1 00:00:00,080 --> 00:00:01,670 The following content is provided 2 00:00:01,670 --> 00:00:03,820 under a Creative Commons license. 3 00:00:03,820 --> 00:00:06,550 Your support will help MIT OpenCourseWare continue 4 00:00:06,550 --> 00:00:10,160 to offer high-quality educational resources for free. 5 00:00:10,160 --> 00:00:12,700 To make a donation or to view additional materials 6 00:00:12,700 --> 00:00:16,620 from hundreds of MIT courses, visit MIT OpenCourseWare 7 00:00:16,620 --> 00:00:17,275 at ocw.mit.edu. 8 00:00:26,006 --> 00:00:26,880 PROFESSOR: All right. 9 00:00:26,880 --> 00:00:28,013 Good afternoon, everyone. 10 00:00:30,590 --> 00:00:35,170 Today, we are going to have two topics. 11 00:00:35,170 --> 00:00:39,050 In the first half we are going to discuss visual illusions, 12 00:00:39,050 --> 00:00:41,240 and in the second half we are going 13 00:00:41,240 --> 00:00:46,550 to talk about prosthetic devices for the blind. 14 00:00:46,550 --> 00:00:49,050 What I would like to start with is, again, 15 00:00:49,050 --> 00:00:53,110 to send around the attendance sheet, 16 00:00:53,110 --> 00:00:56,180 so please put your name to it. 17 00:00:56,180 --> 00:00:58,360 And then, in addition, I'm going to send 18 00:00:58,360 --> 00:01:01,430 around some handouts, one for each person. 19 00:01:02,760 --> 00:01:04,769 I'm going to hand them out in both directions. 20 00:01:04,769 --> 00:01:05,269 Thank you. 21 00:01:08,810 --> 00:01:11,050 The handouts we're going to look at, 22 00:01:11,050 --> 00:01:14,250 I'll refer to them specifically as we go around. 23 00:01:14,250 --> 00:01:18,640 And I also will show pretty much the same handouts, figures 24 00:01:18,640 --> 00:01:21,040 in the handouts, on the screen. 25 00:01:22,060 --> 00:01:22,560 All right. 26 00:01:22,560 --> 00:01:26,820 So for the first half of the presentation today 27 00:01:26,820 --> 00:01:30,380 we are going to talk about visual illusions. 28 00:01:30,380 --> 00:01:35,430 Now, the main emphasis of what I'm going to talk about, 29 00:01:35,430 --> 00:01:39,690 it'll be an attempt to convey to you what 30 00:01:39,690 --> 00:01:44,220 kinds of procedures one can use to get a better sense of why 31 00:01:44,220 --> 00:01:46,935 we see in illusions what we see. 32 00:01:48,100 --> 00:01:51,220 Some of these approaches are strictly behavioral. 33 00:01:51,220 --> 00:01:57,380 And some of them have involved experiments carried out 34 00:01:57,380 --> 00:02:03,720 on a more thorough basis using electrophysiological procedures 35 00:02:03,720 --> 00:02:04,270 and so on. 36 00:02:04,270 --> 00:02:07,310 But those I'm not going to talk too much about. 37 00:02:07,310 --> 00:02:10,800 But I will relate what we can figure out 38 00:02:10,800 --> 00:02:17,380 through how the visual system works. 39 00:02:17,380 --> 00:02:22,330 Now, because of that, there are hundreds of illusions, 40 00:02:22,330 --> 00:02:25,440 and I will emphasize those that allow 41 00:02:25,440 --> 00:02:29,440 us to think about why we see what we see. 42 00:02:29,440 --> 00:02:32,450 Now, one of these illusory effects 43 00:02:32,450 --> 00:02:34,820 is the so-called Hermann grid illusion, 44 00:02:34,820 --> 00:02:36,650 and that's the one I'm going to start 45 00:02:36,650 --> 00:02:39,830 with because it's a fascinating illusion. 46 00:02:39,830 --> 00:02:42,880 If you look at this, what you see 47 00:02:42,880 --> 00:02:49,470 is that at the intersections you see sort of smudges. 48 00:02:49,470 --> 00:02:54,590 And so the big question arose as to why we see these smudges. 49 00:03:00,400 --> 00:03:03,480 Now, does everybody see those smudges? 50 00:03:03,480 --> 00:03:06,190 If you look at the handouts, I also 51 00:03:06,190 --> 00:03:13,700 show you-- pay attention only to the front page, 52 00:03:13,700 --> 00:03:18,340 the one on the left, which says the Hermann grid illusion. 