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 --> 00:00:12,700 To make a donation or to view additional materials 6 00:00:12,700 --> 00:00:16,620 from 100 of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,267 at ocw.mit.edu. 8 00:00:25,155 --> 00:00:26,030 PROFESSOR: All right. 9 00:00:26,030 --> 00:00:29,980 So today then we are going to talk about eye movement 10 00:00:29,980 --> 00:00:30,480 control. 11 00:00:32,369 --> 00:00:37,260 This is a fact-laden set of topics. 12 00:00:37,260 --> 00:00:40,940 We're going to talk about the basics of eye movements, 13 00:00:40,940 --> 00:00:43,440 and then we're going to talk about the various neural 14 00:00:43,440 --> 00:00:49,100 structures that are involved in eye movement control. 15 00:00:49,100 --> 00:00:52,560 The interesting thing about studying eye movement control 16 00:00:52,560 --> 00:00:58,500 here, which is very closely, obviously related to vision. 17 00:00:58,500 --> 00:01:02,410 It's also very closely related to the ocular motor system 18 00:01:02,410 --> 00:01:04,714 because it involves moving the eyes. 19 00:01:06,020 --> 00:01:12,030 Now the basic fact is that we move our eyes 20 00:01:12,030 --> 00:01:14,670 practically endlessly every day. 21 00:01:14,670 --> 00:01:17,440 We make about three saccades a second. 22 00:01:17,440 --> 00:01:21,230 And we make more than 150,000 saccades every day. 23 00:01:21,230 --> 00:01:23,460 And it's not something you ever think about. 24 00:01:23,460 --> 00:01:26,800 Yet every time you look at something, 25 00:01:26,800 --> 00:01:30,250 you have to decide as to where you're going to look next. 26 00:01:30,250 --> 00:01:31,550 You're not even aware of it. 27 00:01:31,550 --> 00:01:33,330 It just happens automatically. 28 00:01:33,330 --> 00:01:37,450 It's a remarkable system, works with incredible rapidity, 29 00:01:37,450 --> 00:01:41,480 and it involves recognizing objects out there, 30 00:01:41,480 --> 00:01:44,590 recognizing the visual scenes, making a selection, 31 00:01:44,590 --> 00:01:47,510 moving your eye there, and you keep doing it, 32 00:01:47,510 --> 00:01:49,630 as I said, three times a second. 33 00:01:49,630 --> 00:01:51,890 So this is, obviously, the eye movements 34 00:01:51,890 --> 00:01:55,530 that you make like that, jumping from one location to another. 35 00:01:57,590 --> 00:02:00,690 And with each of those eye movements, as I've said, 36 00:02:00,690 --> 00:02:03,930 you need to decide where are you going to look next. 37 00:02:03,930 --> 00:02:06,230 So that's quite an amazing feat. 38 00:02:06,230 --> 00:02:09,330 And it's not just something that we do as humans, 39 00:02:09,330 --> 00:02:12,710 but it's also done regularly by animals. 40 00:02:12,710 --> 00:02:15,500 And most of the work I will tell you about 41 00:02:15,500 --> 00:02:19,390 has been conducted actually on monkeys whose eye movements are 42 00:02:19,390 --> 00:02:23,080 remarkably similar to those of our eye movements. 43 00:02:23,080 --> 00:02:26,750 Now let's first of all go over what 44 00:02:26,750 --> 00:02:29,280 I'm going to cover in the next two sessions. 45 00:02:29,280 --> 00:02:33,170 First of all, before I do that, I just 46 00:02:33,170 --> 00:02:37,150 thought I'll show you an amusing cartoon from The New 47 00:02:37,150 --> 00:02:44,320 Yorker, which was made in 2001 which sort of 48 00:02:44,320 --> 00:02:48,590 puts in a nutshell what the nature of all this is. 49 00:02:48,590 --> 00:02:49,840 Here's a cat. 50 00:02:49,840 --> 00:02:52,710 He's going to try to jump up there to get the thing. 51 00:02:52,710 --> 00:02:56,130 And so it has to make a decision where that thing is in space, 52 00:02:56,130 --> 00:02:58,750 has to make the calculations, as to how 53 00:02:58,750 --> 00:03:03,340 to generate the motor activities to jump up there. 54 00:03:03,340 --> 00:03:05,404 Now when it comes to the eye movements, 55 00:03:05,404 --> 00:03:07,070 you don't have to jump up there, but you 56 00:03:07,070 --> 00:03:09,740 have to decide how to jump your eye from one 57 00:03:09,740 --> 00:03:11,590 location to the next. 58 00:03:11,590 --> 00:03:13,780 So what we are going to try to understand 59 00:03:13,780 --> 00:03:17,350 is what are these calculations that we perform 60 00:03:17,350 --> 00:03:20,470 to be able to make accurate eye movements from one location 61 00:03:20,470 --> 00:03:21,530 to another. 62 00:03:21,530 --> 00:03:26,870 And what neural structures are involved in this, 63 00:03:26,870 --> 00:03:31,500 and what are the various rules these neural structures 64 00:03:31,500 --> 00:03:34,700 are involved in to generate those eye movements. 65 00:03:34,700 --> 00:03:36,310 Now here are the topics. 66 00:03:36,310 --> 00:03:37,715 First of all, we're going to look 67 00:03:37,715 --> 00:03:40,880 at just the basics of eye movements. 68 00:03:40,880 --> 00:03:43,940 Then we are going to look at the so-called eye plant 69 00:03:43,940 --> 00:03:47,060 and the brain stem nuclei, which are 70 00:03:47,060 --> 00:03:50,342 involved in moving your eye muscles. 71 00:03:50,342 --> 00:03:52,800 Then, we are going to look at an important structure in eye 72 00:03:52,800 --> 00:03:56,160 movement generation which is called the superior colliculus 73 00:03:56,160 --> 00:03:58,240 that we hadn't talked about yet. 74 00:03:58,240 --> 00:04:01,420 And then we are going to look at the visual input 75 00:04:01,420 --> 00:04:03,310 for saccade generation. 76 00:04:03,310 --> 00:04:07,050 We're going to examine what the various types 77 00:04:07,050 --> 00:04:14,810 of retinal ganglion cells do to be involved in eye movement 78 00:04:14,810 --> 00:04:15,460 generation. 79 00:04:15,460 --> 00:04:16,918 In particular, we are going to look 80 00:04:16,918 --> 00:04:18,870 at the midget and the parasol cells. 81 00:04:20,050 --> 00:04:26,560 Then perhaps a little bit today, but most of the next time, 82 00:04:26,560 --> 00:04:28,820 we can look at the cortical structures 83 00:04:28,820 --> 00:04:30,870 involved in movement control. 84 00:04:30,870 --> 00:04:33,480 Then we're going to look at the effects of paired 85 00:04:33,480 --> 00:04:36,160 electrical and visual stimulation 86 00:04:36,160 --> 00:04:38,970 because that would give us additional insights 87 00:04:38,970 --> 00:04:40,870 about the nature of eye movements. 88 00:04:40,870 --> 00:04:42,370 And then we're going to look at what 89 00:04:42,370 --> 00:04:47,550 happens when you have various deficits in motor control 90 00:04:47,550 --> 00:04:52,320 as a result of having lesions in various parts 91 00:04:52,320 --> 00:04:54,565 of the visual and ocular motor systems. 92 00:04:55,580 --> 00:04:58,570 And then we're going to look at some pharmacological studies 93 00:04:58,570 --> 00:05:03,580 to try to understand what the pharmacology is of eye movement 94 00:05:03,580 --> 00:05:04,080 control. 95 00:05:05,320 --> 00:05:08,780 And I should only say visually guided eye movement control. 96 00:05:08,780 --> 00:05:09,500 All right. 97 00:05:09,500 --> 00:05:12,865 So let's first start then with the basics of eye movements. 98 00:05:14,690 --> 00:05:17,810 We can ask a question, why do we move our eyes. 99 00:05:17,810 --> 00:05:23,380 Now you already have the answer to the first reason 100 00:05:23,380 --> 00:05:26,340 we do that, which is that in the retina, 101 00:05:26,340 --> 00:05:30,180 we have a highly specialized system in higher mammals 102 00:05:30,180 --> 00:05:33,110 and humans where there's only a very small area 103 00:05:33,110 --> 00:05:35,890 where you have a very high density of photoreceptors. 104 00:05:35,890 --> 00:05:38,850 And so to see fine detail, you need 105 00:05:38,850 --> 00:05:42,370 to move your eye to the location to be 106 00:05:42,370 --> 00:05:47,470 able to analyze it at a high level acuity. 107 00:05:47,470 --> 00:05:50,800 So that then involves the directive 108 00:05:50,800 --> 00:05:54,460 to acquire objects for central viewing. 109 00:05:55,800 --> 00:05:57,620 Because it's the central viewing that 110 00:05:57,620 --> 00:05:59,160 allows you to have high acuity. 111 00:06:00,170 --> 00:06:02,590 Now those are accomplished predominantly 112 00:06:02,590 --> 00:06:05,080 by saccadic eye movements, just like the ones 113 00:06:05,080 --> 00:06:07,340 I had mimicked at the very beginning. 114 00:06:07,340 --> 00:06:09,040 You make these very rapid eye movements 115 00:06:09,040 --> 00:06:10,510 fro one location to another. 116 00:06:14,380 --> 00:06:17,340 Another important part of eye movement control 117 00:06:17,340 --> 00:06:21,070 is that when either your are in motion, or whatever you're 118 00:06:21,070 --> 00:06:23,810 looking at, like a bird flying in the sky, 119 00:06:23,810 --> 00:06:27,215 to be able to analyze it, you track that object. 120 00:06:28,450 --> 00:06:31,930 You make what is called smooth pursuit eye movements. 121 00:06:33,550 --> 00:06:37,950 So that is another important mechanism 122 00:06:37,950 --> 00:06:42,920 that enables you to analyze things accurately in the world. 123 00:06:42,920 --> 00:06:49,320 Now yet another factor is that when we move about, 124 00:06:49,320 --> 00:06:52,830 it's important for us to be able for our eyes 125 00:06:52,830 --> 00:06:55,640 to be stable with respect to the world out there. 126 00:06:55,640 --> 00:06:57,910 And one of the mechanisms involved 127 00:06:57,910 --> 00:07:03,850 in that, as we shall see, is the accessory optic system-- 128 00:07:03,850 --> 00:07:07,030 and that's what you're going to be writing a paper about-- that 129 00:07:07,030 --> 00:07:11,610 is involved in controlling the eyes involving 130 00:07:11,610 --> 00:07:17,175 both visual stimuli and the vestibular system. 131 00:07:19,890 --> 00:07:22,140 So that is what we're going to talk about. 132 00:07:22,140 --> 00:07:23,990 Actually that part we're probably 133 00:07:23,990 --> 00:07:28,660 going to talk about when we-- in the next 134 00:07:28,660 --> 00:07:31,420 to last lecture, not next time, but the time after. 135 00:07:31,420 --> 00:07:34,330 We're going to talk about the accessory optic system a bit. 136 00:07:34,330 --> 00:07:34,890 All right. 137 00:07:34,890 --> 00:07:38,020 So now then, just to reiterate then, 138 00:07:38,020 --> 00:07:39,940 we're talking about the vestibular ocular 139 00:07:39,940 --> 00:07:42,795 reflex and the accessory optic system. 140 00:07:43,860 --> 00:07:49,670 So now then, people have classified eye movements 141 00:07:49,670 --> 00:07:51,250 into some very basic types. 142 00:07:52,690 --> 00:07:56,810 We have so-called conjugate eye movements when the two 143 00:07:56,810 --> 00:08:00,450 eyes move in unison, and then vergence eye movements 144 00:08:00,450 --> 00:08:03,030 of an object comes close to you, it goes away from you, 145 00:08:03,030 --> 00:08:06,200 then the two eyes converge or diverge. 146 00:08:06,200 --> 00:08:10,050 And that still has to be done to make sure 147 00:08:10,050 --> 00:08:11,840 that the images you see in the world 148 00:08:11,840 --> 00:08:15,660 fall upon corresponding regions of the two eyes. 149 00:08:15,660 --> 00:08:20,020 Now the conjugate eye movements fall into two basic categories. 150 00:08:20,020 --> 00:08:21,870 They're called saccadic eye movements, 151 00:08:21,870 --> 00:08:24,070 the ones we have talked about already a bit, 152 00:08:24,070 --> 00:08:26,620 and smooth pursuit eye movements that 153 00:08:26,620 --> 00:08:29,450 causes us to be able to track object 154 00:08:29,450 --> 00:08:34,900 so that we can keep that object on the fovea. 155 00:08:34,900 --> 00:08:37,250 Now vergence eye movement-- you go back 156 00:08:37,250 --> 00:08:46,990 to that picture I'd shown you before in which you have 157 00:08:46,990 --> 00:08:50,480 the horopter or the Vieth-Muller circle, if you remember. 