280 --> 00:42:44,210
And so if you add
this all up and record

754
00:42:44,210 --> 00:42:47,790
for many, many cells, you
find that the cells in area

755
00:42:47,790 --> 00:42:50,785
MT, just like in the
lateral geniculate nucleus

756
00:42:50,785 --> 00:42:56,000
and in the retina, respond
quite well at isoluminance--

757
00:42:56,000 --> 00:43:00,650
the parasol cells and the
same cells, of course,

758
00:43:00,650 --> 00:43:04,350
in the lateral geniculate
nucleus, and in area MT.

759
00:43:04,350 --> 00:43:11,620
So this area MT is one that
responds surprisingly well

760
00:43:11,620 --> 00:43:15,920
to anything that's out there
that results in a change,

761
00:43:15,920 --> 00:43:18,440
whether the change is produced
by virtue of chrominance

762
00:43:18,440 --> 00:43:20,550
or by virtue of luminance.

763
00:43:20,550 --> 00:43:25,760
And that, in fact, is one of
the very important attributes

764
00:43:25,760 --> 00:43:31,500
of the parasol system.

765
00:43:31,500 --> 00:43:35,690
Namely, and that's so
numerous in the periphery,

766
00:43:35,690 --> 00:43:38,920
is to be able to detect just
about anything that happens--

767
00:43:38,920 --> 00:43:47,680
motion, flicker, just the onset
of a single stimulus, whatever.

768
00:43:47,680 --> 00:43:49,730
But that system
is very sensitive

769
00:43:49,730 --> 00:43:51,710
and can tell something
has happened there.

770
00:43:53,280 --> 00:43:55,300
That's what that system
is very good for.

771
00:43:55,300 --> 00:43:57,820
It's not that good,
obviously, for seeing

772
00:43:57,820 --> 00:43:59,880
very, very fine detail.

773
00:43:59,880 --> 00:44:01,320
But it's very sensitive.

774
00:44:01,320 --> 00:44:02,990
And it's very, very
good for detecting

775
00:44:02,990 --> 00:44:04,155
motion and appearances.

776
00:44:05,710 --> 00:44:11,010
So now we're going to move
on and talk about a topic

777
00:44:11,010 --> 00:44:13,270
that I'm sure many of
you have an interest in.

778
00:44:13,270 --> 00:44:18,340
And that has to do with
deficiencies in color,

779
00:44:18,340 --> 00:44:20,333
often referred to
as color blindness.

780
00:44:21,770 --> 00:44:26,890
And the first fact
is that if you

781
00:44:26,890 --> 00:44:31,320
look at the incidence of
color deficits in humans,

782
00:44:31,320 --> 00:44:37,660
8 out of 100 males among
Caucasians, 5 in a 100

783
00:44:37,660 --> 00:44:40,440
in Asians, 3 in a
100 in Africans.

784
00:44:40,440 --> 00:44:43,070
In females, it's
much, much less.

785
00:44:43,070 --> 00:44:45,100
It's 10 times less frequent.

786
00:44:45,100 --> 00:44:48,400
But still overall,
that's quite a number

787
00:44:48,400 --> 00:44:53,000
of people who have some
sort of deficiency in color.

788
00:44:53,000 --> 00:44:55,970
So now that we know that, we
can ask the next question-- what

789
00:44:55,970 --> 00:44:59,580
kinds of color
deficits can we denote?

790
00:44:59,580 --> 00:45:03,160
And the type, these are
given fancy names, which

791
00:45:03,160 --> 00:45:05,860
are called protanopes,
deuteranopes, and tritanopes.

792
00:45:05,860 --> 00:45:08,050
And that simply
refers to the fact

793
00:45:08,050 --> 00:45:12,120
that protanopes lack
long-wavelength cones, which

794
00:45:12,120 --> 00:45:13,670
are of course the red cones.

795
00:45:13,670 --> 00:45:16,042
The deuteranopes lack
medium-wavelength cones,

796
00:45:16,042 --> 00:45:17,000
if you talk green ones.

797
00:45:17,000 --> 00:45:20,410
And the tritanopes lack
the short-wavelength cones,

798
00:45:20,410 --> 00:45:22,220
which are the blue ones.

799
00:45:22,220 --> 00:45:25,000
So that is the basic
types of deficits.

800
00:45:25,000 --> 00:45:27,130
Now, some people have
a combination of these.

801
00:45:27,130 --> 00:45:30,830
Some people have no color vision
at all, but that's very rare.

802
00:45:32,110 --> 00:45:37,460
Quite common are
these three types.

803
00:45:38,610 --> 00:45:41,327
You somehow don't have one
particular kind of cone

804
00:45:41,327 --> 00:45:46,500
or you have very few of
them, or you have them,

805
00:45:46,500 --> 00:45:47,910
but they don't function right.

806
00:45:49,140 --> 00:45:56,010
So now, how do we establish
our ability to see colors

807
00:45:56,010 --> 00:45:58,050
and whether we have
normal color vision?

808
00:45:58,050 --> 00:45:59,730
Now, that's very interesting.

809
00:45:59,730 --> 00:46:01,860
A number of tests
have been developed.

810
00:46:01,860 --> 00:46:04,465
And the most famous of
those, the oldest one--

811
00:46:04,465 --> 00:46:11,330
let me go back, sorry-- are
the so-called Ishihara plates.

812
00:46:11,330 --> 00:46:14,882
And the next one's the
Farnsworth-Munsell Hue Test.

813
00:46:14,882 --> 00:46:16,840
And the third one I was
going to tell you about

814
00:46:16,840 --> 00:46:19,140
is the dynamic computer test.

815
00:46:19,140 --> 00:46:21,360
So let's look at
the Ishihara plates

816
00:46:21,360 --> 00:46:24,720
If you look at that,
how many of you

817
00:46:24,720 --> 00:46:28,270
can see what is written there?

818
00:46:30,020 --> 00:46:31,006
What is it?

819
00:46:31,006 --> 00:46:31,900
AUDIENCE: Eight.

820
00:46:31,900 --> 00:46:32,230
PROFESSOR: Eight.

821
00:46:32,230 --> 00:46:32,730
Very good.

