1 00:00:00 --> 00:00:01 2 00:00:01 --> 00:00:03 The following content is provided by MIT OpenCourseWare 3 00:00:03 --> 00:00:06 under a Creative Commons license. 4 00:00:06 --> 00:00:09 Additional information about our license and MIT 5 00:00:09 --> 00:00:13 OpenCourseWare in general is available at ocw.mit.edu. 6 00:00:13 --> 00:00:18 PROFESSOR: What I'm going to do for the next three lectures 7 00:00:18 --> 00:00:23 is to talk about how we see. 8 00:00:23 --> 00:00:26 What I'm going to do today is talk about how we 9 00:00:26 --> 00:00:29 take information in. 10 00:00:29 --> 00:00:33 Next Tuesday, I'll talk about the fact that we take in too 11 00:00:33 --> 00:00:35 much information -- more information than we can 12 00:00:35 --> 00:00:40 use and we have to select amongst that information. 13 00:00:40 --> 00:00:43 14 00:00:43 --> 00:00:48 On Thursday of next week, what I'll talk about is the fact 15 00:00:48 --> 00:00:55 that perception is never just a direct, somehow registration 16 00:00:55 --> 00:00:56 of the outside world. 17 00:00:56 --> 00:00:59 It's always an active interpretation of the world, 18 00:00:59 --> 00:01:02 and I'll talk about how we go about interpreting the 19 00:01:02 --> 00:01:06 information that we've gotten from the outside. 20 00:01:06 --> 00:01:09 I will be barely able to scratch the 21 00:01:09 --> 00:01:11 surface of the topic. 22 00:01:11 --> 00:01:14 I will abandon all discussion of the other senses, 23 00:01:14 --> 00:01:16 for instance, which is a great pity. 24 00:01:16 --> 00:01:19 Now, it's true that I can barely scratch the surface of 25 00:01:19 --> 00:01:23 almost any topic in this course, but here the danger is 26 00:01:23 --> 00:01:25 greater because this is actually what I 27 00:01:25 --> 00:01:27 do for a living. 28 00:01:27 --> 00:01:35 So, on any little bit of this, I can talk forever. 29 00:01:35 --> 00:01:39 So, exactly how much of the surface I scratch will 30 00:01:39 --> 00:01:43 be determined by how far afield I end up going. 31 00:01:43 --> 00:01:49 This slide is up there to illustrate, in some 32 00:01:49 --> 00:01:51 sense, those three bits. 33 00:01:51 --> 00:01:56 Let's see, if I take the stage, chalkboard light off -- there 34 00:01:56 --> 00:01:59 we go, you want to go off? 35 00:01:59 --> 00:02:00 Then you can see a little bit better. 36 00:02:00 --> 00:02:04 37 00:02:04 --> 00:02:05 You can sort of see stuff. 38 00:02:05 --> 00:02:09 39 00:02:09 --> 00:02:16 That's this Bruegel's painting of the battle between the good 40 00:02:16 --> 00:02:20 angels and the bad angels. 41 00:02:20 --> 00:02:23 For me, the purpose is to say look, it's clear that you see 42 00:02:23 --> 00:02:26 something, it's clear that what you are seeing is an 43 00:02:26 --> 00:02:28 active interpretation. 44 00:02:28 --> 00:02:30 In fact, you may not have particularly known that it was 45 00:02:30 --> 00:02:34 a battle between good angels and bad angels before 46 00:02:34 --> 00:02:35 you look at it. 47 00:02:35 --> 00:02:39 It's also clear that even though you, in some sense, see 48 00:02:39 --> 00:02:42 the whole thing, you're not processing it fully, at 49 00:02:42 --> 00:02:44 least not all at once. 50 00:02:44 --> 00:02:47 So, for instance, you may not have particularly noticed -- I 51 00:02:47 --> 00:02:49 don't know what he is, he's a really cool beast -- I'm not 52 00:02:49 --> 00:02:52 sure if that's a bad angel falling or something. 53 00:02:52 --> 00:02:55 But you probably hadn't particularly noticed 54 00:02:55 --> 00:02:56 this fish guy. 55 00:02:56 --> 00:02:59 Actually, in this picture you're probably still not 56 00:02:59 --> 00:03:02 noticing the fish guy -- he just looks like a blob. 57 00:03:02 --> 00:03:06 It's a great picture, look at it sometime. 58 00:03:06 --> 00:03:10 And this guy's got very bad stomach problems, 59 00:03:10 --> 00:03:12 whatever he's doing. 60 00:03:12 --> 00:03:14 OK. 61 00:03:14 --> 00:03:18 Rather than continuing to try to look at a murky picture, 62 00:03:18 --> 00:03:22 let's go to a less murky picture and let's start by 63 00:03:22 --> 00:03:27 talking about how you actually get information in from the 64 00:03:27 --> 00:03:36 visual system, which obviously involves going through the eye. 65 00:03:36 --> 00:03:39 On the handout, which -- do I have a copy 66 00:03:39 --> 00:03:42 of it here, yes, OK. 67 00:03:42 --> 00:03:44 A lot of terminology. 68 00:03:44 --> 00:03:46 Are you responsible for it? 69 00:03:46 --> 00:03:47 Yes, of course. 70 00:03:47 --> 00:03:51 If you miss some little bit of terminology, that's not going 71 00:03:51 --> 00:03:54 to sink you in the course, either on this topic or 72 00:03:54 --> 00:03:56 on any other topic. 73 00:03:56 --> 00:04:01 You want to put your effort into thinking about the broader 74 00:04:01 --> 00:04:05 topic, but it's useful in being able to talk intelligently 75 00:04:05 --> 00:04:09 about it to get a grip on some of this terminology, and hence, 76 00:04:09 --> 00:04:13 the tour of the visual system that I will now embark on. 77 00:04:13 --> 00:04:19 From the outside-in, the purpose of the eye, or at least 78 00:04:19 --> 00:04:23 the front half of the eye is to form an image on the 79 00:04:23 --> 00:04:25 back half of the eye. 80 00:04:25 --> 00:04:29 So all of this, sometimes called the anterior segment of 81 00:04:29 --> 00:04:33 the eye, is there to form an image, like a camera would, on 82 00:04:33 --> 00:04:36 the back surface of the eye. 83 00:04:36 --> 00:04:40 Most of the optical power of the eye is at the very 84 00:04:40 --> 00:04:41 front surface of it. 85 00:04:41 --> 00:04:45 That front surface is called the cornea. 86 00:04:45 --> 00:04:51 It has, apart from its virtues as a lens focusing light, it 87 00:04:51 --> 00:04:55 is also one of the two surfaces in the body 88 00:04:55 --> 00:04:58 most sensitive to pain. 89 00:04:58 --> 00:05:02 You may try this out if you want at home. 90 00:05:02 --> 00:05:06 I don't recommend it, but if you scratch your cornea 91 00:05:06 --> 00:05:09 you will know it and you will be happy. 92 00:05:09 --> 00:05:11 What's the other one? 93 00:05:11 --> 00:05:15 What's the other surface, do you think? 94 00:05:15 --> 00:05:19 Anybody care to venture a guess? 95 00:05:19 --> 00:05:19 Thumbs? 96 00:05:19 --> 00:05:23 97 00:05:23 --> 00:05:26 No, that's a novel thought. 98 00:05:26 --> 00:05:30 No, look -- he's thinking like the cartoons, right, you hit 99 00:05:30 --> 00:05:32 the thumb with the hammer and ow, it hurts. 100 00:05:32 --> 00:05:34 Let me tell you, if you hit your cornea with the hammer 101 00:05:34 --> 00:05:36 it hurts a lot more. 102 00:05:36 --> 00:05:37 Yes? 103 00:05:37 --> 00:05:38 AUDIENCE: Some place inside the mouth? 104 00:05:38 --> 00:05:40 PROFESSOR: Some place inside the mouth. 105 00:05:40 --> 00:05:40 No. 106 00:05:40 --> 00:05:44 Again, you think about that inside the mouth is a place 107 00:05:44 --> 00:05:48 where pain happens, but that's because you do stupid things 108 00:05:48 --> 00:05:52 with your mouth, like hey, that pizza looks good. 109 00:05:52 --> 00:05:52 Ow. 110 00:05:52 --> 00:05:56 You would never stick the pizza on your eyeball. 111 00:05:56 --> 00:05:59 112 00:05:59 --> 00:06:05 So, again, nicely innervated for pain but not particularly. 113 00:06:05 --> 00:06:07 Not the other great location. 114 00:06:07 --> 00:06:08 I'll take one more here. 115 00:06:08 --> 00:06:10 AUDIENCE: [INAUDIBLE] 116 00:06:10 --> 00:06:12 at the bottom of your foot? 117 00:06:12 --> 00:06:14 PROFESSOR: Yeah, yeah, same sort of thing, right. 118 00:06:14 --> 00:06:16 You step on nails or something. 119 00:06:16 --> 00:06:18 The bottom of your foot's a good place. 120 00:06:18 --> 00:06:21 121 00:06:21 --> 00:06:23 There's two people here, I've just gotta take their answers 122 00:06:23 --> 00:06:24 because they're so eager. 123 00:06:24 --> 00:06:26 AUDIENCE: Yeah, that's what I was going to say, 124 00:06:26 --> 00:06:27 somewhere inside your ear. 125 00:06:27 --> 00:06:31 PROFESSOR: Somewhere inside your ear is the point, not the 126 00:06:31 --> 00:06:33 chunk that so many people are thrilled with. 127 00:06:33 --> 00:06:35 Oh, let's put a hole in this -- oh, that was 128 00:06:35 --> 00:06:37 fun, let's do it again. 129 00:06:37 --> 00:06:40 That obviously hurts, I guess. 130 00:06:40 --> 00:06:41 I've never gone in for this myself. 131 00:06:41 --> 00:06:45 132 00:06:45 --> 00:06:48 One of the reasons your mother said don't put anything inside 133 00:06:48 --> 00:06:52 your ear that's bigger than your elbow is because if you 134 00:06:52 --> 00:06:55 put something small and pointy in there, it eventually reaches 135 00:06:55 --> 00:06:58 your eardrum and that's the other surface. 136 00:06:58 --> 00:07:05 When you had an ear infection as a kid and the pressure from 137 00:07:05 --> 00:07:09 your middle ear, the fluid in your ear, stretched that, it 138 00:07:09 --> 00:07:11 hurt, and that hurts a lot. 139 00:07:11 --> 00:07:16 So, don't put the hot pizza in your ear either. 140 00:07:16 --> 00:07:16 OK. 141 00:07:16 --> 00:07:20 The cornea is just a relatively thin layer. 142 00:07:20 --> 00:07:25 It's held in shape by fluid, a watery-like fluid, which is 143 00:07:25 --> 00:07:28 known technically as the aqueous humor -- humor in 144 00:07:28 --> 00:07:30 this case has nothing to do with being funny. 145 00:07:30 --> 00:07:38 It's an old word for a fluid, It forms a space that separates 146 00:07:38 --> 00:07:41 the cornea from the iris. 147 00:07:41 --> 00:07:46 The iris is the bit that gives your eye its color, but it's 148 00:07:46 --> 00:07:50 basically a sphincter of muscle with a hole in it, and that 149 00:07:50 --> 00:07:55 hold is the pupil of the eye, which is the aperture 150 00:07:55 --> 00:07:56 of the camera. 151 00:07:56 --> 00:08:03 That's where light's getting in from the outside into the 152 00:08:03 --> 00:08:06 inner portions of the eye. 153 00:08:06 --> 00:08:08 OK. 154 00:08:08 --> 00:08:11 The pupil changes size, because you've got this sphincter 155 00:08:11 --> 00:08:14 of muscle that either relaxes or contracts. 156 00:08:14 --> 00:08:16 What makes it change size? 157 00:08:16 --> 00:08:17 AUDIENCE: Light. 158 00:08:17 --> 00:08:18 PROFESSOR: Light. 159 00:08:18 --> 00:08:18 We know that. 160 00:08:18 --> 00:08:20 More light makes it do what? 161 00:08:20 --> 00:08:21 AUDIENCE: Get smaller. 162 00:08:21 --> 00:08:22 PROFESSOR: Get smaller. 163 00:08:22 --> 00:08:23 Why? 164 00:08:23 --> 00:08:25 AUDIENCE: [INAUDIBLE]. 165 00:08:25 --> 00:08:27 PROFESSOR: Yeah, so there's less light. 166 00:08:27 --> 00:08:29 Sure, it's going to be less light, but why 167 00:08:29 --> 00:08:31 do you have this? 168 00:08:31 --> 00:08:33 What did they teach you in 3rd grade? 169 00:08:33 --> 00:08:34 AUDIENCE: [INAUDIBLE]. 170 00:08:34 --> 00:08:36 PROFESSOR: Never stick your elbow in your eye. 171 00:08:36 --> 00:08:38 Yes, I know that. 172 00:08:38 --> 00:08:39 No, I need a hand here. 173 00:08:39 --> 00:08:41 There's too many mutterings. 174 00:08:41 --> 00:08:44 There's a hand. 175 00:08:44 --> 00:08:44 AUDIENCE: Because you don't want to go blind. 176 00:08:44 --> 00:08:46 PROFESSOR: Because you don't want to go blind. 177 00:08:46 --> 00:08:47 Why would you go blind? 178 00:08:47 --> 00:08:48 AUDIENCE: There's too much light. 179 00:08:48 --> 00:08:49 PROFESSOR: There's too much light. 180 00:08:49 --> 00:08:51 See, that's what they teach you in 3rd grade. 181 00:08:51 --> 00:08:53 It's bogus. 