158 00:08:50,480 --> 00:08:54,710 If any object is along the Vieth-Muller circle, 159 00:08:54,710 --> 00:08:59,000 like going from 1 to 2, say an object jumps from here to here, 160 00:08:59,000 --> 00:09:01,140 then when the eyes track it, they 161 00:09:01,140 --> 00:09:05,630 follow in corresponding points into two retinae. 162 00:09:05,630 --> 00:09:09,970 However, if there is an object that falls outside or inside 163 00:09:09,970 --> 00:09:16,070 the Vieth-Muller circle, as shown here, then the object, 164 00:09:16,070 --> 00:09:19,460 the second object follows an non-corresponding point. 165 00:09:19,460 --> 00:09:23,930 And so therefore you have to bring about eye movements that 166 00:09:23,930 --> 00:09:27,120 involved vergence-- either divergence or convergence. 167 00:09:30,240 --> 00:09:34,230 For the most part, we get, today especially 168 00:09:34,230 --> 00:09:36,020 and the next time as well, we're going 169 00:09:36,020 --> 00:09:40,280 to be talking about saccadic eye movements. 170 00:09:40,280 --> 00:09:45,910 And then as I said, after we have covered movement, 171 00:09:45,910 --> 00:09:48,310 we're going to look at the movements that 172 00:09:48,310 --> 00:09:50,780 involve pursuit and vergence. 173 00:09:52,680 --> 00:09:59,090 Let's now take a look at a human subject 174 00:09:59,090 --> 00:10:02,879 and see the eye movements the subject makes. 175 00:10:02,879 --> 00:10:04,420 I mean, you're perfectly aware of it, 176 00:10:04,420 --> 00:10:06,960 but I thought it would be fun for you to look at it. 177 00:10:06,960 --> 00:10:10,765 So here's a subject, and he's looking at something, right? 178 00:10:12,290 --> 00:10:14,590 And all these eye movements obvious there's 179 00:10:14,590 --> 00:10:16,370 nothing moving out there, so these are all 180 00:10:16,370 --> 00:10:17,750 saccadic eye movements. 181 00:10:17,750 --> 00:10:18,625 The head is fixed. 182 00:10:24,700 --> 00:10:27,827 And so now once you've seen this, 183 00:10:27,827 --> 00:10:30,285 there are a number of questions that comes into one's mind. 184 00:10:31,550 --> 00:10:35,170 The first one, on the not too scientific end, 185 00:10:35,170 --> 00:10:37,490 is well was that a male or female. 186 00:10:37,490 --> 00:10:40,620 Or secondly, what was a person looking at? 187 00:10:41,670 --> 00:10:44,580 And then thirdly, way down the list you say, but how does 188 00:10:44,580 --> 00:10:45,705 the brain do this. 189 00:10:47,190 --> 00:10:52,230 So let's see if you can answer these three questions. 190 00:10:52,230 --> 00:10:54,115 The third one will take the two lectures. 191 00:10:55,460 --> 00:10:58,395 But the first and second one can be answers fairly quickly. 192 00:10:59,460 --> 00:11:00,545 Was it a male or female? 193 00:11:02,220 --> 00:11:04,380 Anybody want commit themselves? 194 00:11:04,380 --> 00:11:06,250 AUDIENCE: Male, male, male. 195 00:11:06,250 --> 00:11:07,525 PROFESSOR: Male, male, male. 196 00:11:07,525 --> 00:11:08,775 Anybody think it was a female? 197 00:11:09,910 --> 00:11:10,820 You guys are good. 198 00:11:10,820 --> 00:11:11,900 It was a male. 199 00:11:11,900 --> 00:11:13,091 Very good. 200 00:11:13,091 --> 00:11:13,590 All right. 201 00:11:13,590 --> 00:11:15,560 Now the second thing, of course, is 202 00:11:15,560 --> 00:11:18,780 what on earth was this person looking at. 203 00:11:18,780 --> 00:11:25,140 Now there's no way you could glean that until I tell you. 204 00:11:25,140 --> 00:11:27,980 So let me tell you what this person is looking at. 205 00:11:27,980 --> 00:11:31,080 And what this person looking at is up 206 00:11:31,080 --> 00:11:35,500 picture which is created by Rene Magritte. 207 00:11:35,500 --> 00:11:38,260 And actually resides in at the National Gallery of Art 208 00:11:38,260 --> 00:11:39,305 in Washington DC. 209 00:11:40,680 --> 00:11:44,570 That's quite an interesting painting, 210 00:11:44,570 --> 00:11:47,420 and that's what I'm showing it to you. 211 00:11:47,420 --> 00:11:49,420 And it relates closely to perception-- 212 00:11:49,420 --> 00:11:51,210 the curious aspects of it. 213 00:11:51,210 --> 00:11:55,570 And you can see this doesn't make sense quite right. 214 00:11:55,570 --> 00:11:57,100 But it's because of that, because 215 00:11:57,100 --> 00:11:59,770 of the puzzling nature of this picture that it 216 00:11:59,770 --> 00:12:02,780 is in the fabulous museum. 217 00:12:02,780 --> 00:12:05,390 Because it is so unusual and different that it makes 218 00:12:05,390 --> 00:12:06,460 you think. 219 00:12:06,460 --> 00:12:09,930 So anyway, what happens then suppose you started here 220 00:12:09,930 --> 00:12:11,597 when picture came on, then you're 221 00:12:11,597 --> 00:12:13,180 going to have to make a decision where 222 00:12:13,180 --> 00:12:14,630 you're going to look next. 223 00:12:14,630 --> 00:12:16,660 So what would you look at next? 224 00:12:16,660 --> 00:12:20,856 Most, commonly, would look at the eyes Of the rider. 225 00:12:20,856 --> 00:12:22,730 Then you would look at the eyes of the horse. 226 00:12:24,040 --> 00:12:27,340 And so I look at various outstanding features. 227 00:12:27,340 --> 00:12:31,380 Now the way this looks, more or less-- looks like in real time, 228 00:12:31,380 --> 00:12:32,340 looks like this. 229 00:12:43,440 --> 00:12:45,880 So that's what this person was looking at. 230 00:12:45,880 --> 00:12:48,109 And that makes you aware of the fact 231 00:12:48,109 --> 00:12:50,650 that whenever you look at the pictures, you look at anything, 232 00:12:50,650 --> 00:12:54,150 you make all these eye movements in this very short time for you 233 00:12:54,150 --> 00:12:58,680 to comprehend the scene and to understand it in some detail. 234 00:13:03,090 --> 00:13:09,710 When this kind of eye movement series is made, 235 00:13:09,710 --> 00:13:13,540 you wonder is this something that animals also do. 236 00:13:13,540 --> 00:13:14,700 And the answer is yes. 237 00:13:14,700 --> 00:13:15,720 Monkeys certainly do. 238 00:13:15,720 --> 00:13:19,150 Most mammals do, and even birds do this kind of stuff. 239 00:13:19,150 --> 00:13:22,290 So what I want to show you next is, 240 00:13:22,290 --> 00:13:25,370 first of all, what the pattern of eye movements 241 00:13:25,370 --> 00:13:29,470 is in humans and the initial person who 242 00:13:29,470 --> 00:13:31,520 did beautiful work on this who's name is Yarbus. 243 00:13:33,150 --> 00:13:36,340 Now this is a famous sculpture. 244 00:13:36,340 --> 00:13:39,560 This sculpture the so-called Bust of Nefertiti. 245 00:13:41,200 --> 00:13:45,090 And what Yarbus did is he looked at the kinds of eye movements 246 00:13:45,090 --> 00:13:47,420 a person makes when they look at this figure 247 00:13:47,420 --> 00:13:49,142 to be to comprehend it. 248 00:13:49,142 --> 00:13:50,600 And you can see what is interesting 249 00:13:50,600 --> 00:13:53,620 about this is there is a lot of saccades along the edges, 250 00:13:53,620 --> 00:13:56,440 and there's a lot of detail, and not too many saccades 251 00:13:56,440 --> 00:14:00,260 made to regions where there's smooth areas. 252 00:14:00,260 --> 00:14:02,540 So what Yarbus did, he actually published a book 253 00:14:02,540 --> 00:14:07,350 on this analyzing the nature of our eye movements 254 00:14:07,350 --> 00:14:11,140 just looking at its behavior like this. 255 00:14:11,140 --> 00:14:13,290 Now if you do the same thing with monkeys, 256 00:14:13,290 --> 00:14:17,090 they are going to show you that monkey, a bunch of monkeys, 257 00:14:17,090 --> 00:14:20,180 they also move their eyes just like we do. 258 00:14:20,180 --> 00:14:22,400 They also have foveas, of course, as we discussed. 259 00:14:23,780 --> 00:14:26,730 And so as they move around and look around in the world, 260 00:14:26,730 --> 00:14:30,410 they make even more eye movements than we do. 261 00:14:30,410 --> 00:14:32,660 They're a little bit more quick about it, 262 00:14:32,660 --> 00:14:34,720 and so they make many, many eye movements 263 00:14:34,720 --> 00:14:37,445 and make about maybe almost four saccades a second. 264 00:14:38,480 --> 00:14:43,940 Now to see whether they also actually select objects 265 00:14:43,940 --> 00:14:46,550 in a sensible way, what you can do 266 00:14:46,550 --> 00:14:49,350 is you can present a bunch of displays. 267 00:14:49,350 --> 00:14:54,000 Here is a bunch of round ones-- in round locations, 268 00:14:54,000 --> 00:14:55,830 I should say-- different objects. 269 00:14:55,830 --> 00:14:58,530 And here we have a bunch of them arranged 270 00:14:58,530 --> 00:15:00,350 in a rectangular fashion. 271 00:15:00,350 --> 00:15:02,790 Then on the right, with the [INAUDIBLE] fixed again, 272 00:15:02,790 --> 00:15:06,010 you see the kinds of eye movements the monkey makes. 273 00:15:06,010 --> 00:15:09,140 And what you can readily do, even if you didn't see these 274 00:15:09,140 --> 00:15:11,350 here, you can tell that there must 275 00:15:11,350 --> 00:15:14,930 be a whole bunch of objects at these locations, 276 00:15:14,930 --> 00:15:17,570 including one in the center, and here, 277 00:15:17,570 --> 00:15:21,100 that the objects must be-- are aligned the way they are there. 278 00:15:21,100 --> 00:15:23,900 So clearly the monkeys do the same thing we do. 279 00:15:23,900 --> 00:15:29,970 They tend to look at particular objects in the scene, 280 00:15:29,970 --> 00:15:32,440 and make accurate saccades to them 281 00:15:32,440 --> 00:15:34,000 to be able to analyze them. 282 00:15:35,490 --> 00:15:42,470 So because of that, it is natural to take monkeys 283 00:15:42,470 --> 00:15:46,810 and to study their eye movements and to study 284 00:15:46,810 --> 00:15:50,570 the underlying neural mechanisms in these animals, which 285 00:15:50,570 --> 00:15:54,510 would then make us more capable of understanding 286 00:15:54,510 --> 00:15:57,960 just how the brain moves your eyes about. 287 00:16:03,520 --> 00:16:10,480 So now, let us becoming more concerned with the neural 288 00:16:10,480 --> 00:16:14,200 mechanisms and the machinery involved in eye movement 289 00:16:14,200 --> 00:16:15,310 generation. 290 00:16:15,310 --> 00:16:18,650 And so we are going to refer to this as eye plant 291 00:16:18,650 --> 00:16:22,060 and the brain stem nuclei, which are 292 00:16:22,060 --> 00:16:27,300 in the mid and lower portions of the brain, not in the cortex. 293 00:16:27,300 --> 00:16:30,460 We'll talk about the cortical factors later on. 294 00:16:30,460 --> 00:16:33,335 So let's start first of all, with the muscles. 295 00:16:34,850 --> 00:16:38,730 Each eye have six extraocular muscles, 296 00:16:38,730 --> 00:16:40,405 as they are delineated here. 297 00:16:41,780 --> 00:16:45,550 They're called the superior and inferior recti, 298 00:16:45,550 --> 00:16:49,400 the medial and lateral recti, and the obliques, 299 00:16:49,400 --> 00:16:52,640 the superior and inferior oblique muscles. 300 00:16:52,640 --> 00:16:57,000 Now the remarkable feature about these muscles 301 00:16:57,000 --> 00:16:59,610 that renders them different from the muscles 302 00:16:59,610 --> 00:17:01,700 that you have in the rest of your body 303 00:17:01,700 --> 00:17:04,920 is that the fibers of each muscle 304 00:17:04,920 --> 00:17:08,680 run the entire length of the muscle itself. 305 00:17:08,680 --> 00:17:09,980 It's not segmented. 306 00:17:09,980 --> 00:17:12,730 Most muscles in the body are segmented. 307 00:17:12,730 --> 00:17:16,849 Because of that, they're often difficult to understand 308 00:17:16,849 --> 00:17:18,599 the nature of the operation. 309 00:17:18,599 --> 00:17:23,010 But here we have a situation that since all the fibers 310 00:17:23,010 --> 00:17:27,849 on the entire length, it is a relatively easy 311 00:17:27,849 --> 00:17:31,090 to comprehend the basic mechanisms that generate 312 00:17:31,090 --> 00:17:34,566 the eye movements at this low level. 313 00:17:37,280 --> 00:17:40,860 The next thing that's important to know 314 00:17:40,860 --> 00:17:47,180 is that the eye is a balanced structure. 315 00:17:47,180 --> 00:17:48,775 It doesn't have any weights anywhere. 