822
00:46:32,730 --> 00:46:34,560
Anybody who doesn't see it?

823
00:46:34,560 --> 00:46:35,735
OK, you don't see that.

824
00:46:36,870 --> 00:46:38,550
We'll get back to
you in a minute.

825
00:46:39,750 --> 00:46:45,340
So now another test
is a dynamic one.

826
00:46:45,340 --> 00:46:47,740
The reason for
using a dynamic test

827
00:46:47,740 --> 00:46:53,830
is that the
so-called isoluminant

828
00:46:53,830 --> 00:46:56,750
point of individuals
is not the same.

829
00:46:56,750 --> 00:46:59,440
Expect a lot of variations
from person to person.

830
00:46:59,440 --> 00:47:01,630
And so this test
is the dynamic one.

831
00:47:01,630 --> 00:47:03,800
They presented for
the background,

832
00:47:03,800 --> 00:47:09,650
as you can see here, different
luminance levels in gray.

833
00:47:09,650 --> 00:47:11,630
And when the computer
starts running,

834
00:47:11,630 --> 00:47:14,490
these keep exchanging each
other in randomized fashions.

835
00:47:14,490 --> 00:47:15,977
It's the dynamic view.

836
00:47:15,977 --> 00:47:17,560
And then you have a
central area here.

837
00:47:17,560 --> 00:47:19,560
Everybody can read this, right?

838
00:47:19,560 --> 00:47:20,520
What's the word?

839
00:47:20,520 --> 00:47:21,146
AUDIENCE: Lite.

840
00:47:21,146 --> 00:47:21,811
PROFESSOR: Lite.

841
00:47:21,811 --> 00:47:22,470
Very good.

842
00:47:22,470 --> 00:47:27,570
So now instead of presenting
these letters here

843
00:47:27,570 --> 00:47:33,070
in a high brightness overall,
we can present them in color.

844
00:47:33,070 --> 00:47:34,930
So I'm now going to
show you an easy test.

845
00:47:36,540 --> 00:47:37,950
Anybody can read this?

846
00:47:37,950 --> 00:47:39,280
Anybody who cannot read it?

847
00:47:40,560 --> 00:47:41,270
Can you read it?

848
00:47:43,898 --> 00:47:44,971
AUDIENCE: Ish.

849
00:47:44,971 --> 00:47:45,595
PROFESSOR: Ish.

850
00:47:46,790 --> 00:47:49,277
So what's the word?

851
00:47:49,277 --> 00:47:50,211
AUDIENCE: MIT.

852
00:47:50,211 --> 00:47:50,980
PROFESSOR: MIT.

853
00:47:50,980 --> 00:47:51,490
What's that?

854
00:47:54,940 --> 00:47:56,795
So now I'm going to
make it more difficult.

855
00:47:56,795 --> 00:47:57,810
You guys ready?

856
00:47:59,090 --> 00:48:00,207
What is this one?

857
00:48:00,207 --> 00:48:01,430
AUDIENCE: Fit.

858
00:48:01,430 --> 00:48:03,249
PROFESSOR: Are you having a--

859
00:48:03,249 --> 00:48:04,790
AUDIENCE: Yeah, it's
a little harder.

860
00:48:04,790 --> 00:48:06,290
PROFESSOR: You're
having a fit, huh?

861
00:48:06,790 --> 00:48:08,026
Can you read that one?

862
00:48:08,026 --> 00:48:08,519
AUDIENCE: No, I can't.

863
00:48:08,519 --> 00:48:09,102
PROFESSOR: No.

864
00:48:11,480 --> 00:48:14,930
So now it seems like we
do have one person here

865
00:48:14,930 --> 00:48:17,550
who has perhaps a
mild color deficiency.

866
00:48:17,550 --> 00:48:21,330
And so now the question
comes up, even if all of you

867
00:48:21,330 --> 00:48:24,510
want to do this, what can
you do to test yourself?

868
00:48:26,050 --> 00:48:27,740
Well, so let me
tell you about that.

869
00:48:27,740 --> 00:48:32,310
There is a so-called
Farnsworth-Munsell color test.

870
00:48:32,310 --> 00:48:35,200
So what you want to do here is--
can you read this down here?

871
00:48:40,600 --> 00:48:45,831
Get on Google and just type in
Farnsworth-Munsell color test

872
00:48:45,831 --> 00:48:46,330
online.

873
00:48:47,730 --> 00:48:51,450
If you type that in, all
kinds of things come up.

874
00:48:51,450 --> 00:48:54,000
Click on the topmost one.

875
00:48:54,000 --> 00:48:58,000
And then what you see
here is a set of colors.

876
00:48:58,000 --> 00:48:59,570
Actually, there are four sets.

877
00:48:59,570 --> 00:49:00,720
And each of these has 20.

878
00:49:00,720 --> 00:49:02,610
And I just drew
a few of them in.

879
00:49:02,610 --> 00:49:05,490
And your task is
then-- each of these

880
00:49:05,490 --> 00:49:09,780
can be moved-- to
arrange them in an order,

881
00:49:09,780 --> 00:49:13,140
going from this color
to that color in order.

882
00:49:13,140 --> 00:49:15,400
So you do that for
all four of them.

883
00:49:15,400 --> 00:49:19,100
And after did that, you
can click on the bottom.

884
00:49:19,100 --> 00:49:24,930
And it will tell you what your
score is for each of these.

885
00:49:26,060 --> 00:49:29,480
And so if your color
vision is very good,

886
00:49:29,480 --> 00:49:32,840
it gives you sort of
a set of histograms.

887
00:49:32,840 --> 00:49:35,960
And if the histogram is very,
very low, then you're good.

888
00:49:35,960 --> 00:49:38,590
And if it's high,
then you're not.

889
00:49:38,590 --> 00:49:41,357
What you can actually do is
when this first comes on,

890
00:49:41,357 --> 00:49:42,440
these are in random order.

891
00:49:43,870 --> 00:49:46,570
Your task is to
put them in order.

892
00:49:46,570 --> 00:49:50,150
But at the bottom
here, it says score.