182 00:08:53 --> 00:09:01 It sounds good and it seems to make sense, but ask yourself 183 00:09:01 --> 00:09:06 what level of protection that pupil can give you. 184 00:09:06 --> 00:09:09 Right, so how much light is it going to cut out? 185 00:09:09 --> 00:09:14 The answer is it cuts out a bit more than a factor of 10. 186 00:09:14 --> 00:09:17 From the largest it can be to the smallest it'll be if you 187 00:09:17 --> 00:09:20 stare at -- you know, if you're out in really bright light. 188 00:09:20 --> 00:09:23 So it can take it can change light levels by about 189 00:09:23 --> 00:09:24 a factor of 10. 190 00:09:24 --> 00:09:27 If you go from in here right now -- actually, it's fairly 191 00:09:27 --> 00:09:32 dim in here -- but if you go from here to just go outside 192 00:09:32 --> 00:09:37 into the courtyard, the change in light level is going to be 193 00:09:37 --> 00:09:40 on the order of a factor of 1,000 right there. 194 00:09:40 --> 00:09:43 The change in light level that your visual system can deal 195 00:09:43 --> 00:09:49 with from, say, a moonlit night to bright sun on a snow field 196 00:09:49 --> 00:09:52 is on the order of 10 to the twelfth or 10 to 197 00:09:52 --> 00:09:53 the thirteenth. 198 00:09:53 --> 00:09:59 So there's no way that the need to protect yourself is going 199 00:09:59 --> 00:10:02 to be taken care of by the pupil of the eye. 200 00:10:02 --> 00:10:05 There's just not enough range there. 201 00:10:05 --> 00:10:09 The pupillary constriction, what it's probably doing for 202 00:10:09 --> 00:10:13 you, those of you who are photographers, is it's 203 00:10:13 --> 00:10:15 increasing the depth of field on your camera. 204 00:10:15 --> 00:10:19 A pinhole camera will focus many different 205 00:10:19 --> 00:10:21 distances at once. 206 00:10:21 --> 00:10:26 A larger aperature will only focus one plane at one time. 207 00:10:26 --> 00:10:30 And so if you've got lots of light, you crank down the pupil 208 00:10:30 --> 00:10:33 in order to be able to focus -- in order to get better 209 00:10:33 --> 00:10:35 focus, basically. 210 00:10:35 --> 00:10:39 But the protection thing just doesn't -- it's not a stupid 211 00:10:39 --> 00:10:43 answer because it's what everybody believes, but 212 00:10:43 --> 00:10:46 it doesn't turn out to be a correct answer. 213 00:10:46 --> 00:10:49 Oh, now there's several other things that will change the 214 00:10:49 --> 00:10:52 size of your pupil, but one other of interest, what else 215 00:10:52 --> 00:10:53 changes the size of your pupil? 216 00:10:53 --> 00:10:56 Anybody know anything else that they can change the size--? 217 00:10:56 --> 00:10:56 Yup? 218 00:10:56 --> 00:10:58 AUDIENCE: Looking all the way down there at you. 219 00:10:58 --> 00:11:01 PROFESSOR: Looking all the way down there at me. 220 00:11:01 --> 00:11:07 Well, yes, but for reasons that are--. 221 00:11:07 --> 00:11:07 Why? 222 00:11:07 --> 00:11:12 223 00:11:12 --> 00:11:15 Well, no, well you're changing -- sorry, that's the 224 00:11:15 --> 00:11:16 next bullet here. 225 00:11:16 --> 00:11:19 What you're changing when you change focus of the eye is 226 00:11:19 --> 00:11:22 you're changing the shape of the lens -- at least you are. 227 00:11:22 --> 00:11:28 I'm not because at my age the lens has basically become a 228 00:11:28 --> 00:11:34 rigid lump, and so when I want to look from there to here, 229 00:11:34 --> 00:11:36 I use an external lens. 230 00:11:36 --> 00:11:37 Yes, back to the pupil. 231 00:11:37 --> 00:11:38 AUDIENCE: When you lie. 232 00:11:38 --> 00:11:39 PROFESSOR: When you lie. 233 00:11:39 --> 00:11:44 Yes, that will work a little bit maybe, not reliably enough 234 00:11:44 --> 00:11:47 to -- you know, the CIA or something every now and then 235 00:11:47 --> 00:11:50 gets the idea that that really works. 236 00:11:50 --> 00:11:52 It's on the right general track. 237 00:11:52 --> 00:11:52 Yeah? 238 00:11:52 --> 00:11:55 AUDIENCE: Some drugs do that? 239 00:11:55 --> 00:11:55 PROFESSOR: Yup. 240 00:11:55 --> 00:12:02 Some drugs will certainly modulate it. 241 00:12:02 --> 00:12:03 All right, [UNINTELLIGIBLE]. 242 00:12:03 --> 00:12:05 AUDIENCE: When you look at the object of your affection. 243 00:12:05 --> 00:12:06 PROFESSOR: There you go. 244 00:12:06 --> 00:12:11 That's particularly the one I was fishing for. 245 00:12:11 --> 00:12:16 Your level of arousal will change the size of your pupil. 246 00:12:16 --> 00:12:20 Notably, if you look at something that you like or 247 00:12:20 --> 00:12:25 someone that you like, your pupils get bigger. 248 00:12:25 --> 00:12:28 Obviously, not everybody here knew that explicitly, but you 249 00:12:28 --> 00:12:32 sort of knew that implicitly, because the result of that is 250 00:12:32 --> 00:12:36 that you think that things with big pupils are cuter than 251 00:12:36 --> 00:12:39 things a little pinhole pupils, right. 252 00:12:39 --> 00:12:44 That's why sappy cartoons or cards have big pupils. 253 00:12:44 --> 00:12:50 It is also why in years gone by, women would put a drug 254 00:12:50 --> 00:12:54 known as atropine or known as a cosmetic as belladonna for 255 00:12:54 --> 00:12:57 pretty woman, pretty girl. 256 00:12:57 --> 00:13:01 In their eyes what it did was it paralyzed and relaxed the 257 00:13:01 --> 00:13:04 muscles of the irish and made a great big pupil. 258 00:13:04 --> 00:13:06 It didn't do anything good for their vision because it 259 00:13:06 --> 00:13:08 made things kind of blurry. 260 00:13:08 --> 00:13:10 It also paralyzing the lens. 261 00:13:10 --> 00:13:14 But it gave them great big pupils, and so they're looking 262 00:13:14 --> 00:13:18 at the guy who may or may not be the object of their desire 263 00:13:18 --> 00:13:23 -- they're looking at some guy, they can't see him so well, 264 00:13:23 --> 00:13:25 which who knows, that may have helped. 265 00:13:25 --> 00:13:29 But he's looking at her thinking implicitly her 266 00:13:29 --> 00:13:33 pupils are really big, I know what that means. 267 00:13:33 --> 00:13:35 Well, it turns out, following up on whoever it was 268 00:13:35 --> 00:13:37 over there, it actually means she's on drugs. 269 00:13:37 --> 00:13:40 270 00:13:40 --> 00:13:47 In any case, pupil size is modulated by emotional state 271 00:13:47 --> 00:13:50 and it's one of these things that we sort of knew 272 00:13:50 --> 00:13:53 implicitly, people know implicitly, and think big 273 00:13:53 --> 00:13:57 pupils are attractive for that reason. 274 00:13:57 --> 00:13:58 It's only when you show up here you find out these 275 00:13:58 --> 00:14:00 things explicitly. 276 00:14:00 --> 00:14:03 All right, I've already said something about the lens. 277 00:14:03 --> 00:14:06 The most notable thing about the lens is it gets harder 278 00:14:06 --> 00:14:09 with age and -- you could try this out. 279 00:14:09 --> 00:14:15 In fact, it's a good time in your life to start 280 00:14:15 --> 00:14:17 this experiment. 281 00:14:17 --> 00:14:21 Take something like the handout and ask yourself how close you 282 00:14:21 --> 00:14:26 can get and have the letters still be in sharp focus. 283 00:14:26 --> 00:14:29 This is so sad it like makes me weep. 284 00:14:29 --> 00:14:32 But I know you're getting there, too. 285 00:14:32 --> 00:14:35 Now what you can do -- you could actually, if you're 286 00:14:35 --> 00:14:38 compulsive about these sort of things, write it down somewhere 287 00:14:38 --> 00:14:40 and just check every year. 288 00:14:40 --> 00:14:43 This is one of those things where the signs of aging 289 00:14:43 --> 00:14:46 start showing up now. 290 00:14:46 --> 00:14:49 You will be able to see, even during the course of your 291 00:14:49 --> 00:14:52 twenties that that so-called near point is moving 292 00:14:52 --> 00:14:54 further out. 293 00:14:54 --> 00:14:59 For me without my glasses we're out here somewhere. 294 00:14:59 --> 00:15:03 And eventually you're locked at one plane of focus, and 295 00:15:03 --> 00:15:06 depending on where that is, determines which particular 296 00:15:06 --> 00:15:07 set of glasses you need -- 297 00:15:07 --> 00:15:10 do you need glasses to read, do you need bifocals, whatever. 298 00:15:10 --> 00:15:12 It's because you've lost the ability to 299 00:15:12 --> 00:15:15 fine-tune that focus. 300 00:15:15 --> 00:15:20 Now, you need something to hold the rest of the eye in shape. 301 00:15:20 --> 00:15:26 What you've got in there is it's really a jelly called 302 00:15:26 --> 00:15:31 the vitreous humor for the glass-like substance. 303 00:15:31 --> 00:15:34 For our purposes it's not desperately interesting, it's 304 00:15:34 --> 00:15:38 there to hold your eye out so that the front end of the eye 305 00:15:38 --> 00:15:40 can make an image on this back surface. 306 00:15:40 --> 00:15:49 The back surface of the eye where the transduction from 307 00:15:49 --> 00:15:52 photons, the light energy, into signals in the nervous system 308 00:15:52 --> 00:15:55 where that takes place, that tissue is known as the retina 309 00:15:55 --> 00:15:58 -- the word comes from the word for a net. 310 00:15:58 --> 00:16:04 311 00:16:04 --> 00:16:10 The fovea is the point -- fovea comes the word for pit -- it's 312 00:16:10 --> 00:16:16 the point on the retina where you have the highest visual 313 00:16:16 --> 00:16:22 aquity, the greatest ability to resolve small details. 314 00:16:22 --> 00:16:28 You don't particularly realize in the course of your normal 315 00:16:28 --> 00:16:32 day-to-day existence just how bad vision gets away 316 00:16:32 --> 00:16:36 from the fovea. 317 00:16:36 --> 00:16:39 You have this notion of a highly detailed world that's 318 00:16:39 --> 00:16:42 in focus all over the place. 319 00:16:42 --> 00:16:44 But that, to jump ahead to the third lecture in this 320 00:16:44 --> 00:16:46 series, is a construction. 321 00:16:46 --> 00:16:47 It's not reality. 322 00:16:47 --> 00:16:49 And that you can prove to yourself. 323 00:16:49 --> 00:16:53 Take the handout again, hold it at some comfortable reading 324 00:16:53 --> 00:16:58 distance, fixate on the l on that second line there 325 00:16:58 --> 00:17:01 where it says Lecture 4. 326 00:17:01 --> 00:17:05 If you're fixating on the l, the l is forming an 327 00:17:05 --> 00:17:06 image on your fovea. 328 00:17:06 --> 00:17:10 Ask yourself while you're fixated on the l, how many 329 00:17:10 --> 00:17:13 letters off to the right you can actually read. 330 00:17:13 --> 00:17:18 And the answer is going to be not many. 331 00:17:18 --> 00:17:20 If you think you can read to the end of line it's because 332 00:17:20 --> 00:17:24 you're not fixating -- you're moving your eyes around. 333 00:17:24 --> 00:17:26 Again, like the pupil thing, this is something 334 00:17:26 --> 00:17:27 you knew implicitly. 335 00:17:27 --> 00:17:33 Nobody sits there and says I'm going to read this handout. 336 00:17:33 --> 00:17:38 I can see it, it's there, but I can't read it because the 337 00:17:38 --> 00:17:43 outside of the fovea, I simply don't have the resolving 338 00:17:43 --> 00:17:47 ability to do it. 339 00:17:47 --> 00:17:48 Why not? 340 00:17:48 --> 00:17:50 Well, I can give you at least one reason. 341 00:17:50 --> 00:17:53 You have about a hundred million photoreceptors 342 00:17:53 --> 00:17:55 in your retina. 343 00:17:55 --> 00:18:00 Photoreceptors are the cells that convert light energy 344 00:18:00 --> 00:18:01 into a signal in the nervous system, they're the 345 00:18:01 --> 00:18:04 transduction elements. 