1 00:00:00,080 --> 00:00:01,670 The following content is provided 2 00:00:01,670 --> 00:00:03,820 under a Creative Commons license. 3 00:00:03,820 --> 00:00:06,550 Your support will help MIT OpenCourseWare continue 4 00:00:06,550 --> 00:00:10,160 to offer high quality educational resources for free. 5 00:00:10,160 --> 00:00:12,710 To make a donation or to view additional materials 6 00:00:12,710 --> 00:00:16,620 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,275 at ocw.mit.edu. 8 00:00:25,560 --> 00:00:27,060 PROFESSOR: All right so today, we're 9 00:00:27,060 --> 00:00:32,020 going to have a review of the visual and oculomotor systems 10 00:00:32,020 --> 00:00:34,030 that I've covered so far. 11 00:00:34,030 --> 00:00:35,820 And what I'm going to do is I'm going 12 00:00:35,820 --> 00:00:41,030 to go over many, many basic facts in a rather 13 00:00:41,030 --> 00:00:47,710 quick fashion, which will sort of refresh your memory of what 14 00:00:47,710 --> 00:00:50,450 we have covered so far and also will make you 15 00:00:50,450 --> 00:00:55,790 more aware of what you want to look at carefully when you look 16 00:00:55,790 --> 00:01:03,290 at the material on the website, on Stellar, 17 00:01:03,290 --> 00:01:07,805 and also when you read the assigned readings. 18 00:01:09,380 --> 00:01:12,270 I want to remind all of you again that you 19 00:01:12,270 --> 00:01:15,700 will have to put together that paper 20 00:01:15,700 --> 00:01:18,960 on the accessory optic system that I will mention very 21 00:01:18,960 --> 00:01:21,075 briefly at the end of the review. 22 00:01:22,680 --> 00:01:25,230 And your prime task there will be-- 23 00:01:25,230 --> 00:01:28,800 that's an old paper published in the 1960s, which 24 00:01:28,800 --> 00:01:31,080 was a major discovery at the time-- 25 00:01:31,080 --> 00:01:33,620 and your task will be predominantly 26 00:01:33,620 --> 00:01:36,700 to say what-- well, first of all what has been discovered there, 27 00:01:36,700 --> 00:01:40,580 that you can cover in a paragraph and then to add to it 28 00:01:40,580 --> 00:01:45,230 what people have contributed to the study 29 00:01:45,230 --> 00:01:50,990 of that area since that original paper. 30 00:01:50,990 --> 00:01:54,340 All right, so anyway then I will mention 31 00:01:54,340 --> 00:01:57,720 at the end or so a bit more about the exam, which 32 00:01:57,720 --> 00:02:01,240 is going to take place on Wednesday right in here, which 33 00:02:01,240 --> 00:02:04,220 is going to consist of multiple choice questions. 34 00:02:04,220 --> 00:02:06,090 All right, so to begin with then, 35 00:02:06,090 --> 00:02:11,390 let's talk about the basic wiring of the visual system 36 00:02:11,390 --> 00:02:12,730 that we have covered. 37 00:02:12,730 --> 00:02:18,845 And that is outlined here for the primate and for the human. 38 00:02:20,540 --> 00:02:25,450 And I should mention as I had gone in the initial lecture 39 00:02:25,450 --> 00:02:29,400 that this is different from many of the lower level 40 00:02:29,400 --> 00:02:32,750 species in which the two eyes look sideways, 41 00:02:32,750 --> 00:02:38,210 and each eye sends all of its retinal ganglion cell axons 42 00:02:38,210 --> 00:02:43,760 across to the other hemisphere in the brain. 43 00:02:43,760 --> 00:02:45,410 Now this big change occurred when 44 00:02:45,410 --> 00:02:47,060 the eyes move to the front. 45 00:02:47,060 --> 00:02:50,200 And we have discussed already why that may have happened. 46 00:02:50,200 --> 00:02:55,220 And as a result of this, if you imagine cutting your vertically 47 00:02:55,220 --> 00:02:58,380 in half, you divide it into the nasal and temporal hemi 48 00:02:58,380 --> 00:02:59,210 retinae. 49 00:02:59,210 --> 00:03:01,170 And it so happens that the nasal hemi 50 00:03:01,170 --> 00:03:03,530 retina of one eye and the temporal retina 51 00:03:03,530 --> 00:03:05,340 of the other eye goes to one side, 52 00:03:05,340 --> 00:03:07,780 and the obvious happens to the other side. 53 00:03:07,780 --> 00:03:11,390 The connections are made to several areas, most notably 54 00:03:11,390 --> 00:03:14,530 for our purposes, was the lateral geniculate nucleus 55 00:03:14,530 --> 00:03:17,770 but also the superior colliculus and several other structures 56 00:03:17,770 --> 00:03:19,660 that we have talked about that include 57 00:03:19,660 --> 00:03:21,790 the accessory optic system. 58 00:03:21,790 --> 00:03:24,930 Once the connections come up to the cortex, 59 00:03:24,930 --> 00:03:28,950 several cortical areas-- we'll talk about that in a minute-- 60 00:03:28,950 --> 00:03:34,060 have evolved that are involved in progressively higher levels 61 00:03:34,060 --> 00:03:36,690 of visual analysis. 62 00:03:37,810 --> 00:03:42,310 Now this circle here-- hopefully you guys 63 00:03:42,310 --> 00:03:46,200 remember-- is either called the Vieth-Muller circle, 64 00:03:46,200 --> 00:03:48,260 or it's called the horopter. 65 00:03:48,260 --> 00:03:50,980 And it was shown by a clever experimentalist 66 00:03:50,980 --> 00:03:55,110 that if you put any spot along that circle 67 00:03:55,110 --> 00:03:58,800 when the person fixes at this point in the circle, 68 00:03:58,800 --> 00:04:03,740 all of those points impinge on corresponding points in the two 69 00:04:03,740 --> 00:04:04,240 retinae. 70 00:04:05,390 --> 00:04:07,250 However, if there's an object that 71 00:04:07,250 --> 00:04:11,850 seemed to be beyond or closer to the eye than the circle, 72 00:04:11,850 --> 00:04:16,370 then they hit non equivalent points in the retina. 73 00:04:16,370 --> 00:04:20,149 And that non equivalency is actually used 74 00:04:20,149 --> 00:04:22,400 for depth perception, as we have discussed, 75 00:04:22,400 --> 00:04:27,560 and I will mention again when it comes to a stereopsis. 76 00:04:27,560 --> 00:04:32,060 So that is the very, very basic wiring arrangement. 77 00:04:32,060 --> 00:04:34,130 And then if you proceed from here 78 00:04:34,130 --> 00:04:36,780 and look at the retina and the lateral geniculate nucleus 79 00:04:36,780 --> 00:04:39,830 in a bit more detail, first starting with the retina. 80 00:04:39,830 --> 00:04:42,230 I wanted to point out to you first of all 81 00:04:42,230 --> 00:04:45,300 that there are two different kinds of photoreceptors. 82 00:04:45,300 --> 00:04:47,380 You all know this very well by now. 83 00:04:47,380 --> 00:04:49,910 You knew this before you came to class. 84 00:04:49,910 --> 00:04:51,900 You have the rods and the cones. 85 00:04:51,900 --> 00:04:56,470 There are three basic types of cones, red, green, and blue, 86 00:04:56,470 --> 00:05:00,200 which more appropriately refer to short, medium, and long 87 00:05:00,200 --> 00:05:02,640 wavelength selective cones. 88 00:05:02,640 --> 00:05:04,910 And then we have the rods. 89 00:05:04,910 --> 00:05:10,690 Now what happens is that the light comes in, 90 00:05:10,690 --> 00:05:14,470 in this case from the bottom if you look at yourself. 91 00:05:14,470 --> 00:05:17,870 The light comes in, and it goes through many 92 00:05:17,870 --> 00:05:19,710 of the cells in the retina. 93 00:05:19,710 --> 00:05:23,040 And it impinges on the receptors, 94 00:05:23,040 --> 00:05:27,490 which are facing away from the light against the pigment 95 00:05:27,490 --> 00:05:28,710 epithelium. 96 00:05:28,710 --> 00:05:31,670 And as I've mentioned to you, there 97 00:05:31,670 --> 00:05:33,620 have been some interesting questions 98 00:05:33,620 --> 00:05:37,450 as to why this strange arrangement had emerged. 99 00:05:37,450 --> 00:05:41,150 Nobody had predicted that before we had any anatomy, 100 00:05:41,150 --> 00:05:43,150 people just thought if there were any receptors, 101 00:05:43,150 --> 00:05:44,730 they would face the light. 102 00:05:44,730 --> 00:05:48,280 So that's certainly a strange arrangement, an unusual one. 103 00:05:48,280 --> 00:05:51,030 And a lot of speculation have been 104 00:05:51,030 --> 00:05:53,780 advanced as to why this has happened. 105 00:05:53,780 --> 00:05:56,240 And I will just briefly mention two of those. 106 00:05:56,240 --> 00:05:58,950 One is that when these photoreceptors 107 00:05:58,950 --> 00:06:01,640 are right against the pigment epithelium, which 108 00:06:01,640 --> 00:06:12,050 in diurnal animals is black, and absorbs the photons thereby 109 00:06:12,050 --> 00:06:14,480 preventing scattering of light. 110 00:06:14,480 --> 00:06:16,990 As a result of which, you can gain high acuity. 111 00:06:18,410 --> 00:06:23,130 And that is known by the fact that if you assess 112 00:06:23,130 --> 00:06:27,510 the visual capacities of albinos who don't have a black pigment 113 00:06:27,510 --> 00:06:30,120 epithelium because they lack pigment-- that's 114 00:06:30,120 --> 00:06:32,140 the definition of being an albino-- 115 00:06:32,140 --> 00:06:35,300 most people have very poor vision because the photons 116 00:06:35,300 --> 00:06:37,820 come into the eye, and they scatter all over the place 117 00:06:37,820 --> 00:06:42,730 and activate many photoreceptors rather than just those 118 00:06:42,730 --> 00:06:47,010 which the incoming photon would hit directly. 119 00:06:47,010 --> 00:06:49,760 So that is the arrangement for these. 120 00:06:49,760 --> 00:06:54,110 Another factor, which I don't think I may not have mentioned 121 00:06:54,110 --> 00:07:02,050 is that what you have in these photoreceptors 122 00:07:02,050 --> 00:07:05,770 are little packets, if you talk about the rods in particular, 123 00:07:05,770 --> 00:07:07,710 you have little packets, each of which 124 00:07:07,710 --> 00:07:12,680 has the molecules, which are sensitive 125 00:07:12,680 --> 00:07:14,470 to the incoming light. 126 00:07:14,470 --> 00:07:17,603 And each of these packets-- there 127 00:07:17,603 --> 00:07:20,435 are about 1,000 packets in each of these rods. 128 00:07:21,890 --> 00:07:26,190 And each of those 1,000 packets is about 10,000 molecules. 129 00:07:26,190 --> 00:07:28,470 So we're talking about gigantic numbers. 130 00:07:28,470 --> 00:07:34,440 Now what happens is that given this 1,000 of-- about 1,000 131 00:07:34,440 --> 00:07:38,380 packets, they are not there for your life. 132 00:07:38,380 --> 00:07:42,220 What happens is that the packets gradually 133 00:07:42,220 --> 00:07:45,570 disintegrate and get replaced by a new one. 134 00:07:46,920 --> 00:07:51,490 It's about once every 10 days you lose a packet, 135 00:07:51,490 --> 00:07:53,430 and you gain a new one. 136 00:07:53,430 --> 00:07:56,300 Now that means that some of this stuff gets sloughed off. 137 00:07:56,300 --> 00:08:01,620 And one of the reasons people think that the photoreceptors 138 00:08:01,620 --> 00:08:04,430 ended up facing away from the light is that they could 139 00:08:04,430 --> 00:08:07,310 be close to this inner part of the retina 140 00:08:07,310 --> 00:08:11,690 where anything that's sloughed off can be absorbed 141 00:08:11,690 --> 00:08:15,860 rather than being just thrown into the eye itself, 142 00:08:15,860 --> 00:08:17,920 into the vitreous, because if that 143 00:08:17,920 --> 00:08:20,250 were to happen over many, many years, 144 00:08:20,250 --> 00:08:23,690 the vitreous would become cloudy, and you couldn't see. 145 00:08:23,690 --> 00:08:25,900 So those are two possible reasons 146 00:08:25,900 --> 00:08:28,820 why this strange arrangement has evolved. 147 00:08:28,820 --> 00:08:31,020 And you see this in virtually all species. 148 00:08:31,020 --> 00:08:35,450 There are just a few species that have-- and most of those 149 00:08:35,450 --> 00:08:41,580 are actually in the sea-- who have photoreceptors 150 00:08:41,580 --> 00:08:43,440 that face towards the light. 151 00:08:43,440 --> 00:08:45,790 All right, and then if you proceed here, 152 00:08:45,790 --> 00:08:48,700 the other amazing thing that had been discovered 153 00:08:48,700 --> 00:08:54,410 is that all the photoreceptors hyper polarized to light. 154 00:08:54,410 --> 00:08:57,060 Again, they do the opposite of what people had thought. 155 00:08:57,060 --> 00:08:59,150 You'd think that when photons come in, 156 00:08:59,150 --> 00:09:03,000 they would activate the photoreceptors, 157 00:09:03,000 --> 00:09:08,400 and they would send the signal down the stream 158 00:09:08,400 --> 00:09:10,010 through the eye. 159 00:09:10,010 --> 00:09:14,630 Turns out the opposite happens that the discharge 160 00:09:14,630 --> 00:09:17,800 on the neurotransmitter here occurs 161 00:09:17,800 --> 00:09:20,421 when there's a darkening rather than an increase in light. 162 00:09:20,421 --> 00:09:22,045 That's an important factor to remember. 163 00:09:23,310 --> 00:09:25,570 That's true for all the forests accept 164 00:09:25,570 --> 00:09:28,250 as all photoreceptors hyperpolarized to light. 165 00:09:28,250 --> 00:09:29,560 You know this well already. 166 00:09:29,560 --> 00:09:32,120 I must have said that about 10 times by now. 167 00:09:32,120 --> 00:09:33,960 Now the amazing thing is that when 168 00:09:33,960 --> 00:09:38,830 you come to the bipolar cells, the next set of receptors here, 169 00:09:38,830 --> 00:09:41,600 it was discovered that two basic types 170 00:09:41,600 --> 00:09:44,420 of several different types like from for the major parasol 171 00:09:44,420 --> 00:09:48,970 cells, but there are two basic types, the on and the off. 172 00:09:48,970 --> 00:09:55,450 And this is accomplished by having two kinds of synapses 173 00:09:55,450 --> 00:09:59,510 in the on and off bipolar cells, sign conserving ones and sign 174 00:09:59,510 --> 00:10:00,560 inverting ones. 175 00:10:01,650 --> 00:10:04,210 This is accomplished in the on bipolars 176 00:10:04,210 --> 00:10:08,620 by virtue of the [INAUDIBLE] six receptor site 177 00:10:08,620 --> 00:10:14,160 and the [INAUDIBLE] one and two for the off bipolar cells. 