893
00:49:50,150 --> 00:49:53,710
So if you click on
score, it will give you

894
00:49:53,710 --> 00:49:57,290
the histograms uncorrected.

895
00:49:57,290 --> 00:49:58,680
And they're all
going to be high.

896
00:49:58,680 --> 00:49:59,910
Then you do this work.

897
00:49:59,910 --> 00:50:02,060
And then you click
on it again and see

898
00:50:02,060 --> 00:50:04,897
how good your color
vision is based on that.

899
00:50:04,897 --> 00:50:05,980
It's a bit time-consuming.

900
00:50:07,580 --> 00:50:10,790
But if you are interested in
getting a sense of just how

901
00:50:10,790 --> 00:50:14,560
good your color vision is,
this is a rather good test,

902
00:50:14,560 --> 00:50:16,655
which is readily
available on the internet.

903
00:50:18,070 --> 00:50:21,345
Does anybody have any
questions about this portion?

904
00:50:23,450 --> 00:50:24,190
Good.

905
00:50:24,190 --> 00:50:28,220
So now as a result
of this, we're

906
00:50:28,220 --> 00:50:34,250
going to move on and spend the
remainder of our time talking

907
00:50:34,250 --> 00:50:40,420
about adaptation because that is
pretty closely relevant, as we

908
00:50:40,420 --> 00:50:45,040
shall see, to color
vision as well.

909
00:50:45,040 --> 00:50:47,020
So I'm going to talk
about adaptation.

910
00:50:47,020 --> 00:50:50,660
First of all, again we come up
with a number of basic facts.

911
00:50:50,660 --> 00:50:53,560
Now let me at this
point interject

912
00:50:53,560 --> 00:50:59,850
and just tell you
that all the material

913
00:50:59,850 --> 00:51:07,190
I'm talking about today
will be posted on Stellar.

914
00:51:08,380 --> 00:51:12,470
What's on Stellar now is
not an updated version.

915
00:51:12,470 --> 00:51:16,350
But what I'm talking about
today will be on Stellar

916
00:51:16,350 --> 00:51:17,225
I think by tomorrow.

917
00:51:19,080 --> 00:51:19,940
So basic facts.

918
00:51:22,200 --> 00:51:24,253
We talk about overall
levels of illumination.

919
00:51:25,370 --> 00:51:27,770
That's what's so remarkable
about the visual system.

920
00:51:27,770 --> 00:51:28,970
It's actually unbelievable.

921
00:51:28,970 --> 00:51:30,385
10 log units overall.

922
00:51:32,170 --> 00:51:36,360
But the reflected light varies
over much smaller range.

923
00:51:36,360 --> 00:51:39,775
In other words, you don't want
to look at directly at light.

924
00:51:40,790 --> 00:51:44,320
The reflected light
varies only about 20 fold.

925
00:51:46,010 --> 00:51:48,285
So now the question is,
how do we handle this?

926
00:51:48,285 --> 00:51:51,990
Well, it turns out
that the pupil, which

927
00:51:51,990 --> 00:51:55,220
does play a role
in this, can only

928
00:51:55,220 --> 00:51:58,240
adjust over a range of 16 to 1.

929
00:51:59,530 --> 00:52:02,600
So that's a long cry
from 10 log units.

930
00:52:02,600 --> 00:52:04,520
And so much of the
adaptation I would

931
00:52:04,520 --> 00:52:07,450
say, even more so the
adaptation that takes place,

932
00:52:07,450 --> 00:52:09,590
occurs in your photoreceptors.

933
00:52:11,610 --> 00:52:15,200
So here I'm saying this
again-- most light adaptation

934
00:52:15,200 --> 00:52:17,790
takes place in the
photoreceptors.

935
00:52:17,790 --> 00:52:19,310
Now, how does it take place?

936
00:52:19,310 --> 00:52:23,510
Well, the way it takes place is
that-- I mentioned this to you

937
00:52:23,510 --> 00:52:28,340
before-- the photoreceptor
molecules, like rhodopsin

938
00:52:28,340 --> 00:52:32,400
in the rods, comes
in two basic forms.

939
00:52:32,400 --> 00:52:36,590
You don't need to know
the chemistry of it,

940
00:52:36,590 --> 00:52:39,780
but you make two or simply
that it comes in two forms.

941
00:52:39,780 --> 00:52:41,780
We can call it bleached
and unbleached.

942
00:52:41,780 --> 00:52:44,725
Some people call
it open and closed.

943
00:52:44,725 --> 00:52:46,475
But let's call it
bleached and unbleached.

944
00:52:50,080 --> 00:52:51,335
This is very dynamic process.

945
00:52:51,335 --> 00:52:57,800
At any level of illumination a
certain percentage of molecules

946
00:52:57,800 --> 00:53:02,930
is bleached in each receptor
and a certain number

947
00:53:02,930 --> 00:53:04,300
is not bleached.

948
00:53:04,300 --> 00:53:07,250
And the brighter the
illumination, the more

949
00:53:07,250 --> 00:53:08,515
are bleached.

950
00:53:08,515 --> 00:53:13,600
It's dynamic, which I mean is
that the molecules constantly

951
00:53:13,600 --> 00:53:14,710
keep changing.

952
00:53:14,710 --> 00:53:17,936
So it's the overall
percentage of the ratios

953
00:53:17,936 --> 00:53:19,560
between the beached
and the unbleached.

954
00:53:20,840 --> 00:53:24,805
So that means that any increase
in the rate of which quanta

955
00:53:24,805 --> 00:53:28,256
is delivered to the eye results
in a proportional decrease

956
00:53:28,256 --> 00:53:31,150
in the number of pigment
molecules available to absorb

957
00:53:31,150 --> 00:53:33,150
those quanta because
they are bleached.

958
00:53:35,020 --> 00:53:41,880
Now, this arrangement happens
to be extremely clever.

959
00:53:41,880 --> 00:53:45,040
And this is
reflected in the fact

960
00:53:45,040 --> 00:53:49,990
that the retinal ganglion cells
are sensitive to local contrast

961
00:53:49,990 --> 00:53:50,540
differences.

962
00:53:50,540 --> 00:53:53,039
Remember, I told you there's a
center-surround organization.