316 00:17:50,010 --> 00:17:52,460 And so when you move the eyes, it's 317 00:17:52,460 --> 00:17:54,980 not like when you have to pick up an object. 318 00:17:54,980 --> 00:17:56,840 If you were to pick up a heavy object 319 00:17:56,840 --> 00:17:59,920 and you thought it was a feather then 320 00:17:59,920 --> 00:18:02,280 you would practically hit yourself in the face. 321 00:18:03,580 --> 00:18:07,220 But if you know what the object is, you can correct for the way 322 00:18:07,220 --> 00:18:11,180 you're going to lift it, which adds 323 00:18:11,180 --> 00:18:15,910 another huge dimension on how you move your body about, 324 00:18:15,910 --> 00:18:18,240 and how you operate your muscles. 325 00:18:18,240 --> 00:18:22,670 Luckily when it comes to the eyes, that is not a factor. 326 00:18:22,670 --> 00:18:25,320 And because of that, it's easier to understand 327 00:18:25,320 --> 00:18:26,395 the way it operates. 328 00:18:28,140 --> 00:18:31,430 So now that you have this, the next question one can ask 329 00:18:31,430 --> 00:18:35,320 is how are these muscles innervated. 330 00:18:37,290 --> 00:18:39,690 I mean obviously, you have to have some nerves that 331 00:18:39,690 --> 00:18:43,310 connect to it that's cause the muscles to contract 332 00:18:43,310 --> 00:18:45,360 or to let go. 333 00:18:46,470 --> 00:18:51,590 The way that works is that the nerves that 334 00:18:51,590 --> 00:18:56,700 connect to the muscles of the eye release 335 00:18:56,700 --> 00:19:00,570 acetylcholine that causes the muscle to contract. 336 00:19:00,570 --> 00:19:04,060 OK so that's a basic-- that happens everywhere in the body. 337 00:19:04,060 --> 00:19:05,140 So that's very basic. 338 00:19:05,140 --> 00:19:07,360 You already know all that. 339 00:19:07,360 --> 00:19:09,040 But what you probably don't know yet 340 00:19:09,040 --> 00:19:13,660 is what are the nuerons-- where do they come from, 341 00:19:13,660 --> 00:19:18,410 in other words, to innervate these muscles. 342 00:19:18,410 --> 00:19:23,140 Well it turns out that there are three cranial nerves-- 343 00:19:23,140 --> 00:19:25,900 how many cranial nerves are there in the brain? 344 00:19:25,900 --> 00:19:26,760 AUDIENCE: 12. 345 00:19:26,760 --> 00:19:27,357 12? 346 00:19:27,357 --> 00:19:27,940 PROFESSOR: 12. 347 00:19:30,400 --> 00:19:32,755 3 of those, believe it or not, are 348 00:19:32,755 --> 00:19:34,880 involved in the control of eye movement. 349 00:19:34,880 --> 00:19:36,710 And so we can designate that. 350 00:19:36,710 --> 00:19:40,380 We can tell you that the lateral rectus 351 00:19:40,380 --> 00:19:44,795 is innervated by the abducens nerve. 352 00:19:44,795 --> 00:19:46,920 The abducens nerve comes from the abducens nucleus. 353 00:19:51,680 --> 00:19:57,150 The superior oblique muscle is innervated 354 00:19:57,150 --> 00:19:59,980 by trochlear nerve, which is the fourth cranial nerve. 355 00:20:01,870 --> 00:20:05,990 And the rest of them are innervated 356 00:20:05,990 --> 00:20:21,050 by sub-nuclei of the third nerve, which 357 00:20:21,050 --> 00:20:24,040 is called the oculomotor nerve. 358 00:20:24,040 --> 00:20:27,400 So we have these three oculomotor is a third, 359 00:20:27,400 --> 00:20:31,520 trochlear is the fourth, and abducens is the sixth. 360 00:20:31,520 --> 00:20:38,980 So it's these three nuclei which-- note again 361 00:20:38,980 --> 00:20:41,370 that the oculomotor has sub-nuclei-- 362 00:20:41,370 --> 00:20:43,845 that innervate these muscles. 363 00:20:45,790 --> 00:20:50,110 So now that we know that, the next question 364 00:20:50,110 --> 00:20:58,480 is how do these neurons from these three nuclei 365 00:20:58,480 --> 00:21:00,220 act to do this. 366 00:21:00,220 --> 00:21:02,450 But before I tell you that, I want 367 00:21:02,450 --> 00:21:09,620 to make sure that you learn what the 12 cranial nuclei are. 368 00:21:09,620 --> 00:21:13,540 What you can do here-- and I'm using 369 00:21:13,540 --> 00:21:18,310 a so-called clean mnemonic device. 370 00:21:18,310 --> 00:21:22,950 On old Olympus towering tops a fat armed girl vends 371 00:21:22,950 --> 00:21:24,710 snowy hops. 372 00:21:24,710 --> 00:21:26,630 That's easy enough to remember. 373 00:21:26,630 --> 00:21:29,990 The first letter of each designates 374 00:21:29,990 --> 00:21:32,985 each of the 12 cranial nerves. 375 00:21:34,190 --> 00:21:36,560 So if we then look at that, here they are. 376 00:21:37,620 --> 00:21:41,550 The first one is the olfactory one, the second is the optic. 377 00:21:41,550 --> 00:21:43,630 Then we have the ones in blue here, 378 00:21:43,630 --> 00:21:45,287 which innervate the eye muscles-- 379 00:21:45,287 --> 00:21:46,620 oculomotor, trochlear, abducens. 380 00:21:48,070 --> 00:21:50,200 And then one important ones that you're 381 00:21:50,200 --> 00:21:53,470 going to hear a lot about is the auditory one-- 382 00:21:53,470 --> 00:21:54,420 that's number eight. 383 00:21:55,610 --> 00:21:58,390 So those are 12 cranial nerves. 384 00:21:58,390 --> 00:22:00,850 And then, you don't have to remember this, 385 00:22:00,850 --> 00:22:02,650 it might help to know that, of course, 386 00:22:02,650 --> 00:22:05,560 you have a whole bunch of spinal nerves as well. 387 00:22:05,560 --> 00:22:07,655 You have 31 of them. 388 00:22:09,500 --> 00:22:16,284 And those 31, of course, each doubles-- one 389 00:22:16,284 --> 00:22:17,950 on the left side, one on the right side. 390 00:22:19,420 --> 00:22:23,904 You have 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 391 00:22:23,904 --> 00:22:24,570 and 1 coccygeal. 392 00:22:26,435 --> 00:22:29,250 Now you don't-- as I say, you don't have to remember this. 393 00:22:29,250 --> 00:22:32,460 And I'm not even tell you have to remember these. 394 00:22:32,460 --> 00:22:33,900 But it's good to know that. 395 00:22:33,900 --> 00:22:36,360 It's something that you can commit to memory, 396 00:22:36,360 --> 00:22:39,980 so that when you go to a party and talk to somebody, 397 00:22:39,980 --> 00:22:42,810 you can say, hey, you know about the 12 cranial nerves. 398 00:22:42,810 --> 00:22:44,420 And they go, [YAWNS], see you later. 399 00:22:49,150 --> 00:22:49,650 All right. 400 00:22:49,650 --> 00:22:53,910 So anyway, we can move on now and see 401 00:22:53,910 --> 00:22:59,615 how these neurons that innervate the muscles, that 402 00:22:59,615 --> 00:23:04,450 are so often referred to as a final common path-- how 403 00:23:04,450 --> 00:23:05,070 they operate. 404 00:23:06,560 --> 00:23:09,250 Actually that underwent quite a bit of debate 405 00:23:09,250 --> 00:23:10,590 for a long, long time. 406 00:23:10,590 --> 00:23:12,540 We're not going to go into the debate itself, 407 00:23:12,540 --> 00:23:14,373 but I'm just going to tell you how it works. 408 00:23:15,940 --> 00:23:20,030 But the fact is that these neurons 409 00:23:20,030 --> 00:23:24,070 are involved in generating all sorts of eye-- all the eye 410 00:23:24,070 --> 00:23:27,000 movements we talk about, meaning smooth pursuit, 411 00:23:27,000 --> 00:23:30,830 meaning maintained eye position, and saccade. 412 00:23:30,830 --> 00:23:32,470 And so here's an example. 413 00:23:32,470 --> 00:23:34,757 This show vertical eye movements, just 414 00:23:34,757 --> 00:23:35,715 vertical eye movements. 415 00:23:37,060 --> 00:23:42,090 And what you can see here is that the more they eye 416 00:23:42,090 --> 00:23:45,570 looks down, like here, the more sustained activity 417 00:23:45,570 --> 00:23:49,230 to keep the muscle contracted, to keep the eye down. 418 00:23:49,230 --> 00:23:52,500 Now but you also have to remember that while this 419 00:23:52,500 --> 00:23:55,440 is happening here, the opposite muscle 420 00:23:55,440 --> 00:23:58,750 on top, the superior rectus, has to let go. 421 00:23:58,750 --> 00:24:02,660 So there would be no activity to contract 422 00:24:02,660 --> 00:24:06,320 the muscle in the superior rectus-- no muscle 423 00:24:06,320 --> 00:24:08,620 fibers in the superior rectus. 424 00:24:08,620 --> 00:24:11,340 Secondly, the important thing to notice here 425 00:24:11,340 --> 00:24:13,350 is that whenever there's a saccade-- 426 00:24:13,350 --> 00:24:15,170 saccade, saccade, saccade, saccade, 427 00:24:15,170 --> 00:24:17,670 and so on, when there's a little saccade, 428 00:24:17,670 --> 00:24:20,250 there's a high frequency burst. 429 00:24:20,250 --> 00:24:22,510 And the high frequency is short. 430 00:24:22,510 --> 00:24:24,320 When you have a big saccade, there's 431 00:24:24,320 --> 00:24:26,810 a much longer high frequency burst. 432 00:24:26,810 --> 00:24:29,070 And when there's a saccade in the opposite direction, 433 00:24:29,070 --> 00:24:30,072 there is a pause. 434 00:24:30,072 --> 00:24:32,155 So what do you think happens when there's a pause? 435 00:24:34,290 --> 00:24:38,250 In that case, the superior rectus would get a burst 436 00:24:38,250 --> 00:24:41,550 and would contract rapidly to get an upward eye movement. 437 00:24:41,550 --> 00:24:46,140 So this muscle will cause your downward eye movement, 438 00:24:46,140 --> 00:24:50,030 and the superior rectus records an upward movement, of course. 439 00:24:51,240 --> 00:24:53,480 So that is the basic layout. 440 00:24:53,480 --> 00:24:56,900 Now this is schematic, but to show you that this is real, 441 00:24:56,900 --> 00:25:02,320 I'm going to show you some data here of a monkey doing 442 00:25:02,320 --> 00:25:08,600 the actual eye movements, recording from the ocular motor 443 00:25:08,600 --> 00:25:09,530 nucleus. 444 00:25:09,530 --> 00:25:12,680 And in this case again, invading the inferior rectus. 445 00:25:12,680 --> 00:25:15,410 And what you can see here are a bunch of saccades. 446 00:25:15,410 --> 00:25:20,620 And here the monkey is tracking an apple as it moved down. 447 00:25:20,620 --> 00:25:22,360 And what you can see-- let's look first 448 00:25:22,360 --> 00:25:24,130 at this at the bottom. 449 00:25:24,130 --> 00:25:26,672 What you can see here is that somewhere along the line 450 00:25:26,672 --> 00:25:28,130 here is what the monkey's tracking. 451 00:25:28,130 --> 00:25:30,430 This particular neuron begins to cut in, 452 00:25:30,430 --> 00:25:32,750 and then gradually it increases the rate 453 00:25:32,750 --> 00:25:34,934 of its activity as a result of which, 454 00:25:34,934 --> 00:25:36,475 you get smooth pursuit eye movements. 455 00:25:37,610 --> 00:25:39,450 Now if you look at the upper part, 456 00:25:39,450 --> 00:25:43,770 once again as I've shown you, the frequency of activity 457 00:25:43,770 --> 00:25:48,260 is proportional to the angular deviation of the eye, this case 458 00:25:48,260 --> 00:25:50,410 because you're talking about the inferior rectus, 459 00:25:50,410 --> 00:25:52,010 in the vertical dimension. 460 00:25:52,010 --> 00:25:55,470 And whenever there's a saccade downward, 461 00:25:55,470 --> 00:25:57,520 there's a high frequency burst. 462 00:25:57,520 --> 00:26:00,335 And whenever there's a saccade upward, there's a pause. 463 00:26:02,260 --> 00:26:04,820 So that is the basic nature of this. 464 00:26:04,820 --> 00:26:07,890 And what is so lovely about this, is it's like a machine. 465 00:26:07,890 --> 00:26:10,300 This is so lovely that if you then 466 00:26:10,300 --> 00:26:14,240 collect data from several cells, and here's an example of it, 467 00:26:14,240 --> 00:26:17,310 and you look at the number of spikes per second 468 00:26:17,310 --> 00:26:19,700 and the angular deviation of the eye, 469 00:26:19,700 --> 00:26:26,980 it shows you that each of these four neurons 470 00:26:26,980 --> 00:26:29,020 acts in a linear fashion. 471 00:26:29,020 --> 00:26:31,540 As the eye deviates more and more, 472 00:26:31,540 --> 00:26:35,140 the frequency, the activity, gradually increases. 473 00:26:35,140 --> 00:26:36,610 This is not saccades, obviously. 