178 00:10:14,160 --> 00:10:20,570 So that means now that you have signals in some of these cells 179 00:10:20,570 --> 00:10:24,260 when there's an increase in light and the signals in some 180 00:10:24,260 --> 00:10:26,220 where there's a decrease in light. 181 00:10:26,220 --> 00:10:30,030 So that's the situation for the on and off bipolars. 182 00:10:30,030 --> 00:10:35,860 And then when you come into the level of the ganglion cells, 183 00:10:35,860 --> 00:10:40,360 two major classes of ganglion cells are the on and the off. 184 00:10:40,360 --> 00:10:43,570 Now I'll talk about that in more detail in just a minute. 185 00:10:43,570 --> 00:10:45,570 Now the other interesting, curious thing 186 00:10:45,570 --> 00:10:49,090 is that when you look at the rods, 187 00:10:49,090 --> 00:10:57,480 the rods-- and they connect to their bipolar cells. 188 00:10:57,480 --> 00:11:00,380 They are all sign inverting synapses. 189 00:11:01,440 --> 00:11:03,920 They only come in one type, at least in humans 190 00:11:03,920 --> 00:11:05,530 and in primates. 191 00:11:05,530 --> 00:11:10,370 So what happens then to create on and off section 192 00:11:10,370 --> 00:11:12,350 done in the inner retina by virtue 193 00:11:12,350 --> 00:11:16,540 of having a synapse here [INAUDIBLE] to amacrine cell, 194 00:11:16,540 --> 00:11:20,590 which is a glycinergic synapse, and it also makes connections 195 00:11:20,590 --> 00:11:23,180 to the on bipolar, which is a gap junction. 196 00:11:23,180 --> 00:11:25,640 And this way, it becomes a double ended system 197 00:11:25,640 --> 00:11:28,140 for the rods as well as for the cones. 198 00:11:28,140 --> 00:11:29,911 So hopefully you guys all remember this. 199 00:11:29,911 --> 00:11:30,910 I know it's complicated. 200 00:11:32,110 --> 00:11:34,890 But that is something that one doesn't have a choice about. 201 00:11:34,890 --> 00:11:36,690 That's how it simply is. 202 00:11:36,690 --> 00:11:38,590 All right, so now we move on. 203 00:11:38,590 --> 00:11:41,660 And we are going to look at the lateral geniculate nucleus. 204 00:11:41,660 --> 00:11:42,899 Here's a cross section of it. 205 00:11:42,899 --> 00:11:43,815 I've shown you before. 206 00:11:44,950 --> 00:11:47,400 It was discovered that the lateral geniculate 207 00:11:47,400 --> 00:11:50,680 nucleus in central retina-- this is a monkey retina. 208 00:11:50,680 --> 00:11:53,130 The human is very similar, so input 209 00:11:53,130 --> 00:11:57,620 from a monkey retina to the lateral geniculate nucleus. 210 00:11:57,620 --> 00:12:01,410 And the six layers consist of two major types, 211 00:12:01,410 --> 00:12:04,770 the parvocellular so called the magnocellular layers. 212 00:12:04,770 --> 00:12:08,160 And what was discovered is that the parvocellular layers 213 00:12:08,160 --> 00:12:10,940 get input from the midget cells that we'll 214 00:12:10,940 --> 00:12:12,330 talk about in just a minute. 215 00:12:12,330 --> 00:12:18,260 And the bottom two layers, which are the magnocellular layers 216 00:12:18,260 --> 00:12:20,550 get input from the parasol cells. 217 00:12:20,550 --> 00:12:25,140 And then what happens is that when you go from central vision 218 00:12:25,140 --> 00:12:27,980 to peripheral vision, you have a huge change 219 00:12:27,980 --> 00:12:32,150 in the productive percentage of midget and parasol cells 220 00:12:32,150 --> 00:12:35,440 that you have in the retina and in the lateral geniculate 221 00:12:35,440 --> 00:12:38,930 nucleus near the fovea. 222 00:12:38,930 --> 00:12:42,980 In the foveola itself, you don't have any parasol cells, 223 00:12:42,980 --> 00:12:45,930 but in the fovea itself you do. 224 00:12:45,930 --> 00:12:48,070 And there's a ratio of about eight to one. 225 00:12:48,070 --> 00:12:49,870 And then as you go to the periphery, 226 00:12:49,870 --> 00:12:52,040 eventually they're equally in number. 227 00:12:52,040 --> 00:12:55,860 So there's a huge emphasis on the midget system 228 00:12:55,860 --> 00:13:00,110 in central vision and the much increased 229 00:13:00,110 --> 00:13:04,060 emphasis on the parasol cells in peripheral vision. 230 00:13:04,060 --> 00:13:05,160 So that's the arrangement. 231 00:13:05,160 --> 00:13:08,090 And this is reflected in the geniculate, which 232 00:13:08,090 --> 00:13:13,760 has six layers in central vision after about 18 degrees. 233 00:13:13,760 --> 00:13:16,720 And it has four layers in the periphery where 234 00:13:16,720 --> 00:13:20,000 the midget and parasol inputs are pretty much 235 00:13:20,000 --> 00:13:23,710 equal as reflected by these four layers. 236 00:13:23,710 --> 00:13:26,950 So that's the basic arrangements for the lateral geniculate 237 00:13:26,950 --> 00:13:27,800 nucleus. 238 00:13:27,800 --> 00:13:31,360 Now if we move on-- let me say one more thing. 239 00:13:31,360 --> 00:13:36,970 The receptive field properties of cells in the retina, 240 00:13:36,970 --> 00:13:39,220 in retinal ganglion cells, I should say, 241 00:13:39,220 --> 00:13:43,049 and in the lateral geniculate nucleus are highly similar. 242 00:13:43,049 --> 00:13:44,215 They're virtually identical. 243 00:13:45,550 --> 00:13:48,130 You have circular receptive fields 244 00:13:48,130 --> 00:13:50,340 with centers around antagonism. 245 00:13:50,340 --> 00:13:52,600 All right, now if you move on and move up 246 00:13:52,600 --> 00:13:55,170 to the visual cortex, what happens 247 00:13:55,170 --> 00:14:00,450 is that there's a huge change that arises, 248 00:14:00,450 --> 00:14:02,720 the beautiful discoveries made by Hubel and Wiesel 249 00:14:02,720 --> 00:14:04,920 for which they had received the Nobel Prize. 250 00:14:04,920 --> 00:14:08,790 And this is just a quick view of the monkey brain. 251 00:14:08,790 --> 00:14:10,465 Here is area V1. 252 00:14:12,150 --> 00:14:14,560 I'll come back to the other areas in a minute. 253 00:14:14,560 --> 00:14:16,390 The nice thing about this in the monkey 254 00:14:16,390 --> 00:14:19,760 is that this area is [INAUDIBLE] as I had told you. 255 00:14:19,760 --> 00:14:22,590 And because of that, it's easy to study 256 00:14:22,590 --> 00:14:28,350 the cells and their properties in area V1. 257 00:14:28,350 --> 00:14:32,240 All right, so now if one examines the properties 258 00:14:32,240 --> 00:14:35,750 of single cells in area V1, it was 259 00:14:35,750 --> 00:14:41,210 discovered some major transformations had occurred 260 00:14:41,210 --> 00:14:44,340 in the inputs from the lateral geniculate nucleus. 261 00:14:44,340 --> 00:14:48,720 And these major transforms can be summarized in just a second. 262 00:14:48,720 --> 00:14:53,100 But I will first tell you that there is a differential 263 00:14:53,100 --> 00:14:56,680 input from the parvocellular and magnocellular layers, which 264 00:14:56,680 --> 00:15:00,374 project respectively to [? 4C ?] beta and [? 4C ?] alpha. 265 00:15:00,374 --> 00:15:01,790 And then there's yet another class 266 00:15:01,790 --> 00:15:07,360 of cells that originates in the retina that are project 267 00:15:07,360 --> 00:15:09,530 into the inter lamina layers, and they 268 00:15:09,530 --> 00:15:14,630 project into the upper portions of the visual cortex. 269 00:15:14,630 --> 00:15:17,650 So now if one looks in detail at the properties 270 00:15:17,650 --> 00:15:21,120 of these cells, which we have discussed quite a bit. 271 00:15:21,120 --> 00:15:23,560 You can refer to these as transforms. 272 00:15:23,560 --> 00:15:28,690 The transforms of the visual input into the cortical cells. 273 00:15:28,690 --> 00:15:30,880 So when you're record from these cortical cells, 274 00:15:30,880 --> 00:15:34,980 you'll find one big transform is that these cells, 275 00:15:34,980 --> 00:15:37,190 the overwhelming majority of these cells, 276 00:15:37,190 --> 00:15:38,870 become orientation selective. 277 00:15:43,750 --> 00:15:46,680 Many cells become direction selective, 278 00:15:46,680 --> 00:15:51,630 virtually all simple cells and about half the complex cells. 279 00:15:51,630 --> 00:15:53,780 So direction selectively becomes very important. 280 00:15:53,780 --> 00:15:56,550 We'll talk about that in a bit more detail later on. 281 00:15:56,550 --> 00:15:59,870 Then, some cells are spatial frequency selective. 282 00:15:59,870 --> 00:16:02,620 Many cells get an input from both eyes. 283 00:16:04,720 --> 00:16:07,640 And there's a convergence of input 284 00:16:07,640 --> 00:16:10,580 from the on and off channels. 285 00:16:10,580 --> 00:16:12,940 This is also true for some of the cells that 286 00:16:12,940 --> 00:16:16,100 get a convergent input from the midget and parasol cells. 287 00:16:16,100 --> 00:16:18,590 So those are the major transforms 288 00:16:18,590 --> 00:16:21,860 that you see in the visual cortex. 289 00:16:21,860 --> 00:16:26,700 All right, so now as a result of having made these discoveries, 290 00:16:26,700 --> 00:16:28,530 people came up with a question of how 291 00:16:28,530 --> 00:16:32,050 is this organized in the visual cortex. 292 00:16:32,050 --> 00:16:34,000 And the first point that I had made 293 00:16:34,000 --> 00:16:37,490 is that there's a topographic layout 294 00:16:37,490 --> 00:16:40,590 of the visual field in visual cortex. 295 00:16:41,700 --> 00:16:43,750 But with much more area allocated 296 00:16:43,750 --> 00:16:46,560 for central vision than peripheral vision simply 297 00:16:46,560 --> 00:16:49,370 copying the relative percentage of cells 298 00:16:49,370 --> 00:16:52,790 already in the retina that exist in central vision 299 00:16:52,790 --> 00:16:56,100 and peripheral vision and because the thickness 300 00:16:56,100 --> 00:17:00,050 of the gray matter in cortex is about two millimeters roughly. 301 00:17:02,010 --> 00:17:03,510 And it's constant. 302 00:17:03,510 --> 00:17:06,300 More space has to be allocated for central vision 303 00:17:06,300 --> 00:17:07,960 than peripheral vision. 304 00:17:07,960 --> 00:17:09,670 And as a result of these people that 305 00:17:09,670 --> 00:17:14,380 studied the spacial arrangement and organization 306 00:17:14,380 --> 00:17:17,680 of the visual cortex-- and the initial model 307 00:17:17,680 --> 00:17:19,300 that was proposed, if you remember, 308 00:17:19,300 --> 00:17:21,780 is the Hubel and Wiesel model, according 309 00:17:21,780 --> 00:17:23,640 to which in one direction you have 310 00:17:23,640 --> 00:17:25,900 the alternation of left and right eyes. 311 00:17:25,900 --> 00:17:29,210 You have column, left, right, left, right. 312 00:17:29,210 --> 00:17:32,590 And in the other direction, you have a systematic change 313 00:17:32,590 --> 00:17:34,770 in the orientation of cells. 314 00:17:34,770 --> 00:17:37,620 Now this model didn't fare that well 315 00:17:37,620 --> 00:17:40,330 because it's not as neat as has been proposed. 316 00:17:40,330 --> 00:17:42,880 An alternative model was the Raidial model. 317 00:17:42,880 --> 00:17:44,400 And the last one I'm showing here, 318 00:17:44,400 --> 00:17:47,740 which I call the Swirl model is not really a model 319 00:17:47,740 --> 00:17:50,203 because some very clever experiments that 320 00:17:50,203 --> 00:17:53,610 have been carried out by [INAUDIBLE] 321 00:17:53,610 --> 00:17:57,680 actually did optical recording and demonstrated 322 00:17:57,680 --> 00:18:00,040 that the visual cortex from the top 323 00:18:00,040 --> 00:18:01,840 looks something like this where you 324 00:18:01,840 --> 00:18:06,000 have indeed systematic arrangement of orientations 325 00:18:06,000 --> 00:18:09,060 in left and right eye columns. 326 00:18:09,060 --> 00:18:12,430 But it's not a straight linear factor, 327 00:18:12,430 --> 00:18:14,130 but it's kind of a swirly arrangement. 328 00:18:15,300 --> 00:18:20,230 So that then established what is the layout 329 00:18:20,230 --> 00:18:22,490 of the primary visual cortex. 330 00:18:22,490 --> 00:18:27,960 OK now the other important thing that we had emphasized 331 00:18:27,960 --> 00:18:32,080 is that contrary to some of the popular ideas 332 00:18:32,080 --> 00:18:37,090 that people have had that the cells in the brain 333 00:18:37,090 --> 00:18:40,870 are feature selective, meaning that they'll extract 334 00:18:40,870 --> 00:18:46,140 specific features from the visual scene like say one cell 335 00:18:46,140 --> 00:18:49,610 extracts color, another cell extracts a particular face, 336 00:18:49,610 --> 00:18:50,400 and so on. 337 00:18:50,400 --> 00:18:53,330 It turns out that that's a false impression 338 00:18:53,330 --> 00:18:54,820 that people had gained. 339 00:18:54,820 --> 00:18:58,150 And instead what is happening that any given one 340 00:18:58,150 --> 00:19:03,685 cell processes many different kinds of visual information. 341 00:19:04,770 --> 00:19:08,450 And it's the activity of thousands and thousands 342 00:19:08,450 --> 00:19:15,840 of cells in a network that can come up with the percepts 343 00:19:15,840 --> 00:19:18,150 that you perceive. 344 00:19:18,150 --> 00:19:21,640 Now that's extremely complicated, 10 times more 345 00:19:21,640 --> 00:19:23,660 complicated than any computer. 346 00:19:23,660 --> 00:19:26,720 And it is something that to a larger extent 347 00:19:26,720 --> 00:19:28,450 still has not been solved. 348 00:19:28,450 --> 00:19:31,970 You don't know how does a person recognize a face. 349 00:19:31,970 --> 00:19:34,960 You can tell oh, it takes place in various parts of the brain 350 00:19:34,960 --> 00:19:35,680 and so on. 351 00:19:35,680 --> 00:19:38,540 But exactly physically how that's done 352 00:19:38,540 --> 00:19:42,150 is something that's still remains largely a mystery. 