963
00:53:53,039 --> 00:53:55,400
This is one of the prime
reasons we have that.

964
00:53:58,450 --> 00:54:01,110
The overwhelming
majority, like 95%

965
00:54:01,110 --> 00:54:04,650
of the cells in the retina,
the retinal ganglion cells,

966
00:54:04,650 --> 00:54:08,150
respond to contrast
differences, not

967
00:54:08,150 --> 00:54:10,395
to absolute levels
of illumination.

968
00:54:10,395 --> 00:54:13,310
And that's how we often
talk about contrast.

969
00:54:13,310 --> 00:54:16,320
And if you remember what
the contrast formula is,

970
00:54:16,320 --> 00:54:22,330
it's the contrast level
of the stimulus itself

971
00:54:22,330 --> 00:54:24,390
and the contrast level
of the background.

972
00:54:24,390 --> 00:54:28,700
You subtract one from the other,
divided by the sum of the two,

973
00:54:28,700 --> 00:54:30,250
multiplied by 100.

974
00:54:30,250 --> 00:54:32,510
I showed that to you before.

975
00:54:32,510 --> 00:54:34,580
So now here we are.

976
00:54:34,580 --> 00:54:36,930
If you talk about light
and dark adaptation,

977
00:54:36,930 --> 00:54:40,990
this is the basic outline
of the retinal connections

978
00:54:40,990 --> 00:54:42,280
that we have talked about.

979
00:54:42,280 --> 00:54:44,610
We have presented this
several times before.

980
00:54:44,610 --> 00:54:47,170
And then if you
are light adapted,

981
00:54:47,170 --> 00:54:50,910
you essentially have
non-functional rods,

982
00:54:50,910 --> 00:54:52,490
so I took them off here.

983
00:54:52,490 --> 00:54:55,360
But then when you darken
that, the opposite happens.

984
00:54:55,360 --> 00:54:57,005
And you have the rods active.

985
00:54:58,440 --> 00:55:01,230
But they all fit into
the same ganglion cells.

986
00:55:01,230 --> 00:55:04,280
And that's why at night, the
receptor fields are bigger.

987
00:55:04,280 --> 00:55:06,570
And you don't see color
at night because this

988
00:55:06,570 --> 00:55:07,990
is what the picture is.

989
00:55:07,990 --> 00:55:11,470
Now what you can do is
let's ask the question,

990
00:55:11,470 --> 00:55:15,350
how do the neurons, like
the cells in the retina,

991
00:55:15,350 --> 00:55:18,360
the retinal ganglion
cells how do they

992
00:55:18,360 --> 00:55:22,100
fire at different
levels of illumination?

993
00:55:22,100 --> 00:55:24,370
And that's quite an
interesting story

994
00:55:24,370 --> 00:55:25,890
and a very straightforward one.

995
00:55:25,890 --> 00:55:29,230
Here we have a cell
that had been adapted

996
00:55:29,230 --> 00:55:33,990
to these different levels
of background illumination.

997
00:55:36,290 --> 00:55:39,760
At minus 5, your
rods are functional.

998
00:55:39,760 --> 00:55:43,230
And what is
important to see here

999
00:55:43,230 --> 00:55:46,140
is that as you change quite
dramatically the background

1000
00:55:46,140 --> 00:55:50,280
level, the eye adapts.

1001
00:55:50,280 --> 00:55:53,140
And I should say that it's the
photoreceptors predominately

1002
00:55:53,140 --> 00:55:54,170
that do so.

1003
00:55:54,170 --> 00:55:56,850
And what they look at
are local differences.

1004
00:55:56,850 --> 00:56:01,360
And so each of these
then sees a fresh,

1005
00:56:01,360 --> 00:56:06,590
the contrast that is
created, rather than looking

1006
00:56:06,590 --> 00:56:08,400
at absolute illumination levels.

1007
00:56:08,400 --> 00:56:09,900
And that's what you
want, of course.

1008
00:56:09,900 --> 00:56:11,740
You want to be able to
drive well at night.

1009
00:56:11,740 --> 00:56:14,220
You want to be able to
drive well in the daytime.

1010
00:56:14,220 --> 00:56:18,370
And by having this system,
you can look at predominantly

1011
00:56:18,370 --> 00:56:21,800
at contrast
differences rather than

1012
00:56:21,800 --> 00:56:23,560
absolute levels of contrast.

1013
00:56:23,560 --> 00:56:25,250
So that's the arrangement here.

1014
00:56:25,250 --> 00:56:27,180
And now what we
are going to do is

1015
00:56:27,180 --> 00:56:33,297
to move on and talk about
the so-called after-effects

1016
00:56:33,297 --> 00:56:33,880
of adaptation.

1017
00:56:36,010 --> 00:56:38,980
So let me tell you how
this initially was done.

1018
00:56:40,200 --> 00:56:45,120
People ask the question, what
happens if I fix something

1019
00:56:45,120 --> 00:56:50,345
on your retina for
a period of time?

1020
00:56:51,800 --> 00:56:54,840
If that previous set of
data that I've shown you

1021
00:56:54,840 --> 00:56:57,260
is correct, if I present
something to the retina

1022
00:56:57,260 --> 00:56:59,980
and leave it there, pretty
soon you won't see anything.

1023
00:57:03,100 --> 00:57:05,280
A famous series
of experiments was

1024
00:57:05,280 --> 00:57:07,660
done, by now many,
many years ago,

1025
00:57:07,660 --> 00:57:11,100
in which they-- very
clever experiment--

1026
00:57:11,100 --> 00:57:13,780
had subjects lie down.

1027
00:57:13,780 --> 00:57:16,950
And they put a contact
lens in the eye.

1028
00:57:16,950 --> 00:57:22,930
And in the contact lens, they
put a miniature projector on,

1029
00:57:22,930 --> 00:57:26,940
which meant that when you
turn that projector light on,

1030
00:57:26,940 --> 00:57:30,830
it went to a fixed position
on the retinal surface.

1031
00:57:30,830 --> 00:57:32,510
Why was this necessary?