346 00:18:04 --> 00:18:10 You have only about a quarter of a million to a million 347 00:18:10 --> 00:18:16 axons, nerve fibers running from the eye to the brain. 348 00:18:16 --> 00:18:21 So that hundred million has got to get boiled down to the much 349 00:18:21 --> 00:18:25 smaller number that you've got available to you. 350 00:18:25 --> 00:18:46 So that means you can't treat each photoreceptor as a pixel 351 00:18:46 --> 00:18:49 with a private line to the brain. 352 00:18:49 --> 00:18:52 You do have something approximating this in the 353 00:18:52 --> 00:18:56 fovea where you have fine detailed resolution. 354 00:18:56 --> 00:19:01 But in the periphery, away from the fovea, what you have is 355 00:19:01 --> 00:19:06 many, many, many, many photoreceptors that are all 356 00:19:06 --> 00:19:10 ganged together to send their signal off to the 357 00:19:10 --> 00:19:11 central nervous system. 358 00:19:11 --> 00:19:20 So you lose resolution out there in the periphery. 359 00:19:20 --> 00:19:22 Let's say a little more about the details of that retina. 360 00:19:22 --> 00:19:27 Oh, the optic nerve is the bundle of fibers going off 361 00:19:27 --> 00:19:31 from the eye to the brain. 362 00:19:31 --> 00:19:34 I'll say a bit more about that in a minute or so. 363 00:19:34 --> 00:19:39 A quick tour of the retina itself. 364 00:19:39 --> 00:19:43 Up at the top there you've got the photoreceptors, these 365 00:19:43 --> 00:19:45 transduction elements. 366 00:19:45 --> 00:19:49 There are two flavors of them called rods and cones -- named 367 00:19:49 --> 00:19:52 rods and cones because under the microscope rods are 368 00:19:52 --> 00:19:53 rod-shaped and cones are cone-shaped. 369 00:19:53 --> 00:19:56 370 00:19:56 --> 00:20:01 They are different in their functions and capabilities. 371 00:20:01 --> 00:20:06 Cones operate in bright light at daylight levels. 372 00:20:06 --> 00:20:11 They do mediate color vision and they are concentrated 373 00:20:11 --> 00:20:13 at the fovea. 374 00:20:13 --> 00:20:18 They are most densely represented at the fovea. 375 00:20:18 --> 00:20:22 Rods work in dim light levels. 376 00:20:22 --> 00:20:29 They are not able to mediate color vision, which is why on a 377 00:20:29 --> 00:20:31 moonlit night you don't see colors -- it's not because 378 00:20:31 --> 00:20:33 color's somehow drained out of the world. 379 00:20:33 --> 00:20:36 It's because the visual system that's looking at that world 380 00:20:36 --> 00:20:42 can no longer analyze the input for color information, but only 381 00:20:42 --> 00:20:46 for grey scaled information. 382 00:20:46 --> 00:20:49 Rod photoreceptors are concentrated away from 383 00:20:49 --> 00:20:53 the retina -- I'm sorry, away from the fovea. 384 00:20:53 --> 00:20:57 They are, in fact, absent from the central fovea. 385 00:20:57 --> 00:21:00 386 00:21:00 --> 00:21:04 The best place to see this actually is if your room gets 387 00:21:04 --> 00:21:11 dim enough at night that you can't see color, you're working 388 00:21:11 --> 00:21:15 in the rod realm, and then you will discover that if you 389 00:21:15 --> 00:21:18 fixate on a dim spot of light you can't see it. 390 00:21:18 --> 00:21:20 That there might be something out here -- you notice it and 391 00:21:20 --> 00:21:23 then you move your eye to look at it and it'll disappear, 392 00:21:23 --> 00:21:26 because the image, if it's small enough, will now fall 393 00:21:26 --> 00:21:33 into the fovea and there are no rods there, you can't see it. 394 00:21:33 --> 00:21:36 This is why if you're a star gazer and you're looking at a 395 00:21:36 --> 00:21:40 dim star, you want to look at that star out of the so-called 396 00:21:40 --> 00:21:41 corner of your eye. 397 00:21:41 --> 00:21:45 The corner of your eye is about 20 degrees away from straight 398 00:21:45 --> 00:21:48 ahead because that's where the density of rod photoreceptors 399 00:21:48 --> 00:21:50 turns out to be greatest. 400 00:21:50 --> 00:21:54 401 00:21:54 --> 00:21:57 I was going to say something else about rod photoreceptors, 402 00:21:57 --> 00:21:59 but that has disappeared from my mind. 403 00:21:59 --> 00:22:06 So I will say that the rest of this is a collection of 404 00:22:06 --> 00:22:08 nerve cells, of neurons. 405 00:22:08 --> 00:22:10 It's really a little chunk of brain. 406 00:22:10 --> 00:22:13 Developmentally the retina is a chunk of brain that's pushed 407 00:22:13 --> 00:22:18 out into the eye, and this is the start of the visual 408 00:22:18 --> 00:22:20 nervous system. 409 00:22:20 --> 00:22:25 For present purposes it's worth thinking about two aspects 410 00:22:25 --> 00:22:28 of this organization. 411 00:22:28 --> 00:22:38 One is a through path that runs from the photoreceptors through 412 00:22:38 --> 00:22:42 so-called bipolar cells to ganglion cells. 413 00:22:42 --> 00:22:46 There's sort of a vertical path in this picture heading from 414 00:22:46 --> 00:22:49 the photoreceptors out to the brain. 415 00:22:49 --> 00:22:55 Then there are cells whose processes are sitting here and 416 00:22:55 --> 00:22:59 here making horizontal connections so that 417 00:22:59 --> 00:23:03 photoreceptors don't behave like isolated pixels so that 418 00:23:03 --> 00:23:06 they can talk to each other, and so you can start doing some 419 00:23:06 --> 00:23:09 sort calculations, if you like. 420 00:23:09 --> 00:23:12 Those are cells like horizontal cells that have their processes 421 00:23:12 --> 00:23:15 up here, and amacrine cells that have their 422 00:23:15 --> 00:23:16 processes down here. 423 00:23:16 --> 00:23:19 So those are things making horizontal connections 424 00:23:19 --> 00:23:20 across the retina. 425 00:23:20 --> 00:23:24 Photoreceptors, bipolar cells, ganglion cells are forming this 426 00:23:24 --> 00:23:29 through pathway going from eye to brain. 427 00:23:29 --> 00:23:35 Now, if you were to look at this and I was to say where is 428 00:23:35 --> 00:23:39 light coming from in this picture, the reasonable 429 00:23:39 --> 00:23:41 answer would be? 430 00:23:41 --> 00:23:42 AUDIENCE: [INAUDIBLE]. 431 00:23:42 --> 00:23:49 432 00:23:49 --> 00:23:51 PROFESSOR: It would seem like a reasonable answer would be like 433 00:23:51 --> 00:23:53 it's coming from the top hit in the photoreceptors and heading 434 00:23:53 --> 00:23:54 off to the brain and out the bottom. 435 00:23:54 --> 00:23:57 But in fact, light's coming from there. 436 00:23:57 --> 00:24:00 The retina is put on backwards in an interesting kind of a 437 00:24:00 --> 00:24:05 way, or intuitively backwards at least, so that light shines 438 00:24:05 --> 00:24:11 through all of this stuff before actually getting 439 00:24:11 --> 00:24:13 to the photoreceptors. 440 00:24:13 --> 00:24:15 Why is that? 441 00:24:15 --> 00:24:17 Well, there are a variety of possible answers. 442 00:24:17 --> 00:24:22 One of the answers is -- let's see, the remaining term on the 443 00:24:22 --> 00:24:28 list that I haven't mentioned in that retina list, which is 444 00:24:28 --> 00:24:30 actually at the top ofo the list, it says 445 00:24:30 --> 00:24:30 pigment epithelium. 446 00:24:30 --> 00:24:36 447 00:24:36 --> 00:24:42 Up here there would be a layer of what boils down to black 448 00:24:42 --> 00:24:46 gunk coating the back surface of the eye, and the 449 00:24:46 --> 00:24:49 photoreceptors are stuck into that. 450 00:24:49 --> 00:24:51 That's the pigment epithelium. 451 00:24:51 --> 00:24:57 What it's there for is -- well, let's suppose that all of you 452 00:24:57 --> 00:25:05 guys are photoreceptors and here I am, I'm a photon and I'm 453 00:25:05 --> 00:25:08 part of this image of the world on the outside and I'm coming 454 00:25:08 --> 00:25:12 flying in, and with luck I'll be absorbed by Mr. 455 00:25:12 --> 00:25:15 Photoreceptor here. 456 00:25:15 --> 00:25:16 But suppose I don't? 457 00:25:16 --> 00:25:18 Suppose I hit over here. 458 00:25:18 --> 00:25:24 Well, photons don't have much conscious life 459 00:25:24 --> 00:25:25 as far as we know. 460 00:25:25 --> 00:25:28 You, the owner of this visual system, don't want that photon 461 00:25:28 --> 00:25:30 bouncing around anywhere else. 462 00:25:30 --> 00:25:33 You want it either to be absorbed by a photographers 463 00:25:33 --> 00:25:34 or to be gone. 464 00:25:34 --> 00:25:37 Because if it bounces and hits up there somewhere it's going 465 00:25:37 --> 00:25:38 to degrade the image, right. 466 00:25:38 --> 00:25:41 It's going to be like haze across the image if it's not 467 00:25:41 --> 00:25:47 landing where it should to make a nice starp image. 468 00:25:47 --> 00:25:55 So this black gunk is there to soak up extra photoreceptors. 469 00:25:55 --> 00:26:00 A very good idea if you are a diurnal animal, an animal who 470 00:26:00 --> 00:26:02 works out in bright light. 471 00:26:02 --> 00:26:07 Not such a good idea if you're an animal who is nocturnal or 472 00:26:07 --> 00:26:09 running around in dim light. 473 00:26:09 --> 00:26:12 There what you want to do is catch every photon that 474 00:26:12 --> 00:26:14 you can possibly catch. 475 00:26:14 --> 00:26:17 So if you're a cat, for example, you do 476 00:26:17 --> 00:26:18 this differently. 477 00:26:18 --> 00:26:21 Instead of a pigment epithelium at the back of your eye, you 478 00:26:21 --> 00:26:29 have a structure known as the tapetum -- it's actually very 479 00:26:29 --> 00:26:31 beautiful, it looks like Mother of Pearl, but you only get to 480 00:26:31 --> 00:26:36 see that if you take it out of the cat, which makes the 481 00:26:36 --> 00:26:39 cat less beautiful. 482 00:26:39 --> 00:26:41 But it's a reflective surface. 483 00:26:41 --> 00:26:48 This is why if you get a cat in your flashlight beam, its eyes 484 00:26:48 --> 00:26:53 seem to glow at you, because the light comes in -- if it 485 00:26:53 --> 00:26:56 doesn't get absorbed by, say, the photoreceptors, it can 486 00:26:56 --> 00:26:59 bounce and an amount of it will bounce back out. 487 00:26:59 --> 00:27:02 Some amount of light will bounce back out of your eyes. 488 00:27:02 --> 00:27:07 That's what you see as red eye in an unlucky 489 00:27:07 --> 00:27:09 photograph, for example. 490 00:27:09 --> 00:27:10 Is that a hand? 491 00:27:10 --> 00:27:15 AUDIENCE: Wouldn't that distort the image? 492 00:27:15 --> 00:27:17 PROFESSOR: To the cat. 493 00:27:17 --> 00:27:17 AUDIENCE: Yeah. 494 00:27:17 --> 00:27:18 PROFESSOR: Yeah, absolutely. 495 00:27:18 --> 00:27:21 That's why you almost never see cats reading 496 00:27:21 --> 00:27:23 The New York Times. 497 00:27:23 --> 00:27:27 No, cat visual aquity is pretty lousy. 498 00:27:27 --> 00:27:33 A cat has different visual desires than you and I do. 499 00:27:33 --> 00:27:38 The cat's not big on sweating the visual aquity thing. 500 00:27:38 --> 00:27:40 The cat wants to know, did it move? 501 00:27:40 --> 00:27:41 Can I jump on it? 502 00:27:41 --> 00:27:43 Can I bite it? 503 00:27:43 --> 00:27:44 Is it another cat? 504 00:27:44 --> 00:27:46 Can we do stuff? 505 00:27:46 --> 00:27:47 All this sort of stuff. 506 00:27:47 --> 00:27:50 This sort of thing doesn't require a lot of visual aquity 507 00:27:50 --> 00:27:54 and the cat wants to be able to do it in dim light. 508 00:27:54 --> 00:27:59 So, we have this notion that cats can see in the dark, 509 00:27:59 --> 00:28:04 which, of course, is not really true -- no light, no vision. 510 00:28:04 --> 00:28:07 People think cats can see in the dark because cats can see 511 00:28:07 --> 00:28:10 stuff in dim light that you and I can't, and part of the reason 512 00:28:10 --> 00:28:15 for that is that they are willing to let stray photons 513 00:28:15 --> 00:28:16 go where they will. 514 00:28:16 --> 00:28:20 Because, in fact, I gave a sort of a radical example. 