353 00:19:42,150 --> 00:19:45,780 All right, so now let's move on and talk 354 00:19:45,780 --> 00:19:47,960 about extrastriate cortex. 355 00:19:47,960 --> 00:19:53,190 And extrastriate cortex-- here's a diagram of the monkey brain 356 00:19:53,190 --> 00:19:54,040 again. 357 00:19:54,040 --> 00:19:57,170 Now I'll point out here's area V1. 358 00:19:57,170 --> 00:20:00,270 And once you get close to the lunate sulcus here, 359 00:20:00,270 --> 00:20:03,190 V2 begins and folds under. 360 00:20:03,190 --> 00:20:06,290 And then inside there we have V3. 361 00:20:06,290 --> 00:20:09,500 And then actually make folds back out again. 362 00:20:09,500 --> 00:20:10,995 You have area V4 here. 363 00:20:12,320 --> 00:20:16,140 And then you have areas MT and MST right here. 364 00:20:16,140 --> 00:20:18,400 And then, in addition, you have, of course, 365 00:20:18,400 --> 00:20:20,990 your infertemporal cortex area, which 366 00:20:20,990 --> 00:20:25,255 plays a very important role in complex analyses such as faces. 367 00:20:26,650 --> 00:20:30,390 And then you come to the frontal lobe 368 00:20:30,390 --> 00:20:33,010 here, in which you have the frontal eye fields 369 00:20:33,010 --> 00:20:36,890 and medial eye fields that also process visual information 370 00:20:36,890 --> 00:20:40,410 but mostly for eye movements that I 371 00:20:40,410 --> 00:20:42,700 will talk about later on. 372 00:20:42,700 --> 00:20:46,840 So that then is in a nutshell the arrangement. 373 00:20:46,840 --> 00:20:51,480 And much of the work that has been done in the past dozen 374 00:20:51,480 --> 00:20:56,055 years or so was to examine what these extrastriate areas do 375 00:20:56,055 --> 00:20:57,000 for vision. 376 00:20:57,000 --> 00:20:59,460 And I'll come back to that when we 377 00:20:59,460 --> 00:21:01,970 talk about higher level visual processing. 378 00:21:01,970 --> 00:21:05,030 Now basically the fact is that there 379 00:21:05,030 --> 00:21:11,010 are more than 30 visual areas and that there 380 00:21:11,010 --> 00:21:13,810 are more than 300 interconnections among them. 381 00:21:15,240 --> 00:21:18,890 Initially the idea was-- the feature detection idea-- 382 00:21:18,890 --> 00:21:22,800 that each of these areas is specific for analyzing 383 00:21:22,800 --> 00:21:25,000 a particular type of percept. 384 00:21:25,000 --> 00:21:28,180 But then it became more evident, increasingly more evident, 385 00:21:28,180 --> 00:21:32,690 that these areas tremendously interact with each other 386 00:21:32,690 --> 00:21:37,020 and perform these very complex analyses 387 00:21:37,020 --> 00:21:41,290 based on networks being active. 388 00:21:41,290 --> 00:21:44,670 Now the basic major cortical visual areas, 389 00:21:44,670 --> 00:21:49,420 V1 I just talked about, V2 I mentioned, V3, V4, MT. 390 00:21:49,420 --> 00:21:52,900 Then when you come to the temporal cortex, 391 00:21:52,900 --> 00:21:56,740 you come to infertemporal region that I just mentioned. 392 00:21:56,740 --> 00:21:59,020 And then in the parietal cortex, we 393 00:21:59,020 --> 00:22:00,900 have the lateral parietal sulcrus, 394 00:22:00,900 --> 00:22:04,340 the ventral interparietal, and the medial superior temporal 395 00:22:04,340 --> 00:22:05,140 sulcrus. 396 00:22:05,140 --> 00:22:07,060 So those are some of the major areas. 397 00:22:07,060 --> 00:22:10,270 And then as I've already noted, in the frontal cortex, 398 00:22:10,270 --> 00:22:11,950 we have the frontal eye fields. 399 00:22:11,950 --> 00:22:15,560 And then even we had the medial eye fields, which are not 400 00:22:15,560 --> 00:22:19,210 listed here that also play a role 401 00:22:19,210 --> 00:22:22,140 in eye movements, perhaps a lesser 402 00:22:22,140 --> 00:22:24,030 extent in visual analysis as such. 403 00:22:24,030 --> 00:22:28,260 But many of the cells there too have visual receptive fields, 404 00:22:28,260 --> 00:22:31,180 although they are very hard to discern. 405 00:22:31,180 --> 00:22:35,900 They much more clearly have motor fields than visual field. 406 00:22:35,900 --> 00:22:38,100 All right so now what we are going to do is 407 00:22:38,100 --> 00:22:40,540 we are going to go back to the beginning 408 00:22:40,540 --> 00:22:44,490 and look at the so-called on and off channels briefly. 409 00:22:44,490 --> 00:22:45,890 We talked about that a lot. 410 00:22:46,980 --> 00:22:49,930 Again, to reemphasize, all photoreceptors 411 00:22:49,930 --> 00:22:51,690 hyperpolarize to light. 412 00:22:51,690 --> 00:22:57,650 And then because the two major classes of neurotransmitter 413 00:22:57,650 --> 00:23:00,050 receptor sites in the bipolar cells, 414 00:23:00,050 --> 00:23:02,440 you create a double ended system from a single 415 00:23:02,440 --> 00:23:05,340 ended one creating the so-called on and off. 416 00:23:05,340 --> 00:23:08,240 Now these systems were discovered initially 417 00:23:08,240 --> 00:23:10,480 by Keffer Hartline, who received the Nobel 418 00:23:10,480 --> 00:23:12,590 Prize for that remarkable discovery. 419 00:23:12,590 --> 00:23:16,250 And he thought at the time that the on system 420 00:23:16,250 --> 00:23:18,590 signaled when a stimulus came on. 421 00:23:18,590 --> 00:23:21,610 And the off channel signaled when it went off. 422 00:23:21,610 --> 00:23:23,540 That was his idea, which turned out 423 00:23:23,540 --> 00:23:28,160 to be all wrong because that's not what these cells are about. 424 00:23:28,160 --> 00:23:31,390 What these cells are about, as I've pointed out repeatedly, 425 00:23:31,390 --> 00:23:36,920 is that they can process both light increment and light 426 00:23:36,920 --> 00:23:40,660 decrement with an excitatory response. 427 00:23:40,660 --> 00:23:44,060 That means because of the nature, the physical nature, 428 00:23:44,060 --> 00:23:48,630 of light that some objects in the world reflect light, 429 00:23:48,630 --> 00:23:51,650 and some objects in the world absorb light. 430 00:23:53,860 --> 00:23:56,340 Because of this, as you look around, 431 00:23:56,340 --> 00:23:58,270 some objects look black, and some objects 432 00:23:58,270 --> 00:23:59,940 look white, or whatever. 433 00:23:59,940 --> 00:24:04,980 And because of that, to be able to rapidly process 434 00:24:04,980 --> 00:24:09,200 something that is a dark object as well as a light object, 435 00:24:09,200 --> 00:24:11,610 you need to have excitatory signals 436 00:24:11,610 --> 00:24:14,520 to go to the central nervous system to process that. 437 00:24:14,520 --> 00:24:17,440 So therefore, we can say, first of all, 438 00:24:17,440 --> 00:24:19,370 that we have these cell types. 439 00:24:19,370 --> 00:24:21,910 And they won't have sensor surround antagonism. 440 00:24:21,910 --> 00:24:23,990 And let me add one more fact here 441 00:24:23,990 --> 00:24:26,770 is that they're comfortable with adaptation, 442 00:24:26,770 --> 00:24:31,760 that the average firing rate, average maximum firing rate, 443 00:24:31,760 --> 00:24:38,395 of a retinal ganglion cell is maybe about 400 to 600 hertz. 444 00:24:39,670 --> 00:24:43,380 And that is a rather limited frequency range. 445 00:24:43,380 --> 00:24:46,710 And yet, you have to analyze practically 446 00:24:46,710 --> 00:24:50,630 over 10 log units of light information. 447 00:24:50,630 --> 00:24:53,530 And because of that, the sensor surround antagonism 448 00:24:53,530 --> 00:24:55,780 has evolved so that these cells always 449 00:24:55,780 --> 00:25:03,380 look at local contrast changes rather than absolutes. 450 00:25:03,380 --> 00:25:07,190 So then, if you look at the on and off cells, 451 00:25:07,190 --> 00:25:10,400 I've told you, in accordance with the sensor surround 452 00:25:10,400 --> 00:25:13,834 antagonism, if you split a small spot of light 453 00:25:13,834 --> 00:25:15,250 in the center of receptive fields, 454 00:25:15,250 --> 00:25:17,460 on cells fire when you increase it. 455 00:25:17,460 --> 00:25:20,080 Off cells fire when you decrease it. 456 00:25:20,080 --> 00:25:22,400 But when you use a much larger spot, 457 00:25:22,400 --> 00:25:24,240 you get a lesser response because 458 00:25:24,240 --> 00:25:26,100 of the surround antagonism. 459 00:25:26,100 --> 00:25:30,420 So that's the basic principle of these two types of cells. 460 00:25:30,420 --> 00:25:34,500 And then, I told you about these experiments, 461 00:25:34,500 --> 00:25:37,465 in which two 2-amino-4-phosphonobutyrate had 462 00:25:37,465 --> 00:25:42,470 been used, which is for brief purposes, called APB. 463 00:25:42,470 --> 00:25:45,140 And I told you about two types of experiments, 464 00:25:45,140 --> 00:25:49,290 one doing single cell recordings in various parts of the brain 465 00:25:49,290 --> 00:25:51,920 and the others to do behavioral studies. 466 00:25:51,920 --> 00:25:55,230 And what the signal cell recordings had shown 467 00:25:55,230 --> 00:26:02,060 is that the-- let me first say what APB does. 468 00:26:02,060 --> 00:26:03,190 APB is what? 469 00:26:03,190 --> 00:26:04,540 Anybody remember? 470 00:26:04,540 --> 00:26:07,215 It's a neurotransmitter analog. 471 00:26:08,780 --> 00:26:10,460 And what neurotransmitter is it? 472 00:26:12,010 --> 00:26:12,885 AUDIENCE: [INAUDIBLE] 473 00:26:14,340 --> 00:26:15,310 PROFESSOR: Very good. 474 00:26:15,310 --> 00:26:15,910 Glutamate. 475 00:26:15,910 --> 00:26:20,520 All right so what you do is when you inject this substance 476 00:26:20,520 --> 00:26:25,460 into the eye-- this is an artificial substance-- 477 00:26:25,460 --> 00:26:30,130 it blocks the on cells from being 478 00:26:30,130 --> 00:26:32,940 able to respond to incoming light 479 00:26:32,940 --> 00:26:35,240 but does nothing to the off cells. 480 00:26:36,910 --> 00:26:42,260 So if you do this and study the responses of single neurons 481 00:26:42,260 --> 00:26:44,250 in various parts of the brain-- there 482 00:26:44,250 --> 00:26:46,090 have been all these different hypotheses 483 00:26:46,090 --> 00:26:50,730 as to why we had the on and off channels. 484 00:26:50,730 --> 00:26:53,366 One of them was to create sensor surround antagonism. 485 00:26:55,280 --> 00:27:02,110 And the other one was to create orientation direction 486 00:27:02,110 --> 00:27:03,400 selectivism in the cortex. 487 00:27:04,550 --> 00:27:09,070 But it turned out that when you injected APB into the eye, 488 00:27:09,070 --> 00:27:13,870 and you blocked the on channel, the off input to the cells, 489 00:27:13,870 --> 00:27:16,670 and the off cells therefore, still 490 00:27:16,670 --> 00:27:18,970 had sensor surround antagonism. 491 00:27:18,970 --> 00:27:21,005 And the cells in the cortex still 492 00:27:21,005 --> 00:27:23,080 had orientation and direction selectivities. 493 00:27:24,100 --> 00:27:26,710 So these two systems did not arise 494 00:27:26,710 --> 00:27:30,040 for the purpose of creating those basic attributes, which 495 00:27:30,040 --> 00:27:33,350 are so central for being able to analyze the visual scene. 496 00:27:33,350 --> 00:27:36,410 Now the second important finding was 497 00:27:36,410 --> 00:27:39,000 that when you did a behavioral study 498 00:27:39,000 --> 00:27:41,035 and asked monkeys to detect light increment 499 00:27:41,035 --> 00:27:43,450 and detect light decrement, there 500 00:27:43,450 --> 00:27:48,590 was a huge deficit in detecting light increments but no deficit 501 00:27:48,590 --> 00:27:50,860 in detecting light decrement. 502 00:27:50,860 --> 00:27:53,480 So these observations and many other studies 503 00:27:53,480 --> 00:27:56,570 analyzing why there are on and off channels 504 00:27:56,570 --> 00:27:58,990 came up with the conclusion, which I think 505 00:27:58,990 --> 00:28:02,280 is quite valid that these two systems 506 00:28:02,280 --> 00:28:07,620 have evolved to enable organisms to quickly respond 507 00:28:07,620 --> 00:28:11,130 to both light detrimental and light incremental input. 508 00:28:14,220 --> 00:28:18,530 And you probably remember the little quick movie 509 00:28:18,530 --> 00:28:21,040 I showed you that you have a fish in the ocean. 510 00:28:21,040 --> 00:28:23,140 Fish also have on and off channels of course. 511 00:28:24,410 --> 00:28:29,760 If there's a bird in the sky, like an osprey that 512 00:28:29,760 --> 00:28:32,220 is seen by virtue of light decrement, 513 00:28:32,220 --> 00:28:34,630 your off system tells that fish, oh, there's 514 00:28:34,630 --> 00:28:36,810 a bird up there so it can escape. 515 00:28:36,810 --> 00:28:39,670 And if a predator from below that is 516 00:28:39,670 --> 00:28:45,480 lit up by the sunshine, the on system in response to that 517 00:28:45,480 --> 00:28:48,470 and enables this fish to escape. 518 00:28:48,470 --> 00:28:54,770 So that's one example of the function, the prime function, 519 00:28:54,770 --> 00:28:57,430 of the on and off channels. 520 00:28:57,430 --> 00:28:59,360 All right so that's the basic fact then. 521 00:28:59,360 --> 00:29:01,960 So to conclude then, the on and off channels 522 00:29:01,960 --> 00:29:03,970 have emerged in the course of evolution 523 00:29:03,970 --> 00:29:08,370 to enable organisms to process both light incremental and 524 00:29:08,370 --> 00:29:12,016 light decremental information rapidly and effectively. 525 00:29:13,670 --> 00:29:17,080 So that's the conclusion then in a nutshell 526 00:29:17,080 --> 00:29:20,240 of the on and the off channels. 527 00:29:20,240 --> 00:29:23,170 Now we can move on and look at the so-called midget 528 00:29:23,170 --> 00:29:29,120 and parasol cells that had been discovered initially 529 00:29:29,120 --> 00:29:30,190 in the cat. 530 00:29:30,190 --> 00:29:31,950 And they were called the x and y cells. 