53 00:03:18,340 --> 00:03:22,440 So if you look at that either on your handout 54 00:03:22,440 --> 00:03:24,870 or you look at it on the screen here, 55 00:03:24,870 --> 00:03:29,190 you see these smudges at the intersections, not where you're 56 00:03:29,190 --> 00:03:33,650 looking at, but in the smudges other than where 57 00:03:33,650 --> 00:03:34,725 you're fixating. 58 00:03:34,725 --> 00:03:36,100 Does everybody see those smudges? 59 00:03:38,030 --> 00:03:38,940 OK. 60 00:03:38,940 --> 00:03:42,780 So the question arose why we have these smudges. 61 00:03:42,780 --> 00:03:49,640 And back in the 1960s, an investigator 62 00:03:49,640 --> 00:03:52,380 came up with a rather clever idea 63 00:03:52,380 --> 00:03:54,580 of why we see these smudges. 64 00:03:54,580 --> 00:03:57,350 And the person who came up with this theory 65 00:03:57,350 --> 00:03:59,330 was a fellow called Baumgartner. 66 00:03:59,330 --> 00:04:03,380 And so what he proposed, that this illusory effect 67 00:04:03,380 --> 00:04:07,000 is not directly attributable to the center-surround 68 00:04:07,000 --> 00:04:11,010 organization or the receptive fields or retinal ganglion 69 00:04:11,010 --> 00:04:11,780 cells. 70 00:04:11,780 --> 00:04:17,579 So what he proposed is that when a retinal ganglion cell falls 71 00:04:17,579 --> 00:04:21,709 in the center like this, there's more inhibition in the surround 72 00:04:21,709 --> 00:04:25,720 than when it falls along not the intersections, 73 00:04:25,720 --> 00:04:27,350 but along the straight lines. 74 00:04:28,400 --> 00:04:31,540 And because of that, there's more inhibition here than here, 75 00:04:31,540 --> 00:04:33,770 and that's why we see the smudges. 76 00:04:33,770 --> 00:04:36,630 Now, that was a very attractive theory. 77 00:04:36,630 --> 00:04:41,940 And in fact, it has appeared and still exists in many textbooks 78 00:04:41,940 --> 00:04:46,610 that you read about illusions and about vision in general. 79 00:04:46,610 --> 00:04:50,460 Now, OK, so here is the exact description 80 00:04:50,460 --> 00:04:52,930 that I just made to tell you this. 81 00:04:52,930 --> 00:04:56,000 And of course, you would expect that if this is the case, 82 00:04:56,000 --> 00:05:00,240 you should also get the smudges if you'd reverse the contrast. 83 00:05:00,240 --> 00:05:02,390 And in fact, that is true. 84 00:05:02,390 --> 00:05:05,420 So here, once again, you see the smudges, maybe not quite 85 00:05:05,420 --> 00:05:06,310 as sharp. 86 00:05:06,310 --> 00:05:09,850 But in this case, it's whiter smudges, 87 00:05:09,850 --> 00:05:14,080 whereas in the previous case, this one, 88 00:05:14,080 --> 00:05:15,790 you have darker smudges. 89 00:05:15,790 --> 00:05:19,460 So that kind of fits with that basic idea. 90 00:05:19,460 --> 00:05:23,840 But then, subsequently, people began 91 00:05:23,840 --> 00:05:27,930 to play around with various conditions to see when you do 92 00:05:27,930 --> 00:05:32,100 and when you don't get the smudge effect 93 00:05:32,100 --> 00:05:35,960 and to see whether that would fit with this theory. 94 00:05:35,960 --> 00:05:38,810 Now, that's the basic tenet in just about everything 95 00:05:38,810 --> 00:05:42,230 that psychologists and neuroscientists 96 00:05:42,230 --> 00:05:44,050 working in the visual system do. 97 00:05:44,050 --> 00:05:48,480 They try to critically examine various hypotheses. 98 00:05:49,610 --> 00:05:51,200 And doesn't have to be just illusions, 99 00:05:51,200 --> 00:05:53,370 it can be a whole bunch of other things. 100 00:05:53,370 --> 00:05:56,940 And they do this then by systematically manipulating 101 00:05:56,940 --> 00:06:02,050 things in experiments that can present these things just 102 00:06:02,050 --> 00:06:05,580 under free viewing conditions, as I'm doing it 103 00:06:05,580 --> 00:06:08,250 in terms of these handouts, or you 104 00:06:08,250 --> 00:06:10,640 can do it under more controlled conditions. 