515 00:28:20 --> 00:28:25 If my hypothetical photon missed my hypothetical 516 00:28:25 --> 00:28:29 photoreceptor here, odds are it's not going to go out to 517 00:28:29 --> 00:28:32 outer Mongolia, it's going to get absorbed someplace nearby, 518 00:28:32 --> 00:28:35 and so the effect will be a blurring, not a complete 519 00:28:35 --> 00:28:37 degradation of the image. 520 00:28:37 --> 00:28:39 I think the cat can cope. 521 00:28:39 --> 00:28:42 The cat doesn't mind too much. 522 00:28:42 --> 00:28:51 By the way, if you're interested in design, taking a 523 00:28:51 --> 00:28:58 look at the design of eyes across the animal kingdom 524 00:28:58 --> 00:29:01 is a beautiful--. 525 00:29:01 --> 00:29:06 The number of different ways that evolution or the hand of 526 00:29:06 --> 00:29:09 God, you can take your pick, has chosen to solve the problem 527 00:29:09 --> 00:29:14 of collecting light, an amazing array of possibilities. 528 00:29:14 --> 00:29:17 Once you start -- cats are one thing, but you go off 529 00:29:17 --> 00:29:24 into insects and other beasties and it's great. 530 00:29:24 --> 00:29:25 Wonderful stuff. 531 00:29:25 --> 00:29:31 If you want to be an engineer, probably lots of good 532 00:29:31 --> 00:29:33 industrial design there. 533 00:29:33 --> 00:29:37 All right, let me take a quick trip up into the rest of 534 00:29:37 --> 00:29:40 the visual system here. 535 00:29:40 --> 00:29:45 This picture that's up on the screen now is mimicking the 536 00:29:45 --> 00:29:49 picture on the upper left of the second page of the handout. 537 00:29:49 --> 00:29:53 It's showing the connections -- oh, I just remembered. 538 00:29:53 --> 00:29:55 I want to do a quick demo here before I do that. 539 00:29:55 --> 00:29:56 Let's go back and do that. 540 00:29:56 --> 00:29:59 541 00:29:59 --> 00:30:03 If the retina is on backwards, you got this 542 00:30:03 --> 00:30:07 interesting problem. 543 00:30:07 --> 00:30:12 So, there are those nerve fibers to the optic nerve. 544 00:30:12 --> 00:30:15 How are they going to get out of the eye if light 545 00:30:15 --> 00:30:18 is coming from there? 546 00:30:18 --> 00:30:27 What they do is they run across the surface of the retina until 547 00:30:27 --> 00:30:29 they reach the optic nerve and then they go out 548 00:30:29 --> 00:30:31 up to the brain. 549 00:30:31 --> 00:30:34 Where are the photoreceptors at the optic nerve? 550 00:30:34 --> 00:30:36 The answer is there aren't any photo receptors 551 00:30:36 --> 00:30:37 at the optic nerve. 552 00:30:37 --> 00:30:39 If there aren't any photoreceptors at the optic 553 00:30:39 --> 00:30:42 nerve, what do you see there? 554 00:30:42 --> 00:30:46 The answer isn't nothing because you're not aware of the 555 00:30:46 --> 00:30:50 fact that there's some big hole in your visual field. 556 00:30:50 --> 00:30:53 But there is. 557 00:30:53 --> 00:30:57 You will automatically fill it in because if it's green here 558 00:30:57 --> 00:31:00 and green here and there's no information there, it's a good 559 00:31:00 --> 00:31:02 bet that it's probably green all the way across and your 560 00:31:02 --> 00:31:04 visual system can figure this out. 561 00:31:04 --> 00:31:07 But it's worth it to prove it to yourself that there 562 00:31:07 --> 00:31:09 is a blind spot there. 563 00:31:09 --> 00:31:11 Let's see, on the handout -- well, flip to the back of the 564 00:31:11 --> 00:31:19 handout where you got a little extra white space, and put an x 565 00:31:19 --> 00:31:25 on handout here like this, and a little ways away like about 566 00:31:25 --> 00:31:27 like that, put a little black dot or a little dot -- I 567 00:31:27 --> 00:31:28 don't care if it's black. 568 00:31:28 --> 00:31:32 569 00:31:32 --> 00:31:37 So the dot is to the right of the x. 570 00:31:37 --> 00:31:40 Close the left eye, cover your left eye. 571 00:31:40 --> 00:31:44 Look at the x, hold it so that the dot is straight out to the 572 00:31:44 --> 00:31:49 right and then move the paper back and forth and you will 573 00:31:49 --> 00:31:53 find that there's a spot where the dot completely disappears. 574 00:31:53 --> 00:31:59 What you've done at that point is move the dot into the optic 575 00:31:59 --> 00:32:01 nerve head -- the place where the optic nerve is 576 00:32:01 --> 00:32:02 leaving the eye. 577 00:32:02 --> 00:32:07 578 00:32:07 --> 00:32:11 If you're saying I can't do this, I don't see a blind spot, 579 00:32:11 --> 00:32:15 it's not because you're special and don't have one, it's 580 00:32:15 --> 00:32:16 because you're doing it wrong. 581 00:32:16 --> 00:32:19 582 00:32:19 --> 00:32:22 Well, there is an alternative. 583 00:32:22 --> 00:32:25 I'm given to understand that the octopus retina is not put 584 00:32:25 --> 00:32:28 on backwards in this way and so perhaps you're an octopus. 585 00:32:28 --> 00:32:33 586 00:32:33 --> 00:32:38 OK, let's go back to this wiring diagram here. 587 00:32:38 --> 00:32:46 588 00:32:46 --> 00:32:49 You can play with this later. 589 00:32:49 --> 00:32:50 It's cool. 590 00:32:50 --> 00:32:53 Do things like -- well, if you flip it around you can find 591 00:32:53 --> 00:32:55 that you got one in the other eye, too. 592 00:32:55 --> 00:32:59 You can also do things like draw a line with a hole in it. 593 00:32:59 --> 00:33:01 If you draw a line with a gap in it and put the gap in the 594 00:33:01 --> 00:33:04 hole, the line will appear to complete. 595 00:33:04 --> 00:33:07 If you get really adept at this you can do what King Charles 596 00:33:07 --> 00:33:10 the Second of England was reputed to have liked to do, 597 00:33:10 --> 00:33:14 which is you can look at your friends and put their heads 598 00:33:14 --> 00:33:19 in blind spots and watch their heads disappear. 599 00:33:19 --> 00:33:23 Charles the Second didn't get to chop off as many heads as 600 00:33:23 --> 00:33:25 his predecessors had, so this was the best he 601 00:33:25 --> 00:33:26 could do I guess. 602 00:33:26 --> 00:33:28 All right. 603 00:33:28 --> 00:33:36 How do the axons from the optic nerve feed the visual 604 00:33:36 --> 00:33:39 centers of the brain? 605 00:33:39 --> 00:33:43 You will recall, from the first lecture perhaps, that 606 00:33:43 --> 00:33:51 everything from the right side of my body on the skin senses 607 00:33:51 --> 00:33:55 gets represented in the sensory homunculus on the left, and 608 00:33:55 --> 00:33:57 everything from the left in the sensory homunculus 609 00:33:57 --> 00:34:00 on the right. 610 00:34:00 --> 00:34:04 The way to not do this individual system would be to 611 00:34:04 --> 00:34:08 have let's have everything from the right, that's my right eye, 612 00:34:08 --> 00:34:10 my right eye go to my left hemisphere, and everything from 613 00:34:10 --> 00:34:12 the left eye go to the right hemisphere. 614 00:34:12 --> 00:34:14 That wouldn't be any good. 615 00:34:14 --> 00:34:17 Isaac Newton figured out without bothering to do 616 00:34:17 --> 00:34:19 any anatomy, Isaac Newton figured out that that 617 00:34:19 --> 00:34:20 wouldn't be any good. 618 00:34:20 --> 00:34:24 Because I want to look out at the world with two eyes and 619 00:34:24 --> 00:34:25 I want to see one world. 620 00:34:25 --> 00:34:28 So I'm going to have to do something that brings the 621 00:34:28 --> 00:34:31 information from the two eyes together. 622 00:34:31 --> 00:34:36 What you actually do is the following. 623 00:34:36 --> 00:34:41 Everything from the left side of each retina ends up in the 624 00:34:41 --> 00:34:44 left hemisphere, and everything from the right side of each 625 00:34:44 --> 00:34:48 retina ends up in the right hemisphere. 626 00:34:48 --> 00:34:56 The image on eye is flipped -- simple lenses do that, right. 627 00:34:56 --> 00:35:00 So I've got a simple lens in that anterior 628 00:35:00 --> 00:35:01 portion of my eye. 629 00:35:01 --> 00:35:06 That means that the image of you guys landing on my retain 630 00:35:06 --> 00:35:09 has your feet up, your head down and left and 631 00:35:09 --> 00:35:10 right reversed. 632 00:35:10 --> 00:35:19 So what that means here is if I stare at the person wearing 633 00:35:19 --> 00:35:23 yellow here -- if I stare at the person wearing yellow 634 00:35:23 --> 00:35:25 she'll look embarrassed. 635 00:35:25 --> 00:35:27 It always works but that's a different phenomenon. 636 00:35:27 --> 00:35:29 All right, but I'm staring at you but I'm not actually 637 00:35:29 --> 00:35:30 paying any attention to you. 638 00:35:30 --> 00:35:33 What I'm actually doing is looking at -- my aquity's 639 00:35:33 --> 00:35:37 so bad, it's a black blob right about there. 640 00:35:37 --> 00:35:42 I assume she's a person -- yeah, she's a person, OK. 641 00:35:42 --> 00:35:44 Anyway, acquity's very bad. 642 00:35:44 --> 00:35:47 643 00:35:47 --> 00:35:52 If you're on my fovea, the woman to the right is in the 644 00:35:52 --> 00:35:55 right half of my visual field landing on the left 645 00:35:55 --> 00:35:57 side of my retina. 646 00:35:57 --> 00:36:00 She lands on the left side in both eyes, and as a result, 647 00:36:00 --> 00:36:02 she's landing in my left hemisphere. 648 00:36:02 --> 00:36:05 If I'm looking at you, some arbitrary blob -- 649 00:36:05 --> 00:36:07 there's a blob that just moved over there. 650 00:36:07 --> 00:36:10 Oh, she's a person, too. 651 00:36:10 --> 00:36:13 This person in the left visual field ends up on the right 652 00:36:13 --> 00:36:17 retain in each eye and in the right hemisphere. 653 00:36:17 --> 00:36:20 So there's only one of her, in a sense, up in my brain, and 654 00:36:20 --> 00:36:26 only one of her up in my brain and they're getting 655 00:36:26 --> 00:36:27 together in that--. 656 00:36:27 --> 00:36:30 And the result is that half the fibers from each retina have 657 00:36:30 --> 00:36:32 to cross to the other side. 658 00:36:32 --> 00:36:40 That's what this cute little map on page 2 is telling you. 659 00:36:40 --> 00:36:44 But the important thing not to get confused about is don't 660 00:36:44 --> 00:36:48 tell us on some exam that your left eye goes to your right 661 00:36:48 --> 00:36:50 hemisphere or something. 662 00:36:50 --> 00:36:56 It's left half of each retina goes to left hemisphere, right 663 00:36:56 --> 00:36:59 half of each retina goes to the right hemisphere. 664 00:36:59 --> 00:37:09 What it does once it gets there is -- this is a monkey brain, 665 00:37:09 --> 00:37:15 and what you're looking at is the cortical surface here. 666 00:37:15 --> 00:37:17 That must be the outside surface and this is the inner 667 00:37:17 --> 00:37:21 surface of the hemisphere, if there are two hemispheres. 668 00:37:21 --> 00:37:23 The one on the bottom is looking at the inside surface. 669 00:37:23 --> 00:37:26 The colored areas are ones that are important in vision. 670 00:37:26 --> 00:37:30 I talked at the beginning about the fact that primary visual 671 00:37:30 --> 00:37:32 cortex is right here at the back of the brain, but it's 672 00:37:32 --> 00:37:38 amazing how much of the brain is devoted to vision. 673 00:37:38 --> 00:37:40 It's very hard to look at in a picture -- well, it's 674 00:37:40 --> 00:37:42 very hard look at, period. 