1032
00:57:32,510 --> 00:57:35,300
Well, the reason it
happened is because it

1033
00:57:35,300 --> 00:57:38,850
was discovered that
your eye actually

1034
00:57:38,850 --> 00:57:41,130
is not really stable on purpose.

1035
00:57:41,130 --> 00:57:43,020
Your eye ha a
so-called eye tremor.

1036
00:57:44,100 --> 00:57:46,770
And of course, you move
your eyes all the time.

1037
00:57:46,770 --> 00:57:51,000
So this procedure of
having a contact lens

1038
00:57:51,000 --> 00:57:54,870
with a projector
attached to it kind of

1039
00:57:54,870 --> 00:57:56,435
got rid of the eye tremor.

1040
00:57:57,830 --> 00:58:04,710
So when they did
that, they found that,

1041
00:58:04,710 --> 00:58:08,070
depending on the contrast, in
a matter of a minute or less,

1042
00:58:08,070 --> 00:58:11,370
maybe even 30 seconds,
you would stop

1043
00:58:11,370 --> 00:58:16,020
seeing what was
presented to the eye

1044
00:58:16,020 --> 00:58:20,470
because you then
change the adaptation

1045
00:58:20,470 --> 00:58:22,570
level in your photoreceptors.

1046
00:58:23,710 --> 00:58:26,830
So then subsequently,
people had a clever idea.

1047
00:58:26,830 --> 00:58:29,590
Said, we don't need to go
through this incredible trouble

1048
00:58:29,590 --> 00:58:33,526
of having to have people with
contact lenses and a projector,

1049
00:58:33,526 --> 00:58:35,150
and having them lie
down because that's

1050
00:58:35,150 --> 00:58:36,500
the only way it would work.

1051
00:58:36,500 --> 00:58:38,830
We can do this much more simply.

1052
00:58:38,830 --> 00:58:42,820
And so to do that, I'm going
to have a demonstration here.

1053
00:58:42,820 --> 00:58:44,990
What I would like
each of you to do

1054
00:58:44,990 --> 00:58:48,930
is to-- you see it's a light
spot here and dark spot there.

1055
00:58:48,930 --> 00:58:53,280
So I want you to fixate
here and count to about 30.

1056
00:58:54,810 --> 00:58:56,980
And be very relaxed about it.

1057
00:58:56,980 --> 00:58:57,990
This is a Gaussian.

1058
00:58:57,990 --> 00:59:00,730
Therefore there's no sharp
edge and the eye tremor

1059
00:59:00,730 --> 00:59:01,820
doesn't matter.

1060
00:59:01,820 --> 00:59:04,270
Then after you counted
to 30, shift your gaze

1061
00:59:04,270 --> 00:59:05,370
to the bottom one.

1062
00:59:10,675 --> 00:59:12,180
The first thing
that happens if you

1063
00:59:12,180 --> 00:59:14,340
keep looking at it,
the two on the top

1064
00:59:14,340 --> 00:59:18,660
here disappear if you
fixate very tightly.

1065
00:59:18,660 --> 00:59:21,600
And once they disappear,
then you can look down.

1066
00:59:23,975 --> 00:59:25,100
Everybody see it disappear?

1067
00:59:26,490 --> 00:59:27,030
Good.

1068
00:59:27,030 --> 00:59:29,112
And what happened when
you looked at the bottom?

1069
00:59:29,112 --> 00:59:30,028
AUDIENCE: [INAUDIBLE].

1070
00:59:31,000 --> 00:59:32,840
PROFESSOR: You got
a reversal, right?

1071
00:59:32,840 --> 00:59:35,505
You got a dark spot here
and a light spot here.

1072
00:59:35,505 --> 00:59:38,310
And you say oh my god,
what's going on here?

1073
00:59:39,660 --> 00:59:42,440
So now let's do
another experiment.

1074
00:59:42,440 --> 00:59:44,290
I want you to do this again.

1075
00:59:44,290 --> 00:59:47,610
But what I want you to do is
to cover one eye up and then

1076
00:59:47,610 --> 00:59:48,210
do it again.

1077
00:59:48,210 --> 00:59:49,400
Count to 30.

1078
00:59:49,400 --> 00:59:53,470
And after you did so, make
[INAUDIBLE] down the bottom,

1079
00:59:53,470 --> 00:59:54,880
but switch your eye.

1080
00:59:59,950 --> 01:00:04,790
And if you switch the eye,
cover the one that you looked at

1081
01:00:04,790 --> 01:00:07,650
and uncover the one
that you didn't look at.

1082
01:00:07,650 --> 01:00:10,450
And if you do that, you
won't get any effect.

1083
01:00:10,450 --> 01:00:12,240
So what does that prove?

1084
01:00:12,240 --> 01:00:14,690
That prove that this is
happening in the retina.

1085
01:00:15,820 --> 01:00:17,170
Everybody agree?

1086
01:00:18,470 --> 01:00:19,290
Very clear cut.

1087
01:00:20,430 --> 01:00:22,970
So what's going on here?

1088
01:00:22,970 --> 01:00:25,600
So let's diagram this.

1089
01:00:25,600 --> 01:00:27,310
Here we have the situation.

1090
01:00:27,310 --> 01:00:28,735
You turned on these two stimuli.

1091
01:00:30,200 --> 01:00:34,180
And then when we turn it down,
initially their sensitivity

1092
01:00:34,180 --> 01:00:36,350
of your photoreceptors
is pretty much the same

1093
01:00:36,350 --> 01:00:38,260
because it was a
homogeneous background.

1094
01:00:38,260 --> 01:00:41,650
Then after you looked at this
for awhile-- no, one more

1095
01:00:41,650 --> 01:00:42,450
thing.

1096
01:00:42,450 --> 01:00:44,900
In this case, the ON cells
fired, and in this case,

1097
01:00:44,900 --> 01:00:48,770
the OFF cells fired, saying,
oh, dark spot, oh, light spot.

1098
01:00:48,770 --> 01:00:52,030
Now, if you keep looking at
this for awhile, what happens

1099
01:00:52,030 --> 01:00:53,550
is it begins to disappear.