105 00:06:10,640 --> 00:06:13,690 The subjects can look through a tachistoscope or something 106 00:06:13,690 --> 00:06:14,640 like that. 107 00:06:14,640 --> 00:06:15,230 All right. 108 00:06:15,230 --> 00:06:19,780 So anyway, let's now look at this in a bit more detail 109 00:06:19,780 --> 00:06:24,050 and see what's wrong with this theory. 110 00:06:24,050 --> 00:06:27,780 Well, first of all, if you look at this figure, 111 00:06:27,780 --> 00:06:30,370 what you can see here that you can see the smudges 112 00:06:30,370 --> 00:06:33,645 irrespective of the size of the overall display. 113 00:06:34,780 --> 00:06:37,290 Now, that's a problem because, of course, 114 00:06:37,290 --> 00:06:40,640 the size of the receptive fields in the retina, 115 00:06:40,640 --> 00:06:44,530 in their center-surround arrangement, is constant. 116 00:06:44,530 --> 00:06:47,700 So if it were the sharp arrangement 117 00:06:47,700 --> 00:06:51,510 that we described about the way the center and the surround 118 00:06:51,510 --> 00:06:53,810 are activated by the intersections 119 00:06:53,810 --> 00:06:57,880 and the non-intersections in the display, that should not 120 00:06:57,880 --> 00:06:59,460 apply to different sizes. 121 00:06:59,460 --> 00:07:03,260 It should only work then for a particular size 122 00:07:03,260 --> 00:07:05,400 where you approximate that. 123 00:07:05,400 --> 00:07:09,640 Now, I will elaborate on that in more detail in just a minute. 124 00:07:09,640 --> 00:07:12,440 Now, another bug that arose here is 125 00:07:12,440 --> 00:07:17,130 that if you now turn this display just 45 degrees, 126 00:07:17,130 --> 00:07:20,045 all right, so that it's a diamond rather than a square, 127 00:07:20,045 --> 00:07:23,765 the illusory effect has declined considerably. 128 00:07:24,960 --> 00:07:27,370 And then to make this even more problematic, 129 00:07:27,370 --> 00:07:32,750 and this appears in your handout on that backside 130 00:07:32,750 --> 00:07:37,110 of the first page, what you can see here 131 00:07:37,110 --> 00:07:40,980 is that when you make the lines arranged 132 00:07:40,980 --> 00:07:45,060 in such a fashion at slight angles to each other, then 133 00:07:45,060 --> 00:07:47,540 you don't see the illusory effect at all. 134 00:07:47,540 --> 00:07:56,690 Whereas here, we also see it a little bit less but still 135 00:07:56,690 --> 00:07:58,470 slightly visible. 136 00:07:58,470 --> 00:08:00,350 But here it's not visible at all. 137 00:08:00,350 --> 00:08:04,600 And you can compare that even better on the handout there. 138 00:08:04,600 --> 00:08:07,520 So the question is, why would that be? 139 00:08:08,880 --> 00:08:10,580 Well, we give you another example here. 140 00:08:10,580 --> 00:08:12,260 If you make serrated edges-- I don't 141 00:08:12,260 --> 00:08:15,090 know if the people in the back can see the serrations. 142 00:08:15,090 --> 00:08:17,430 Those of you in front can see the serrations. 143 00:08:17,430 --> 00:08:20,150 And when you do that, also you fail to see it. 144 00:08:20,150 --> 00:08:22,290 And the problem here, of course, is 145 00:08:22,290 --> 00:08:26,490 that if you make these slight changes in the angles 146 00:08:26,490 --> 00:08:32,909 of the intersecting lines or you make them serrated, 147 00:08:32,909 --> 00:08:36,860 the center-surround antagonism should be the same. 148 00:08:36,860 --> 00:08:41,289 And yet such a small change causes the illusory effect 149 00:08:41,289 --> 00:08:42,590 to disappear. 150 00:08:42,590 --> 00:08:46,830 So that then, in itself, questions the Baumgartner 151 00:08:46,830 --> 00:08:47,330 theory. 152 00:08:49,050 --> 00:08:52,040 Now, there are several other factors that enter into this. 153 00:08:52,040 --> 00:08:53,850 This is a very subtle one. 154 00:08:53,850 --> 00:08:57,710 If you look at this side, OK, we have different shades 155 00:08:57,710 --> 00:08:58,991 for the lines. 