675 00:37:42 --> 00:37:46 What vision researchers like to do is to make maps like this 676 00:37:46 --> 00:37:52 where you take the cortical surface and flatten it -- it's 677 00:37:52 --> 00:37:59 all this wrinkley involuted structure -- use a variety of 678 00:37:59 --> 00:38:01 techniques to flatten it out like a map. 679 00:38:01 --> 00:38:04 Then you see that this is the piece that's at the back 680 00:38:04 --> 00:38:06 of the head, the one primary visual cortex. 681 00:38:06 --> 00:38:11 Then you've got all these gazillion other regions that 682 00:38:11 --> 00:38:16 are important in visual processing, some of them for 683 00:38:16 --> 00:38:17 rather precise sort of things. 684 00:38:17 --> 00:38:19 Where did mt go? 685 00:38:19 --> 00:38:23 686 00:38:23 --> 00:38:28 I think right there is mt, for example, which is important 687 00:38:28 --> 00:38:30 in motion processing. 688 00:38:30 --> 00:38:32 Lots of work being done trying to figure out what 689 00:38:32 --> 00:38:34 different bits of it do. 690 00:38:34 --> 00:38:38 Look, let me give you a broad organizing scheme for 691 00:38:38 --> 00:38:41 how to think about this. 692 00:38:41 --> 00:38:44 It's not perfect but it ain't bad. 693 00:38:44 --> 00:38:48 From primary visual cortex where very basic information 694 00:38:48 --> 00:38:53 about spots and lines and primitive bits of motion gets 695 00:38:53 --> 00:38:59 pulled out, more advanced processing can be thought of 696 00:38:59 --> 00:39:01 as in two large streams. 697 00:39:01 --> 00:39:07 One large stream going up towards the parietal lobe is 698 00:39:07 --> 00:39:12 telling you things about where stuff is in the world. 699 00:39:12 --> 00:39:14 How is the world laid out? 700 00:39:14 --> 00:39:20 So, my notion that this space is laid out as a big slanted 701 00:39:20 --> 00:39:27 plane in front of me is probably a product of 702 00:39:27 --> 00:39:30 this sort of a pathway. 703 00:39:30 --> 00:39:37 A second pathway going down into the temporal lobe is 704 00:39:37 --> 00:39:42 very concerned with what stimuli it might be. 705 00:39:42 --> 00:39:48 So there, if you're a monkey and I spear individual cells, 706 00:39:48 --> 00:39:50 it might be where I would find a cell that was particularly 707 00:39:50 --> 00:39:56 interested in a hand, a monkey hand, or a hairy leg or 708 00:39:56 --> 00:39:58 something like that. 709 00:39:58 --> 00:40:01 It turns out that the hairy part can be important. 710 00:40:01 --> 00:40:03 711 00:40:03 --> 00:40:07 Some of the work on things like monkey hand detectors was being 712 00:40:07 --> 00:40:14 done by one of my professors at Princeton, and actually my TA 713 00:40:14 --> 00:40:18 in intro psych, when I was an undergrad, was a grad student 714 00:40:18 --> 00:40:21 who was working on this, and he reported to us that he was 715 00:40:21 --> 00:40:24 recording -- you go spear a cell. 716 00:40:24 --> 00:40:26 It's one thing if you're just trying to figure out if it 717 00:40:26 --> 00:40:30 likes spots of light, but how do you figure out if the cell 718 00:40:30 --> 00:40:32 likes something like a hand. 719 00:40:32 --> 00:40:37 Well, they had in those days drawers full of stuff and you 720 00:40:37 --> 00:40:40 waved all this stuff in front of the monkey and see 721 00:40:40 --> 00:40:43 what makes the cell go. 722 00:40:43 --> 00:40:46 These experiments in the good old days tended to run for very 723 00:40:46 --> 00:40:50 long time, because this was an anesthetized monkey and the 724 00:40:50 --> 00:40:52 monkey wasn't going to wake up from this particular operation. 725 00:40:52 --> 00:40:56 You ran the experiment for days and days and days and you 726 00:40:56 --> 00:40:58 stayed up the whole time. 727 00:40:58 --> 00:41:03 So, Bob Desimone, now a famous visual neuroscientist is up 728 00:41:03 --> 00:41:08 late at night with this stupid monkey with his stupid cell and 729 00:41:08 --> 00:41:10 the cell isn't doing anything. 730 00:41:10 --> 00:41:11 He's waving everything at it. 731 00:41:11 --> 00:41:16 So finally, he took off his pants, wrapped a towel around 732 00:41:16 --> 00:41:20 his waist and did a little dance in front of the monkey. 733 00:41:20 --> 00:41:21 The cell went nuts. 734 00:41:21 --> 00:41:25 735 00:41:25 --> 00:41:29 Being a good scientist, he then narrowed this down to the 736 00:41:29 --> 00:41:31 notion that this was a cell that actually seem to be 737 00:41:31 --> 00:41:36 interested in hairy legs, and the hairy part turned 738 00:41:36 --> 00:41:38 out to be important. 739 00:41:38 --> 00:41:43 I feel free to tell this story about Bob Desimone because Bob 740 00:41:43 --> 00:41:47 Desimone is running around the country with a talk at the 741 00:41:47 --> 00:41:50 moment that has a picture of me in it kissing a wombat. 742 00:41:50 --> 00:41:53 743 00:41:53 --> 00:41:55 Don't ask. 744 00:41:55 --> 00:41:58 It all seems fair to me. 745 00:41:58 --> 00:42:03 All right, so if you go up into, way down here in the 746 00:42:03 --> 00:42:09 temporal lobe, you might find cells that respond only to 747 00:42:09 --> 00:42:11 something like a hairy leg or a monkey hand or 748 00:42:11 --> 00:42:12 something like that. 749 00:42:12 --> 00:42:19 If you're hanging out back here in primary visual cortex -- 750 00:42:19 --> 00:42:22 well, actually let's go back and hang out 751 00:42:22 --> 00:42:24 in the retina even. 752 00:42:24 --> 00:42:26 Look at that, we'll hang out right there. 753 00:42:26 --> 00:42:33 If you're hanging out in the retina, if you like much 754 00:42:33 --> 00:42:38 simpler needs than this hairy leg detector somewhere up in 755 00:42:38 --> 00:42:46 the temporal lobe, I mentioned before that you might be 756 00:42:46 --> 00:42:53 bringing together a whole bunch of photoreceptors through, say, 757 00:42:53 --> 00:42:59 a bipolar cell to a ganglion cell, to bring together a 758 00:42:59 --> 00:43:03 whole bunch of them to send a signal off to the brain. 759 00:43:03 --> 00:43:06 But what I didn't mention is that not all the photoreceptors 760 00:43:06 --> 00:43:09 that are hooked up to stimulated ganglion cell are 761 00:43:09 --> 00:43:11 going to excite that cell. 762 00:43:11 --> 00:43:15 Let's draw some pictures over here. 763 00:43:15 --> 00:43:21 What actually happens, and I'm just going to mimic this over 764 00:43:21 --> 00:43:30 here, is that you'd have some photoreceptors who are set up 765 00:43:30 --> 00:43:35 to excite a ganglion cell. 766 00:43:35 --> 00:43:39 Then some other guys around them who would be set 767 00:43:39 --> 00:43:45 up to inhibit it. 768 00:43:45 --> 00:43:52 769 00:43:52 --> 00:43:56 The result is that what this cell -- that's a 770 00:43:56 --> 00:43:59 one-dimensional slice and this would be a two-dimensional 771 00:43:59 --> 00:44:02 picture -- the retina's after all a two-dimensional surface. 772 00:44:02 --> 00:44:12 So what you'd really get is some region of photoreceptors 773 00:44:12 --> 00:44:17 that excite the cell, and some surrounding region where light 774 00:44:17 --> 00:44:21 inhibits the subsequent ganglion cell. 775 00:44:21 --> 00:44:27 Then light out here does nothing. 776 00:44:27 --> 00:44:30 So this is the receptive field of this cell -- this is the 777 00:44:30 --> 00:44:33 only part of the retina that this cell over 778 00:44:33 --> 00:44:37 here cares about. 779 00:44:37 --> 00:44:41 It's going to send its signal off to the brain. 780 00:44:41 --> 00:44:42 What does this cell do? 781 00:44:42 --> 00:44:45 Well, among the things that this cell can do is it can tell 782 00:44:45 --> 00:44:52 you I am most excited by a spot that's exacty this size. 783 00:44:52 --> 00:44:55 If the spot gets any bigger it's going to encroach on 784 00:44:55 --> 00:44:58 this inhibitory stuff and reduce my excitation. 785 00:44:58 --> 00:45:04 If the spot's any smaller, I'm not going to get all of my 786 00:45:04 --> 00:45:08 excitatory guys excited, and the response will get smaller. 787 00:45:08 --> 00:45:12 So I'm most excited by a spot of exactly this size. 788 00:45:12 --> 00:45:17 If I build receptors with different size centers, this 789 00:45:17 --> 00:45:20 guy's only going to be excited by a little spot and I could 790 00:45:20 --> 00:45:23 make one that would be excited by a bigger spot. 791 00:45:23 --> 00:45:27 So, even at the level of the retina, you can start to get 792 00:45:27 --> 00:45:31 cells that are telling you more than just light on, light off, 793 00:45:31 --> 00:45:34 they're starting to tell you something about the size of 794 00:45:34 --> 00:45:39 things in the visual field. 795 00:45:39 --> 00:45:46 When you get up to the cortex, you find that now cells come to 796 00:45:46 --> 00:45:51 be interested in things like lines -- not just a 797 00:45:51 --> 00:45:53 spot but a line. 798 00:45:53 --> 00:45:58 And this cell will way, I like lines in this orientation. 799 00:45:58 --> 00:46:01 I don't like lines in this orientation. 800 00:46:01 --> 00:46:03 In fact, I like lines in this orientation only if they 801 00:46:03 --> 00:46:05 move this direction. 802 00:46:05 --> 00:46:08 So you can imagine that what you start to do early in the 803 00:46:08 --> 00:46:16 visual system is to pull out a whole bunch of little features, 804 00:46:16 --> 00:46:21 if you like, that you can then use subsequently to try to 805 00:46:21 --> 00:46:25 figure out is there a monkey hand here. 806 00:46:25 --> 00:46:31 Is there a laptop computer or something like that. 807 00:46:31 --> 00:46:37 Early in the system you've got basic features being 808 00:46:37 --> 00:46:38 pulled out of the image. 809 00:46:38 --> 00:46:41 Now, this presents a problem. 810 00:46:41 --> 00:46:45 Suppose that what you want to do -- Let's start 811 00:46:45 --> 00:46:46 with orientation. 812 00:46:46 --> 00:46:49 Suppose you want to decide that a line is a 813 00:46:49 --> 00:46:50 particular orientation. 814 00:46:50 --> 00:46:52 How good are you at that? 815 00:46:52 --> 00:46:58 The answer turns out to be that you are good enough to be 816 00:46:58 --> 00:47:03 able to tell roughly 1 degree increments. 817 00:47:03 --> 00:47:04 How are you going to do that? 818 00:47:04 --> 00:47:07 Well you could have a cell specifically designed to look 819 00:47:07 --> 00:47:11 for zero degrees vertical, 1 degree, 2 degree, 3 degree, 4 820 00:47:11 --> 00:47:14 degree, and you need one of those everywhere in the visual 821 00:47:14 --> 00:47:18 field, and at all different sizes -- a little line that's 822 00:47:18 --> 00:47:19 vertical, a big line that's vertical. 823 00:47:19 --> 00:47:22 You're going to need an awful lot of cells then. 824 00:47:22 --> 00:47:25 That's not what we do. 825 00:47:25 --> 00:47:29 What we seem to do all over the place in the visual system, and 826 00:47:29 --> 00:47:35 in sensory systems in general, is to make comparisons. 827 00:47:35 --> 00:47:40 That comparisons are very powerful in this game. 828 00:47:40 --> 00:47:43 So how do you decide that something's vertical? 829 00:47:43 --> 00:47:46 To a first approximation what you do is you look at whether 830 00:47:46 --> 00:47:49 or not the lift-tilted guys and the right-tilted guys, the 831 00:47:49 --> 00:47:54 cells that like stuff -- the cells that look for something 832 00:47:54 --> 00:47:56 that's tilted a little to the right, cells that are 833 00:47:56 --> 00:47:58 interested in stuff that are tilted to the left. 834 00:47:58 --> 00:48:01 Are they roughly equally excited? 835 00:48:01 --> 00:48:04 If they're roughly equally excited in balance one with the 836 00:48:04 --> 00:48:14 other, then odds are that you're looking at a 837 00:48:14 --> 00:48:16 vertical stimulus. 