531 00:29:33,840 --> 00:29:36,440 In the monkey, it's called midget and parasol 532 00:29:36,440 --> 00:29:39,420 because when you look at them anatomically, 533 00:29:39,420 --> 00:29:40,910 the midget cells are small. 534 00:29:40,910 --> 00:29:42,900 And they're very small dendritic arbors 535 00:29:42,900 --> 00:29:45,230 where the parasols cells are much bigger 536 00:29:45,230 --> 00:29:48,634 and have much larger dendritic arbors that 537 00:29:48,634 --> 00:29:49,550 look like an umbrella. 538 00:29:51,070 --> 00:29:53,880 So those two systems were discovered, 539 00:29:53,880 --> 00:29:57,080 and statistical analysis revealed that they are totally 540 00:29:57,080 --> 00:29:59,230 separate types of cells. 541 00:29:59,230 --> 00:30:00,800 They're not a continuum. 542 00:30:00,800 --> 00:30:06,430 So the question then became why did these two systems evolve? 543 00:30:06,430 --> 00:30:08,800 And why did nature go to such trouble 544 00:30:08,800 --> 00:30:11,276 as to make sure that they were separate in retina 545 00:30:11,276 --> 00:30:13,150 and separate in the geniculate to the monkey? 546 00:30:14,680 --> 00:30:17,360 And then, in the cortex, sometimes it 547 00:30:17,360 --> 00:30:20,840 remains separate-- sometimes the two systems remain separate-- 548 00:30:20,840 --> 00:30:23,730 and sometimes they converge as I had 549 00:30:23,730 --> 00:30:26,450 noted in those transforms in area V1. 550 00:30:28,320 --> 00:30:30,100 So now if you look at that, you've 551 00:30:30,100 --> 00:30:31,630 seen this several times now. 552 00:30:31,630 --> 00:30:34,310 The midget system, the center in central retina 553 00:30:34,310 --> 00:30:36,320 consists of just a single cone. 554 00:30:36,320 --> 00:30:38,370 And therefore, this system should 555 00:30:38,370 --> 00:30:41,400 be able to tell you about color whereas the parasol system has 556 00:30:41,400 --> 00:30:44,420 mixed inputs both in the center and the surround. 557 00:30:44,420 --> 00:30:46,890 Furthermore, the parasol system response much more 558 00:30:46,890 --> 00:30:49,490 trangently than the midget system. 559 00:30:49,490 --> 00:30:54,440 So temporal information can be processed more effectively 560 00:30:54,440 --> 00:30:57,120 by the parasol system than the midget system. 561 00:30:57,120 --> 00:31:00,670 So those are the initial observations at the single cell 562 00:31:00,670 --> 00:31:01,380 level. 563 00:31:01,380 --> 00:31:05,760 And then behavioral studies were carried out 564 00:31:05,760 --> 00:31:08,600 in which either the midget or the parasol system 565 00:31:08,600 --> 00:31:10,580 were selectively blocked. 566 00:31:10,580 --> 00:31:14,810 And then performance was tested where those systems had 567 00:31:14,810 --> 00:31:17,900 been blocked and where the systems were intact. 568 00:31:17,900 --> 00:31:23,050 And when this was done, some major findings emerged. 569 00:31:23,050 --> 00:31:25,700 Before I tell you about that, let me just reiterate 570 00:31:25,700 --> 00:31:28,365 again what these connections are. 571 00:31:28,365 --> 00:31:30,240 Here we have the midget and the parasol cells 572 00:31:30,240 --> 00:31:33,490 as well as the cornea cellular cells. 573 00:31:33,490 --> 00:31:35,240 They project through the geniculate 574 00:31:35,240 --> 00:31:37,030 up to the visual cortex. 575 00:31:37,030 --> 00:31:38,545 And then from there, there has been 576 00:31:38,545 --> 00:31:42,430 lots of debate as to what is the nature of the connections 577 00:31:42,430 --> 00:31:45,060 to higher areas in the brain. 578 00:31:45,060 --> 00:31:47,082 And we talked about that quite a bit. 579 00:31:47,082 --> 00:31:48,540 And some beautiful studies had been 580 00:31:48,540 --> 00:31:52,260 carried out showing that the input to area 581 00:31:52,260 --> 00:31:55,110 MT in the parietal lobe is dominated 582 00:31:55,110 --> 00:31:58,130 by the parasol system, but the input 583 00:31:58,130 --> 00:32:00,540 to V4 in the temporal lobe is about 584 00:32:00,540 --> 00:32:02,760 equal for the two systems. 585 00:32:02,760 --> 00:32:05,110 So that was the basic factor then. 586 00:32:05,110 --> 00:32:07,280 And so now the question then comes up, 587 00:32:07,280 --> 00:32:09,620 what is the contribution of these two 588 00:32:09,620 --> 00:32:11,670 systems, the midget and the parasol? 589 00:32:11,670 --> 00:32:13,681 And so experiments are carried out 590 00:32:13,681 --> 00:32:16,305 where lesions are made in either parvocellular or magnocellular 591 00:32:16,305 --> 00:32:17,570 geniculate. 592 00:32:17,570 --> 00:32:22,441 And then the monkey was tested, as I've said, in intact areas, 593 00:32:22,441 --> 00:32:24,440 in areas where the midget system and areas where 594 00:32:24,440 --> 00:32:26,070 the parasol system had been blocked. 595 00:32:27,590 --> 00:32:30,430 Now one additional fact is that when 596 00:32:30,430 --> 00:32:33,030 you block both of these by lesion 597 00:32:33,030 --> 00:32:35,850 in the lateral geniculate nucleus, for the most part, 598 00:32:35,850 --> 00:32:37,930 the monkey becomes blind. 599 00:32:37,930 --> 00:32:40,350 OK, so these two systems are really 600 00:32:40,350 --> 00:32:45,000 central for being able to process visual information. 601 00:32:45,000 --> 00:32:49,380 All right so now, if one looks at what kinds of deficits 602 00:32:49,380 --> 00:32:52,470 arise, a monkey can be trained in a whole bunch 603 00:32:52,470 --> 00:32:53,960 of different tasks. 604 00:32:53,960 --> 00:32:55,420 It talked about these. 605 00:32:55,420 --> 00:32:57,900 Color vision, texture perception, pattern perception, 606 00:32:57,900 --> 00:33:01,510 shape perception, brightness, [INAUDIBLE] scotopic vision, 607 00:33:01,510 --> 00:33:04,680 contrast sensitivity, stereopsis, motion perception, 608 00:33:04,680 --> 00:33:05,660 flicker perception. 609 00:33:05,660 --> 00:33:07,260 We'll talk about those first. 610 00:33:07,260 --> 00:33:11,890 So it was found that there was severe deficits 611 00:33:11,890 --> 00:33:17,350 after a parvocellular lesion, meaning when the midget 612 00:33:17,350 --> 00:33:20,690 system was blocked in color vision, and texture perception, 613 00:33:20,690 --> 00:33:23,170 pattern perception, and shape perception. 614 00:33:23,170 --> 00:33:26,690 Also in contrast sensitivity and severe in stereopsis. 615 00:33:27,830 --> 00:33:34,180 None of those cause a deficit with the magnocellular region, 616 00:33:34,180 --> 00:33:36,820 mean eliminating the parasol system. 617 00:33:36,820 --> 00:33:41,680 But when examine motion perception and flicker 618 00:33:41,680 --> 00:33:47,840 perception, there was a moderate to major deficit 619 00:33:47,840 --> 00:33:50,470 where that system was missing. 620 00:33:50,470 --> 00:33:52,500 So that's then established. 621 00:33:52,500 --> 00:33:53,780 I'll come back to these later. 622 00:33:54,850 --> 00:33:58,980 Established at least in some people's mind 623 00:33:58,980 --> 00:34:00,890 why these two systems have emerged 624 00:34:00,890 --> 00:34:02,420 in the course of evolution. 625 00:34:02,420 --> 00:34:07,300 And so a summary statement to that effect is shown here. 626 00:34:07,300 --> 00:34:11,670 If you look at the ability to process spatial frequency 627 00:34:11,670 --> 00:34:13,659 by the midget and parasol system, 628 00:34:13,659 --> 00:34:15,989 the midget system can process it up 629 00:34:15,989 --> 00:34:17,935 to much higher spatial frequencies. 630 00:34:19,710 --> 00:34:23,170 The obverse is the case when it comes to temporal frequency. 631 00:34:23,170 --> 00:34:25,929 The parasol system can process to much higher levels 632 00:34:25,929 --> 00:34:31,159 of rapid motion or flicker, as you 633 00:34:31,159 --> 00:34:32,659 can see in this little diagram. 634 00:34:32,659 --> 00:34:35,380 So the midget system extends the range 635 00:34:35,380 --> 00:34:39,719 of vision in the spatial frequency wavelength range. 636 00:34:39,719 --> 00:34:41,560 And the parasol system extends it 637 00:34:41,560 --> 00:34:43,510 in the temporal frequency range. 638 00:34:43,510 --> 00:34:46,120 So that's why these two systems have evolved. 639 00:34:46,120 --> 00:34:49,100 And then if you look at this in terms 640 00:34:49,100 --> 00:34:54,449 of the relative number of cells in the retina that are devoted 641 00:34:54,449 --> 00:34:56,960 to these two attributes-- I told you that 642 00:34:56,960 --> 00:35:05,540 in the foveola, there no input at all to the parasol system. 643 00:35:05,540 --> 00:35:07,940 So therefore, what about this fine vision 644 00:35:07,940 --> 00:35:12,740 that the fovea makes possible for you is due to the fact 645 00:35:12,740 --> 00:35:18,740 that area is dominated by the midget system. 646 00:35:18,740 --> 00:35:21,430 Then as you go progressive to the periphery, 647 00:35:21,430 --> 00:35:24,200 that ratio changes as I had just shown you 648 00:35:24,200 --> 00:35:27,020 because increased emphasis has to be placed 649 00:35:27,020 --> 00:35:29,955 a seeing motion and rapid changes in the periphery. 650 00:35:31,080 --> 00:35:38,470 So that's what happens with the parasol system's increased 651 00:35:38,470 --> 00:35:41,230 number of cells in the periphery that 652 00:35:41,230 --> 00:35:43,150 can handle that requirement. 653 00:35:44,290 --> 00:35:46,450 So now we're going to move on and talk 654 00:35:46,450 --> 00:35:51,670 about various aspects of visual processing. 655 00:35:51,670 --> 00:35:54,405 And we'll start first with color vision and adaptation. 656 00:35:56,090 --> 00:36:01,040 As I've shown you before, one of the beautiful advances that 657 00:36:01,040 --> 00:36:03,410 had been made initially actually, believe it or not, 658 00:36:03,410 --> 00:36:06,460 by Newton-- I mentioned that I think-- 659 00:36:06,460 --> 00:36:09,760 was the discovery of-- I shouldn't say discovery-- 660 00:36:09,760 --> 00:36:13,180 the invention on the color circle. 661 00:36:13,180 --> 00:36:18,970 Now this invention arose in part because it was established-- 662 00:36:18,970 --> 00:36:21,140 it's a well-known fact-- that we don't 663 00:36:21,140 --> 00:36:24,950 have opposites along these axes. 664 00:36:24,950 --> 00:36:28,450 You don't have a yellowish blue color. 665 00:36:28,450 --> 00:36:30,930 You don't have a reddish green color. 666 00:36:30,930 --> 00:36:33,920 But anything that's not an opposite in this color circle, 667 00:36:33,920 --> 00:36:34,540 you do have. 668 00:36:35,550 --> 00:36:39,520 So you have yellowish red, or you have yellowish green, 669 00:36:39,520 --> 00:36:40,780 and so on. 670 00:36:40,780 --> 00:36:45,180 So the color circle was then elaborated 671 00:36:45,180 --> 00:36:47,020 upon over many years. 672 00:36:47,020 --> 00:36:49,340 This is a slightly modified version 673 00:36:49,340 --> 00:36:52,310 from what Newton had invented. 674 00:36:52,310 --> 00:36:54,680 And this is set up in such a fashion 675 00:36:54,680 --> 00:36:57,690 that when you go around this circle-- 676 00:36:57,690 --> 00:37:01,951 I should say disk I suppose-- you change the hue of course. 677 00:37:01,951 --> 00:37:04,200 And then when you go from the center of the periphery, 678 00:37:04,200 --> 00:37:05,575 you increase saturation. 679 00:37:07,450 --> 00:37:10,510 This is not the perfect display, especially 680 00:37:10,510 --> 00:37:13,580 because the projector isn't perfect. 681 00:37:13,580 --> 00:37:16,510 But the center is supposed to be white. 682 00:37:16,510 --> 00:37:19,140 And all this is fairly equal luminent. 683 00:37:19,140 --> 00:37:21,560 And so you go from unsaturated to saturated. 684 00:37:22,940 --> 00:37:26,990 Now I will say already at this point 685 00:37:26,990 --> 00:37:32,250 another very important factor in appreciating 686 00:37:32,250 --> 00:37:37,740 the beauty of the color circle is 687 00:37:37,740 --> 00:37:42,250 that when you analyze after images, 688 00:37:42,250 --> 00:37:49,814 it was found that if you adapt to something that's yellow, 689 00:37:49,814 --> 00:37:51,355 you adapt the eye to this wavelength. 690 00:37:52,550 --> 00:37:56,110 And then you shift it to white, then 691 00:37:56,110 --> 00:37:58,880 you get an after-effect, which is blue. 692 00:37:58,880 --> 00:38:01,830 And if you do that for red, the after-effect is green. 693 00:38:01,830 --> 00:38:04,560 And the same thing is all the way around the circle. 694 00:38:04,560 --> 00:38:07,010 If you have this one, the after-effect is here. 695 00:38:07,010 --> 00:38:09,970 So the color circle perfectly predicts 696 00:38:09,970 --> 00:38:16,870 what you're after images are due to adaptation, which 697 00:38:16,870 --> 00:38:23,060 occurs as a result of having bleached selectively 698 00:38:23,060 --> 00:38:31,530 the molecules in the various cone types that we have, 699 00:38:31,530 --> 00:38:33,560 the three cones, red, green, and blue. 700 00:38:33,560 --> 00:38:36,030 So that's the basic rule of the color circle, 701 00:38:36,030 --> 00:38:37,545 which can be used extensively. 702 00:38:37,545 --> 00:38:38,920 And I think you yourself can have 703 00:38:38,920 --> 00:38:43,560 a lot of fun studying this in your off time, which you 704 00:38:43,560 --> 00:38:45,220 don't have too much of I'm sure. 705 00:38:45,220 --> 00:38:48,310 But it's really a wonderful thing to play around with. 706 00:38:50,840 --> 00:38:55,840 Since this course is rather heavily fact oriented, 707 00:38:55,840 --> 00:38:59,060 I want you to remember these basic facts 708 00:38:59,060 --> 00:39:02,430 that I had listed before. 