156 00:08:58,991 --> 00:08:59,490 OK? 157 00:09:01,180 --> 00:09:05,610 On this side, we have the darkened lines, OK, in front. 158 00:09:05,610 --> 00:09:07,760 And in this case, they have them in the back. 159 00:09:07,760 --> 00:09:10,930 And here you see the illusory effect, and here you don't. 160 00:09:10,930 --> 00:09:14,330 And once again, how can that be if the illusory effect is 161 00:09:14,330 --> 00:09:16,770 due to the center-surround antagonism? 162 00:09:16,770 --> 00:09:20,600 Even more dramatic is when you do this with colors. 163 00:09:20,600 --> 00:09:23,782 Here you have, again, the colored lines in front, 164 00:09:23,782 --> 00:09:25,240 and here you have them in the back. 165 00:09:25,240 --> 00:09:30,500 And here you can see homonymous shades, all right? 166 00:09:30,500 --> 00:09:33,760 This is darker yellow and so on. 167 00:09:34,800 --> 00:09:37,860 But it's the same color but a little bit darker 168 00:09:37,860 --> 00:09:39,310 at the intersections. 169 00:09:39,310 --> 00:09:41,580 So here you get a pretty good illusory effect. 170 00:09:41,580 --> 00:09:44,800 But once those white lines are in front, 171 00:09:44,800 --> 00:09:47,960 we get very little effect at all. 172 00:09:47,960 --> 00:09:53,020 So that then denotes a number of problems with the theory. 173 00:09:53,020 --> 00:09:57,340 And then one more fact is that when you present the stimuli 174 00:09:57,340 --> 00:10:01,210 at isoluminance, you don't get an effect at all. 175 00:10:01,210 --> 00:10:03,940 So that then raised some serious questions 176 00:10:03,940 --> 00:10:05,720 about the illusory effect. 177 00:10:05,720 --> 00:10:08,390 And then a quantitative study was 178 00:10:08,390 --> 00:10:13,150 undertaken to see just what is the layout of the receptive 179 00:10:13,150 --> 00:10:16,790 fields in this display. 180 00:10:16,790 --> 00:10:20,090 And so here what we have is a display. 181 00:10:20,090 --> 00:10:25,830 This square here is exactly five degrees from your fixation, OK, 182 00:10:25,830 --> 00:10:31,610 and you can calculate the number of midget and parasol cells 183 00:10:31,610 --> 00:10:35,250 that are activated by that square area. 184 00:10:35,250 --> 00:10:39,570 And you can see in total there are more than 360 cells that 185 00:10:39,570 --> 00:10:42,390 are being activated in that little region. 186 00:10:42,390 --> 00:10:48,540 And here is a display, in this case, of only the ON cells, 187 00:10:48,540 --> 00:10:50,760 and you can double that by having the OFF 188 00:10:50,760 --> 00:10:52,570 cells in the same location. 189 00:10:52,570 --> 00:10:57,100 Saying that there's just no good correspondence 190 00:10:57,100 --> 00:11:02,210 whatsoever in the number of ganglion cells 191 00:11:02,210 --> 00:11:05,220 that are being activated and how the center-surround is truly 192 00:11:05,220 --> 00:11:11,230 laid out at a point where you get a very good smudge 193 00:11:11,230 --> 00:11:15,030 effect with this illusion. 194 00:11:15,030 --> 00:11:17,850 So this then, taken all together, 195 00:11:17,850 --> 00:11:27,380 raises the issue then if indeed there's a retina phenomenon, 196 00:11:27,380 --> 00:11:29,240 why is there no fit here? 197 00:11:29,240 --> 00:11:31,660 And so it does look like that it's unlikely 198 00:11:31,660 --> 00:11:33,760 that this illusion can be explained 199 00:11:33,760 --> 00:11:36,070 by the center-surround antagonism theory. 200 00:11:39,780 --> 00:11:42,460 So then another theory was advanced 201 00:11:42,460 --> 00:11:45,150 claiming that this doesn't take place in the retina, 202 00:11:45,150 --> 00:11:47,930 but instead that it takes place in the cortex. 203 00:11:49,200 --> 00:11:51,560 And so another theory that was advanced 204 00:11:51,560 --> 00:11:54,780 is saying that this happens in the cortex due to the fact 205 00:11:54,780 --> 00:11:58,090 that you have these simple cells-- we talked 206 00:11:58,090 --> 00:12:05,530 about those before-- that had these elongated inhibitory, 207 00:12:05,530 --> 00:12:10,500 supposedly inhibitory, surrounds. 