474 00:26:36,610 --> 00:26:41,170 This is maintained activity-- let me go back again-- 475 00:26:41,170 --> 00:26:45,460 like here, like here, like here, and like here. 476 00:26:45,460 --> 00:26:49,320 So you get this beautiful linear function, which again as I've 477 00:26:49,320 --> 00:26:53,960 said, makes it relatively easy to understand quantitatively 478 00:26:53,960 --> 00:26:57,405 the process of moving the eyes. 479 00:26:58,570 --> 00:27:01,960 So that is the basic mechanism that you 480 00:27:01,960 --> 00:27:09,750 see in the oculomotor nuclei. 481 00:27:09,750 --> 00:27:12,750 And the important thing to understand then 482 00:27:12,750 --> 00:27:16,920 is that the activity of these neurons 483 00:27:16,920 --> 00:27:20,230 is involved both in the making saccades 484 00:27:20,230 --> 00:27:24,516 and in maintaining various eye positions. 485 00:27:24,516 --> 00:27:29,290 The reason I emphasize this is because maybe 486 00:27:29,290 --> 00:27:31,990 about 30 years ago, there was a big debate. 487 00:27:33,010 --> 00:27:36,060 Some people argued, and they initially 488 00:27:36,060 --> 00:27:39,660 began to record from these oculomotor nuclei 489 00:27:39,660 --> 00:27:46,040 that we have two different sets of neurons 490 00:27:46,040 --> 00:27:49,420 in the final common path, one of which controls saccades, 491 00:27:49,420 --> 00:27:54,400 and one of which controls maintained position, 492 00:27:54,400 --> 00:27:56,820 or smooth pursuit. 493 00:27:56,820 --> 00:27:58,390 But that's not the case. 494 00:27:58,390 --> 00:28:01,105 The case is that these neurons do both. 495 00:28:02,160 --> 00:28:04,880 Now why did this confusion arise? 496 00:28:04,880 --> 00:28:08,840 Well, it arose because it was found 497 00:28:08,840 --> 00:28:12,080 that if you looked at the so-called supranuclear 498 00:28:12,080 --> 00:28:16,740 complex, which are neurons which are sort of above, if you will, 499 00:28:16,740 --> 00:28:20,040 the oculomotor, trochlear, and abducens nuclei, 500 00:28:20,040 --> 00:28:23,124 there are a whole bunch of subnuclei there-- 501 00:28:23,124 --> 00:28:25,040 we're not going to go into details about them. 502 00:28:25,040 --> 00:28:26,890 You don't have to know that. 503 00:28:26,890 --> 00:28:30,740 But at any rate, in those nuclei there 504 00:28:30,740 --> 00:28:33,920 are neurons that are indeed separately coding 505 00:28:33,920 --> 00:28:35,360 different attributes. 506 00:28:35,360 --> 00:28:37,410 I'm going to show this, but before I 507 00:28:37,410 --> 00:28:40,350 do I want to make one more important point. 508 00:28:40,350 --> 00:28:45,360 If you take a microelectrode, and you put it 509 00:28:45,360 --> 00:28:50,660 in one of these nuclei that controls a final common path, 510 00:28:50,660 --> 00:28:54,590 if you stimulate there at a high frequency, in this case 511 00:28:54,590 --> 00:29:00,680 500 Hertz, if you increase the duration of the stimulation, 512 00:29:00,680 --> 00:29:03,860 you get progressively bigger saccades, 513 00:29:03,860 --> 00:29:07,993 Which proves, with stimulation that it's indeed 514 00:29:07,993 --> 00:29:12,560 the duration of the high frequency burst that defines 515 00:29:12,560 --> 00:29:15,025 the size of the saccade that you elicit. 516 00:29:16,300 --> 00:29:18,500 So now we can move on. 517 00:29:18,500 --> 00:29:21,890 And in this case as I say, you don't have to know this, 518 00:29:21,890 --> 00:29:24,290 but I want to just mention it briefly. 519 00:29:24,290 --> 00:29:27,680 We have a [INAUDIBLE] the eye-- the eye muscle. 520 00:29:27,680 --> 00:29:30,980 And then we have, in addition to the final common path, 521 00:29:30,980 --> 00:29:35,620 we have a supranuclear complex, which have various neuron, some 522 00:29:35,620 --> 00:29:39,449 which pause, in other words, that don't cause saccades, 523 00:29:39,449 --> 00:29:40,990 and some which fire only to saccades. 524 00:29:42,200 --> 00:29:44,590 I think that's the only ones I need to point out. 525 00:29:44,590 --> 00:29:46,350 But at that level, they're separate. 526 00:29:46,350 --> 00:29:51,170 But once they come together in the oculomotor complex, 527 00:29:51,170 --> 00:29:54,440 then they impinge on these neurons 528 00:29:54,440 --> 00:29:58,400 of the final common path that carry both of these signals. 529 00:30:00,640 --> 00:30:05,550 So that then is the essence of the oculomotor complex 530 00:30:05,550 --> 00:30:07,090 that I want to cover. 531 00:30:07,090 --> 00:30:10,770 And so we can a quick, summary diagram here. 532 00:30:10,770 --> 00:30:12,410 We're going to make a diagram that's 533 00:30:12,410 --> 00:30:16,070 going to grow over in this session and the next one, 534 00:30:16,070 --> 00:30:19,510 and that's going to be very complex in the end, 535 00:30:19,510 --> 00:30:20,590 unfortunately. 536 00:30:20,590 --> 00:30:24,340 But here we have the brain stem oculomotor complex. 537 00:30:24,340 --> 00:30:26,790 And the way to put this-- because you 538 00:30:26,790 --> 00:30:30,680 talk about what kinds of coding operations are involved-- this 539 00:30:30,680 --> 00:30:33,760 carries what you're going to call a rate code. 540 00:30:33,760 --> 00:30:36,750 The higher the rate, the greater the angular deviation 541 00:30:36,750 --> 00:30:37,680 of the eye. 542 00:30:37,680 --> 00:30:40,450 And of course, the longer the high frequency burst, 543 00:30:40,450 --> 00:30:41,840 the bigger the saccade. 544 00:30:41,840 --> 00:30:44,570 So that is the so-called rate code. 545 00:30:44,570 --> 00:30:46,830 So this is the basic layout then. 546 00:30:46,830 --> 00:30:49,340 And of course, if there's a lot of activity here, 547 00:30:49,340 --> 00:30:50,780 this muscle is going to contract, 548 00:30:50,780 --> 00:30:53,624 the eye is going to deviate downward. 549 00:30:53,624 --> 00:30:55,540 At the same time, you're going to get a signal 550 00:30:55,540 --> 00:30:57,980 to the upper part that's going to let go. 551 00:30:57,980 --> 00:30:59,990 In other words, the signal is going 552 00:30:59,990 --> 00:31:02,510 to be terminated so that you have 553 00:31:02,510 --> 00:31:07,730 no activity in the upper part, so that that muscle can expand. 554 00:31:07,730 --> 00:31:09,830 So that's basically it at this point. 555 00:31:09,830 --> 00:31:13,060 And as I say, this will grow, and grow, and grow, and grow, 556 00:31:13,060 --> 00:31:15,310 until by the end of next time, it's 557 00:31:15,310 --> 00:31:16,610 going to be really complicated. 558 00:31:16,610 --> 00:31:18,487 But that's how the brain works. 559 00:31:18,487 --> 00:31:19,195 It's complicated. 560 00:31:21,100 --> 00:31:24,910 So with that then, we are next going 561 00:31:24,910 --> 00:31:28,595 to move to the so-called superior colliculus. 562 00:31:29,890 --> 00:31:33,140 Now the first thing I want to say about this structure 563 00:31:33,140 --> 00:31:37,410 is that this region of the brain-- I'll show you 564 00:31:37,410 --> 00:31:39,130 a picture of it in just a minute-- 565 00:31:39,130 --> 00:31:42,416 is one that has undergone tremendous changes 566 00:31:42,416 --> 00:31:43,540 in the course of evolution. 567 00:31:44,590 --> 00:31:49,940 In more primitive animals, like in fish 568 00:31:49,940 --> 00:31:55,120 and in amphibia, what you find is 569 00:31:55,120 --> 00:32:02,040 that this structure is the prime structure for analyzing vision. 570 00:32:02,040 --> 00:32:04,230 So it does vision and then generates 571 00:32:04,230 --> 00:32:07,000 commands of various sorts, including 572 00:32:07,000 --> 00:32:10,199 commands to make eye movements, or generate 573 00:32:10,199 --> 00:32:12,115 other kinds of movements in different animals. 574 00:32:14,370 --> 00:32:17,510 And this is the case because in these animals, 575 00:32:17,510 --> 00:32:19,660 there is very little cortex. 576 00:32:19,660 --> 00:32:22,880 But with the process of encephalization 577 00:32:22,880 --> 00:32:27,440 more and more of the analytical undertakings 578 00:32:27,440 --> 00:32:32,200 the brain was involved in had been relegated to the cortex. 579 00:32:32,200 --> 00:32:34,260 And that also meant that more and more 580 00:32:34,260 --> 00:32:38,700 of the way we analyze vision became more central, 581 00:32:38,700 --> 00:32:42,980 as we have discussed already, with cortical mechanisms. 582 00:32:42,980 --> 00:32:45,890 Now to make this clear, let me show 583 00:32:45,890 --> 00:32:54,640 you three drawings of a so-called toad, a rabbit, 584 00:32:54,640 --> 00:32:55,990 and a monkey. 585 00:32:55,990 --> 00:32:57,672 And what you see here, the toad-- 586 00:32:57,672 --> 00:32:59,005 here is the superior colliculus. 587 00:33:00,030 --> 00:33:01,950 This structure is quite large relative 588 00:33:01,950 --> 00:33:03,500 to the rest of the brain. 589 00:33:03,500 --> 00:33:05,690 Then when you come to the rabbit, 590 00:33:05,690 --> 00:33:07,910 there's some degree of encephalization. 591 00:33:07,910 --> 00:33:09,650 So this structure is much smaller 592 00:33:09,650 --> 00:33:11,200 relative to the rest of the brain. 593 00:33:11,200 --> 00:33:14,630 And then when it comes to the monkey, 594 00:33:14,630 --> 00:33:18,090 it looks like the superior colliculus is actually 595 00:33:18,090 --> 00:33:19,640 quite tiny. 596 00:33:19,640 --> 00:33:22,640 And in monkeys and humans, the superior colliculus 597 00:33:22,640 --> 00:33:24,010 is very small. 598 00:33:24,010 --> 00:33:27,500 It's maybe about five or six millimeters in diameter. 599 00:33:27,500 --> 00:33:29,870 And yet, it's an extremely important structure 600 00:33:29,870 --> 00:33:32,950 and we're going to examine just what the structure does. 601 00:33:32,950 --> 00:33:35,300 And later on, we're going to examine 602 00:33:35,300 --> 00:33:37,135 what happens if you lose that structure. 603 00:33:38,290 --> 00:33:45,170 So now here is a sagittal midline section of the monkey. 604 00:33:45,170 --> 00:33:46,780 So you can see this real. 605 00:33:47,910 --> 00:33:49,930 And here we have the lunate again. 606 00:33:49,930 --> 00:33:51,860 This is what part of the brain here? 607 00:33:51,860 --> 00:33:53,574 This is a cephalic area. 608 00:33:53,574 --> 00:33:54,490 AUDIENCE: [INAUDIBLE]. 609 00:33:54,490 --> 00:33:55,323 PROFESSOR: OK, good. 610 00:33:55,323 --> 00:33:57,570 This is area V1 here. 611 00:33:57,570 --> 00:34:02,090 Then if you look down here, you see this little bubble 612 00:34:02,090 --> 00:34:03,410 looking thing here? 613 00:34:03,410 --> 00:34:04,825 That's the superior colliculus. 614 00:34:06,200 --> 00:34:08,150 So that is a structure that we are 615 00:34:08,150 --> 00:34:10,260 going to look at in some detail. 616 00:34:10,260 --> 00:34:15,560 Now if one enlarges this-- this in a cat actually-- 617 00:34:15,560 --> 00:34:18,560 and one takes a coronal cross section 618 00:34:18,560 --> 00:34:20,520 of the superior colliculus, they can 619 00:34:20,520 --> 00:34:22,760 see it's a fairly complicated structure. 620 00:34:22,760 --> 00:34:24,679 It has several layers. 621 00:34:24,679 --> 00:34:27,009 People have distinguished at least seven clear layers. 622 00:34:28,929 --> 00:34:31,590 And you can see the upper part here 623 00:34:31,590 --> 00:34:34,025 is often referred to as superficial gray. 624 00:34:35,500 --> 00:34:37,550 There are two layers of that. 625 00:34:37,550 --> 00:34:41,730 The very top layer gets an input directly from the retina. 626 00:34:41,730 --> 00:34:45,007 The one little further down, I'll 627 00:34:45,007 --> 00:34:46,340 elaborate on that in the minute. 628 00:34:47,420 --> 00:34:50,050 It gets input from the visual cortex. 629 00:34:50,050 --> 00:34:52,560 And then we have a whole bunch of other layers 630 00:34:52,560 --> 00:34:56,199 that we are going to examine in some detail in just a minute. 631 00:34:56,199 --> 00:34:58,040 So that's the nature of the structure-- 632 00:34:58,040 --> 00:34:59,960 the superior colliculus. 633 00:34:59,960 --> 00:35:03,060 And the fact is that in more primitive animals, actually, 634 00:35:03,060 --> 00:35:05,470 it is a structure with many more layers. 