709 00:39:06,610 --> 00:39:09,520 I just noted along the color circle, 710 00:39:09,520 --> 00:39:12,480 you have three attributes, hue, brightness, and saturation. 711 00:39:14,780 --> 00:39:17,300 And then I also mentioned to you that there 712 00:39:17,300 --> 00:39:21,230 is a distinction between the psychological 713 00:39:21,230 --> 00:39:26,575 and the physical attribute of images. 714 00:39:28,610 --> 00:39:34,310 And this arrangement is such that I gave you an example of. 715 00:39:34,310 --> 00:39:40,050 For example, when a tree falls in the forest, 716 00:39:40,050 --> 00:39:44,700 is there a sound when there's nobody around? 717 00:39:45,920 --> 00:39:49,110 And the answer is a distinctive no. 718 00:39:50,420 --> 00:39:51,160 Why? 719 00:39:51,160 --> 00:39:53,970 Because sound is a psychological attribute. 720 00:39:53,970 --> 00:39:55,390 If you're on the other hand, you'd 721 00:39:55,390 --> 00:39:59,400 have said well, if a tree falls in the forest, 722 00:39:59,400 --> 00:40:03,430 through some wavelength result that 723 00:40:03,430 --> 00:40:05,699 are in the range of hearing. 724 00:40:05,699 --> 00:40:06,740 And that, of course, yes. 725 00:40:07,900 --> 00:40:11,380 But if you say sound, that's something you hear. 726 00:40:11,380 --> 00:40:14,400 It's not something that's a physical thing. 727 00:40:14,400 --> 00:40:20,670 So that applies to many aspects of vision, 728 00:40:20,670 --> 00:40:23,450 as well as audition and many other senses 729 00:40:23,450 --> 00:40:26,890 that you must make a distinction between what 730 00:40:26,890 --> 00:40:30,160 your psychological disposition is as opposed 731 00:40:30,160 --> 00:40:32,040 to what a physical fact is. 732 00:40:34,280 --> 00:40:37,120 The next thing here is that we have 733 00:40:37,120 --> 00:40:40,770 three types of photoreceptors, a short, medium, 734 00:40:40,770 --> 00:40:43,920 and long wavelength for the cones. 735 00:40:43,920 --> 00:40:46,740 And then we have also a different wavelengths 736 00:40:46,740 --> 00:40:48,210 peak for the rods. 737 00:40:48,210 --> 00:40:52,460 All of these peaks are broadly tuned 738 00:40:52,460 --> 00:40:57,480 to enable you to-- some are in the brain-- 739 00:40:57,480 --> 00:40:59,837 examine the relative amounts of information 740 00:40:59,837 --> 00:41:01,045 from these three wavelengths. 741 00:41:04,870 --> 00:41:07,230 That then enables you to perceive 742 00:41:07,230 --> 00:41:13,150 many, many other colors partly because of color [INAUDIBLE] 743 00:41:13,150 --> 00:41:16,180 and partly because of variety of amount of activity of them. 744 00:41:16,180 --> 00:41:18,410 And then I mentioned Grassman's laws. 745 00:41:18,410 --> 00:41:21,780 Every color has a complementary, which when mixed 746 00:41:21,780 --> 00:41:23,920 properly yields gray. 747 00:41:23,920 --> 00:41:28,100 That should do with again with the color circle. 748 00:41:28,100 --> 00:41:30,330 So in other words, to go back to that, 749 00:41:30,330 --> 00:41:34,770 if you mix yellow and blue in equal amounts, 750 00:41:34,770 --> 00:41:37,650 you get white or gray I should say. 751 00:41:37,650 --> 00:41:40,050 And the same thing for anything that's and opposite. 752 00:41:40,050 --> 00:41:43,480 But then if you mix things which are not 753 00:41:43,480 --> 00:41:48,220 at diagonals to each other, then you get an in between color. 754 00:41:48,220 --> 00:41:52,230 So if you mix yellow and green, you get yellowish green. 755 00:41:52,230 --> 00:41:54,650 All right so that's Grassman's laws. 756 00:41:54,650 --> 00:41:58,190 So if you have non complementary colors, you get intermediate. 757 00:41:58,190 --> 00:42:01,460 And if you get complementary colors, you get gray. 758 00:42:01,460 --> 00:42:04,700 Again to make sure that you understand this, 759 00:42:04,700 --> 00:42:07,600 complementary means this and that, this and that, 760 00:42:07,600 --> 00:42:16,020 this and that, which are on the opposites on the lines that 761 00:42:16,020 --> 00:42:20,100 intersect the center of the color circle. 762 00:42:22,700 --> 00:42:23,740 We move on. 763 00:42:23,740 --> 00:42:25,560 And we talk about Abney's law. 764 00:42:25,560 --> 00:42:27,450 That is not very important. 765 00:42:27,450 --> 00:42:28,980 And you don't have to remember that. 766 00:42:28,980 --> 00:42:31,313 The luminance of a mixture of differently colored lights 767 00:42:31,313 --> 00:42:33,950 is equal to the sum of the luminance of its components. 768 00:42:33,950 --> 00:42:37,472 That another fact, but you don't need to know that. 769 00:42:37,472 --> 00:42:39,620 The last thing that I want to mention 770 00:42:39,620 --> 00:42:46,570 is so-called metamers, which are stimuli which look the same 771 00:42:46,570 --> 00:42:52,050 but are the product of different subcompositions of wavelengths. 772 00:42:52,050 --> 00:43:00,260 So because we only have three different cone photoreceptors, 773 00:43:00,260 --> 00:43:01,710 you can, in a sense, if you will, 774 00:43:01,710 --> 00:43:04,780 fool them a little bit by very carefully mixing things 775 00:43:04,780 --> 00:43:08,810 up with different wavelengths to activate them equally. 776 00:43:08,810 --> 00:43:11,450 So that is what is called a metamer when 777 00:43:11,450 --> 00:43:15,330 you can't tell the difference between two stimuli. 778 00:43:15,330 --> 00:43:18,310 They look identical even though their wavelength compositions 779 00:43:18,310 --> 00:43:19,410 are different. 780 00:43:19,410 --> 00:43:23,620 OK so now another factor that I should note here 781 00:43:23,620 --> 00:43:27,590 when it comes to color vision is that when 782 00:43:27,590 --> 00:43:32,290 you look at the response characteristics of cells 783 00:43:32,290 --> 00:43:36,630 in the retina-- and I'm talking about the retinal ganglion 784 00:43:36,630 --> 00:43:39,470 cells-- when we look at the cells in the geniculate, which 785 00:43:39,470 --> 00:43:46,820 is here, what you find actually is just a few major categories. 786 00:43:46,820 --> 00:43:49,100 This is a color circle here. 787 00:43:49,100 --> 00:43:52,090 And one presents the stimuli around the circle 788 00:43:52,090 --> 00:43:54,640 and see how the cell responds. 789 00:43:54,640 --> 00:43:56,590 And what you're see here is one cell, 790 00:43:56,590 --> 00:44:01,860 which is a blue on cell, a green off cell, a yellow on cell, 791 00:44:01,860 --> 00:44:03,080 and a green on cell. 792 00:44:04,310 --> 00:44:07,290 Now it turns out if you record from hundreds and hundreds 793 00:44:07,290 --> 00:44:09,780 of cells, you only get these categories. 794 00:44:09,780 --> 00:44:13,410 You don't ever get any cells which are at the diagonals. 795 00:44:13,410 --> 00:44:16,050 So to see the diagonals, something 796 00:44:16,050 --> 00:44:20,420 has to be taking place in the cortex on the basis of what 797 00:44:20,420 --> 00:44:24,250 is coming in from the retina and the lateral geniculate nucleus. 798 00:44:26,510 --> 00:44:30,870 Now when you come to adaptation, we talk about that quite a bit. 799 00:44:30,870 --> 00:44:33,740 And also with after images, I'll come back to that in a minute. 800 00:44:34,930 --> 00:44:37,720 It was discovered in some very nice example 801 00:44:37,720 --> 00:44:40,520 that you take a cell, and you adapt it 802 00:44:40,520 --> 00:44:43,710 to various levels of overall illumination, 803 00:44:43,710 --> 00:44:45,470 and then see how the cell responds to it, 804 00:44:45,470 --> 00:44:48,100 and then you stimulate the receptive field. 805 00:44:48,100 --> 00:44:51,590 What you find is that here's the same cell. 806 00:44:51,590 --> 00:44:56,210 Here's a background illumination of a huge range over five log 807 00:44:56,210 --> 00:44:56,710 units. 808 00:44:58,080 --> 00:45:00,680 And what you find is that when most of the cell is adapted, 809 00:45:00,680 --> 00:45:02,910 it responds always the same. 810 00:45:02,910 --> 00:45:07,410 So it's looking not at over all levels of illumination, 811 00:45:07,410 --> 00:45:10,039 it's looking at differences in illumination. 812 00:45:10,039 --> 00:45:11,080 It's looking at contrast. 813 00:45:14,970 --> 00:45:18,380 Now how many of you remember the formula for contrast? 814 00:45:20,491 --> 00:45:20,990 Anybody? 815 00:45:22,974 --> 00:45:26,805 All right, I think that's a really good thing to remember. 816 00:45:26,805 --> 00:45:28,430 I'm sure when you go to a party, people 817 00:45:28,430 --> 00:45:29,950 would be fascinated by you knowing 818 00:45:29,950 --> 00:45:31,375 the formula for contrast. 819 00:45:32,485 --> 00:45:36,370 OK, so contrast equals-- you take 820 00:45:36,370 --> 00:45:38,885 the stimulus, which is call it x. 821 00:45:41,160 --> 00:45:44,820 And you take the background, which you called y. 822 00:45:44,820 --> 00:45:47,010 You subtract one from the other. 823 00:45:47,010 --> 00:45:48,510 Then you add the two up. 824 00:45:50,380 --> 00:45:53,700 And then you multiply this by 100. 825 00:45:53,700 --> 00:45:55,290 So that your contrast. 826 00:45:55,290 --> 00:45:59,030 What this formula means that this applies 827 00:45:59,030 --> 00:46:02,630 to endless levels of overall illumination. 828 00:46:02,630 --> 00:46:04,020 You can do this in the sunshine. 829 00:46:04,020 --> 00:46:06,680 You can do this in the moonshine because you're 830 00:46:06,680 --> 00:46:09,770 looking at the differences between the background 831 00:46:09,770 --> 00:46:13,025 and the stimulus itself. 832 00:46:16,566 --> 00:46:17,915 AUDIENCE: What is x and y? 833 00:46:20,190 --> 00:46:22,730 PROFESSOR: As I've said, x is the illumination level 834 00:46:22,730 --> 00:46:24,795 for the target. 835 00:46:25,880 --> 00:46:34,290 Suppose you take a cell, and you shine a spot of light on it 836 00:46:34,290 --> 00:46:35,130 like that. 837 00:46:36,960 --> 00:46:43,990 Then you remove it, and you measure the background level. 838 00:46:43,990 --> 00:46:46,567 And so x is a visual stimulus. 839 00:46:46,567 --> 00:46:47,400 y is the background. 840 00:47:03,040 --> 00:47:05,330 We talk about light adaptation. 841 00:47:05,330 --> 00:47:07,865 Again, I want you to know a few basic facts. 842 00:47:09,010 --> 00:47:11,500 The overall level of illumination 843 00:47:11,500 --> 00:47:12,915 is close to 10 log units. 844 00:47:14,730 --> 00:47:17,460 But in contrast to that, if you just 845 00:47:17,460 --> 00:47:19,860 look at reflected light, that varies 846 00:47:19,860 --> 00:47:22,930 over a much smaller range because on the very 847 00:47:22,930 --> 00:47:25,810 bright illumination conditions, even a black object 848 00:47:25,810 --> 00:47:27,120 will reflect some light. 849 00:47:27,120 --> 00:47:31,000 So you're talking about direct light versus reflected light. 850 00:47:31,000 --> 00:47:34,270 So when you do reflected light, you get a smaller range. 851 00:47:34,270 --> 00:47:38,400 Now the pupil plays a role in the amount of light 852 00:47:38,400 --> 00:47:41,550 it controls getting into the eye. 853 00:47:41,550 --> 00:47:44,800 But it can only do that over a range of 16 to one. 854 00:47:44,800 --> 00:47:48,200 Now because of that, the major role of adaptation 855 00:47:48,200 --> 00:47:53,660 has to do with the photoreceptors 856 00:47:53,660 --> 00:47:55,720 in your rods and your cones. 857 00:47:55,720 --> 00:47:58,270 And the way that works is, if you remember, 858 00:47:58,270 --> 00:48:03,660 is that you can think of your molecules 859 00:48:03,660 --> 00:48:07,785 in your photoreceptors as existing in two forms, bleached 860 00:48:07,785 --> 00:48:08,410 and unbleached. 861 00:48:09,500 --> 00:48:11,260 And because of the millions and millions 862 00:48:11,260 --> 00:48:13,700 of them that I told you already about, 863 00:48:13,700 --> 00:48:19,450 1,000 times 10,000 in just a single cone, 864 00:48:19,450 --> 00:48:24,670 there's a relative percentage of bleached and unbleached 865 00:48:24,670 --> 00:48:29,085 molecules in each cone and in each rod. 866 00:48:31,050 --> 00:48:36,170 And so what is happening is that during dark adaptation, 867 00:48:36,170 --> 00:48:38,070 there's a huge increase in the unbleached, 868 00:48:38,070 --> 00:48:39,760 and during light adaptation, there's 869 00:48:39,760 --> 00:48:41,900 an increase in the bleached molecules. 870 00:48:43,590 --> 00:48:47,450 So therefore, any increase in the rate 871 00:48:47,450 --> 00:48:49,840 of at which quanta are delivered to the eye 872 00:48:49,840 --> 00:48:51,780 is also in the proportion of decrease 873 00:48:51,780 --> 00:48:53,500 in the number of pigment molecules 874 00:48:53,500 --> 00:48:56,020 available to absorb those quanta. 875 00:48:56,020 --> 00:48:58,980 Retina ganglion cells are selected 876 00:48:58,980 --> 00:49:01,360 sensitive to local contrast differences 877 00:49:01,360 --> 00:49:03,470 not absolute levels of illumination. 878 00:49:03,470 --> 00:49:05,720 I've said that many times over again. 879 00:49:05,720 --> 00:49:09,980 OK, and that's why this formula, this contrast formula, 880 00:49:09,980 --> 00:49:16,110 is one that's the most useful in being able to depict 881 00:49:16,110 --> 00:49:20,460 what kind of input these cells are getting. 882 00:49:20,460 --> 00:49:25,030 So that then is the arrangement about light adaptation. 883 00:49:26,720 --> 00:49:30,550 Now let's move on to depth perception, which is one 884 00:49:30,550 --> 00:49:35,410 the most intriguing capacities that we have since our retinae 885 00:49:35,410 --> 00:49:37,750 essentially are like a two dimensional surface. 