838 00:48:16 --> 00:48:18 OK? 839 00:48:18 --> 00:48:22 If the right-tilted stuff is stronger than the left-tilted 840 00:48:22 --> 00:48:24 stuff, so that balance gets tiled, you're going to say 841 00:48:24 --> 00:48:27 that the stimulus is tilted. 842 00:48:27 --> 00:48:36 It turns out that if you do something to make -- let's say 843 00:48:36 --> 00:48:42 you go off and you make the left-tilted stuff weak by 844 00:48:42 --> 00:48:44 tiring it out -- how do you tire it out, you show it a 845 00:48:44 --> 00:48:47 bunch of left, show it a bunch of left lines, that 846 00:48:47 --> 00:48:48 makes this weaker. 847 00:48:48 --> 00:48:50 Then something that's vertical's going to turn out to 848 00:48:50 --> 00:48:53 look tilted to the -- you're not going to believe that. 849 00:48:53 --> 00:48:55 Let me show this to you. 850 00:48:55 --> 00:49:01 I'll show it to you in -- I'm going to jump to here. 851 00:49:01 --> 00:49:05 Well, we'll jump here first. 852 00:49:05 --> 00:49:05 OK. 853 00:49:05 --> 00:49:11 Here are a very boring picture. 854 00:49:11 --> 00:49:13 It will be even more boring if I turn out a little 855 00:49:13 --> 00:49:15 more of the lights here. 856 00:49:15 --> 00:49:18 857 00:49:18 --> 00:49:20 OK. 858 00:49:20 --> 00:49:24 How you know that something's white or grey or achromatic? 859 00:49:24 --> 00:49:27 Again, in this case you've got color mechanisms that say 860 00:49:27 --> 00:49:33 how red or green is this? 861 00:49:33 --> 00:49:36 If the answer is well, it's balanced between red and 862 00:49:36 --> 00:49:44 green, and you also ask how blue or yellow is this? 863 00:49:44 --> 00:49:46 If it's not blue and it's not yellow and it's not red and 864 00:49:46 --> 00:49:50 it's not green, well, you know what it's probably white. 865 00:49:50 --> 00:49:53 OK, well, so here let's take this red/green thing and 866 00:49:53 --> 00:49:59 let's make the green side weak by wearing it out. 867 00:49:59 --> 00:50:02 We'll show you some green. 868 00:50:02 --> 00:50:04 Now I'm going to show you something white -- the green is 869 00:50:04 --> 00:50:08 weak, the red is strong, the balance goes this way, and you 870 00:50:08 --> 00:50:11 think that something that was once white now looks 871 00:50:11 --> 00:50:14 kind of green. 872 00:50:14 --> 00:50:17 There, see, isn't that amazing? 873 00:50:17 --> 00:50:19 They're not convinced. 874 00:50:19 --> 00:50:22 The ones who are convinced are confused. 875 00:50:22 --> 00:50:26 Stare at the black dot at the center here. 876 00:50:26 --> 00:50:30 You just want to stare at the black dot and think about, say, 877 00:50:30 --> 00:50:32 that red dot up at 12 o'clock. 878 00:50:32 --> 00:50:35 What you're doing is you're tiring out the red side of this 879 00:50:35 --> 00:50:38 red/green equation in this particular case, and just keep 880 00:50:38 --> 00:50:43 staring there, and then I'm going to go boink. 881 00:50:43 --> 00:50:51 882 00:50:51 --> 00:50:56 So, the people who went "oh," presumably saw colors. 883 00:50:56 --> 00:50:59 Let's do that again, here. 884 00:50:59 --> 00:51:10 What you want to notice is that the color is this opposite 885 00:51:10 --> 00:51:11 color, it's going the other direction. 886 00:51:11 --> 00:51:14 So the red one will turn green, the yellow one will turn 887 00:51:14 --> 00:51:16 bluish, the green one will turn reddish, right. 888 00:51:16 --> 00:51:18 So stare at the center, boink. 889 00:51:18 --> 00:51:21 890 00:51:21 --> 00:51:23 It works. 891 00:51:23 --> 00:51:26 892 00:51:26 --> 00:51:29 That's what's known as a negative after image. 893 00:51:29 --> 00:51:31 894 00:51:31 --> 00:51:33 You can just toggle back and forth. 895 00:51:33 --> 00:51:41 If you look at the black dot on the right and then move your 896 00:51:41 --> 00:51:44 eyes to the left, you'll see the colors show up in the left 897 00:51:44 --> 00:51:47 hand circles, because that after image is essentially 898 00:51:47 --> 00:51:50 cooked onto your retina -- it's there in retinal coordinates 899 00:51:50 --> 00:51:54 and moves with your eyes. 900 00:51:54 --> 00:51:58 Alright, now let me illustrate to you that this is a 901 00:51:58 --> 00:52:02 ubiquitous phenomenon that shows up for basic phenomena 902 00:52:02 --> 00:52:08 all over the place by turning this back on. 903 00:52:08 --> 00:52:09 Is that going to be enough light for this? 904 00:52:09 --> 00:52:12 905 00:52:12 --> 00:52:15 So, I won't show you the orientation one, even though I 906 00:52:15 --> 00:52:19 managed to get a PhD studying it, but I'll show you the 907 00:52:19 --> 00:52:22 motion equivalent of it. 908 00:52:22 --> 00:52:23 Here's what you want to do. 909 00:52:23 --> 00:52:26 910 00:52:26 --> 00:52:28 You'll still see it over there, except for the guy who's 911 00:52:28 --> 00:52:30 asleep behind the pillar, that's his tough luck. 912 00:52:30 --> 00:52:38 913 00:52:38 --> 00:52:42 So, look at this, and we're doing the same game. 914 00:52:42 --> 00:52:45 We're just taking the game and transferring from the color 915 00:52:45 --> 00:52:47 realm, or the orientation realm, down to the 916 00:52:47 --> 00:52:48 motion realm. 917 00:52:48 --> 00:52:51 So this is contracting, right? 918 00:52:51 --> 00:52:53 Stare at the center, that's important. 919 00:52:53 --> 00:52:54 So stare at the center. 920 00:52:54 --> 00:52:57 Again, think about 12 o'clock. 921 00:52:57 --> 00:52:59 All the contours are moving down. 922 00:52:59 --> 00:53:02 You know that something's stationary because it's not 923 00:53:02 --> 00:53:04 moving up, down, left or right. 924 00:53:04 --> 00:53:07 But at 12 o'clock I'm wearing out your down detectors, and 925 00:53:07 --> 00:53:11 at 6 o'clock I'm wearing out your up detectors and so on. 926 00:53:11 --> 00:53:17 The consequences of this evil act of mine will be seen 927 00:53:17 --> 00:53:20 vividly if, when I take it away, what you do is 928 00:53:20 --> 00:53:22 look at my nose. 929 00:53:22 --> 00:53:41 930 00:53:41 --> 00:53:47 Of course, what really worries me here is several people's 931 00:53:47 --> 00:53:48 pupils got much larger, which means they think 932 00:53:48 --> 00:53:51 I look better that way. 933 00:53:51 --> 00:53:53 934 00:53:53 --> 00:53:57 Now, a word of warning. 935 00:53:57 --> 00:53:58 Two words of warning. 936 00:53:58 --> 00:54:00 This is the best demo in the whole class. 937 00:54:00 --> 00:54:02 It's all downhill from here. 938 00:54:02 --> 00:54:06 The other word of warning is it's a very salient demo, but 939 00:54:06 --> 00:54:10 it's there for a purpose. 940 00:54:10 --> 00:54:14 It's really lame on the mid-term or the final when some 941 00:54:14 --> 00:54:16 motion -- this is called a motion after effect -- when a 942 00:54:16 --> 00:54:20 motion after effect question shows up to say oh, that was 943 00:54:20 --> 00:54:24 cool, man, his head was shrinking and stuff. 944 00:54:24 --> 00:54:26 Because you don't get much points for that. 945 00:54:26 --> 00:54:30 What you want to know is why this is actually interesting, 946 00:54:30 --> 00:54:33 which I will reiterate by going the other direction since it 947 00:54:33 --> 00:54:36 seems a pity not to, right? 948 00:54:36 --> 00:54:40 OK, so again, the way you figure out that something is, 949 00:54:40 --> 00:54:43 in this case, stationary is by making a set of comparisons, 950 00:54:43 --> 00:54:48 and Iam using this clever little tool to systematically 951 00:54:48 --> 00:54:51 distort the comparisons that you're making. 952 00:54:51 --> 00:54:53 By the way, I'm also causing you to make one of these 953 00:54:53 --> 00:54:57 inferences that we'll talk about next week. 954 00:54:57 --> 00:55:00 By seeing all these various bits of odd motion, you are 955 00:55:00 --> 00:55:03 making the conclusion that my head is shrinking or growing 956 00:55:03 --> 00:55:04 or something like that. 957 00:55:04 --> 00:55:07 So this is expanding, right, so this is the amazing 958 00:55:07 --> 00:55:08 pinhead version. 959 00:55:08 --> 00:55:09 Ready? 960 00:55:09 --> 00:55:19 961 00:55:19 --> 00:55:25 The original version of this, by the way, is known in the 962 00:55:25 --> 00:55:29 literature as sometimes it's the waterfall illusion, because 963 00:55:29 --> 00:55:33 it gets described multiple times in the literature, but 964 00:55:33 --> 00:55:40 one of them is by a Scotsman named Adams who was staring at 965 00:55:40 --> 00:55:44 a waterfall and then noticed that the rocks on the side of 966 00:55:44 --> 00:55:46 the waterfall seem to be drifting up. 967 00:55:46 --> 00:55:50 This is a known waterfall in a known spot in Scotland and 968 00:55:50 --> 00:55:52 there's a group of vision researchers who meet there 969 00:55:52 --> 00:55:57 every year and the entry requirement for joining this 970 00:55:57 --> 00:56:01 particular elite group is a bottle of scotch of a variety 971 00:56:01 --> 00:56:04 that they have not tried before, which I think helps 972 00:56:04 --> 00:56:08 make the rocks move in all sorts of interesting ways. 973 00:56:08 --> 00:56:13 They've never invited me so how would I know. 974 00:56:13 --> 00:56:15 OK, this is what I'm going to do. 975 00:56:15 --> 00:56:17 Let me follow up. 976 00:56:17 --> 00:56:21 So you've seen a negative color after image. 977 00:56:21 --> 00:56:25 You've seen a negative motion after effect. 978 00:56:25 --> 00:56:29 I've asserted that these exist in orientation. 979 00:56:29 --> 00:56:32 You can show this often -- you show somebody big stuff and 980 00:56:32 --> 00:56:33 then medium stuff looks small. 981 00:56:33 --> 00:56:34 It's ubiquitous. 982 00:56:34 --> 00:56:36 It shows up all over the place. 983 00:56:36 --> 00:56:39 I want to show you one more version of this kind of a 984 00:56:39 --> 00:56:42 effect, and that's going to take a certain amount 985 00:56:42 --> 00:56:45 of build-up here. 986 00:56:45 --> 00:56:48 So the first thing to do is say the vertical and horizontal 987 00:56:48 --> 00:56:52 stripes, do they look different to you? 988 00:56:52 --> 00:56:54 If they look different somebody raise their hand and tell 989 00:56:54 --> 00:56:55 me what the difference is. 990 00:56:55 --> 00:56:56 What's the difference? 991 00:56:56 --> 00:56:56 AUDIENCE: The 992 00:56:56 --> 00:56:57 horizontal ones are horizontal. 993 00:56:57 --> 00:57:02 PROFESSOR: Yeah, alright. 994 00:57:02 --> 00:57:05 In their color or shading in some fashion. 995 00:57:05 --> 00:57:05 Yeah? 996 00:57:05 --> 00:57:07 AUDIENCE: The vertical ones look blurry. 997 00:57:07 --> 00:57:09 PROFESSOR: The vertical ones look a little -- they do 998 00:57:09 --> 00:57:12 look a little blurry. 999 00:57:12 --> 00:57:12 Yeah? 1000 00:57:12 --> 00:57:15 AUDIENCE: The vertical ones -- the black lines look 1001 00:57:15 --> 00:57:16 like they're thicker than the ones--. 1002 00:57:16 --> 00:57:18 PROFESSOR: Well, we're not getting a lot of vivid 1003 00:57:18 --> 00:57:19 color reports here. 1004 00:57:19 --> 00:57:22 AUDIENCE: The horizontal lines look brighter. 1005 00:57:22 --> 00:57:23 PROFESSOR: Brighter? 1006 00:57:23 --> 00:57:26 Nobody knows what the word color means. 1007 00:57:26 --> 00:57:27 Last try. 1008 00:57:27 --> 00:57:33 AUDIENCE: The horizontal lines are greyer and the 1009 00:57:33 --> 00:57:34 vertical lines are black. 1010 00:57:34 --> 00:57:38 PROFESSOR: OK, look, this makes the point just fine. 1011 00:57:38 --> 00:57:40 Never mind guys. 1012 00:57:40 --> 00:57:42 There is no in general consensus here. 1013 00:57:42 --> 00:57:45 I'm about to produce a general consensus. 1014 00:57:45 --> 00:57:49 1015 00:57:49 --> 00:57:51 That's the motivational part of today's lecture. 