635 00:35:05,470 --> 00:35:07,230 Some of these more primitive animals 636 00:35:07,230 --> 00:35:11,020 have as many as 14 layers in this structure 637 00:35:11,020 --> 00:35:14,800 because it is heavily involved in eye movement control, 638 00:35:14,800 --> 00:35:18,805 as well as in the analysis of vision. 639 00:35:19,930 --> 00:35:23,450 Now the interesting fact about the way 640 00:35:23,450 --> 00:35:27,910 this is laid out in the cat, and in higher animals, 641 00:35:27,910 --> 00:35:32,840 and in humans, that most of the cells in the various layers 642 00:35:32,840 --> 00:35:34,570 reside in those layers. 643 00:35:34,570 --> 00:35:36,510 They don't talk to each other that much. 644 00:35:37,560 --> 00:35:40,920 But they get a lot of input from many other structures 645 00:35:40,920 --> 00:35:43,070 that control those layers. 646 00:35:43,070 --> 00:35:45,420 And I'll go into that in some detail in a minute. 647 00:35:46,470 --> 00:35:50,960 Now the next important fact about the superior colliculus, 648 00:35:50,960 --> 00:35:54,650 which makes it similar to lateral geniculate nucleus 649 00:35:54,650 --> 00:35:58,270 in the cortex, is that the visual field is laid out 650 00:35:58,270 --> 00:36:02,140 in a topographic manner in the superior colliculus. 651 00:36:02,140 --> 00:36:04,990 This is a top view of the colliculus. 652 00:36:04,990 --> 00:36:06,510 This is the visual field. 653 00:36:06,510 --> 00:36:08,800 And once again, the rule, if you remember, 654 00:36:08,800 --> 00:36:13,980 is that the contralateral half of the visual field 655 00:36:13,980 --> 00:36:17,175 is it projects to the superior colliculus. 656 00:36:19,040 --> 00:36:22,840 And of course, the opposite then is the case for the other side. 657 00:36:22,840 --> 00:36:25,720 And then if you look at the nature of the connections, 658 00:36:25,720 --> 00:36:29,200 you find that everything that's close to the fovea, 659 00:36:29,200 --> 00:36:33,830 the foveal representation, projects to the anterior 660 00:36:33,830 --> 00:36:36,070 level of the superior colliculus. 661 00:36:36,070 --> 00:36:38,680 So this is sort of the foveal region. 662 00:36:38,680 --> 00:36:44,420 Then the upper visual field projects 663 00:36:44,420 --> 00:36:46,220 to the medial portion of the colliculus. 664 00:36:48,830 --> 00:36:51,860 The horizontal meridian area projects 665 00:36:51,860 --> 00:36:54,350 to the horizontal meridian of the colliculus in the back. 666 00:36:55,370 --> 00:36:58,380 And the lower part of the visual field 667 00:36:58,380 --> 00:37:03,160 projects to the lateral portion of the colliculus. 668 00:37:03,160 --> 00:37:04,930 Now why do I emphasize this? 669 00:37:04,930 --> 00:37:07,230 You will see in just a minute why that is important. 670 00:37:07,230 --> 00:37:09,110 It's very important for us understand 671 00:37:09,110 --> 00:37:12,000 that there's a lovely topographic layout 672 00:37:12,000 --> 00:37:15,470 of the visual field on to the superior colliculus. 673 00:37:18,080 --> 00:37:22,020 So now we are going to move on and examine 674 00:37:22,020 --> 00:37:24,880 the question of what is the nature of the responses 675 00:37:24,880 --> 00:37:27,555 of neurons in the superior colliculus. 676 00:37:28,930 --> 00:37:33,570 Now I can tell you right off that in the monkey colliculus, 677 00:37:33,570 --> 00:37:36,480 in the cat colliculus, the nature 678 00:37:36,480 --> 00:37:38,490 of the responses of these neurons 679 00:37:38,490 --> 00:37:39,840 is not particularly interesting. 680 00:37:39,840 --> 00:37:43,140 They're small receptive fields, and they 681 00:37:43,140 --> 00:37:45,980 tend to respond both to the onset 682 00:37:45,980 --> 00:37:49,520 and the termination of a visual stimulus, 683 00:37:49,520 --> 00:37:52,425 meaning that they seem to get an input from both on and off. 684 00:37:54,370 --> 00:37:58,150 And then if you make the a stimulus progressively larger. 685 00:37:58,150 --> 00:38:00,180 You get an increase in responses, 686 00:38:00,180 --> 00:38:05,870 but then once you make it quite large, in this case 10 degrees 687 00:38:05,870 --> 00:38:08,150 and 20 degrees, the response greatly 688 00:38:08,150 --> 00:38:11,970 declines, which says that just like in the retina, 689 00:38:11,970 --> 00:38:15,776 these neurons have centers around antagonism. 690 00:38:17,980 --> 00:38:21,120 And they respond vigorously to small spots 691 00:38:21,120 --> 00:38:23,400 and very little to large spots. 692 00:38:23,400 --> 00:38:25,190 Now that's a basic layout. 693 00:38:25,190 --> 00:38:26,730 But then if you ask the question, 694 00:38:26,730 --> 00:38:30,220 well, are these cells in the colliculus orientation 695 00:38:30,220 --> 00:38:31,780 and direction specific? 696 00:38:31,780 --> 00:38:34,210 Are these cells color selective? 697 00:38:34,210 --> 00:38:36,360 For the most part, that answer is 698 00:38:36,360 --> 00:38:39,400 no, not entirely, but mostly no, meaning 699 00:38:39,400 --> 00:38:41,610 that these cells are not that really interesting. 700 00:38:41,610 --> 00:38:43,840 They are very, very, very basic. 701 00:38:43,840 --> 00:38:47,660 So that is the nature of the responses 702 00:38:47,660 --> 00:38:50,620 of these neurons in the superior colliculus 703 00:38:50,620 --> 00:38:53,120 in the superficial layers. 704 00:38:53,120 --> 00:38:55,690 I've told you that the superficial gray, which 705 00:38:55,690 --> 00:38:58,340 consists of two layers, the one that that's 706 00:38:58,340 --> 00:39:01,110 a direct input from the retina, and the other that 707 00:39:01,110 --> 00:39:05,520 gets an input from the cortex, gives you 708 00:39:05,520 --> 00:39:07,563 vigorous visual responses. 709 00:39:08,860 --> 00:39:11,270 But now, when you get into the deeper 710 00:39:11,270 --> 00:39:14,640 layers of the colliculus, you find something very exciting 711 00:39:14,640 --> 00:39:16,440 an extremely interesting. 712 00:39:16,440 --> 00:39:18,540 And that is sort of diagrammed here. 713 00:39:18,540 --> 00:39:20,290 What you have is a bunch of eye movements. 714 00:39:21,320 --> 00:39:22,780 And then look at this cell. 715 00:39:22,780 --> 00:39:26,660 This cell responds-- it's the same cell throughout-- responds 716 00:39:26,660 --> 00:39:29,270 vigorously to a small saccade. 717 00:39:30,720 --> 00:39:34,280 And this responds well before the saccade is made. 718 00:39:35,840 --> 00:39:37,150 That's what happens here. 719 00:39:37,150 --> 00:39:40,970 But then if the monkey makes bigger saccades, 720 00:39:40,970 --> 00:39:44,189 and some even in the same direction, like these here, 721 00:39:44,189 --> 00:39:45,230 you don't get a response. 722 00:39:46,330 --> 00:39:48,320 So what does that mean? 723 00:39:48,320 --> 00:39:49,110 That's curious. 724 00:39:51,850 --> 00:39:59,899 So what that means is that these cells, some have something 725 00:39:59,899 --> 00:40:00,940 to do with eye movements. 726 00:40:01,990 --> 00:40:05,172 This fires vigorously, and then eye movement ensues-- 727 00:40:05,172 --> 00:40:06,880 fires vigorously and eye movement ensues. 728 00:40:06,880 --> 00:40:10,310 Now there's an eye movement that ensues here as well, 729 00:40:10,310 --> 00:40:11,870 but there's no saccade. 730 00:40:11,870 --> 00:40:13,670 So how do we explain this? 731 00:40:13,670 --> 00:40:16,670 Well, what you're do in these kinds of experiments 732 00:40:16,670 --> 00:40:26,210 then is that you collect extensive data to see when 733 00:40:26,210 --> 00:40:30,110 a cell like this fires, and when it doesn't fire. 734 00:40:30,110 --> 00:40:34,550 And you can generate a response curve, if you will, 735 00:40:34,550 --> 00:40:37,910 or a response diagram to see when it fires 736 00:40:37,910 --> 00:40:39,400 and when it doesn't fire. 737 00:40:39,400 --> 00:40:43,040 So now to look at that, here's an example of that. 738 00:40:43,040 --> 00:40:45,640 In this case, all the white spots 739 00:40:45,640 --> 00:40:47,950 are saccadic eye movements generated 740 00:40:47,950 --> 00:40:56,770 from a central point that were not preceded by eye move-- 741 00:40:56,770 --> 00:41:02,480 saccadic-- by neural responses. 742 00:41:02,480 --> 00:41:08,810 And the red ones show the times when 743 00:41:08,810 --> 00:41:11,520 the neurons you're recording from, the single cell, 744 00:41:11,520 --> 00:41:12,595 responded vigorously. 745 00:41:14,100 --> 00:41:16,660 So that meant that whatever mark you 746 00:41:16,660 --> 00:41:19,320 made a saccade in this direction, 747 00:41:19,320 --> 00:41:21,370 there was vigorous activity in this neuron, 748 00:41:21,370 --> 00:41:24,570 meaning the neuron had an important role in generating 749 00:41:24,570 --> 00:41:32,990 an eye movement, whereas all the other saccades did not 750 00:41:32,990 --> 00:41:35,840 generate a responds in this particular neuron. 751 00:41:35,840 --> 00:41:40,900 But to make this clear, there are many different neurons 752 00:41:40,900 --> 00:41:43,990 in many different parts of the colliculus. 753 00:41:43,990 --> 00:41:46,920 And so when you generated other eye movements, 754 00:41:46,920 --> 00:41:47,975 other neurons fired. 755 00:41:49,320 --> 00:41:53,140 So how can we analyze this in more detail? 756 00:41:53,140 --> 00:41:57,590 Well, the way we can-- let me just add one more point here. 757 00:41:57,590 --> 00:42:00,450 This green circle here is what we're 758 00:42:00,450 --> 00:42:05,600 going to call the motor field of this particular neuron. 759 00:42:05,600 --> 00:42:09,260 So that's a new concept for you-- motor field. 760 00:42:09,260 --> 00:42:11,740 These neurons in the intermediate and deep layers 761 00:42:11,740 --> 00:42:14,670 of the colliculus have motor field 762 00:42:14,670 --> 00:42:16,530 that define the size of the saccade. 763 00:42:17,960 --> 00:42:20,810 Now let me say one more thing in anticipation-- 764 00:42:20,810 --> 00:42:25,890 that if you then study where the cells' visual receptive field 765 00:42:25,890 --> 00:42:30,690 is, it's right there relative to the central fixation point. 766 00:42:30,690 --> 00:42:34,670 So it means that this particular cell 767 00:42:34,670 --> 00:42:39,820 fires when a saccade is made into the receptive 768 00:42:39,820 --> 00:42:41,360 field of that neuron. 769 00:42:43,950 --> 00:42:48,700 So how can we verify this interesting, clever arrangement 770 00:42:48,700 --> 00:42:53,450 in the superior colliculus? 771 00:42:53,450 --> 00:42:58,390 Well, the way we can do this is to instead of recording, 772 00:42:58,390 --> 00:43:00,510 or actually I shouldn't say instead, really. 773 00:43:00,510 --> 00:43:01,990 But we can do two things. 774 00:43:01,990 --> 00:43:03,900 We can record and see what happens 775 00:43:03,900 --> 00:43:05,600 in different parts of the colliculus. 776 00:43:05,600 --> 00:43:07,690 Or we can electrically stimulate there. 777 00:43:12,950 --> 00:43:16,930 Well, so here is a schematic. 778 00:43:16,930 --> 00:43:21,160 Here is a colliculus-- this is anterior, this is posterior. 779 00:43:21,160 --> 00:43:22,680 So this, obviously, if you remember, 780 00:43:22,680 --> 00:43:23,804 this is close to the fovea. 781 00:43:26,010 --> 00:43:27,950 And here is medial and here is lateral. 782 00:43:29,520 --> 00:43:32,240 And what we can do then, we can see 783 00:43:32,240 --> 00:43:34,475 where the receptive fields are of these neurons. 784 00:43:37,800 --> 00:43:39,360 And we can plot them out. 785 00:43:39,360 --> 00:43:41,120 Here are the receptive fields. 786 00:43:41,120 --> 00:43:43,360 Here is number 1, close to the fovea, 787 00:43:43,360 --> 00:43:47,810 number 2 is medial is up, and lateral is down. 788 00:43:48,900 --> 00:43:53,917 Now that one has done this, you can switch over 789 00:43:53,917 --> 00:43:55,375 and you can electrically stimulate. 790 00:43:57,820 --> 00:43:59,200 And look what happens. 