886 00:49:37,750 --> 00:49:40,170 So whatever comes onto the retina, 887 00:49:40,170 --> 00:49:43,920 some mechanisms have to be able to tell you 888 00:49:43,920 --> 00:49:46,210 where things are in depth. 889 00:49:46,210 --> 00:49:48,630 And because it's such a complex problem, 890 00:49:48,630 --> 00:49:50,910 quite a number of different mechanisms 891 00:49:50,910 --> 00:49:54,490 have evolved to make it possible for you to do that. 892 00:49:54,490 --> 00:49:58,330 And that means that first we have oculomotor cues. 893 00:49:58,330 --> 00:50:00,210 We don't need to talk about those. 894 00:50:00,210 --> 00:50:02,750 But we have visual cues, which have binocular cues, 895 00:50:02,750 --> 00:50:04,810 stereopsis we talked about quite a bit. 896 00:50:04,810 --> 00:50:08,080 And the binocular cues are motion parallax, shading, 897 00:50:08,080 --> 00:50:10,810 interposition size, and perspective. 898 00:50:10,810 --> 00:50:13,510 All these cues we can utilize to tell us 899 00:50:13,510 --> 00:50:14,935 where things are in depth. 900 00:50:15,940 --> 00:50:19,890 So now if we look at stereopsis-- 901 00:50:19,890 --> 00:50:22,430 I've handed out to you some of these autostereograms. 902 00:50:23,770 --> 00:50:26,250 If you look at these, you can't see it looking at that. 903 00:50:26,250 --> 00:50:28,170 You have to do it on those sheets 904 00:50:28,170 --> 00:50:29,600 that I handed out to you. 905 00:50:29,600 --> 00:50:31,550 You can see something in depth. 906 00:50:31,550 --> 00:50:34,510 And this arises by virtue of the fact 907 00:50:34,510 --> 00:50:38,330 that stimuli are arranged in such a fashion 908 00:50:38,330 --> 00:50:44,020 that they selectively activate neurons in the visual cortex 909 00:50:44,020 --> 00:50:46,790 that code depth by virtue of the fact 910 00:50:46,790 --> 00:50:50,550 that they get disparity inputs from the two eyes. 911 00:50:50,550 --> 00:50:54,330 Now another central mechanism-- I 912 00:50:54,330 --> 00:50:56,747 should add one more thing about stereopsis. 913 00:50:56,747 --> 00:50:57,830 I think I've mentioned it. 914 00:50:57,830 --> 00:51:02,360 In fact, 10% of the population in the United States lack 915 00:51:02,360 --> 00:51:09,590 stereopsis in most cases due to either misalignment of the two 916 00:51:09,590 --> 00:51:16,320 eyes or do to ambliopia, meaning one eye doesn't see 917 00:51:16,320 --> 00:51:18,160 as well as the other. 918 00:51:18,160 --> 00:51:22,510 But those people can still do many things 919 00:51:22,510 --> 00:51:24,370 and do depth quite well. 920 00:51:24,370 --> 00:51:26,730 They can't thread needles, but they 921 00:51:26,730 --> 00:51:29,420 can do courser depth quite well. 922 00:51:29,420 --> 00:51:32,010 And one of those is due to motion parallax. 923 00:51:32,010 --> 00:51:34,520 Now the basic rule about motion parallax 924 00:51:34,520 --> 00:51:37,840 that cause the brain to evolve to analyze it 925 00:51:37,840 --> 00:51:39,980 is that when objects are different distances 926 00:51:39,980 --> 00:51:42,730 from the eyes as depicted here, the objects that 927 00:51:42,730 --> 00:51:44,950 are closer to the eye when this object 928 00:51:44,950 --> 00:51:49,940 moves over a greater range on the retinal surface 929 00:51:49,940 --> 00:51:51,400 than those that are further apart. 930 00:51:51,400 --> 00:51:55,590 You can see the green versus the red. 931 00:51:55,590 --> 00:52:03,470 So therefore, the system is such that it 932 00:52:03,470 --> 00:52:06,920 has evolved to be able to see small differences 933 00:52:06,920 --> 00:52:11,130 in the relative motion of objects in the retinal surface. 934 00:52:11,130 --> 00:52:13,530 And I showed you an example of that. 935 00:52:13,530 --> 00:52:16,660 And I'll show it to you again because this is fun. 936 00:52:16,660 --> 00:52:20,440 This is essentially similar to the random autostereogram 937 00:52:20,440 --> 00:52:23,730 except it's just a single bunch of random dots. 938 00:52:23,730 --> 00:52:25,810 And as soon as I set this in motion, 939 00:52:25,810 --> 00:52:29,630 you see them in three dimensions beautifully because these move 940 00:52:29,630 --> 00:52:32,490 over a greater range than these. 941 00:52:32,490 --> 00:52:34,500 And these move even less so. 942 00:52:34,500 --> 00:52:38,440 So this differential motion commands 943 00:52:38,440 --> 00:52:39,760 you to see it in depth. 944 00:52:41,170 --> 00:52:43,900 So that's quite a remarkable ability. 945 00:52:43,900 --> 00:52:47,880 And monkeys are even better at it than we are. 946 00:52:47,880 --> 00:52:51,630 And even fish have this kind of capacity 947 00:52:51,630 --> 00:52:53,440 as do many, many other species. 948 00:52:53,440 --> 00:52:57,150 It's so central to our ability to process depth. 949 00:52:57,150 --> 00:53:00,660 OK 950 00:53:00,660 --> 00:53:03,910 Studies have been carried out to determine 951 00:53:03,910 --> 00:53:08,760 where and how these are analyzed. 952 00:53:08,760 --> 00:53:10,650 And when we came to where, here's 953 00:53:10,650 --> 00:53:14,350 an example of looking at a brain in a normal stereo-blind 954 00:53:14,350 --> 00:53:14,850 subject. 955 00:53:16,020 --> 00:53:18,690 When you only present motion parallax, 956 00:53:18,690 --> 00:53:19,970 you only presents stereo. 957 00:53:21,210 --> 00:53:24,040 And when you do the stereo, monocularly, you 958 00:53:24,040 --> 00:53:27,370 don't see depth, and the brain is not active. 959 00:53:27,370 --> 00:53:29,410 So this tells you which part of the brain 960 00:53:29,410 --> 00:53:35,110 is active and involved in analysis of stereopsis 961 00:53:35,110 --> 00:53:38,850 and which one is involved in the analysis of motion parallax. 962 00:53:38,850 --> 00:53:41,800 I showed you this picture and several others telling you 963 00:53:41,800 --> 00:53:43,240 which areas it is. 964 00:53:43,240 --> 00:53:47,520 The limitation of that is that it can tell you 965 00:53:47,520 --> 00:53:49,030 where it takes place in the brain, 966 00:53:49,030 --> 00:53:51,460 but it doesn't tell you how it takes place. 967 00:53:51,460 --> 00:53:53,150 So because of that, many studies have 968 00:53:53,150 --> 00:53:55,040 been carried out doing single cell 969 00:53:55,040 --> 00:53:58,630 recordings in these cortical areas. 970 00:53:58,630 --> 00:54:00,990 And it was discovered that the there are indeed 971 00:54:00,990 --> 00:54:03,730 cells already in area 17 that get 972 00:54:03,730 --> 00:54:05,345 disparate input from the two eyes. 973 00:54:06,840 --> 00:54:11,810 Beautiful work by [INAUDIBLE] showing this. 974 00:54:11,810 --> 00:54:15,930 And establish therefore that already in area 17, 975 00:54:15,930 --> 00:54:19,105 you have neurons that tell you about stereoscopic depth. 976 00:54:20,520 --> 00:54:24,635 And then it was also discovered, especially in area MT, 977 00:54:24,635 --> 00:54:27,370 to a lesser extent already in V1 also, 978 00:54:27,370 --> 00:54:31,630 that you have cells that respond to differential motion. 979 00:54:31,630 --> 00:54:33,400 And so those cells are presumptively 980 00:54:33,400 --> 00:54:37,820 involved in the processing of depth information 981 00:54:37,820 --> 00:54:42,100 based on motion parallax. 982 00:54:42,100 --> 00:54:44,000 Now another mechanism involved that we 983 00:54:44,000 --> 00:54:46,790 talked about is shading. 984 00:54:46,790 --> 00:54:50,510 Light coming from above like from the sun 985 00:54:50,510 --> 00:54:55,580 had been incorporated into the visual system 986 00:54:55,580 --> 00:55:01,660 to tell you whether an object is towards you or away from you. 987 00:55:01,660 --> 00:55:03,310 And this is an example of that. 988 00:55:03,310 --> 00:55:06,950 Here the light is from above, and the darkness is below. 989 00:55:06,950 --> 00:55:08,390 This is reversed here. 990 00:55:08,390 --> 00:55:10,760 And because of that, you see this as protruding, 991 00:55:10,760 --> 00:55:12,640 and you see this as receding. 992 00:55:12,640 --> 00:55:16,945 And so I showed you several examples, some in the handout, 993 00:55:16,945 --> 00:55:22,200 of the fact that even shading is a hue that's used quite 994 00:55:22,200 --> 00:55:28,540 extensively in depth perception. 995 00:55:29,570 --> 00:55:31,390 Now we come to form perception. 996 00:55:31,390 --> 00:55:33,290 I'll talk about this briefly. 997 00:55:33,290 --> 00:55:35,610 I mentioned three kinds of theories. 998 00:55:35,610 --> 00:55:38,790 One is that the former is due to the fact 999 00:55:38,790 --> 00:55:40,670 that neurons respond selectively to line 1000 00:55:40,670 --> 00:55:43,340 segments of different orientations in V1. 1001 00:55:43,340 --> 00:55:47,170 Another theory was that they have a spatial mapping 1002 00:55:47,170 --> 00:55:49,760 of the stimuli on to the visual cortex since you 1003 00:55:49,760 --> 00:55:50,650 have topography. 1004 00:55:50,650 --> 00:55:53,390 And the third one is that form perceptions 1005 00:55:53,390 --> 00:55:55,500 are accomplished by Fourier analysis. 1006 00:55:55,500 --> 00:55:57,170 We talked about each of these. 1007 00:55:57,170 --> 00:56:00,020 And I pointed out to you that even when 1008 00:56:00,020 --> 00:56:03,345 there are no orientation segments in the display, 1009 00:56:03,345 --> 00:56:08,340 you can still see and identify faces quite well, 1010 00:56:08,340 --> 00:56:13,490 as seen in the Wall Street Journal where 1011 00:56:13,490 --> 00:56:18,780 these kinds of pictures appear every day in the paper itself. 1012 00:56:18,780 --> 00:56:21,360 Now then if you move on, and you look 1013 00:56:21,360 --> 00:56:24,710 at the layout of how the cortexes-- this is a monkey 1014 00:56:24,710 --> 00:56:25,340 cortex here. 1015 00:56:25,340 --> 00:56:26,435 This is a visual field. 1016 00:56:27,480 --> 00:56:30,760 If you present these three stimuli in the visual field, 1017 00:56:30,760 --> 00:56:33,600 this is the area that's activated in the cortex 1018 00:56:33,600 --> 00:56:37,140 because more area is allocated to central vision 1019 00:56:37,140 --> 00:56:38,210 and peripheral vision. 1020 00:56:38,210 --> 00:56:40,461 And so you say oh, this is much bigger than those. 1021 00:56:40,461 --> 00:56:41,460 But that's not the case. 1022 00:56:41,460 --> 00:56:42,959 You can tell that they're identical. 1023 00:56:43,680 --> 00:56:45,440 Now even more dramatic is the fact 1024 00:56:45,440 --> 00:56:50,320 that if you put these three disks centered-- OK, so half 1025 00:56:50,320 --> 00:56:55,080 of it goes to the ipsilater and half of it 1026 00:56:55,080 --> 00:56:58,290 to the contralateral visual hemisphere. 1027 00:56:58,290 --> 00:57:01,010 What you get are a bunch of half circles like that. 1028 00:57:01,010 --> 00:57:03,200 And it doesn't look anything like that. 1029 00:57:03,200 --> 00:57:07,070 So the idea that somehow images are laid down 1030 00:57:07,070 --> 00:57:09,580 in the visual cortex, and the mind then looks at it 1031 00:57:09,580 --> 00:57:10,700 is totally wrong. 1032 00:57:10,700 --> 00:57:12,920 It's wrong to the extent that it's ridiculous. 1033 00:57:14,160 --> 00:57:20,310 The last analysis theory is accepted by some people. 1034 00:57:20,310 --> 00:57:26,290 And doing computer analyses has revealed that system actually 1035 00:57:26,290 --> 00:57:28,670 can be mimicked extremely well based 1036 00:57:28,670 --> 00:57:30,750 on what we know about the organization 1037 00:57:30,750 --> 00:57:35,455 of the visual cortex. 1038 00:57:36,990 --> 00:57:38,950 It has all the basic attributes that you need, 1039 00:57:38,950 --> 00:57:41,840 orientation, direction selectivity, 1040 00:57:41,840 --> 00:57:44,990 and phase that enable you to break down 1041 00:57:44,990 --> 00:57:48,710 the visual scene in an analytical fashion, which 1042 00:57:48,710 --> 00:57:50,891 is kind of foreign to our thinking, namely Fourier 1043 00:57:50,891 --> 00:57:51,390 analysis. 1044 00:57:53,570 --> 00:57:57,670 Then we spent some time talking about prosthesis, which you're 1045 00:57:57,670 --> 00:57:59,380 going to hear quite a bit about actually 1046 00:57:59,380 --> 00:58:03,570 when Chris Brown is going to lecture because that has been 1047 00:58:03,570 --> 00:58:09,030 so successful in the auditory system 1048 00:58:09,030 --> 00:58:13,980 with the cochlear implant, which is a remarkable achievement. 1049 00:58:13,980 --> 00:58:17,780 We have many more than 50,000 by now 1050 00:58:17,780 --> 00:58:20,240 in the United States who have cochlear implants. 1051 00:58:20,240 --> 00:58:23,034 And they can talk and do all kinds of remarkable things. 1052 00:58:23,034 --> 00:58:24,700 We don't have this in the visual system. 1053 00:58:25,950 --> 00:58:28,600 And I've mentioned to you that one of the big differences 1054 00:58:28,600 --> 00:58:32,450 is that in the retina, there are more than a million fibers 1055 00:58:32,450 --> 00:58:36,780 in each eye that come from the ganglion cells that project 1056 00:58:36,780 --> 00:58:43,130 into the brain, whereas when you talk about the auditory system, 1057 00:58:43,130 --> 00:58:46,260 you only have about 30,000 fibers. 1058 00:58:46,260 --> 00:58:49,960 So the magnitude is much less. 1059 00:58:49,960 --> 00:58:51,400 But also there are other factors, 1060 00:58:51,400 --> 00:58:54,060 namely the retina is a very difficult structure 1061 00:58:54,060 --> 00:58:55,190 to work with. 1062 00:58:55,190 --> 00:59:00,580 And also when people become blind, most cases the retina 1063 00:59:00,580 --> 00:59:01,520 degenerates. 1064 00:59:01,520 --> 00:59:06,430 So you can't put a device into the eye 1065 00:59:06,430 --> 00:59:11,580 very effectively in most blind people to create vision. 1066 00:59:11,580 --> 00:59:15,090 So another alternative is to try some other regions. 