208 00:12:10,500 --> 00:12:15,600 And if one calculates this, that can explain the illusion 209 00:12:15,600 --> 00:12:19,390 a little bit better, especially since these cells, 210 00:12:19,390 --> 00:12:22,310 even at any given point in the visual cortex, 211 00:12:22,310 --> 00:12:23,755 come in several different sizes. 212 00:12:24,940 --> 00:12:26,970 So that's an alternate theory. 213 00:12:26,970 --> 00:12:28,780 But still, further research needs 214 00:12:28,780 --> 00:12:34,980 to be done to be able to verify whether this theory is more 215 00:12:34,980 --> 00:12:41,040 valid than the center-surround theory in the retina, which 216 00:12:41,040 --> 00:12:43,390 certainly is totally wrong. 217 00:12:43,390 --> 00:12:47,200 So I think maybe in another 100 years, 218 00:12:47,200 --> 00:12:50,280 they're going to drop the Baumgartner theory out 219 00:12:50,280 --> 00:12:58,190 from textbooks and general reviews of illusions 220 00:12:58,190 --> 00:12:59,150 and vision. 221 00:12:59,150 --> 00:12:59,650 OK. 222 00:12:59,650 --> 00:13:03,560 So now I want to show you one more illusory effect, which 223 00:13:03,560 --> 00:13:13,370 is a variant of the illusion that I have just shown you. 224 00:13:13,370 --> 00:13:17,180 And this one is something that can drive you nuts, I suppose. 225 00:13:17,180 --> 00:13:23,970 And that appears on the second front page, the second sheet, 226 00:13:23,970 --> 00:13:26,330 I should say, of the handout. 227 00:13:26,330 --> 00:13:29,100 You can look at it there, or you can look at it here. 228 00:13:29,100 --> 00:13:34,020 And you can see this scintillating effect, 229 00:13:34,020 --> 00:13:38,380 and therefore it has been called the scintillating grid 230 00:13:38,380 --> 00:13:39,310 illusion. 231 00:13:39,310 --> 00:13:41,320 It was invented by fellow called Lingelbach. 232 00:13:42,340 --> 00:13:43,080 All right? 233 00:13:43,080 --> 00:13:46,450 That's a very strong illusory effect, very dramatic. 234 00:13:46,450 --> 00:13:49,360 And whether this can be explained 235 00:13:49,360 --> 00:13:54,070 by any particular theory about the visual cortex 236 00:13:54,070 --> 00:13:55,810 still remains to be settled. 237 00:13:55,810 --> 00:14:00,450 Right now I don't think there's a specific theory that 238 00:14:00,450 --> 00:14:03,730 can explain this. 239 00:14:03,730 --> 00:14:08,380 Now, I will now move on and talk about yet 240 00:14:08,380 --> 00:14:12,740 another illusory effect, or actually a set of them, 241 00:14:12,740 --> 00:14:15,960 that has a much better explanation. 242 00:14:15,960 --> 00:14:18,080 This one we don't have a handout for. 243 00:14:18,080 --> 00:14:20,630 If you look at this display here, 244 00:14:20,630 --> 00:14:24,560 you'll agree that the left and right bars here are identical, 245 00:14:24,560 --> 00:14:27,740 right, going in shade from light to dark. 246 00:14:27,740 --> 00:14:28,330 All right? 247 00:14:28,330 --> 00:14:32,520 So now what I'm going to do here-- what I want to do 248 00:14:32,520 --> 00:14:35,170 is to present these two images here. 249 00:14:35,170 --> 00:14:37,730 I want you to fixate in the fixation spot, 250 00:14:37,730 --> 00:14:41,230 and we'll count to about, I don't know, 20, say. 251 00:14:41,230 --> 00:14:42,570 Keep fixating. 252 00:14:42,570 --> 00:14:45,300 1, 2, 3, 4, 5, 6, 7, 8, 9, 10. 253 00:14:45,300 --> 00:14:47,640 1, 2, 3, 4, 5, 6, 7, 8, 9, 20. 254 00:14:47,640 --> 00:14:49,390 And then I'm going to go back here, 255 00:14:49,390 --> 00:14:51,890 and you can see that this side is much darker than that one. 256 00:14:51,890 --> 00:14:53,105 Everybody got that effect? 