791 00:43:59,200 --> 00:44:01,150 If you electrically stimulate at 1, 792 00:44:01,150 --> 00:44:04,090 you get a saccade that moves the eye into the receptive 793 00:44:04,090 --> 00:44:08,280 field of that neuron-- that set of neurons, I should say. 794 00:44:08,280 --> 00:44:11,980 Then if you stimulate in 2, you get saccades that move it to 2. 795 00:44:11,980 --> 00:44:16,010 And when you simulate in 3, it moves to location 3. 796 00:44:18,540 --> 00:44:25,360 Now what more do we need to verify given this situation? 797 00:44:25,360 --> 00:44:29,730 How can one believe this hypothesis, if you will, 798 00:44:29,730 --> 00:44:30,500 at this stage? 799 00:44:31,820 --> 00:44:34,200 What did I tell you about when you electrically 800 00:44:34,200 --> 00:44:40,680 stimulate the neurons in the final common paths, 801 00:44:40,680 --> 00:44:45,270 back in abducens, in the oculomotor nucleus, 802 00:44:45,270 --> 00:44:46,650 for example? 803 00:44:46,650 --> 00:44:48,000 What did I tell you? 804 00:44:48,000 --> 00:44:51,620 That this duration of the simulation 805 00:44:51,620 --> 00:44:55,800 define the size of the saccade elicited, right? 806 00:44:58,250 --> 00:45:01,960 Now if that's the case, if that's were the case, 807 00:45:01,960 --> 00:45:04,550 then this would not mean a damn thing, would it? 808 00:45:04,550 --> 00:45:07,470 Because you could take that same location and stimulated longer, 809 00:45:07,470 --> 00:45:09,630 and you would get a bigger saccade. 810 00:45:09,630 --> 00:45:11,620 So what you need to do? 811 00:45:11,620 --> 00:45:15,850 You need to systematically vary the duration 812 00:45:15,850 --> 00:45:18,750 of the high frequency burst in the colliculus 813 00:45:18,750 --> 00:45:23,290 to compare it with what happens when you stimulate 814 00:45:23,290 --> 00:45:26,480 in the abducens or oculomotor nuclei. 815 00:45:26,480 --> 00:45:28,350 OK so let's look at that. 816 00:45:28,350 --> 00:45:30,790 Here is a picture I've shown you before. 817 00:45:30,790 --> 00:45:37,370 As we increase the duration of the high frequency burst, 818 00:45:37,370 --> 00:45:39,950 you get progressively biggest saccades. 819 00:45:39,950 --> 00:45:42,550 So now we do the same thing in the colliculus. 820 00:45:42,550 --> 00:45:45,200 And lo and behold, you get something totally different 821 00:45:45,200 --> 00:45:47,750 because you have a totally different code 822 00:45:47,750 --> 00:45:49,120 in the colliculus. 823 00:45:49,120 --> 00:45:52,750 What you get here is that you get 824 00:45:52,750 --> 00:45:55,947 after you exceed a very short time-- just 10 825 00:45:55,947 --> 00:45:57,780 milliseconds-- when you don't get a saccade. 826 00:45:58,940 --> 00:46:03,470 From 25 to about 120 milliseconds, you get saccades, 827 00:46:03,470 --> 00:46:06,550 but they're all the same size, so 828 00:46:06,550 --> 00:46:10,180 meaning it's where you stimulate in the colliculus, not how 829 00:46:10,180 --> 00:46:11,440 long you stimulate. 830 00:46:11,440 --> 00:46:13,800 And then to prove this even further, 831 00:46:13,800 --> 00:46:16,940 when you make the stimulation much longer than that, you get 832 00:46:16,940 --> 00:46:20,520 a staircase of saccades, each reaches the same size. 833 00:46:21,790 --> 00:46:24,630 Now yet another proof here is that if you stimulate 834 00:46:24,630 --> 00:46:27,607 an anterior tip of the colliculus, 835 00:46:27,607 --> 00:46:29,440 remember where the receptive fields are very 836 00:46:29,440 --> 00:46:33,930 close to the foveola, then they get a hold array of, 837 00:46:33,930 --> 00:46:38,124 I think there's something like 14 saccadic eye movements-- 838 00:46:38,124 --> 00:46:40,540 bang, bang, bang bang, bang, bang-- because the simulation 839 00:46:40,540 --> 00:46:42,130 goes on for a long time. 840 00:46:42,130 --> 00:46:45,870 So each little saccade is the same size, 841 00:46:45,870 --> 00:46:47,310 you just get a staircase of them. 842 00:46:48,570 --> 00:46:54,820 So that proves that the coding operation in the colliculus 843 00:46:54,820 --> 00:47:04,300 is one that defines the location of a receptive field, 844 00:47:04,300 --> 00:47:07,170 and when you stimulate it, it generates a saccade 845 00:47:07,170 --> 00:47:10,375 that moves the eye into that receptive field. 846 00:47:11,970 --> 00:47:15,350 So that then in a nutshell tells you 847 00:47:15,350 --> 00:47:21,060 what the basic principle is off the functioning 848 00:47:21,060 --> 00:47:23,670 of the colliculus when it comes to eye movement. 849 00:47:23,670 --> 00:47:26,580 Namely it says, a saccade is generated 850 00:47:26,580 --> 00:47:29,410 by computing the size and direction 851 00:47:29,410 --> 00:47:31,950 of the saccadic vector needed to null 852 00:47:31,950 --> 00:47:35,445 the retinal error between the present and intended eye 853 00:47:35,445 --> 00:47:35,945 positions. 854 00:47:37,910 --> 00:47:41,750 That's just putting it in a slightly different way, saying 855 00:47:41,750 --> 00:47:45,070 that, OK, I decide I'm going to make a saccade to here-- bang, 856 00:47:45,070 --> 00:47:46,027 like that. 857 00:47:46,027 --> 00:47:46,860 Just a saccade line. 858 00:47:49,430 --> 00:47:55,730 And but you did that you had an intended location. 859 00:47:55,730 --> 00:48:00,080 And by making an eye movement, you know the error. 860 00:48:00,080 --> 00:48:03,160 Now the colliculus is really quite accurate. 861 00:48:03,160 --> 00:48:07,850 You can make saccades with very small error. 862 00:48:09,360 --> 00:48:15,420 Usually the error is roughly, at most around 10%, 863 00:48:15,420 --> 00:48:17,420 meaning that when you make very small saccades, 864 00:48:17,420 --> 00:48:19,040 is almost negligible. 865 00:48:19,040 --> 00:48:22,860 When you make a large saccade, then maybe there is an error. 866 00:48:22,860 --> 00:48:25,680 And when that happens, sometimes what you have to do 867 00:48:25,680 --> 00:48:28,180 is to make what is called the corrective saccade. 868 00:48:28,180 --> 00:48:33,690 So suppose you're looking here and an object appears here. 869 00:48:34,920 --> 00:48:37,940 And so you say, I intend to go here, 870 00:48:37,940 --> 00:48:41,580 and the colliculus fires in that region of the colliculus 871 00:48:41,580 --> 00:48:42,990 to generate an eye movement. 872 00:48:42,990 --> 00:48:45,350 And say the eye movement goes to here. 873 00:48:45,350 --> 00:48:48,820 Then what you have to do is to make a second saccade, 874 00:48:48,820 --> 00:48:51,955 like that, to correct for that little error. 875 00:48:53,690 --> 00:48:56,010 So sometimes when you make large eye movements, 876 00:48:56,010 --> 00:49:01,300 indeed, you make a secondary saccade for the correction 877 00:49:01,300 --> 00:49:06,065 because your accuracy is roughly around 90% correct. 878 00:49:07,960 --> 00:49:13,420 Does everybody so far follow the basic principle? 879 00:49:13,420 --> 00:49:16,430 It's very important to understand the basic principle 880 00:49:16,430 --> 00:49:18,520 of the operational characteristics 881 00:49:18,520 --> 00:49:19,645 of the superior colliculus. 882 00:49:21,710 --> 00:49:24,650 So again to repeat, a saccade is generated 883 00:49:24,650 --> 00:49:27,270 by computing the size and direction 884 00:49:27,270 --> 00:49:29,700 of the saccadic vector needed to know 885 00:49:29,700 --> 00:49:32,950 the retinal error between the present and intended eye 886 00:49:32,950 --> 00:49:34,100 position. 887 00:49:34,100 --> 00:49:35,970 And what you have in the colliculus 888 00:49:35,970 --> 00:49:38,170 is a cell that is being activated 889 00:49:38,170 --> 00:49:42,260 because a target falls into it. 890 00:49:42,260 --> 00:49:47,150 And that activation then generates a signal 891 00:49:47,150 --> 00:49:48,870 to the lower parts of the colliculus-- 892 00:49:48,870 --> 00:49:52,620 we'll talk about that, how that is done-- which then results 893 00:49:52,620 --> 00:49:56,060 in a saccadic eye movement that nulls that error signal. 894 00:49:58,190 --> 00:50:02,320 Very clever, very basic, very beautiful-- 895 00:50:02,320 --> 00:50:05,880 and almost computer-like at this level. 896 00:50:09,410 --> 00:50:16,200 So what we can do now, we are adding the superior colliculus, 897 00:50:16,200 --> 00:50:19,455 the very basics of it so far, to this diagram. 898 00:50:20,560 --> 00:50:23,820 And that says that the colliculus then 899 00:50:23,820 --> 00:50:25,765 sends a signal to the brain stem. 900 00:50:27,700 --> 00:50:30,610 Some of it initially to the supranuclear complex, 901 00:50:30,610 --> 00:50:33,100 and then from there it sends a signal 902 00:50:33,100 --> 00:50:41,210 to the abducens, or oculomotor, or trochlear nuclei 903 00:50:41,210 --> 00:50:43,585 to generate the appropriate eye movements. 904 00:50:45,340 --> 00:50:48,680 So that then is the very basics of 905 00:50:48,680 --> 00:50:53,850 the operational characteristics of the superior colliculus. 906 00:50:55,040 --> 00:50:57,890 Now what we're going to do next is 907 00:50:57,890 --> 00:51:02,780 we are going to examine what the nature of the visual input 908 00:51:02,780 --> 00:51:08,780 is, predominantly the superior colliculus-- 909 00:51:08,780 --> 00:51:15,895 not entirely, but heavily-- to generate a saccadic eye 910 00:51:15,895 --> 00:51:16,395 movement. 911 00:51:17,750 --> 00:51:23,620 So if you remember, I told you that in the retina, 912 00:51:23,620 --> 00:51:27,080 we make a major distinction between two classes 913 00:51:27,080 --> 00:51:31,190 of retinal ganglion cells-- the midget and the parasol. 914 00:51:31,190 --> 00:51:34,460 The midget project to the parvocellular layers 915 00:51:34,460 --> 00:51:35,980 of the colliculus. 916 00:51:35,980 --> 00:51:42,210 And the parasol cells project to the magnocellular layers. 917 00:51:43,370 --> 00:51:47,525 However, it's also been found that-- I mentioned 918 00:51:47,525 --> 00:51:51,030 that only briefly, that in the retina 919 00:51:51,030 --> 00:51:53,640 there is yet another class of cells 920 00:51:53,640 --> 00:51:56,340 it is called-- some people have called 921 00:51:56,340 --> 00:51:58,370 it the W cells in the cat. 922 00:51:58,370 --> 00:52:01,760 Other people have called it the corneal cellular cells that 923 00:52:01,760 --> 00:52:08,330 reside in the relatively small numbers compared 924 00:52:08,330 --> 00:52:16,450 to the other two throughout the retina, and project, in part, 925 00:52:16,450 --> 00:52:22,320 to interlaminar layers of the lateral geniculate nucleus. 926 00:52:22,320 --> 00:52:26,050 But it's been found that they also project heavily 927 00:52:26,050 --> 00:52:29,500 to the superior colliculus. 928 00:52:29,500 --> 00:52:31,130 Now how do we know this? 929 00:52:31,130 --> 00:52:33,880 Well let me tell you, it's important 930 00:52:33,880 --> 00:52:36,770 always is to understand the experimental procedures. 931 00:52:36,770 --> 00:52:39,510 So let me skip that here. 932 00:52:39,510 --> 00:52:41,850 Just to remind you again are the connections. 933 00:52:43,550 --> 00:52:45,375 This I've shown you several times before. 934 00:52:46,820 --> 00:52:49,780 So it says here the interlaminar layers project 935 00:52:49,780 --> 00:52:54,230 to the upper portions of the visual cortex. 936 00:52:54,230 --> 00:52:56,390 But I didn't tell you before that 937 00:52:56,390 --> 00:52:58,660 these corneal cellular cells also 938 00:52:58,660 --> 00:53:00,295 project to the superior colliculus. 939 00:53:01,700 --> 00:53:03,880 So here's the superior colliculus. 940 00:53:03,880 --> 00:53:06,020 And here, initially, we can ask the question, 941 00:53:06,020 --> 00:53:08,050 what projects look to the colliculus. 942 00:53:08,050 --> 00:53:09,940 So how do we find out? 943 00:53:09,940 --> 00:53:16,280 Suppose you are in a laboratory where the big question is what 944 00:53:16,280 --> 00:53:19,270 kinds of cells project from the retina 945 00:53:19,270 --> 00:53:20,810 to the superior colliculus. 946 00:53:20,810 --> 00:53:22,760 Think about this for a minute. 