1067 00:59:15,090 --> 00:59:18,360 Some people have advocated to do this in the visual cortex. 1068 00:59:18,360 --> 00:59:23,640 And the problem there is we have the huge magnification factor. 1069 00:59:23,640 --> 00:59:27,940 So if you put 256 stimuli like this in the visual scene, 1070 00:59:27,940 --> 00:59:29,825 this is the actual physical activation. 1071 00:59:31,420 --> 00:59:33,570 And once you know what this layout is, 1072 00:59:33,570 --> 00:59:37,180 then you can put electrodes in, which are spaced like this. 1073 00:59:37,180 --> 00:59:39,140 Then, if you were to stimulate these, 1074 00:59:39,140 --> 00:59:42,480 then you would create an image, which is at least moderately 1075 00:59:42,480 --> 00:59:46,480 similar to this that would be in slightly different 1076 00:59:46,480 --> 00:59:49,480 washed out colors, but which would still essentially 1077 00:59:49,480 --> 00:59:50,660 be a square. 1078 00:59:50,660 --> 00:59:54,370 So if you do that then, and you take a camera, 1079 00:59:54,370 --> 00:59:58,180 and you take the input to the camera 1080 00:59:58,180 --> 01:00:04,130 and connect it selectively to this proportional implant. 1081 01:00:04,130 --> 01:00:07,060 If you put the word, fiat lux-- remember what that is? 1082 01:00:07,060 --> 01:00:08,440 Let there be light. 1083 01:00:08,440 --> 01:00:10,440 You get a pretty good reproduction 1084 01:00:10,440 --> 01:00:12,060 of what has been put in there. 1085 01:00:12,060 --> 01:00:19,560 But by contrast, if you take a ray of electrodes, which 1086 01:00:19,560 --> 01:00:23,020 are equally spaced-- then, if you activated all those, 1087 01:00:23,020 --> 01:00:24,670 you would get a butterfly image. 1088 01:00:24,670 --> 01:00:29,200 And if you then put in the fiat lux to the camera, 1089 01:00:29,200 --> 01:00:30,900 it would look like that. 1090 01:00:30,900 --> 01:00:32,460 So that would mean that they would 1091 01:00:32,460 --> 01:00:34,470 get a pretty false impression of the world, 1092 01:00:34,470 --> 01:00:36,390 and you wouldn't be able to even read. 1093 01:00:36,390 --> 01:00:40,990 So therefore, it would be very important to take into account 1094 01:00:40,990 --> 01:00:43,520 the functioning of the visual system, 1095 01:00:43,520 --> 01:00:48,040 as well as functioning of the individual neurons 1096 01:00:48,040 --> 01:00:50,920 if you are going to create a prosthetic device. 1097 01:00:50,920 --> 01:00:53,120 So now we will move on. 1098 01:00:53,120 --> 01:00:55,635 And I will say a few words about illusions. 1099 01:00:56,637 --> 01:00:58,220 We talked about quite a few illusions, 1100 01:00:58,220 --> 01:01:00,190 and you got some of those in the hand out. 1101 01:01:00,190 --> 01:01:04,130 The one I mentioned to you that I think all of you enjoyed 1102 01:01:04,130 --> 01:01:05,990 is the Hermann grid illusion, which 1103 01:01:05,990 --> 01:01:08,840 shows the smudges at the smudges at the intersections. 1104 01:01:08,840 --> 01:01:12,470 And the famous theory that was advanced by Baumgartner 1105 01:01:12,470 --> 01:01:16,770 is that it's due to the fact that if you have a cell that 1106 01:01:16,770 --> 01:01:18,720 is centered around here, as opposed 1107 01:01:18,720 --> 01:01:22,120 to not at the intersections, this cell would 1108 01:01:22,120 --> 01:01:24,150 be inhibited more than this one. 1109 01:01:24,150 --> 01:01:31,070 So this hypothesis had been accepted by many people 1110 01:01:31,070 --> 01:01:34,660 a few years back, and it has appeared 1111 01:01:34,660 --> 01:01:36,430 in many, many textbooks. 1112 01:01:36,430 --> 01:01:38,520 It turns out this theory is all wrong 1113 01:01:38,520 --> 01:01:41,150 if you remember because first of all, 1114 01:01:41,150 --> 01:01:43,780 here you just make a small change 1115 01:01:43,780 --> 01:01:48,380 in the physical layout of the lines. 1116 01:01:48,380 --> 01:01:50,270 And you don't get the effect at all, 1117 01:01:50,270 --> 01:01:56,270 even though if you put a cell here, as opposed to here, 1118 01:01:56,270 --> 01:01:58,110 the arrangement is still the same. 1119 01:01:58,110 --> 01:02:00,950 So consequently, that theory is wrong. 1120 01:02:00,950 --> 01:02:04,110 And it's even further proven by the fact that when you analyze 1121 01:02:04,110 --> 01:02:08,630 it physically to see what the number of cells is in this area 1122 01:02:08,630 --> 01:02:11,360 here-- and this is for parasol cells. 1123 01:02:11,360 --> 01:02:13,060 And this is for the midget cells. 1124 01:02:13,060 --> 01:02:14,830 You have a huge number of cells. 1125 01:02:14,830 --> 01:02:16,810 And this is shown only for the on cells. 1126 01:02:16,810 --> 01:02:20,900 You can double that for the off cells. 1127 01:02:20,900 --> 01:02:23,400 You would activate this teeny area here, 1128 01:02:23,400 --> 01:02:26,380 five degrees from the fixation. 1129 01:02:26,380 --> 01:02:32,690 You would activate 365 midget cells and 50 parasol cells, 1130 01:02:32,690 --> 01:02:35,450 half of which would be on and half of which would be off. 1131 01:02:35,450 --> 01:02:40,270 So this theory is just incorrect. 1132 01:02:40,270 --> 01:02:43,330 And so alternative ideas have been developed still 1133 01:02:43,330 --> 01:02:44,790 sort of under debate. 1134 01:02:44,790 --> 01:02:47,910 And one is that this takes place because of the simple cells 1135 01:02:47,910 --> 01:02:50,107 in the visual cortex as we have talked about it. 1136 01:02:52,970 --> 01:02:58,570 Now then, another set of illusions we talked about 1137 01:02:58,570 --> 01:03:00,540 are the after-effect illusions. 1138 01:03:00,540 --> 01:03:04,490 And the experiments that we so informally did here 1139 01:03:04,490 --> 01:03:07,390 asked if you look at a particular display, 1140 01:03:07,390 --> 01:03:10,597 you fix it for a while, and then you change the display, 1141 01:03:10,597 --> 01:03:11,930 you have an after-effect, right? 1142 01:03:11,930 --> 01:03:13,600 A very dramatic after-effect. 1143 01:03:13,600 --> 01:03:18,325 And one of those was the rotating dots in the circle. 1144 01:03:21,330 --> 01:03:22,970 And I showed it to you. 1145 01:03:22,970 --> 01:03:26,700 The experiment was that you adapt to it with one eye, 1146 01:03:26,700 --> 01:03:30,170 and then you look at the display afterwards with the other eye, 1147 01:03:30,170 --> 01:03:32,070 and then you would have no effect, which 1148 01:03:32,070 --> 01:03:34,980 proves that this takes place in the retina 1149 01:03:34,980 --> 01:03:38,317 and proves it is due to the adaptation that 1150 01:03:38,317 --> 01:03:39,733 takes place in the photoreceptors. 1151 01:03:41,090 --> 01:03:44,170 All right so those were the so-called interlocking 1152 01:03:44,170 --> 01:03:45,475 experiments we had discussed. 1153 01:03:46,900 --> 01:03:50,580 So now let me move on and talk some more 1154 01:03:50,580 --> 01:04:01,010 about the deficits in vision arise as a function of lesions. 1155 01:04:02,300 --> 01:04:04,010 And I already showed you a whole set 1156 01:04:04,010 --> 01:04:08,650 of those when we talked about the lesions 1157 01:04:08,650 --> 01:04:11,390 of the midget and parasol systems. 1158 01:04:11,390 --> 01:04:14,490 And now if you look at this in more detail, 1159 01:04:14,490 --> 01:04:17,400 we add to this, what happens when 1160 01:04:17,400 --> 01:04:20,480 you remove V4 and remove MT? 1161 01:04:20,480 --> 01:04:24,030 And it's quite striking that the deficits are far, 1162 01:04:24,030 --> 01:04:31,425 far less when you take out the midget system. 1163 01:04:32,520 --> 01:04:35,830 You have very mild deficits with V4 lesions 1164 01:04:35,830 --> 01:04:37,420 for most of these up here. 1165 01:04:37,420 --> 01:04:39,650 These are basic visual capacities. 1166 01:04:41,740 --> 01:04:45,430 But MT lesions do give you pretty much the same deficits 1167 01:04:45,430 --> 01:04:51,840 as a magnocellular lesion that blocks the parasol system. 1168 01:04:51,840 --> 01:04:53,810 Now then when higher level visual 1169 01:04:53,810 --> 01:04:56,740 capacities we have now analyzed-- I showed you those 1170 01:04:56,740 --> 01:04:58,950 as well-- you found that there was 1171 01:04:58,950 --> 01:05:06,370 some dramatic deficits with V4 lesions when monkeys 1172 01:05:06,370 --> 01:05:13,230 had to choose less a stimuli and had to learn visual percepts, 1173 01:05:13,230 --> 01:05:17,160 they had severe deficits with a V4 lesion. 1174 01:05:17,160 --> 01:05:21,490 So that suggests that an area like V4 1175 01:05:21,490 --> 01:05:24,410 plays a very important role in higher level visual processing. 1176 01:05:24,410 --> 01:05:25,770 Yes? 1177 01:05:25,770 --> 01:05:27,710 AUDIENCE: What does the pronounced mean? 1178 01:05:27,710 --> 01:05:28,284 Is that-- 1179 01:05:28,284 --> 01:05:29,200 PROFESSOR: Pronounced? 1180 01:05:29,200 --> 01:05:31,340 It means like a strong deficit. 1181 01:05:31,340 --> 01:05:32,980 AUDIENCE: So more than severe? 1182 01:05:32,980 --> 01:05:33,730 PROFESSOR: No, no. 1183 01:05:35,770 --> 01:05:37,860 You can see by the color also. 1184 01:05:37,860 --> 01:05:39,380 Severe is the strongest. 1185 01:05:39,380 --> 01:05:41,170 Pronounced is strong. 1186 01:05:41,170 --> 01:05:42,720 Moderate is weaker. 1187 01:05:42,720 --> 01:05:48,430 And mild is mild. [CHUCKLES] 1188 01:05:48,430 --> 01:05:52,745 OK so now, next I want to turn to eye movement control. 1189 01:05:53,970 --> 01:05:57,680 And when we do that I want to remind you 1190 01:05:57,680 --> 01:06:01,460 that the many cortical areas as well as the subcortical areas 1191 01:06:01,460 --> 01:06:05,570 that play a significant role in eye movement control. 1192 01:06:05,570 --> 01:06:08,370 And one way to test this is to electrically stimulate 1193 01:06:08,370 --> 01:06:10,430 various regions in the brain and see 1194 01:06:10,430 --> 01:06:12,110 if you get any eye movements. 1195 01:06:12,110 --> 01:06:13,830 And this happens in many areas. 1196 01:06:13,830 --> 01:06:17,740 The ones we have here are superior colliculus, of course, 1197 01:06:17,740 --> 01:06:18,830 we talked a lot about. 1198 01:06:18,830 --> 01:06:22,750 V1, LIP, the medial eye fields, and the frontal eye fields. 1199 01:06:24,230 --> 01:06:28,630 Now in all but one of these you get 1200 01:06:28,630 --> 01:06:31,977 a constant vector [INAUDIBLE] at any sight where you stimulate, 1201 01:06:31,977 --> 01:06:34,560 meaning no matter where the eye is looking when you stimulate, 1202 01:06:34,560 --> 01:06:38,700 you've got a particular vector as depicted here. 1203 01:06:38,700 --> 01:06:42,160 The exception to that is the medial eye fields 1204 01:06:42,160 --> 01:06:43,880 where you have a place code. 1205 01:06:43,880 --> 01:06:47,200 The result of stimulating any given area 1206 01:06:47,200 --> 01:06:49,790 is to bring the eye, normally where the eye is, 1207 01:06:49,790 --> 01:06:51,485 into that motor field. 1208 01:06:52,510 --> 01:06:56,310 Now different regions obviously in these areas 1209 01:06:56,310 --> 01:06:57,880 have different vectors. 1210 01:06:57,880 --> 01:06:59,520 So that's the basic layout. 1211 01:06:59,520 --> 01:07:05,200 And then the question arose how do these get down to the brain 1212 01:07:05,200 --> 01:07:10,660 stem ocularmotor complex that drives the eye muscles that we 1213 01:07:10,660 --> 01:07:11,650 had talked about. 1214 01:07:11,650 --> 01:07:14,110 Well, the way the experiment was done then 1215 01:07:14,110 --> 01:07:16,470 is to remove the superior colliculus. 1216 01:07:16,470 --> 01:07:18,670 And when that was done, what you found 1217 01:07:18,670 --> 01:07:21,470 was really quite dramatic, namely 1218 01:07:21,470 --> 01:07:23,820 that you could no longer drive cells 1219 01:07:23,820 --> 01:07:25,870 from the posterior part of the cortex, 1220 01:07:25,870 --> 01:07:28,710 but you could still drive them from the anterior part. 1221 01:07:28,710 --> 01:07:32,390 This led to the idea that you have two major systems 1222 01:07:32,390 --> 01:07:36,500 in saccadic eye movement generation, 1223 01:07:36,500 --> 01:07:39,120 the so-called posterior system and the anterior system. 1224 01:07:41,610 --> 01:07:45,567 And then when people looked at the question of well, 1225 01:07:45,567 --> 01:07:46,650 we have these two systems. 1226 01:07:46,650 --> 01:07:47,730 What do they do? 1227 01:07:47,730 --> 01:07:49,970 It was discovered that the posterior system 1228 01:07:49,970 --> 01:07:53,330 is very important for generating quick saccades, especially 1229 01:07:53,330 --> 01:07:56,250 express saccades, because when you remove the colliculus, 1230 01:07:56,250 --> 01:07:58,510 you never got an express saccade again. 1231 01:07:58,510 --> 01:08:03,770 And the anterior system plays a very important role in stimulus 1232 01:08:03,770 --> 01:08:07,580 selection and the sequencing of eye movements 1233 01:08:07,580 --> 01:08:10,800 because you make so many eye movements in rapid succession, 1234 01:08:10,800 --> 01:08:14,042 you have to make plans ahead to decide 1235 01:08:14,042 --> 01:08:15,750 where you're going to look in a sequence. 1236 01:08:15,750 --> 01:08:18,939 And that was found to be very important for the frontal eye 1237 01:08:18,939 --> 01:08:21,300 fields because when you remove that, there 1238 01:08:21,300 --> 01:08:25,649 was a major deficit in target selection and in sequencing. 1239 01:08:25,649 --> 01:08:28,180 So then when this was done, we also 1240 01:08:28,180 --> 01:08:31,960 examined, if you remember the question, of what 1241 01:08:31,960 --> 01:08:40,050 is the role of these various areas when you block inhibition 1242 01:08:40,050 --> 01:08:43,729 or you increase inhibition. 