1100
01:00:53,550 --> 01:00:56,750
When it does, what happens
is you don't see anything

1101
01:00:56,750 --> 01:01:01,330
and what happens also is that
the sensitivity here decreases

1102
01:01:01,330 --> 01:01:04,530
for white light and [INAUDIBLE]
increases here for white light.

1103
01:01:04,530 --> 01:01:08,470
So therefore, what happens
is once you adapted to this,

1104
01:01:08,470 --> 01:01:14,720
there is no response
in the ganglion cells.

1105
01:01:14,720 --> 01:01:17,450
Then for the third step.

1106
01:01:17,450 --> 01:01:19,930
When you've made
yourself look down,

1107
01:01:19,930 --> 01:01:21,950
there were homogeneous
background.

1108
01:01:21,950 --> 01:01:24,990
But this region is less and
this region's more sensitive.

1109
01:01:24,990 --> 01:01:29,790
And so therefore the
photons come into your eye

1110
01:01:29,790 --> 01:01:33,010
from those two regions hit
more and less sensitive

1111
01:01:33,010 --> 01:01:37,140
regions on the rental
surface, thereby activating

1112
01:01:37,140 --> 01:01:40,600
the opposite cells-- here
activating the OFF cells,

1113
01:01:40,600 --> 01:01:42,040
here activating the ON cells.

1114
01:01:44,290 --> 01:01:47,864
Sorry-- right, yeah.

1115
01:01:49,136 --> 01:01:50,740
We got a reversal.

1116
01:01:50,740 --> 01:01:53,950
So that explains why
you see the after image.

1117
01:01:55,620 --> 01:01:58,900
So now we're going to
continue and get back

1118
01:01:58,900 --> 01:02:05,050
to the question of
the color circle,

1119
01:02:05,050 --> 01:02:07,605
see what happens with
after images with color.

1120
01:02:09,310 --> 01:02:16,170
So this color circle is
unbelievably powerful

1121
01:02:16,170 --> 01:02:21,130
because it explains the after
images that you see with color.

1122
01:02:21,130 --> 01:02:23,520
And I will come
back to this circle.

1123
01:02:23,520 --> 01:02:26,400
But what I want
you to do now again

1124
01:02:26,400 --> 01:02:32,200
is to fixate here, count
until, again, about 30,

1125
01:02:32,200 --> 01:02:33,600
and then fixate on the bottom.

1126
01:02:49,800 --> 01:02:53,640
So if you do that, I think
most of you should see,

1127
01:02:53,640 --> 01:02:54,537
again, a reversal.

1128
01:02:54,537 --> 01:02:55,870
Here you would see some reddish.

1129
01:02:55,870 --> 01:02:57,510
Here you would see
something greenish.

1130
01:02:57,510 --> 01:02:59,280
Does everybody see that?

1131
01:02:59,280 --> 01:03:00,520
Do you see that too?

1132
01:03:00,520 --> 01:03:01,020
Good.

1133
01:03:02,220 --> 01:03:04,490
So you have a very
minor color deficit.

1134
01:03:06,070 --> 01:03:08,250
So that's the case here.

1135
01:03:08,250 --> 01:03:09,870
So now let me show
you another one.

1136
01:03:12,000 --> 01:03:14,790
And again, do the
same experiment.

1137
01:03:26,160 --> 01:03:29,260
And what you see here at the
after images you get-- whoops.

1138
01:03:29,260 --> 01:03:29,760
Sorry.

1139
01:03:30,795 --> 01:03:31,420
Let me go back.

1140
01:03:31,420 --> 01:03:35,950
The after images
you get here are not

1141
01:03:35,950 --> 01:03:38,850
going to be the
complimentaries here.

1142
01:03:38,850 --> 01:03:40,570
So what's going on?

1143
01:03:40,570 --> 01:03:42,360
So let me explain it to you.

1144
01:03:42,360 --> 01:03:43,755
Here again is a color circle.

1145
01:03:45,200 --> 01:03:47,170
And here the prime axes.

1146
01:03:47,170 --> 01:03:53,640
It turns out that if you
adapt to this level here,

1147
01:03:53,640 --> 01:03:58,077
the rule of the color circle
says if you go across,

1148
01:03:58,077 --> 01:03:59,910
this is the after image
you're going to see.

1149
01:04:01,370 --> 01:04:04,590
And if you adapt to here,
this is the after image

1150
01:04:04,590 --> 01:04:05,650
you're going to see.

1151
01:04:05,650 --> 01:04:07,140
So let me draw that up.

1152
01:04:07,140 --> 01:04:08,140
It looks like that.

1153
01:04:10,230 --> 01:04:14,430
And that is true everywhere, as
long as you go across the axis.

1154
01:04:14,430 --> 01:04:18,320
You could do this horizontally
or even diagonally,

1155
01:04:18,320 --> 01:04:19,565
and you get this reversal.

1156
01:04:20,720 --> 01:04:25,610
So it says that an after image
can be perfectly predicted

1157
01:04:25,610 --> 01:04:27,530
by the rules of
the color circle.

1158
01:04:27,530 --> 01:04:31,950
It's on the opposite
side of the image

1159
01:04:31,950 --> 01:04:34,430
that you look at,
going across the center

1160
01:04:34,430 --> 01:04:37,030
along the cardinal axis.

1161
01:04:37,030 --> 01:04:41,240
But now if you do the diagonals,
which I showed you before,

1162
01:04:41,240 --> 01:04:43,540
when it didn't match,
what we have here,

1163
01:04:43,540 --> 01:04:45,120
you have this and this.

1164
01:04:45,120 --> 01:04:46,810
Then the after image
is this and that.

1165
01:04:46,810 --> 01:04:50,500
And so we don't have a
correspondence, of course,

1166
01:04:50,500 --> 01:04:59,490
because you're not along the
axes that would predict this.

1167
01:04:59,490 --> 01:05:01,680
This one gives you that
and this one give you that.

1168
01:05:01,680 --> 01:05:04,380
So if I did this and
this to begin with,

1169
01:05:04,380 --> 01:05:06,130
it would be the same
as that, but rotated.