947 00:53:22,760 --> 00:53:24,510 What kind of experiment would you do? 948 00:53:24,510 --> 00:53:25,468 How would you find out? 949 00:53:28,540 --> 00:53:32,100 Well, that's quite an interesting question. 950 00:53:32,100 --> 00:53:36,940 And so to understand that, I'm going 951 00:53:36,940 --> 00:53:40,790 to tell you about a couple of techniques. 952 00:53:40,790 --> 00:53:43,620 One technique would be that you could inject-- 953 00:53:43,620 --> 00:53:45,280 anatomical technique-- you would inject 954 00:53:45,280 --> 00:53:49,560 a substance in the colliculus that would be then retrogradely 955 00:53:49,560 --> 00:53:53,107 transported to the retina, and would light up the cells there. 956 00:53:53,107 --> 00:53:54,315 That's would be one approach. 957 00:53:56,050 --> 00:53:58,630 Another approach is that you could actually 958 00:53:58,630 --> 00:54:02,870 record from the retina itself with microelectrodes. 959 00:54:04,040 --> 00:54:06,940 And you could electrically stimulate the colliculus. 960 00:54:06,940 --> 00:54:10,610 That way we would be backdriving cells. 961 00:54:10,610 --> 00:54:13,420 And when you find the cell in the retina that 962 00:54:13,420 --> 00:54:15,750 is backdriven from the colliculus, 963 00:54:15,750 --> 00:54:19,540 you know that particular cell does project to the colliculus. 964 00:54:19,540 --> 00:54:21,540 So that's a very strong technique 965 00:54:21,540 --> 00:54:26,900 because that enables you then to identify the cell type 966 00:54:26,900 --> 00:54:30,340 and also you could, if you do intercellular recording, 967 00:54:30,340 --> 00:54:31,140 to label it. 968 00:54:32,430 --> 00:54:35,430 So they can do the same thing also-- 969 00:54:35,430 --> 00:54:37,600 because I'm going to talk about that as well-- is 970 00:54:37,600 --> 00:54:41,500 to see what happens in terms of inputs 971 00:54:41,500 --> 00:54:43,025 to the colliculus from the cortex. 972 00:54:45,630 --> 00:54:47,570 So let's talk about each of those, 973 00:54:47,570 --> 00:54:51,260 and that'll give us a better sense of what 974 00:54:51,260 --> 00:54:53,322 the projections are from the retina 975 00:54:53,322 --> 00:54:54,905 and from the cortex to the colliculus. 976 00:54:56,440 --> 00:54:59,080 So here's an example of how you do it again. 977 00:55:00,460 --> 00:55:04,710 So we are recording from the retina here, 978 00:55:04,710 --> 00:55:09,480 or in a different experiment, you can record from the cortex. 979 00:55:09,480 --> 00:55:11,820 You can then electrically stimulate here. 980 00:55:11,820 --> 00:55:14,550 You can backdrive the cells in the retina, 981 00:55:14,550 --> 00:55:17,900 or we can backdrive the cells in the visual cortex. 982 00:55:19,930 --> 00:55:23,630 And then once we do so, and record it from a single cell, 983 00:55:23,630 --> 00:55:26,250 you can study to see what it is like. 984 00:55:26,250 --> 00:55:28,360 So that is the approach. 985 00:55:28,360 --> 00:55:32,770 And if you do that, you come up with some very nice answers. 986 00:55:32,770 --> 00:55:36,300 First of all, these cells are W-like, 987 00:55:36,300 --> 00:55:39,320 which means that they're like the corneal cellular cells 988 00:55:39,320 --> 00:55:40,990 that we have seen the retina. 989 00:55:40,990 --> 00:55:44,540 These are these small cells that are not 990 00:55:44,540 --> 00:55:48,740 particularly rapidly conducting, obviously 991 00:55:48,740 --> 00:55:50,720 and are not color selective. 992 00:55:53,110 --> 00:55:58,400 And then if we look at the cells up here, 993 00:55:58,400 --> 00:56:00,680 we find that all those cells are complex cells, 994 00:56:00,680 --> 00:56:03,320 and they all reside in layer five. 995 00:56:03,320 --> 00:56:05,430 That's already known anatomically 996 00:56:05,430 --> 00:56:08,640 that the cells in layer five are the ones 997 00:56:08,640 --> 00:56:11,260 that project-- many of them project 998 00:56:11,260 --> 00:56:12,740 to the superior colliculus. 999 00:56:14,350 --> 00:56:16,090 So that then is the basic procedure. 1000 00:56:17,220 --> 00:56:22,035 So now we can expand on this basic procedure. 1001 00:56:24,820 --> 00:56:27,612 What we can do here is we can record 1002 00:56:27,612 --> 00:56:28,820 from the superior colliculus. 1003 00:56:29,840 --> 00:56:35,250 And then we can eliminate the inputs from the cortex 1004 00:56:35,250 --> 00:56:36,300 by cooling it. 1005 00:56:37,500 --> 00:56:38,455 That's one approach. 1006 00:56:40,420 --> 00:56:42,920 So if you do that, you record from a cell here, 1007 00:56:42,920 --> 00:56:44,810 you drive it by electric stimulation-- 1008 00:56:44,810 --> 00:56:45,985 by visual stimulation. 1009 00:56:48,980 --> 00:56:54,350 We identify that cell in V1 by backdriving it. 1010 00:56:54,350 --> 00:56:59,050 And then once you cool it, cool the visual cortex, 1011 00:56:59,050 --> 00:57:02,820 you see what happens to that cell in the various layers 1012 00:57:02,820 --> 00:57:04,690 of the superior colliculus. 1013 00:57:04,690 --> 00:57:09,360 Well when you do that, you get a very dramatic and, luckily, 1014 00:57:09,360 --> 00:57:11,120 clear cut result. 1015 00:57:11,120 --> 00:57:13,110 The result is the following. 1016 00:57:13,110 --> 00:57:18,400 If you look at the superficial layers of the colliculus, 1017 00:57:18,400 --> 00:57:22,830 and look at what happens before you cool, when you cool, 1018 00:57:22,830 --> 00:57:26,030 and after you cool, the cells keep firing. 1019 00:57:26,030 --> 00:57:30,520 So in these upper layers in superficial gray, 1020 00:57:30,520 --> 00:57:32,864 those cells keep firing. 1021 00:57:32,864 --> 00:57:34,280 They're not interested in anything 1022 00:57:34,280 --> 00:57:36,190 that comes down from the cortex. 1023 00:57:36,190 --> 00:57:40,370 That's because the inputs from the retina 1024 00:57:40,370 --> 00:57:47,050 goes to these upper layers of the superior colliculus, 1025 00:57:47,050 --> 00:57:50,060 and goes predominantly to the superficial gray. 1026 00:57:51,630 --> 00:57:55,880 However, once you get down to the intermediate layers, 1027 00:57:55,880 --> 00:57:57,980 and the lower layers as well, every time 1028 00:57:57,980 --> 00:58:03,140 you cool, the cell stops firing, as you can see here. 1029 00:58:03,140 --> 00:58:09,065 Which means that these cells in the intermediate and lower 1030 00:58:09,065 --> 00:58:11,770 layers of the colliculus are driven 1031 00:58:11,770 --> 00:58:13,780 by the cortical down flow. 1032 00:58:13,780 --> 00:58:18,300 They're not driven by the direct input from the retina. 1033 00:58:21,490 --> 00:58:26,330 So now that we have seen this, one can say, OK, that's fine. 1034 00:58:26,330 --> 00:58:35,050 But is this really something that happens over time, 1035 00:58:35,050 --> 00:58:36,480 or is this just something peculiar 1036 00:58:36,480 --> 00:58:38,970 to the particular experiment that's been run? 1037 00:58:38,970 --> 00:58:41,790 So what do you do to see if this is really 1038 00:58:41,790 --> 00:58:45,540 something-- a permanent effect? 1039 00:58:46,560 --> 00:58:51,710 Well, what you can do here is you can take a monkey, 1040 00:58:51,710 --> 00:58:56,240 and you can remove the visual cortex on one side, 1041 00:58:56,240 --> 00:59:00,550 so the monkey can see fine in half the visual field. 1042 00:59:01,580 --> 00:59:04,750 And then you can record on either side-- 1043 00:59:04,750 --> 00:59:07,840 the intact side or the affected side-- 1044 00:59:07,840 --> 00:59:09,900 in the superior colliculus. 1045 00:59:09,900 --> 00:59:10,910 Everybody follow that? 1046 00:59:12,230 --> 00:59:12,730 All right. 1047 00:59:12,730 --> 00:59:18,540 So if you do that, what you find is this is the intact side, 1048 00:59:18,540 --> 00:59:21,350 and this is the side where there's no V1. 1049 00:59:21,350 --> 00:59:29,170 And this was taken months after the area V1 was removed. 1050 00:59:29,170 --> 00:59:32,720 And here you see these red spots here, and the red spots here? 1051 00:59:32,720 --> 00:59:37,240 That spot is the location after which you no longer 1052 00:59:37,240 --> 00:59:39,150 could drive any cells visually. 1053 00:59:40,210 --> 00:59:42,600 So in the intact side, you can drive it 1054 00:59:42,600 --> 00:59:44,570 to very deep layers of the colliculus. 1055 00:59:44,570 --> 00:59:47,700 They had responded very well to visual stimuli. 1056 00:59:47,700 --> 00:59:50,140 But on this side, they stopped responding here. 1057 00:59:50,140 --> 00:59:54,040 So none of these deeper layers could 1058 00:59:54,040 --> 00:59:57,930 be activated by visual stimuli because those deep layer, 1059 00:59:57,930 --> 01:00:02,760 indeed, are driven by visual inputs 1060 01:00:02,760 --> 01:00:04,605 from the downflow from the cortex. 1061 01:00:06,700 --> 01:00:09,580 So that then is the basic rule of what 1062 01:00:09,580 --> 01:00:13,400 has happened in the superior colliculus. 1063 01:00:14,320 --> 01:00:14,820 All right. 1064 01:00:14,820 --> 01:00:24,320 Now let's see-- the next big question we can ask 1065 01:00:24,320 --> 01:00:28,570 is this downflow from the cortex that 1066 01:00:28,570 --> 01:00:31,680 drives these important intermediate layers, which 1067 01:00:31,680 --> 01:00:36,320 are involved in eye movement generation, to what degree are 1068 01:00:36,320 --> 01:00:39,180 those cortical tactile cells driven 1069 01:00:39,180 --> 01:00:42,240 by either the parvocellular or the magnocellular 1070 01:00:42,240 --> 01:00:45,670 layer of cells, meaning by the midget cells, 1071 01:00:45,670 --> 01:00:47,190 or the parasol cells. 1072 01:00:49,610 --> 01:00:51,600 Well how do you do that? 1073 01:00:51,600 --> 01:00:53,850 I've already told you about experiments 1074 01:00:53,850 --> 01:00:57,230 in which what you can do is you can put a recording 1075 01:00:57,230 --> 01:00:59,410 electrode, in this case in the colliculus. 1076 01:00:59,410 --> 01:01:03,810 And then you can inactivate either the parvocellular 1077 01:01:03,810 --> 01:01:10,330 or the magnocellular of the lateral geniculate nucleus, 1078 01:01:10,330 --> 01:01:14,665 while you record in the superior colliculus. 1079 01:01:15,840 --> 01:01:19,360 So what happens is that these cells then, obviously as shown 1080 01:01:19,360 --> 01:01:23,540 here, project to V1, and then layer five cells project down. 1081 01:01:23,540 --> 01:01:25,690 So what we are going to identify then 1082 01:01:25,690 --> 01:01:29,970 is what is the nature of these cells in layer five 1083 01:01:29,970 --> 01:01:31,360 that drive down there. 1084 01:01:31,360 --> 01:01:34,730 So first of all, what this shows is 1085 01:01:34,730 --> 01:01:38,250 that if you record from-- in this case, 1086 01:01:38,250 --> 01:01:41,220 from two cells in the geniculate shown-- sorry, 1087 01:01:41,220 --> 01:01:43,560 two cells in the colliculus shown, 1088 01:01:43,560 --> 01:01:45,760 this is under normal conditions, and this 1089 01:01:45,760 --> 01:01:48,240 is when the parvocell layers are blocked. 1090 01:01:48,240 --> 01:01:50,490 Here's another cell-- normal condition, 1091 01:01:50,490 --> 01:01:52,910 and then magnocellular layers are blocked. 1092 01:01:52,910 --> 01:01:55,340 So this clearly shows-- and this was done with dozens, 1093 01:01:55,340 --> 01:01:57,350 and dozens, and dozens of cells-- 1094 01:01:57,350 --> 01:02:02,680 that blocking the parvocellular layers, 1095 01:02:02,680 --> 01:02:04,720 meaning blocking the midget system, 1096 01:02:04,720 --> 01:02:12,210 had no effect on the cortical tactile driving of cells, 1097 01:02:12,210 --> 01:02:15,740 whereas magnocellular block eliminated it. 1098 01:02:15,740 --> 01:02:22,390 So that meant that these cells in layer five that 1099 01:02:22,390 --> 01:02:24,340 project to the superior colliculus 1100 01:02:24,340 --> 01:02:30,570 are driven exclusively by the parasol system. 