1243 01:08:43,729 --> 01:08:46,790 So we use muscimal and bicuculline to do 1244 01:08:46,790 --> 01:08:48,950 that as shown here. 1245 01:08:48,950 --> 01:08:54,930 And it showed that with V1, you get a strong interference 1246 01:08:54,930 --> 01:08:58,510 with both, and you also get a strong deficit 1247 01:08:58,510 --> 01:09:00,910 in visual discrimination because to be 1248 01:09:00,910 --> 01:09:02,960 able to analyze the visual scene, 1249 01:09:02,960 --> 01:09:05,960 you need to have interaction between excitation 1250 01:09:05,960 --> 01:09:07,960 and inhibition both for eye movements 1251 01:09:07,960 --> 01:09:10,250 and for visual discrimination. 1252 01:09:11,649 --> 01:09:14,970 And then with a frontal eye field lesion, 1253 01:09:14,970 --> 01:09:18,750 you've got facilitation as you did in the colliculus 1254 01:09:18,750 --> 01:09:23,220 when you put in bicuculline, which eliminates inhibition. 1255 01:09:23,220 --> 01:09:25,286 The monkey couldn't help but makes saccades. 1256 01:09:25,286 --> 01:09:27,035 But you've got interference with muscimal. 1257 01:09:28,300 --> 01:09:30,149 LIP had no effect. 1258 01:09:30,149 --> 01:09:33,905 So that's then in a summary was what we had discussed. 1259 01:09:33,905 --> 01:09:35,279 And this is something, of course, 1260 01:09:35,279 --> 01:09:38,670 you need to go over again in your notes, 1261 01:09:38,670 --> 01:09:42,660 and in Stellar, and in the assigned readings 1262 01:09:42,660 --> 01:09:46,310 so that you can remember this for the exam. 1263 01:09:46,310 --> 01:09:51,260 OK and then I pointed out to you that even though we never 1264 01:09:51,260 --> 01:09:53,460 think of eye movements, we have an incredible number 1265 01:09:53,460 --> 01:09:56,000 of structures and a number of tasks 1266 01:09:56,000 --> 01:10:00,050 to be able to make each eye movement. 1267 01:10:00,050 --> 01:10:01,560 We have to select a target. 1268 01:10:01,560 --> 01:10:04,810 We have to decide what each-- every time you move your eye, 1269 01:10:04,810 --> 01:10:06,050 there are dozens of targets. 1270 01:10:06,050 --> 01:10:07,460 We have to select one of those. 1271 01:10:07,460 --> 01:10:09,644 Then we have to decide what they are. 1272 01:10:09,644 --> 01:10:11,310 Then we have to decide which one to look 1273 01:10:11,310 --> 01:10:12,840 at which one not to look at. 1274 01:10:12,840 --> 01:10:16,090 And then we have to use our system, which 1275 01:10:16,090 --> 01:10:20,860 is a spatial organization of the motor fields 1276 01:10:20,860 --> 01:10:23,570 to eventually generate an eye movement. 1277 01:10:23,570 --> 01:10:27,280 Now in reality then what happens is that many other systems-- 1278 01:10:27,280 --> 01:10:30,430 I showed you this before, too-- many other systems are involved 1279 01:10:30,430 --> 01:10:33,990 in generational eye movements, hearing, touch, so on. 1280 01:10:35,020 --> 01:10:37,950 And we had generated all sorts of systems 1281 01:10:37,950 --> 01:10:39,830 to enable for you to do this. 1282 01:10:39,830 --> 01:10:43,170 the so-called anterior and the posterior systems 1283 01:10:43,170 --> 01:10:48,580 that reach the brain stem through various channels here. 1284 01:10:48,580 --> 01:10:51,740 This is available for you on the internet. 1285 01:10:51,740 --> 01:10:54,310 It's also available to you on the assigned reading. 1286 01:10:55,390 --> 01:10:59,020 So now lastly we'll turn to motion perception. 1287 01:10:59,020 --> 01:11:01,200 And when we talked about motion perception, 1288 01:11:01,200 --> 01:11:04,615 I pointed out to you that in the area of V1, 1289 01:11:04,615 --> 01:11:08,300 we have simple cells, and complex cells, 1290 01:11:08,300 --> 01:11:10,420 several different classes of simple cells. 1291 01:11:10,420 --> 01:11:13,630 And almost all of these cells, if you look at their responses 1292 01:11:13,630 --> 01:11:15,770 to light increment and light decrement, 1293 01:11:15,770 --> 01:11:18,140 meaning light edges and dark edges, 1294 01:11:18,140 --> 01:11:23,030 almost every one of these cells is direction selective. 1295 01:11:23,030 --> 01:11:25,000 And it's also true for most complex cells. 1296 01:11:25,000 --> 01:11:27,440 About half the complex cells, maybe more, 1297 01:11:27,440 --> 01:11:29,850 are also direction selective. 1298 01:11:29,850 --> 01:11:34,450 So direction selectivity is one of the most central features 1299 01:11:34,450 --> 01:11:38,040 in the visual system that we use extensively 1300 01:11:38,040 --> 01:11:40,230 not just to analyze motion but also 1301 01:11:40,230 --> 01:11:44,000 to be able to see depth by virtue of motion. 1302 01:11:44,000 --> 01:11:45,540 Paradox 1303 01:11:45,540 --> 01:11:49,930 So now we can say, because of all those little experiments 1304 01:11:49,930 --> 01:11:54,040 I had shown you, that the parasol system and because 1305 01:11:54,040 --> 01:11:57,960 of the lesion experiments plays a central role in motion 1306 01:11:57,960 --> 01:11:58,930 analysis. 1307 01:11:58,930 --> 01:12:01,930 And when we do those experiments with a apparent motion, when 1308 01:12:01,930 --> 01:12:08,270 we moved little spots in color or in small differences 1309 01:12:08,270 --> 01:12:13,870 in shape, color and small difference in shape 1310 01:12:13,870 --> 01:12:19,550 didn't matter, indicating that the parasol system plays 1311 01:12:19,550 --> 01:12:24,040 a central role in us seeing apparent motion the way we 1312 01:12:24,040 --> 01:12:25,030 see it. 1313 01:12:25,030 --> 01:12:25,530 All right. 1314 01:12:25,530 --> 01:12:29,330 So now, last very briefly I want to say here 1315 01:12:29,330 --> 01:12:32,770 is about the accessory optic system 1316 01:12:32,770 --> 01:12:35,060 because this is what you're going to be writing about. 1317 01:12:35,060 --> 01:12:38,850 And I just wanted to remind you that the basic discovery was 1318 01:12:38,850 --> 01:12:42,460 that in the retina, the cells of [INAUDIBLE] that 1319 01:12:42,460 --> 01:12:45,960 feed into the accessory optic system come in three 1320 01:12:45,960 --> 01:12:50,440 different direction selectivities as shown here 1321 01:12:50,440 --> 01:12:53,720 and that these three direction selectivities correspond 1322 01:12:53,720 --> 01:12:56,820 to the direction selectivity of semicircular canals. 1323 01:12:56,820 --> 01:12:58,320 That's quite a remarkable discovery. 1324 01:13:00,020 --> 01:13:02,600 And this then enables the organism 1325 01:13:02,600 --> 01:13:04,990 through the system, which by the way 1326 01:13:04,990 --> 01:13:09,120 these cells respond to all the slow movements, that's 1327 01:13:09,120 --> 01:13:12,830 prime function is to-- so they claim, 1328 01:13:12,830 --> 01:13:16,340 and I think that's correct-- is to stabilize 1329 01:13:16,340 --> 01:13:18,230 the eye with respect to the world. 1330 01:13:18,230 --> 01:13:20,490 So when you walk around, what happens 1331 01:13:20,490 --> 01:13:23,820 is you can still see the world very clearly with no blurring 1332 01:13:23,820 --> 01:13:28,220 because the accessory optic system adjusts the eye 1333 01:13:28,220 --> 01:13:31,900 to keep it stable with respect to the world out there. 1334 01:13:31,900 --> 01:13:34,235 And in fact, I can't remember if I told you this story. 1335 01:13:40,400 --> 01:13:45,400 Way back when in Germany when some people were treated 1336 01:13:45,400 --> 01:13:50,950 for pneumonia, they used the drug-- I forget the name of it 1337 01:13:50,950 --> 01:13:57,570 right now-- that caused malfunctioning 1338 01:13:57,570 --> 01:13:59,290 in the semicircular canals. 1339 01:14:00,380 --> 01:14:04,250 As a result of that, that system, 1340 01:14:04,250 --> 01:14:11,260 which co-exists with the accessory optic system 1341 01:14:11,260 --> 01:14:13,830 no longer was able to stabilize the eyes. 1342 01:14:13,830 --> 01:14:19,184 And so here was this guy in Munich living in a neighborhood 1343 01:14:19,184 --> 01:14:20,850 where he had lived for many, many years. 1344 01:14:20,850 --> 01:14:25,490 And he realized that he can't see anything clearly 1345 01:14:25,490 --> 01:14:26,890 when he's walking on the street. 1346 01:14:26,890 --> 01:14:28,190 Everything was blurry. 1347 01:14:28,190 --> 01:14:29,840 And so he said oh, my god. 1348 01:14:29,840 --> 01:14:31,510 I won't be able to recognize my friend. 1349 01:14:31,510 --> 01:14:33,360 I won't be able to say hello to him. 1350 01:14:33,360 --> 01:14:34,990 And so what he learned to do is this. 1351 01:14:38,510 --> 01:14:39,985 Hi, Joe. 1352 01:14:39,985 --> 01:14:41,060 Like that. 1353 01:14:41,060 --> 01:14:44,560 He stabilized his head by holding it. 1354 01:14:44,560 --> 01:14:46,870 So that highlights for you the fact 1355 01:14:46,870 --> 01:14:52,690 that this system of stabilizing the eye 1356 01:14:52,690 --> 01:14:55,200 through the accessory optic system 1357 01:14:55,200 --> 01:14:57,370 plays a very important central role 1358 01:14:57,370 --> 01:14:59,620 in your being able to move around in the world 1359 01:14:59,620 --> 01:15:02,337 and being able to analyze the visual scene in spite 1360 01:15:02,337 --> 01:15:03,920 of the fact that you're moving around. 1361 01:15:05,210 --> 01:15:08,196 So the last thing I wanted to show you-- first, 1362 01:15:08,196 --> 01:15:09,820 I'm going to show you one more picture. 1363 01:15:09,820 --> 01:15:13,360 But first, let me say a couple of words about the exam again. 1364 01:15:13,360 --> 01:15:17,360 I've told you the exam is a multiple choice exam, 1365 01:15:17,360 --> 01:15:20,155 probably something about 100 or so questions. 1366 01:15:23,120 --> 01:15:26,270 Almost all of them deal with basic facts, 1367 01:15:26,270 --> 01:15:27,755 I should say basic facts. 1368 01:15:27,755 --> 01:15:29,130 So you've got to know your facts. 1369 01:15:30,690 --> 01:15:35,180 And what you need to do is read each question, 1370 01:15:35,180 --> 01:15:36,230 circle the choice. 1371 01:15:37,270 --> 01:15:40,391 You don't get punished extra for being wrong. 1372 01:15:40,391 --> 01:15:41,640 If you're wrong, you're wrong. 1373 01:15:41,640 --> 01:15:43,350 But I'm not going to subtract on top 1374 01:15:43,350 --> 01:15:47,405 of that the wrong answers from the right answers. 1375 01:15:48,810 --> 01:15:52,530 So choose an answer, even if you don't know it 1376 01:15:52,530 --> 01:15:53,485 for every question. 1377 01:15:55,070 --> 01:15:59,030 And you'll have a probability of one in four, maybe one in five 1378 01:15:59,030 --> 01:16:03,700 of getting the right answer if you're totally ignorant. 1379 01:16:03,700 --> 01:16:05,650 So that's what the exam is. 1380 01:16:05,650 --> 01:16:08,200 It's going to take about an hour or so, hour and a half 1381 01:16:08,200 --> 01:16:10,050 maybe, depending on how fast you read 1382 01:16:10,050 --> 01:16:12,150 and how fast you make decisions. 1383 01:16:12,150 --> 01:16:18,820 And that's going to take place this coming Wednesday 1384 01:16:18,820 --> 01:16:19,830 right in this room. 1385 01:16:20,900 --> 01:16:24,640 Now the last thing I wanted to show you is-- I 1386 01:16:24,640 --> 01:16:26,750 mean I seem to be so certain about everything 1387 01:16:26,750 --> 01:16:28,230 being right and wrong here. 1388 01:16:28,230 --> 01:16:30,655 I just wanted to tell you one thing, a note of caution. 1389 01:16:31,890 --> 01:16:33,780 And the caution is this one here. 1390 01:16:33,780 --> 01:16:36,325 This is a wonderful sculpture by Naum Gabo. 1391 01:16:39,010 --> 01:16:41,470 I don't know if you've ever heard of Naum Gabo. 1392 01:16:41,470 --> 01:16:45,090 Anyway, this is obviously you can almost instantly 1393 01:16:45,090 --> 01:16:49,350 say it's an upper body and a face, right? 1394 01:16:49,350 --> 01:16:52,640 But the fact is that, as I say here, 1395 01:16:52,640 --> 01:16:55,980 as many scientific hypotheses of brain function 1396 01:16:55,980 --> 01:16:59,590 are appealing but a far cry from the real McCoy. 1397 01:17:00,620 --> 01:17:01,820 So we are still groping. 1398 01:17:02,980 --> 01:17:07,610 And yes, we are a long way from phrenology, 1399 01:17:07,610 --> 01:17:10,920 but still many of the hypotheses and ideas that we have 1400 01:17:10,920 --> 01:17:15,900 are wrong and are more like a cartoon of what it really 1401 01:17:15,900 --> 01:17:16,600 is like. 1402 01:17:19,880 --> 01:17:23,320 And of course, the further up, in my opinion, 1403 01:17:23,320 --> 01:17:26,380 you go from the retina, the higher 1404 01:17:26,380 --> 01:17:28,830 the fancifulness of the ideas. 1405 01:17:28,830 --> 01:17:30,380 At least when it comes to the retina, 1406 01:17:30,380 --> 01:17:32,240 I think we are reasonably comfortable 1407 01:17:32,240 --> 01:17:34,730 that we know a lot about the photoreceptors 1408 01:17:34,730 --> 01:17:36,030 and how they interact. 1409 01:17:36,030 --> 01:17:38,520 And that's fairly close to the way it really is. 1410 01:17:39,700 --> 01:17:44,605 So that may be more like a photograph of Obama. 1411 01:17:46,390 --> 01:17:49,020 But when it comes to the cortex of the higher areas, 1412 01:17:49,020 --> 01:17:51,050 things are a bit more like that. 1413 01:17:51,050 --> 01:17:52,730 So that's the end of it then. 1414 01:17:52,730 --> 01:17:53,930 Thank you very much. 1415 01:17:53,930 --> 01:18:00,170 And I wish you the best of luck on your exam on Wednesday. 1416 01:18:00,170 --> 01:18:01,087 [APPLAUSE] 1417 01:18:01,087 --> 01:18:01,670 Oh, thank you. 1418 01:18:01,670 --> 01:18:01,970 Thank you. 1419 01:18:01,970 --> 01:18:02,678 That's very nice. 1420 01:18:02,678 --> 01:18:04,120 I appreciate it.