1170
01:05:08,090 --> 01:05:15,020
So that then clearly enables
us to use the color circle

1171
01:05:15,020 --> 01:05:18,010
to predict not only some of the
basic effects of what colors

1172
01:05:18,010 --> 01:05:20,400
we see, but also to tell
you exactly what kinds

1173
01:05:20,400 --> 01:05:21,787
of after images you get.

1174
01:05:21,787 --> 01:05:22,495
Quite remarkable.

1175
01:05:23,960 --> 01:05:26,770
So now to drive
this home once more.

1176
01:05:26,770 --> 01:05:29,330
This doesn't work too well
because the colors are crummy

1177
01:05:29,330 --> 01:05:31,037
here, but we can try it.

1178
01:05:31,037 --> 01:05:33,620
Everybody agree this is sort of
a more or less black and white

1179
01:05:33,620 --> 01:05:35,770
display-- a little
bluish, unfortunately,

1180
01:05:35,770 --> 01:05:38,035
here-- of a beautiful castle?

1181
01:05:39,600 --> 01:05:41,590
See that fixation point here?

1182
01:05:41,590 --> 01:05:44,840
What I want you to do
here is to fixate here,

1183
01:05:44,840 --> 01:05:48,210
again, count to 30, and
then I'll switch back to it.

1184
01:05:49,790 --> 01:05:52,150
And if this were
to work right, you

1185
01:05:52,150 --> 01:05:55,840
would see the original black
and white image in color.

1186
01:05:57,210 --> 01:05:58,345
Keep fixating.

1187
01:05:58,345 --> 01:05:59,890
Count to 20 more.

1188
01:05:59,890 --> 01:06:01,256
And then I'll switch back.

1189
01:06:06,940 --> 01:06:08,527
Did you see the colors?

1190
01:06:08,527 --> 01:06:09,690
Yeah.

1191
01:06:09,690 --> 01:06:13,100
So this is a very
clever demo I found.

1192
01:06:13,100 --> 01:06:15,140
I'm afraid I don't
remember the person's name

1193
01:06:15,140 --> 01:06:17,350
who came up with this one.

1194
01:06:17,350 --> 01:06:22,230
But the essence of it is, again,
that indeed what is happening

1195
01:06:22,230 --> 01:06:28,115
is that you're creating an
after effect due to adaptation

1196
01:06:28,115 --> 01:06:28,770
in the retina.

1197
01:06:31,892 --> 01:06:33,350
And when you do
this very cleverly,

1198
01:06:33,350 --> 01:06:37,430
like this particular
castle picture,

1199
01:06:37,430 --> 01:06:40,350
you can actually create
an artificial impression

1200
01:06:40,350 --> 01:06:43,420
of colors which are in
consonance with what

1201
01:06:43,420 --> 01:06:46,170
the real colors would look like
in a black and white picture.

1202
01:06:47,220 --> 01:06:50,300
So that then is
the essence of what

1203
01:06:50,300 --> 01:06:52,460
I wanted to cover
about adaptation.

1204
01:06:52,460 --> 01:06:56,120
And we can now
come and summarize

1205
01:06:56,120 --> 01:06:58,890
what I had covered today.

1206
01:07:02,850 --> 01:07:04,885
So first of all, I
told you that there

1207
01:07:04,885 --> 01:07:08,450
are three qualities of
color-- hue, brightness,

1208
01:07:08,450 --> 01:07:09,100
and saturation.

1209
01:07:10,300 --> 01:07:13,230
And just to go back--
hang on for a minute.

1210
01:07:13,230 --> 01:07:18,230
I want to go back once
more to make this clear,

1211
01:07:18,230 --> 01:07:19,570
which I forgot to mention.

1212
01:07:19,570 --> 01:07:22,820
If you go around the
circle, you change hue.

1213
01:07:25,930 --> 01:07:30,240
And if you go from the
periphery to the center,

1214
01:07:30,240 --> 01:07:31,170
you change saturation.

1215
01:07:34,410 --> 01:07:39,040
And the center here, if this
color circle were 100% correct,

1216
01:07:39,040 --> 01:07:40,805
this would be a white area.

1217
01:07:47,270 --> 01:07:49,660
Now, the basic rules
of color vision

1218
01:07:49,660 --> 01:07:53,960
are explained by the color
circle, as we have amply seen.

1219
01:07:53,960 --> 01:07:57,200
There's something probably--
I have a color circle

1220
01:07:57,200 --> 01:08:01,162
up on my wall in my office
because I'm always fascinated

1221
01:08:01,162 --> 01:08:03,245
by this, even though I've
been doing it for years.

1222
01:08:05,130 --> 01:08:09,360
So the three photoreceptors
we talked about in humans

1223
01:08:09,360 --> 01:08:11,890
and in primates, the red,
green, and blue, which

1224
01:08:11,890 --> 01:08:13,640
shouldn't be called that.

1225
01:08:13,640 --> 01:08:16,130
People get mad when you do
that, even though that's the way

1226
01:08:16,130 --> 01:08:17,010
I call it.

1227
01:08:17,010 --> 01:08:18,840
But people want to call
them short, medium,

1228
01:08:18,840 --> 01:08:20,800
and long-wavelength cones.

1229
01:08:21,910 --> 01:08:25,010
They are broadly tuned,
as I had shown you

1230
01:08:25,010 --> 01:08:26,240
in those spectrograms.

1231
01:08:27,939 --> 01:08:30,910
The color-opponent midget
retinal ganglion cells

1232
01:08:30,910 --> 01:08:34,230
form two cardinal axes, the
red/green and the blue/yellow.

1233
01:08:38,250 --> 01:08:41,189
And those, if you
remember I told you

1234
01:08:41,189 --> 01:08:43,180
at the level of the
retinal ganglion cells

1235
01:08:43,180 --> 01:08:48,330
and at the level of the
lateral geniculate nucleus,

1236
01:08:48,330 --> 01:08:51,109
fall into these two
major categories.

1237
01:08:51,109 --> 01:08:53,029
We won't have any
under diagonals.

1238
01:08:53,029 --> 01:08:56,140
So for us to be properly
able to see diagonals,

1239
01:08:56,140 --> 01:08:58,390
that's done somewhere
in the cortex.