1101 01:02:30,570 --> 01:02:34,040 And indeed then, that was also done-- 1102 01:02:34,040 --> 01:02:36,730 now let me go back to that-- that was also done 1103 01:02:36,730 --> 01:02:39,730 in another set of experiments showing that 1104 01:02:39,730 --> 01:02:44,010 those cells in the cortex in layer five, 1105 01:02:44,010 --> 01:02:48,840 that you could drive from the geniculate by backdriving it 1106 01:02:48,840 --> 01:02:51,340 if you then block the midget or parasol cells, 1107 01:02:51,340 --> 01:02:52,960 those cells would stop firing. 1108 01:02:52,960 --> 01:02:57,140 So those were indeed cells that were driven exclusively 1109 01:02:57,140 --> 01:03:00,550 by the parasol system. 1110 01:03:00,550 --> 01:03:03,420 So the parasol system plays a very important role 1111 01:03:03,420 --> 01:03:05,490 in getting-- in driving the colliculus. 1112 01:03:05,490 --> 01:03:09,210 But doing it so only through the cortex, not directly. 1113 01:03:11,420 --> 01:03:19,510 so to then put this in an easy to remember the diagram, 1114 01:03:19,510 --> 01:03:23,960 is to say that when you come to these high level animals, what 1115 01:03:23,960 --> 01:03:27,330 happened is that due to encephalization, 1116 01:03:27,330 --> 01:03:33,920 the cortex gained control over the superior colliculus, 1117 01:03:33,920 --> 01:03:36,710 except the superficial gray. 1118 01:03:37,980 --> 01:03:39,800 Everything else in the colliculus 1119 01:03:39,800 --> 01:03:42,450 is controlled by the cortical downflow. 1120 01:03:44,210 --> 01:03:46,090 And that, of course, became very important 1121 01:03:46,090 --> 01:03:48,390 because when you make a decision as to where you're 1122 01:03:48,390 --> 01:03:51,940 going to look next, where you're going to identify an object, 1123 01:03:51,940 --> 01:03:56,360 you have to have information from the cortex itself, 1124 01:03:56,360 --> 01:03:58,950 which then has to be transferred, in this case, 1125 01:03:58,950 --> 01:04:02,760 to the colliculus to generate the desired eye movement, 1126 01:04:02,760 --> 01:04:06,020 so that you can analyze in detail 1127 01:04:06,020 --> 01:04:08,140 what you want to analyze. 1128 01:04:08,140 --> 01:04:10,180 So that then is the basic layout. 1129 01:04:10,180 --> 01:04:12,970 And so now what we can do is we can expand 1130 01:04:12,970 --> 01:04:19,040 some more on this diagram, namely-- once again, 1131 01:04:19,040 --> 01:04:20,270 let's go through all this. 1132 01:04:20,270 --> 01:04:22,840 We have the brain stem here, which is a rate code. 1133 01:04:23,960 --> 01:04:26,610 And then we have a superior colliculus. 1134 01:04:26,610 --> 01:04:29,810 And that code that we're going talk about there, 1135 01:04:29,810 --> 01:04:31,890 that I've already described, you can 1136 01:04:31,890 --> 01:04:33,840 refer to that as the vector code. 1137 01:04:36,170 --> 01:04:38,430 And then what we can see here-- some 1138 01:04:38,430 --> 01:04:40,530 of this work I've shown you before-- 1139 01:04:40,530 --> 01:04:43,710 is that we have these three systems from the retina 1140 01:04:43,710 --> 01:04:45,110 that we talked about now. 1141 01:04:45,110 --> 01:04:48,510 The midget, the parasol, and the W system, 1142 01:04:48,510 --> 01:04:51,960 or the corneal cellular system, that goes up the cortex. 1143 01:04:51,960 --> 01:04:58,480 And then the downflow from V1, we now know, 1144 01:04:58,480 --> 01:05:04,310 is driven exclusively, it seems, by the parasol system. 1145 01:05:04,310 --> 01:05:09,990 However, remember also that when we go to higher cortical 1146 01:05:09,990 --> 01:05:14,430 areas-- V2, MT, and V4, and so on-- these areas, some of them 1147 01:05:14,430 --> 01:05:16,860 get a mixed input from two systems, 1148 01:05:16,860 --> 01:05:21,250 like V4 and the temporal lobe. 1149 01:05:21,250 --> 01:05:24,390 And MT, on the other hand, is dominated 1150 01:05:24,390 --> 01:05:28,440 by inputs from the parasol system, but it's not exclusive. 1151 01:05:28,440 --> 01:05:31,180 So in the end, the colliculus also 1152 01:05:31,180 --> 01:05:34,440 gets an input from the midget system. 1153 01:05:34,440 --> 01:05:41,870 But the heaviest input to it is indeed from the parasol system. 1154 01:05:41,870 --> 01:05:46,890 So that then is what the diagram looks like at this point. 1155 01:05:46,890 --> 01:05:49,670 And so what we are going to do next time, 1156 01:05:49,670 --> 01:05:57,104 we're going to expand on this connection in which we 1157 01:05:57,104 --> 01:05:58,770 are going to look at some other cortical 1158 01:05:58,770 --> 01:06:02,470 structures-- namely I've already referred to that a few times-- 1159 01:06:02,470 --> 01:06:05,200 the frontal eye fields and the medial eye fields, 1160 01:06:05,200 --> 01:06:09,140 which are regions in the cortex that control eye movements. 1161 01:06:09,140 --> 01:06:13,090 And that's what we're going to look at in much more detail. 1162 01:06:13,090 --> 01:06:16,965 So now I'm ready to summarize what I told you so far. 1163 01:06:18,450 --> 01:06:22,740 First of all, there are several classes of eye movements 1164 01:06:22,740 --> 01:06:23,990 one can distinguish. 1165 01:06:23,990 --> 01:06:26,850 Those are vergence eye movements and conjugate eye movements. 1166 01:06:28,150 --> 01:06:31,280 And the conjugate eye movements come 1167 01:06:31,280 --> 01:06:34,580 in two major types, which we call the smooth pursuit 1168 01:06:34,580 --> 01:06:36,760 type and the saccadic type. 1169 01:06:37,910 --> 01:06:41,100 Then we're going to talk about eye movements 1170 01:06:41,100 --> 01:06:46,300 as a result of the 6 extraocular muscles that we have. 1171 01:06:46,300 --> 01:06:51,650 And important to note that these extraocular muscles are 1172 01:06:51,650 --> 01:06:53,540 such that the fibers in the muscle 1173 01:06:53,540 --> 01:06:55,160 run the entire length of the muscles. 1174 01:06:55,160 --> 01:06:58,600 They're not segmented, and that they 1175 01:06:58,600 --> 01:07:01,650 are innervated by the third, fourth, and sixth 1176 01:07:01,650 --> 01:07:02,400 cranial nerves. 1177 01:07:03,760 --> 01:07:06,680 Then we say that the discharge rate 1178 01:07:06,680 --> 01:07:09,350 in neurons of the final common path 1179 01:07:09,350 --> 01:07:12,085 is proportional to the angular deviation of the eye. 1180 01:07:13,390 --> 01:07:16,060 Saccade size is a function of the duration 1181 01:07:16,060 --> 01:07:19,480 of the high frequency burst in these neurons. 1182 01:07:19,480 --> 01:07:21,360 And if you look it up and down eye movements, 1183 01:07:21,360 --> 01:07:28,780 as I've shown you, if the more the muscles contract that 1184 01:07:28,780 --> 01:07:35,240 are the inferior recti, the more the idea deviates downwards. 1185 01:07:35,240 --> 01:07:39,880 And because it's reciprocal, as the eye deviates more and more, 1186 01:07:39,880 --> 01:07:44,380 the superior rectus is activated less and less. 1187 01:07:44,380 --> 01:07:46,340 So we have this proportional arrangements. 1188 01:07:46,340 --> 01:07:50,750 It's as beautiful as I've shown you-- a linear system that 1189 01:07:50,750 --> 01:07:55,360 defines very accurately the degree of deviation 1190 01:07:55,360 --> 01:07:58,150 of the eye as a function of the frequency 1191 01:07:58,150 --> 01:07:59,385 of the neuronal activity. 1192 01:08:00,950 --> 01:08:04,900 Then the superior colliculus codes saccadic vectors 1193 01:08:04,900 --> 01:08:09,130 whose amplitude and direction is laid out in an orderly fashion 1194 01:08:09,130 --> 01:08:12,620 and is in register with the visual receptive fields. 1195 01:08:12,620 --> 01:08:15,880 I told you that in the anterior colliculus, 1196 01:08:15,880 --> 01:08:19,590 the receptive fields are very close to the fovea. 1197 01:08:19,590 --> 01:08:22,590 And then you go to the back of the colliculus, 1198 01:08:22,590 --> 01:08:27,580 the progressively further peripheral, and medial is up, 1199 01:08:27,580 --> 01:08:29,060 lateral is down. 1200 01:08:29,060 --> 01:08:30,425 It's a nice orderly arrangement. 1201 01:08:31,470 --> 01:08:36,490 And you have an arrangement where the visual receptive 1202 01:08:36,490 --> 01:08:40,100 fields and the motor fields are in register with each other. 1203 01:08:40,100 --> 01:08:45,210 And every time there's activity, there's a particular receptive 1204 01:08:45,210 --> 01:08:48,240 field location, the result is that a saccade 1205 01:08:48,240 --> 01:08:51,760 will move the eye into the receptive field, thereby 1206 01:08:51,760 --> 01:08:54,180 nulling the retinal error signal. 1207 01:08:55,350 --> 01:08:58,410 And then the retinal input to the superior colliculus 1208 01:08:58,410 --> 01:09:01,160 comes predominantly from w-like cells. 1209 01:09:01,160 --> 01:09:06,069 But the cortical downflow from V1, which comes from layer 5, 1210 01:09:06,069 --> 01:09:08,365 is driven by the parasol system. 1211 01:09:09,470 --> 01:09:11,319 But we should not neglect the fact-- 1212 01:09:11,319 --> 01:09:17,840 I'm not sure if you're-- neglect the fact that they are other 1213 01:09:17,840 --> 01:09:24,045 pathways to which the midget system can also make maybe 1214 01:09:24,045 --> 01:09:27,844 a more limited contribution in generating eye movements. 1215 01:09:30,979 --> 01:09:35,020 And as a diagram I had shown you, 1216 01:09:35,020 --> 01:09:38,180 I demonstrated that the superior colliculus 1217 01:09:38,180 --> 01:09:43,140 is on the cortical control in higher primates. 1218 01:09:43,140 --> 01:09:48,620 So that then is the essence of what I wanted to cover today. 1219 01:09:48,620 --> 01:09:51,990 And then next time, we are going to talk about-- let me see, 1220 01:09:51,990 --> 01:09:54,100 do I have a-- we're going to talk 1221 01:09:54,100 --> 01:09:57,640 about the cortical areas involved in saccadic eye 1222 01:09:57,640 --> 01:10:00,140 movements control, which we will then 1223 01:10:00,140 --> 01:10:06,270 highlight the fact that this incessant activity of us moving 1224 01:10:06,270 --> 01:10:09,760 the eyes is incredibly elaborate and complicated. 1225 01:10:09,760 --> 01:10:12,610 There are millions of neurons involved in doing it. 1226 01:10:12,610 --> 01:10:16,970 And yet, it's such an incredible system 1227 01:10:16,970 --> 01:10:19,080 that it's something we never even think about. 1228 01:10:20,550 --> 01:10:24,870 Luckily, it's a kind of system-- partly because of the fact 1229 01:10:24,870 --> 01:10:29,000 that there's no wait involved in the muscles, 1230 01:10:29,000 --> 01:10:31,730 and that the fibers run the entire length of each 1231 01:10:31,730 --> 01:10:36,710 of the extraocular muscles-- it is a system that one has been 1232 01:10:36,710 --> 01:10:38,690 fortunate enough to understand reasonably 1233 01:10:38,690 --> 01:10:43,230 well in how it works in the brain. 1234 01:10:43,230 --> 01:10:46,760 So that, in essence, is what I wanted to cover today. 1235 01:10:46,760 --> 01:10:49,050 If any of you have any questions, 1236 01:10:49,050 --> 01:10:51,720 anything that-- I know this is kind of complicated, 1237 01:10:51,720 --> 01:10:55,610 but I hope that you will understand 1238 01:10:55,610 --> 01:10:57,080 this reasonably well. 1239 01:10:57,080 --> 01:10:59,480 And if it's not clear to you, I will 1240 01:10:59,480 --> 01:11:02,786 be very happy to answer any questions that you might have. 1241 01:11:08,100 --> 01:11:10,000 Well, I was reasonably clear then. 1242 01:11:13,080 --> 01:11:17,780 So next time-- here's my to-do-- the cortex, heavily. 1243 01:11:17,780 --> 01:11:21,555 I will also show you some interesting movies about that. 1244 01:11:22,720 --> 01:11:25,701 And I think you're going to find that it's 1245 01:11:25,701 --> 01:11:30,430 a really fascinating story of how the cortex has evolved 1246 01:11:30,430 --> 01:11:33,990 to generate more and more areas that are involved 1247 01:11:33,990 --> 01:11:39,760 in an incredible number of functions 1248 01:11:39,760 --> 01:11:42,990 in the generation of eye movements, as well, of course, 1249 01:11:42,990 --> 01:11:45,470 as in processing visual information. 1250 01:11:46,921 --> 01:11:47,420 OK. 1251 01:11:47,420 --> 01:11:49,670 Well, thank you very much.