257 00:14:53,105 --> 00:14:53,580 AUDIENCE: Yeah. 258 00:14:53,580 --> 00:14:54,913 PROFESSOR: I'll do it once more. 259 00:14:59,050 --> 00:15:00,401 Pretty dramatic, huh? 260 00:15:00,401 --> 00:15:00,900 OK. 261 00:15:00,900 --> 00:15:02,524 Now I'll tell you what I'm going to do. 262 00:15:04,140 --> 00:15:08,670 What I want you to do is to look at this with one eye. 263 00:15:08,670 --> 00:15:09,290 OK? 264 00:15:09,290 --> 00:15:10,500 Don't look at it yet. 265 00:15:10,500 --> 00:15:11,620 Wait a while. 266 00:15:11,620 --> 00:15:12,950 Just let your eye get settled. 267 00:15:14,180 --> 00:15:16,000 You're going to look at it with one eye. 268 00:15:16,000 --> 00:15:19,500 And then when I switch over, I want you to switch eyes. 269 00:15:19,500 --> 00:15:20,000 OK? 270 00:15:21,062 --> 00:15:21,770 All right, ready? 271 00:15:21,770 --> 00:15:25,520 Go ahead now and fixate, and we'll count to 20 again. 272 00:15:25,520 --> 00:15:27,020 5, 6, 7, 8, 9, 10. 273 00:15:27,020 --> 00:15:29,655 1, 2, 3, 4, 5, 6, 7, 8, 9, 20. 274 00:15:29,655 --> 00:15:30,155 Switch. 275 00:15:31,860 --> 00:15:34,500 And if you do that, you don't have an effect. 276 00:15:34,500 --> 00:15:35,000 OK? 277 00:15:35,000 --> 00:15:36,760 So what does that prove? 278 00:15:36,760 --> 00:15:38,846 Well, that obviously proves-- that's why 279 00:15:38,846 --> 00:15:41,780 it's nice to do these little, itty-bitty experiments one can 280 00:15:41,780 --> 00:15:46,490 do in minutes-- it proves that this is a retinal effect. 281 00:15:46,490 --> 00:15:50,020 And the effect is actually due to adaptation 282 00:15:50,020 --> 00:15:55,020 that we have talked about, when was it, a couple of times ago. 283 00:15:55,020 --> 00:15:58,140 Now, I want to show you yet another effect 284 00:15:58,140 --> 00:16:01,440 that has a similar arrangement. 285 00:16:01,440 --> 00:16:06,920 What I'm going to do here is I'm going to put this display on. 286 00:16:06,920 --> 00:16:09,080 And what I'm going to do here is I'm 287 00:16:09,080 --> 00:16:11,040 going to turn these off successively 288 00:16:11,040 --> 00:16:12,780 as we go around the clock. 289 00:16:12,780 --> 00:16:13,440 OK? 290 00:16:13,440 --> 00:16:15,420 And what I want you to do, again, 291 00:16:15,420 --> 00:16:18,660 is to fixate here on the cross, OK, 292 00:16:18,660 --> 00:16:21,490 and then I'm going to start running it. 293 00:16:21,490 --> 00:16:23,740 And what are you going to see? 294 00:16:23,740 --> 00:16:26,334 A pretty amazing thing you're going to see. 295 00:16:26,334 --> 00:16:27,000 What do you see? 296 00:16:27,000 --> 00:16:27,916 AUDIENCE: [INAUDIBLE]. 297 00:16:29,410 --> 00:16:30,740 PROFESSOR: Yeah? 298 00:16:30,740 --> 00:16:33,080 You see a green spot going around? 299 00:16:33,080 --> 00:16:35,061 Everybody see that? 300 00:16:35,061 --> 00:16:35,560 OK. 301 00:16:35,560 --> 00:16:39,650 So now the big question comes up, why on earth 302 00:16:39,650 --> 00:16:41,715 do we see this green spot? 303 00:16:41,715 --> 00:16:42,715 What is the explanation? 304 00:16:44,160 --> 00:16:47,240 Well, this again is a retinal effect. 305 00:16:47,240 --> 00:16:53,410 And to test this in more detail, we 306 00:16:53,410 --> 00:16:57,780 have to go back to what I told you about color vision. 307 00:16:57,780 --> 00:16:59,960 And when I told you about color vision-- 308 00:16:59,960 --> 00:17:04,349 if you remember we did that experiment with Gaussians-- 309 00:17:04,349 --> 00:17:10,220 that if you look at the color circle, all right, and then 310 00:17:10,220 --> 00:17:15,710 you adapt to a stimulus here, which is pretty similar to what 311 00:17:15,710 --> 00:17:18,810 was on the screen, then the afterimage 312 00:17:18,810 --> 00:17:24,740 is on the opposite side cutting across the color circle. 313 00:17:24,740 --> 00:17:30,490 So the afterimages that you see that are in color 314 00:17:30,490 --> 00:17:33,770 can be perfectly explained by the color circle. 315 00:17:33,770 --> 00:17:35,830 If you adapt to this, this is the afterimage. 