1016 00:57:51 --> 00:57:57 No, actually what I have to do now is for the next few minutes 1017 00:57:57 --> 00:57:59 what you want to do is keep your head upright. 1018 00:57:59 --> 00:58:02 You don't have to stare at any one spot. 1019 00:58:02 --> 00:58:03 You don't even have to your head upright, just keep it 1020 00:58:03 --> 00:58:05 in the same orientation. 1021 00:58:05 --> 00:58:08 If you flop all over the place and fall asleep 1022 00:58:08 --> 00:58:10 nothing good happens. 1023 00:58:10 --> 00:58:12 So you're going to look at these green vertical lines and 1024 00:58:12 --> 00:58:14 these red horizontal lines. 1025 00:58:14 --> 00:58:20 1026 00:58:20 --> 00:58:25 All I need to do here is toggle back and forth between them. 1027 00:58:25 --> 00:58:28 Now you can see the negative after image here. 1028 00:58:28 --> 00:58:30 If you stare at this green -- oh, I should have done 1029 00:58:30 --> 00:58:32 that quickly before. 1030 00:58:32 --> 00:58:34 So stare at this green thing for a while. 1031 00:58:34 --> 00:58:38 You get an nice purply red thing? 1032 00:58:38 --> 00:58:39 OK. 1033 00:58:39 --> 00:58:42 So that's just the negative after image. 1034 00:58:42 --> 00:58:48 But we're not interested in the negative after image anymore. 1035 00:58:48 --> 00:58:50 Actually, we are. 1036 00:58:50 --> 00:58:53 There's an aspect of the negative after image that's 1037 00:58:53 --> 00:59:01 really cool here, which you may start to notice shortly. 1038 00:59:01 --> 00:59:05 During the blank period, shout out the orientation 1039 00:59:05 --> 00:59:06 that you see. 1040 00:59:06 --> 00:59:14 AUDIENCE: [INAUDIBLE PHRASE] 1041 00:59:14 --> 00:59:18 PROFESSOR: That's a little weird. 1042 00:59:18 --> 00:59:20 Wouldn't you think? 1043 00:59:20 --> 00:59:26 Now that's a little weird because if you stare at these 1044 00:59:26 --> 00:59:28 horizontal stripes -- in fact, I should be able to get you 1045 00:59:28 --> 00:59:29 back to seeing horizontal. 1046 00:59:29 --> 00:59:32 So, stare at these -- fixate, don't move your eyes around 1047 00:59:32 --> 00:59:37 and we'll just stare at that for a minute or so. 1048 00:59:37 --> 00:59:40 1049 00:59:40 --> 00:59:43 Has it been long enough? 1050 00:59:43 --> 00:59:44 Yeah. 1051 00:59:44 --> 00:59:46 Does it look horizontal? 1052 00:59:46 --> 00:59:49 Still looks vertical, oh boy. 1053 00:59:49 --> 00:59:58 Well, here's the get rich quick part of this lecture, assuming 1054 00:59:58 --> 01:00:02 that you're unscrupulous, you can use this to make your 1055 01:00:02 --> 01:00:05 fortune claiming that you can teach people to be 1056 01:00:05 --> 01:00:07 psychic, right. 1057 01:00:07 --> 01:00:10 What's the next orientation I'm going to show? 1058 01:00:10 --> 01:00:12 What's the next orientation I'm going to show? 1059 01:00:12 --> 01:00:16 1060 01:00:16 --> 01:00:21 If you can make your fortune out of that, just send me 10%. 1061 01:00:21 --> 01:00:27 But what's really going on here, what this is actually 1062 01:00:27 --> 01:00:31 illustrating, this aspect of it is illustrating, is that you're 1063 01:00:31 --> 01:00:34 seeing with your whole visual system, not with your 1064 01:00:34 --> 01:00:37 retina all by itself or something like that. 1065 01:00:37 --> 01:00:41 So, each time you look, say, the red horizontal, you're 1066 01:00:41 --> 01:00:44 building up a green horizontal negative after image 1067 01:00:44 --> 01:00:46 on your retina, right? 1068 01:00:46 --> 01:00:49 And this is building up a red vertical kind of after 1069 01:00:49 --> 01:00:50 image on your retina. 1070 01:00:50 --> 01:00:58 If you go back and forth, you build up sort of a plaid. 1071 01:00:58 --> 01:01:02 So think about your retina as having a plaid on it. 1072 01:01:02 --> 01:01:07 Now you don't see it here, but the plaid is -- when you're 1073 01:01:07 --> 01:01:09 looking at a blank screen, it's just that plaid that's getting 1074 01:01:09 --> 01:01:12 shipped up to your brain, basically. 1075 01:01:12 --> 01:01:14 OK, well what else is happening? 1076 01:01:14 --> 01:01:17 Well, when you're staring at this green vertical thing, up 1077 01:01:17 --> 01:01:20 in your brain the cells that are interested in vertical are 1078 01:01:20 --> 01:01:22 saying we're getting tired. 1079 01:01:22 --> 01:01:26 Go to the blank part, tired vertical guys and more awake 1080 01:01:26 --> 01:01:29 horizontal guys are looking at this weak plaid and 1081 01:01:29 --> 01:01:30 they're only seeing the horizontal part. 1082 01:01:30 --> 01:01:33 Now you look at the horizontal, the horizontal guys get tired 1083 01:01:33 --> 01:01:36 out -- oh, we can only see the vertical, and so on. 1084 01:01:36 --> 01:01:39 1085 01:01:39 --> 01:01:42 You may ask isn't it about time we saw something 1086 01:01:42 --> 01:01:45 interesting here? 1087 01:01:45 --> 01:01:48 The answer is undoubtedly so, but this particular effect 1088 01:01:48 --> 01:01:55 takes a few minutes to build up, and so you gotta 1089 01:01:55 --> 01:01:59 keep looking at this. 1090 01:01:59 --> 01:02:06 Oh, you've got to ask yourself what the effects ought to be. 1091 01:02:06 --> 01:02:08 What I'm building up here is an effect called the McCullough 1092 01:02:08 --> 01:02:13 Effect, named after Celeste McCullough, who unlike most 1093 01:02:13 --> 01:02:18 people who discover visual effects, was actually 1094 01:02:18 --> 01:02:21 looking for this. 1095 01:02:21 --> 01:02:23 She knew what she was looking for and she knew 1096 01:02:23 --> 01:02:24 what she could find. 1097 01:02:24 --> 01:02:26 Now I already showed you that the pattern you're going to 1098 01:02:26 --> 01:02:28 look at again, right, it was that black and white pattern of 1099 01:02:28 --> 01:02:31 vertical and horizontal lines. 1100 01:02:31 --> 01:02:34 What do you think the vertical lines should look 1101 01:02:34 --> 01:02:37 like when we're done? 1102 01:02:37 --> 01:02:38 They should look kind of reddish maybe. 1103 01:02:38 --> 01:02:40 The horizontal should look? 1104 01:02:40 --> 01:02:41 AUDIENCE: Green. 1105 01:02:41 --> 01:02:41 PROFESSOR: Green. 1106 01:02:41 --> 01:02:43 Let's see, well we might have done this long enough. 1107 01:02:43 --> 01:02:46 This is one of these things where if I do it long enough I 1108 01:02:46 --> 01:02:51 can hit absolutely everybody, and if I quit too soon not 1109 01:02:51 --> 01:02:52 everybody will see the effect. 1110 01:02:52 --> 01:02:58 1111 01:02:58 --> 01:02:59 All right, here we go. 1112 01:02:59 --> 01:03:02 So, close your eyes, because that will let me get to the 1113 01:03:02 --> 01:03:05 right slide, but it will also let you perhaps see that 1114 01:03:05 --> 01:03:08 checkerboard floating there in the dark? 1115 01:03:08 --> 01:03:09 Murmur if you can see a checkerboard floating 1116 01:03:09 --> 01:03:11 in the dark. 1117 01:03:11 --> 01:03:11 OK, good. 1118 01:03:11 --> 01:03:13 That's that's the negative after image thing. 1119 01:03:13 --> 01:03:16 OK, now take a look at this test pattern again. 1120 01:03:16 --> 01:03:20 1121 01:03:20 --> 01:03:22 How many people think that the vertical and horizontal look 1122 01:03:22 --> 01:03:27 different now in terms of their color? 1123 01:03:27 --> 01:03:30 How many people think it might look a little sort of maybe 1124 01:03:30 --> 01:03:32 kind of different, but it's not exactly biting me 1125 01:03:32 --> 01:03:35 on the posterier? 1126 01:03:35 --> 01:03:37 Alright, that's a better test. 1127 01:03:37 --> 01:03:39 Now, the way to find out whether you've actually got 1128 01:03:39 --> 01:03:43 the right effect is without knocking out your neighbor, 1129 01:03:43 --> 01:03:45 tilt your head 90 degrees. 1130 01:03:45 --> 01:03:50 1131 01:03:50 --> 01:03:51 So that's cooler. 1132 01:03:51 --> 01:04:10 1133 01:04:10 --> 01:04:13 I'll say a word more about this, but I won't say a word 1134 01:04:13 --> 01:04:17 more about this until you've had a chance to get up and 1135 01:04:17 --> 01:04:18 stretch for a second. 1136 01:04:18 --> 01:05:11 1137 01:05:11 --> 01:05:16 OK. 1138 01:05:16 --> 01:05:28 1139 01:05:28 --> 01:05:37 How many people when they look at these dots still see 1140 01:05:37 --> 01:05:38 reasonably compelling colors? 1141 01:05:38 --> 01:05:42 1142 01:05:42 --> 01:05:43 Not many. 1143 01:05:43 --> 01:05:46 How many people when they look at my head still 1144 01:05:46 --> 01:05:47 see it shrinking? 1145 01:05:47 --> 01:05:50 1146 01:05:50 --> 01:05:52 Yeah, except for the weird people. 1147 01:05:52 --> 01:05:55 1148 01:05:55 --> 01:06:03 That's because your visual system is continuously 1149 01:06:03 --> 01:06:05 re-generating itself. 1150 01:06:05 --> 01:06:10 You go stare at the red thing, you wear out, in a sense, the 1151 01:06:10 --> 01:06:14 red detecting chunk of your visual system, and then if you 1152 01:06:14 --> 01:06:16 flip your eyes over you find out that you see a 1153 01:06:16 --> 01:06:17 white thing is green. 1154 01:06:17 --> 01:06:19 But then you recover from it, right? 1155 01:06:19 --> 01:06:21 Good. 1156 01:06:21 --> 01:06:22 How many people still see this effect? 1157 01:06:22 --> 01:06:26 1158 01:06:26 --> 01:06:29 Tilt your head just to make sure if you think it's there. 1159 01:06:29 --> 01:06:31 A little weak today. 1160 01:06:31 --> 01:06:34 We probably should have gone a little bit longer. 1161 01:06:34 --> 01:06:36 Those of you who are seeing it, and perhaps even those of you 1162 01:06:36 --> 01:06:39 who are not -- did I put a test pattern on the 1163 01:06:39 --> 01:06:41 handout, I forget? 1164 01:06:41 --> 01:06:42 No, OK. 1165 01:06:42 --> 01:06:45 I'll just bring the Powerpoint back next week. 1166 01:06:45 --> 01:06:49 Not only are you still seeing this some minutes afterwards, 1167 01:06:49 --> 01:06:51 but particularly if I don't show you the test pattern 1168 01:06:51 --> 01:06:57 a lot you may see it for days and weeks. 1169 01:06:57 --> 01:07:00 The longest report in the literature for the McCullough 1170 01:07:00 --> 01:07:04 effect is three months. 1171 01:07:04 --> 01:07:10 The only reason it's only three months is because at that point 1172 01:07:10 --> 01:07:13 the undergraduates, who, of course, were the subjects, 1173 01:07:13 --> 01:07:17 went away on vacation and the experiment ended. 1174 01:07:17 --> 01:07:21 The evidence suggests that if we locked you in the dark that 1175 01:07:21 --> 01:07:24 the McCullough effect would last forever. 1176 01:07:24 --> 01:07:26 We won't lock you in the dark. 1177 01:07:26 --> 01:07:30 1178 01:07:30 --> 01:07:32 I teach this up at Harvard where there are many more 1179 01:07:32 --> 01:07:35 pre-law students, and the immediate reaction to this 1180 01:07:35 --> 01:07:38 news is half of them consider filing suit. 1181 01:07:38 --> 01:07:41 1182 01:07:41 --> 01:07:43 This will not, in fact, ruin your life in any fashion. 1183 01:07:43 --> 01:07:47 In fact, it'll only be noticeable if you happen to end 1184 01:07:47 --> 01:07:49 up looking at a test pattern like this or perhaps serving 1185 01:07:49 --> 01:07:55 time in the near future. 1186 01:07:55 --> 01:07:58 But this was something that McCullough was not expecting. 1187 01:07:58 --> 01:08:03 McCullough thought that what she had done here was, OK, you 1188 01:08:03 --> 01:08:06 can fatigue color, you can fatigue orientation, well I'll 1189 01:08:06 --> 01:08:08 fatigue the cells in the brain and there are cells in the 1190 01:08:08 --> 01:08:12 brain that are interested in red vertical lines. 