1240
01:08:58,390 --> 01:09:00,069
It's not done at the
level of the retina

1241
01:09:00,069 --> 01:09:01,485
or the lateral
geniculate nucleus.

1242
01:09:03,450 --> 01:09:05,979
Now, I also pointed out
to you several times

1243
01:09:05,979 --> 01:09:08,255
already that the midget
system's essential for color

1244
01:09:08,255 --> 01:09:08,880
discrimination.

1245
01:09:10,439 --> 01:09:15,210
And the parasol cells can see
stimuli even at isoluminance.

1246
01:09:17,010 --> 01:09:19,620
They just cannot say
what the color is.

1247
01:09:19,620 --> 01:09:23,100
They don't, quotes,
"perceive" different colors.

1248
01:09:23,100 --> 01:09:26,729
But they can see
any kind of change

1249
01:09:26,729 --> 01:09:28,609
that occurs in the
environment, even when it

1250
01:09:28,609 --> 01:09:29,567
occurs at isoluminance.

1251
01:09:34,550 --> 01:09:37,229
Color is processed in
many cortical areas,

1252
01:09:37,229 --> 01:09:41,899
lesions to any single
extrastriate area

1253
01:09:41,899 --> 01:09:44,090
fails to eliminate
the processing

1254
01:09:44,090 --> 01:09:45,630
of chrominance information.

1255
01:09:45,630 --> 01:09:49,390
It can reduce it, but
it doesn't block it out.

1256
01:09:49,390 --> 01:09:50,880
That's true for many things.

1257
01:09:53,410 --> 01:09:56,510
So the cortical areas
are very, very complex.

1258
01:09:57,540 --> 01:10:01,950
And they do interactive analysis
for many different attributes,

1259
01:10:01,950 --> 01:10:03,070
including color.

1260
01:10:05,480 --> 01:10:10,240
However, I can add here that
does not apply to area MT

1261
01:10:10,240 --> 01:10:13,080
because MT does not
seem to be specializing

1262
01:10:13,080 --> 01:10:16,786
in color because lesions there,
you [INAUDIBLE] any deficit.

1263
01:10:16,786 --> 01:10:18,410
But there's several
other visual areas.

1264
01:10:18,410 --> 01:10:22,040
We went through that, V3, and
[INAUDIBLE] cortex, and so on.

1265
01:10:23,066 --> 01:10:28,070
These areas contribute to
the processing of color.

1266
01:10:28,070 --> 01:10:30,300
Now, the perception
of isoluminance

1267
01:10:30,300 --> 01:10:33,680
is categorized for all
categories of vision.

1268
01:10:33,680 --> 01:10:35,760
It's not selected
to only those that

1269
01:10:35,760 --> 01:10:39,940
are processed by the midget
or the parasol systems.

1270
01:10:39,940 --> 01:10:42,330
All aspects of vision--
and the three I

1271
01:10:42,330 --> 01:10:46,916
showed you was,
stereopsis, motion,

1272
01:10:46,916 --> 01:10:49,480
and what was the third one?

1273
01:10:49,480 --> 01:10:50,460
AUDIENCE: Texture.

1274
01:10:50,460 --> 01:10:51,700
PROFESSOR: Texture, right.

1275
01:10:51,700 --> 01:10:57,140
So all three of
those are compromised

1276
01:10:57,140 --> 01:11:00,100
when stimuli are
presented at isoluminance.

1277
01:11:01,400 --> 01:11:03,980
The most significant aspect
of luminance adaptation

1278
01:11:03,980 --> 01:11:06,210
occurs in the photoreceptors.

1279
01:11:06,210 --> 01:11:11,110
And it's explainable by the
relative number at any given

1280
01:11:11,110 --> 01:11:16,930
level of adaptation of bleached
and unbleached photoreceptor

1281
01:11:16,930 --> 01:11:20,570
molecules, as in the
case of rhodopsin.

1282
01:11:20,570 --> 01:11:24,920
Lastly, after images are
a product of photoreceptor

1283
01:11:24,920 --> 01:11:30,360
adaptation and their subsequent
response to the incoming light.

1284
01:11:31,530 --> 01:11:36,190
So that then is the essence of
what you wanted to cover today.

1285
01:11:36,190 --> 01:11:39,740
And I hope you did
find this interesting,

1286
01:11:39,740 --> 01:11:41,240
because certainly
our ability to see

1287
01:11:41,240 --> 01:11:43,140
color is quite a
remarkable thing.

1288
01:11:43,140 --> 01:11:47,930
It's amazing to get a sense
of how the nervous system does

1289
01:11:47,930 --> 01:11:50,260
that, even though
at this stage, we

1290
01:11:50,260 --> 01:11:53,210
are still at a fairly
early level of having

1291
01:11:53,210 --> 01:11:54,710
gained full understanding of it.

1292
01:11:59,240 --> 01:12:02,450
Now the next time,
we are going to move

1293
01:12:02,450 --> 01:12:05,860
on to another fascinating topic,
at least for me fascinating,

1294
01:12:05,860 --> 01:12:09,180
which is depth perception,
which I had mentioned

1295
01:12:09,180 --> 01:12:15,190
before is a remarkable
achievement because images fall

1296
01:12:15,190 --> 01:12:17,400
on a two-dimensional
retinal surface.

1297
01:12:17,400 --> 01:12:19,140
And from that, the
third dimension

1298
01:12:19,140 --> 01:12:20,840
has to be reconstructed.

1299
01:12:20,840 --> 01:12:24,410
And how that is done, we are
going to discuss the next time.

1300
01:12:25,465 --> 01:12:29,235
Now let's make sure that all
of you had signed attendance.

1301
01:12:33,330 --> 01:12:37,910
And the next thing is if any
of you have any questions,

1302
01:12:37,910 --> 01:12:39,515
I will be happy to
try to answer them.

1303
01:12:44,170 --> 01:12:46,308
So once again, I'm
crystal clear, huh?

1304
01:12:49,850 --> 01:12:51,790
Well, thank you very
much for attending.

1305
01:12:51,790 --> 01:12:54,570
And I do hope that
your knowledge

1306
01:12:54,570 --> 01:12:58,900
has increased a bit about how
we process color information.