316 00:17:35,830 --> 00:17:38,420 You adapt to this, this is the afterimage. 317 00:17:38,420 --> 00:17:44,900 It's always straight across the center line, center point, 318 00:17:44,900 --> 00:17:49,780 I should say, where you have the white center, which perfectly 319 00:17:49,780 --> 00:17:53,620 explains your afterimage. 320 00:17:53,620 --> 00:17:57,075 So what I'll do again, I'll show this to you just once more. 321 00:18:00,750 --> 00:18:02,720 OK, here we go, ready? 322 00:18:02,720 --> 00:18:07,360 You can experience it again just to sort of get a sense. 323 00:18:07,360 --> 00:18:09,690 It's amazing how the green circle 324 00:18:09,690 --> 00:18:11,639 becomes more and more and more and more vivid. 325 00:18:11,639 --> 00:18:13,180 Now, one more thing I'm going to tell 326 00:18:13,180 --> 00:18:16,020 you is when I turn this off-- keep fixating-- 327 00:18:16,020 --> 00:18:18,960 and then you're going to see a green afterimage, right? 328 00:18:18,960 --> 00:18:23,270 So it proves that, indeed, it's the afterimage that 329 00:18:23,270 --> 00:18:26,860 appears over a period of time with that rotating circle, 330 00:18:26,860 --> 00:18:30,195 and it's fully explained by the color circle. 331 00:18:31,701 --> 00:18:32,200 All right. 332 00:18:32,200 --> 00:18:35,350 So now we're going to move on, and you 333 00:18:35,350 --> 00:18:37,500 look at yet another illusory effect. 334 00:18:37,500 --> 00:18:41,080 If you look at this display, what do you see here? 335 00:18:41,080 --> 00:18:44,980 You see a light square and a dark square, right? 336 00:18:44,980 --> 00:18:45,990 Everybody see that? 337 00:18:47,350 --> 00:18:51,490 Now, what if I told you that what you're seeing 338 00:18:51,490 --> 00:18:53,696 is a total misconception? 339 00:18:54,567 --> 00:18:56,150 So I'll tell you what I'm going to do. 340 00:18:56,150 --> 00:18:59,160 I'm going to leave these two squares exactly as they are, 341 00:18:59,160 --> 00:19:00,860 but I'm going to change the background. 342 00:19:00,860 --> 00:19:01,360 Ready? 343 00:19:03,386 --> 00:19:04,510 What's the difference here? 344 00:19:05,580 --> 00:19:10,110 The difference is that now the background is uniform, right? 345 00:19:10,110 --> 00:19:12,270 And you can see these two squares are indeed 346 00:19:12,270 --> 00:19:14,530 identical in contrast, right? 347 00:19:14,530 --> 00:19:17,081 But now if I go back to the previous one, 348 00:19:17,081 --> 00:19:18,580 if you look at it more carefully you 349 00:19:18,580 --> 00:19:20,230 can see this goes from light to dark. 350 00:19:21,290 --> 00:19:23,650 Of course, the projector makes this look bluish. 351 00:19:23,650 --> 00:19:25,870 It shouldn't be, but that's OK. 352 00:19:25,870 --> 00:19:30,470 And it's a gradual shift in the illumination level 353 00:19:30,470 --> 00:19:35,320 that fools you to think that one is light, the other is dark. 354 00:19:35,320 --> 00:19:38,050 And that has to do heavily with the fact 355 00:19:38,050 --> 00:19:41,780 that the receptive fields of cells in the retina 356 00:19:41,780 --> 00:19:44,210 have center-surround organization, 357 00:19:44,210 --> 00:19:47,740 and they're sensitive to local contrast differences. 358 00:19:47,740 --> 00:19:48,880 That's their make-up. 359 00:19:48,880 --> 00:19:51,870 And so until I point it out to you, 360 00:19:51,870 --> 00:19:55,960 can't even tell that this is a gradual shift. 361 00:19:55,960 --> 00:19:58,540 And if it did not change in color, 362 00:19:58,540 --> 00:20:00,850 it would be harder to tell. 363 00:20:00,850 --> 00:20:02,600 And it has to do with the fact that you're 364 00:20:02,600 --> 00:20:04,900 making local comparisons. 365 00:20:04,900 --> 00:20:09,240 And because of that, to make a distant comparison 366 00:20:09,240 --> 00:20:11,300 you are inaccurate about it. 367 00:20:11,300 --> 00:20:14,680 So this illusory effect, again, can be readily explained 368 00:20:14,680 --> 00:20:18,410 on the basis of what we know about the center-surround 369 00:20:18,410 --> 00:20:21,951 organization of receptive fields in the retina. 