1191 01:08:12 --> 01:08:15 They'll get tired, I'll see white vertical as green, 1192 01:08:15 --> 01:08:18 they'll recover, I'll see white vertical as white. 1193 01:08:18 --> 01:08:21 But what happened was you looked at red vertical lines 1194 01:08:21 --> 01:08:24 for a while, white vertical looked green and white vertical 1195 01:08:24 --> 01:08:29 looked green for like a really, really long time thereafter. 1196 01:08:29 --> 01:08:34 What seems to be going on here is something like this. 1197 01:08:34 --> 01:08:38 1198 01:08:38 --> 01:09:03 Let's suppose that the way you decide that something is white 1199 01:09:03 --> 01:09:10 vertical is by looking at the balance of -- well here, 1200 01:09:10 --> 01:09:11 let's draw it this way. 1201 01:09:11 --> 01:09:15 1202 01:09:15 --> 01:09:19 You've got red vertical input and you've got green vertical 1203 01:09:19 --> 01:09:28 input, and they're being like let's put a little dial here. 1204 01:09:28 --> 01:09:31 If they're roughly equal you decide something looks white. 1205 01:09:31 --> 01:09:34 So I show you red vertical for a while and this thing gets 1206 01:09:34 --> 01:09:37 weak and cruddy, and now something that should be 1207 01:09:37 --> 01:09:40 producing equal input gets more from there and it ends up 1208 01:09:40 --> 01:09:42 looking a little green. 1209 01:09:42 --> 01:09:45 Alright, that's fine. 1210 01:09:45 --> 01:09:53 What happens if this cell gets sick -- it's not going to be 1211 01:09:53 --> 01:09:57 one cell, but suppose that this process gets weak in some 1212 01:09:57 --> 01:10:02 fashion, not because you were looking at some stimulus, but 1213 01:10:02 --> 01:10:09 because you went out and ate the mushroom that you shouldn't 1214 01:10:09 --> 01:10:13 have eaten or whatever and this cell just took a hit. 1215 01:10:13 --> 01:10:16 And so now this is kind of a cruddy cell. 1216 01:10:16 --> 01:10:19 1217 01:10:19 --> 01:10:21 Well, there are a couple of possibilities. 1218 01:10:21 --> 01:10:26 One of them is that for the rest of time you're going 1219 01:10:26 --> 01:10:31 to end up seeing white things as looking green. 1220 01:10:31 --> 01:10:33 That's not going to be good right. 1221 01:10:33 --> 01:10:38 You don't want that happen. 1222 01:10:38 --> 01:10:45 So if you decided that this was the situation, what you would 1223 01:10:45 --> 01:10:50 want to do is change the gain on here. 1224 01:10:50 --> 01:10:55 So if this was previously just equal gain, let's crank this up 1225 01:10:55 --> 01:10:59 a little bit so that we can drag that back up 1226 01:10:59 --> 01:11:03 to being equal. 1227 01:11:03 --> 01:11:05 That sounds OK. 1228 01:11:05 --> 01:11:06 That seems like a clever enough thing to do. 1229 01:11:06 --> 01:11:09 But how are you gonna do that? 1230 01:11:09 --> 01:11:16 How are you going to figure out that you're broken. 1231 01:11:16 --> 01:11:20 How would you know that your visual system was not 1232 01:11:20 --> 01:11:22 reporting the truth to you. 1233 01:11:22 --> 01:11:25 Well, I mean one way would be you're saying hey, that looks 1234 01:11:25 --> 01:11:27 white to me, and somebody else says that looks green and they 1235 01:11:27 --> 01:11:29 laugh at you or something. 1236 01:11:29 --> 01:11:31 Nah, it doesn't work. 1237 01:11:31 --> 01:11:33 No evidence that that works. 1238 01:11:33 --> 01:11:41 What you do is that you seem to have some sort of notion of the 1239 01:11:41 --> 01:11:46 statistics of the world, the way the world ought to be. 1240 01:11:46 --> 01:11:52 If the world is systematically different from that, you draw 1241 01:11:52 --> 01:11:58 the conclusion that you've got trouble here of some 1242 01:11:58 --> 01:11:59 variety or other. 1243 01:11:59 --> 01:12:02 Well not necessary, you don't necessarily draw the -- let me 1244 01:12:02 --> 01:12:04 give you a non-troubled version of this. 1245 01:12:04 --> 01:12:06 How many people have been to one of these omni 1246 01:12:06 --> 01:12:09 theater kind of movies? 1247 01:12:09 --> 01:12:14 The main reason for going to an omni theater movie is what? 1248 01:12:14 --> 01:12:17 You want to feel like you're moving, right, 1249 01:12:17 --> 01:12:19 when you're not moving. 1250 01:12:19 --> 01:12:21 Because all the movies that they ever show you at the 1251 01:12:21 --> 01:12:26 science museum are things where you go driving around in 1252 01:12:26 --> 01:12:31 race cars, stuff like that. 1253 01:12:31 --> 01:12:33 Why does this work? 1254 01:12:33 --> 01:12:41 You're sitting at this huge screen, and everything 1255 01:12:41 --> 01:12:45 in your visual field is moving in the same way. 1256 01:12:45 --> 01:12:47 You know something about the world. 1257 01:12:47 --> 01:12:52 The world on average is stationary. 1258 01:12:52 --> 01:12:57 If the whole world is moving there's a good explanation 1259 01:12:57 --> 01:12:58 for that typically. 1260 01:12:58 --> 01:13:02 The answer is not the world, it's you. 1261 01:13:02 --> 01:13:04 What is it that causes the whole world to slide 1262 01:13:04 --> 01:13:05 across your retina? 1263 01:13:05 --> 01:13:07 This is what causes the whole world to slide 1264 01:13:07 --> 01:13:09 across the retina. 1265 01:13:09 --> 01:13:13 Sit in the omni and the world is sliding across your retina. 1266 01:13:13 --> 01:13:16 Your brain says uh-huh, the world isn't moving, I'm moving, 1267 01:13:16 --> 01:13:18 and I think I'll throw up now for a while or 1268 01:13:18 --> 01:13:19 something like that. 1269 01:13:19 --> 01:13:23 That's a separate interesting question. 1270 01:13:23 --> 01:13:27 What is it here that could tell you that you're out of whack? 1271 01:13:27 --> 01:13:31 Well, what is the normal relationship between color and 1272 01:13:31 --> 01:13:32 orientation in the world? 1273 01:13:32 --> 01:13:36 1274 01:13:36 --> 01:13:40 Typically in the world if you see something that's vertical, 1275 01:13:40 --> 01:13:43 what do you know about its color? 1276 01:13:43 --> 01:13:45 That's right, nothing. 1277 01:13:45 --> 01:13:49 So what's going on here? 1278 01:13:49 --> 01:13:50 It's vertical. 1279 01:13:50 --> 01:13:51 It's green. 1280 01:13:51 --> 01:13:54 If it's horizontal it's red. 1281 01:13:54 --> 01:13:56 If it's vertical, it's green. 1282 01:13:56 --> 01:13:58 If it's horizontal, it's red. 1283 01:13:58 --> 01:14:01 You do this for long enough and a little chunk of your brain, 1284 01:14:01 --> 01:14:04 rather like the little chunk of your brain that's saying I ate 1285 01:14:04 --> 01:14:08 the cupcakes, I threw up, I'd better do something about this. 1286 01:14:08 --> 01:14:11 This little chunk of your brain is saying, oh, there's 1287 01:14:11 --> 01:14:14 a correlation between color and orientation. 1288 01:14:14 --> 01:14:21 The only way that happens is if I'm broken, and the only thing 1289 01:14:21 --> 01:14:25 to do about that is to adjust the gain on chunks of 1290 01:14:25 --> 01:14:27 my visual system. 1291 01:14:27 --> 01:14:33 So, everything looks green. 1292 01:14:33 --> 01:14:34 All the vertical things look green. 1293 01:14:34 --> 01:14:36 Oh my goodness, what am I going to do? 1294 01:14:36 --> 01:14:40 Well I better crank down the gain on the green side, it's 1295 01:14:40 --> 01:14:43 obviously too high, and I'm want to crank up the 1296 01:14:43 --> 01:14:45 gain on the red side. 1297 01:14:45 --> 01:14:49 Unfortunately, that was a goof. 1298 01:14:49 --> 01:14:52 And it was wrong, so then when you look at a white thing it 1299 01:14:52 --> 01:14:54 now looks red because you cranked the gain up 1300 01:14:54 --> 01:14:56 too much over here. 1301 01:14:56 --> 01:14:58 How do you recover from this? 1302 01:14:58 --> 01:15:03 Well, if I now show you the opposite one -- what 1303 01:15:03 --> 01:15:04 were we doing before? 1304 01:15:04 --> 01:15:05 What was it, green vertical? 1305 01:15:05 --> 01:15:08 If I show you red verticals and green horizontals for a while, 1306 01:15:08 --> 01:15:09 that'll push it back the other way. 1307 01:15:09 --> 01:15:12 But that doesn't typically happen in the world. 1308 01:15:12 --> 01:15:15 Simply being exposed to a world where there's no correlation 1309 01:15:15 --> 01:15:17 will slowly push you back. 1310 01:15:17 --> 01:15:23 But the reason the effect lasts so long is that the evidence 1311 01:15:23 --> 01:15:26 from the real world is kind a weak compared to this 1312 01:15:26 --> 01:15:30 artificial situation that I cooked up. 1313 01:15:30 --> 01:15:32 The way I'm thinking about this is like -- here's another 1314 01:15:32 --> 01:15:33 way of thinking about this. 1315 01:15:33 --> 01:15:45 Suppose you're measuring average weight of people. 1316 01:15:45 --> 01:15:51 I'm just going to keep weighing people and what's the 1317 01:15:51 --> 01:15:52 average weight of people. 1318 01:15:52 --> 01:15:54 It'll bounce around a little bit. 1319 01:15:54 --> 01:15:55 Alright. 1320 01:15:55 --> 01:15:58 This is how much people on average weigh. 1321 01:15:58 --> 01:16:02 Now what happens is the amazing 1,000 pound guy 1322 01:16:02 --> 01:16:04 comes into my experiment. 1323 01:16:04 --> 01:16:06 What happens to the average? 1324 01:16:06 --> 01:16:09 1325 01:16:09 --> 01:16:12 Now I start weighing normal people again. 1326 01:16:12 --> 01:16:16 Does it just go -- no. 1327 01:16:16 --> 01:16:19 Weighing regular people it'll take a comparatively long time 1328 01:16:19 --> 01:16:21 for me to get back to normal. 1329 01:16:21 --> 01:16:23 So that's what's going on here. 1330 01:16:23 --> 01:16:26 The McCullough effect adapting stimulus is the 1,000 pound guy 1331 01:16:26 --> 01:16:29 that's causing you to think that your brain is broken. 1332 01:16:29 --> 01:16:34 You go and re-adjust your brain and now slowly 1333 01:16:34 --> 01:16:37 it will get better. 1334 01:16:37 --> 01:16:40 How's it look there? 1335 01:16:40 --> 01:16:41 Not better yet. 1336 01:16:41 --> 01:16:43 How many people have a nice after effect? 1337 01:16:43 --> 01:16:46 Still, nice McCullough effect? 1338 01:16:46 --> 01:16:50 Celeste McCullough would be so proud. 1339 01:16:50 --> 01:16:53 You will notice that I have not said a word about 1340 01:16:53 --> 01:16:55 signal detection theory. 1341 01:16:55 --> 01:16:58 I noticed that, too. 1342 01:16:58 --> 01:16:59 There is a chunk in [? Gleichman ?] 1343 01:16:59 --> 01:16:59 about it. 1344 01:16:59 --> 01:17:02 I might decide to talk about it on Thursday. 1345 01:17:02 --> 01:17:05 Oh, you may also notice that you have lost your cell phone. 1346 01:17:05 --> 01:17:07 If that is true come and talk to me about it 1347 01:17:07 --> 01:17:09 because we found it. 1348 01:17:09 --> 01:17:11 One last thing. 1349 01:17:11 --> 01:17:13 How many people know what that is? 1350 01:17:13 --> 01:17:16 1351 01:17:16 --> 01:17:18 I didn't ask you what it was, I just wanted to know if 1352 01:17:18 --> 01:17:19 you knew what it was. 1353 01:17:19 --> 01:17:23 How many people recognize this? 1354 01:17:23 --> 01:17:24 Only a few. 1355 01:17:24 --> 01:17:26 OK, now you can shout out. 1356 01:17:26 --> 01:17:27 AUDIENCE: It's a dog. 1357 01:17:27 --> 01:17:28 PROFESSOR: It's a dog. 1358 01:17:28 --> 01:17:29 How many people does that help? 1359 01:17:29 --> 01:17:35 1360 01:17:35 --> 01:17:38 It's head is -- well, wait a second. 1361 01:17:38 --> 01:17:44 There's its head, foot, back paw, it's back, it's sort 1362 01:17:44 --> 01:17:47 of sniffing at a ground plane that's like here. 1363 01:17:47 --> 01:17:55 1364 01:17:55 --> 01:17:58 You can store that one away till next week and 1365 01:17:58 --> 01:18:00 we'll talk about it. 1366 01:18:00 --> 01:18:01