370 00:20:21,951 --> 00:20:22,450 OK. 371 00:20:22,450 --> 00:20:26,780 Now, if you look at the same thing in color, which 372 00:20:26,780 --> 00:20:30,450 doesn't come up too well here because of the projector, 373 00:20:30,450 --> 00:20:34,270 but still you can tell that these are not the same color, 374 00:20:34,270 --> 00:20:37,160 OK, again because of the gradual shift, 375 00:20:37,160 --> 00:20:38,830 color shift, in the background. 376 00:20:38,830 --> 00:20:43,190 But exactly identical they are, as they had been before. 377 00:20:43,190 --> 00:20:46,660 So this works not only for contrast, 378 00:20:46,660 --> 00:20:48,750 but it also works for color. 379 00:20:48,750 --> 00:20:53,441 Now, some illusory effects don't work for color and some do. 380 00:20:53,441 --> 00:20:53,940 All right. 381 00:20:53,940 --> 00:20:58,460 So now yet another related effect 382 00:20:58,460 --> 00:21:00,290 that I'm sure some of you have seen 383 00:21:00,290 --> 00:21:03,870 before is if you look at this circle, 384 00:21:03,870 --> 00:21:06,310 we agree that it's uniform in color. 385 00:21:06,310 --> 00:21:11,119 Now, if I cover half of it with dark and the other with light, 386 00:21:11,119 --> 00:21:12,410 it still looks pretty the same. 387 00:21:12,410 --> 00:21:14,410 Maybe it's a little darker here than here. 388 00:21:14,410 --> 00:21:18,880 But then if I put a partition here, as you can see here, 389 00:21:18,880 --> 00:21:22,750 then this side definitely looks darker than that side. 390 00:21:22,750 --> 00:21:23,820 OK? 391 00:21:23,820 --> 00:21:26,430 So this, again, looks at contrast effects, 392 00:21:26,430 --> 00:21:30,577 and we'll talk about these contrasts in just a bit 393 00:21:30,577 --> 00:21:31,285 with more detail. 394 00:21:32,420 --> 00:21:36,060 Next, let me turn to another illusory effect 395 00:21:36,060 --> 00:21:38,840 that you can see in many, many textbooks, which 396 00:21:38,840 --> 00:21:40,960 is called the wallpaper illusion. 397 00:21:40,960 --> 00:21:44,500 If you look at this-- I'll come to it 398 00:21:44,500 --> 00:21:49,780 in a second in your display, where it will appear 399 00:21:49,780 --> 00:21:55,250 on the back of the second page-- what you see here, 400 00:21:55,250 --> 00:21:59,770 it looks like that these are not parallel lines here. 401 00:21:59,770 --> 00:22:03,300 I shouldn't say lines, but parallel 402 00:22:03,300 --> 00:22:05,480 black and white squares. 403 00:22:05,480 --> 00:22:08,810 It looks like that they're going like back and forth like that, 404 00:22:08,810 --> 00:22:10,640 OK, and it's quite dramatic. 405 00:22:10,640 --> 00:22:13,710 And so now we can ask the question, how can we see this? 406 00:22:13,710 --> 00:22:15,650 What is this due to? 407 00:22:15,650 --> 00:22:18,582 It could be due to the way they're aligned. 408 00:22:18,582 --> 00:22:20,165 It could be due to the fact that there 409 00:22:20,165 --> 00:22:22,670 is space between them, that gray space. 410 00:22:22,670 --> 00:22:26,570 And so one can do experiments like the one 411 00:22:26,570 --> 00:22:29,930 you have on your next page there. 412 00:22:29,930 --> 00:22:32,010 Let me first-- I will prove that, indeed, we 413 00:22:32,010 --> 00:22:33,250 have parallel lines here. 414 00:22:33,250 --> 00:22:36,080 These are fully parallel lines, and they go back and forth. 415 00:22:36,080 --> 00:22:39,900 To convince you, you can see that the lines are identical, 416 00:22:39,900 --> 00:22:43,110 and yet they do not look parallel 417 00:22:43,110 --> 00:22:45,881 at all because of this illusory effect. 418 00:22:45,881 --> 00:22:46,380 OK. 419 00:22:46,380 --> 00:22:50,882 So now what we can do is we can fiddle around with this a bit. 420 00:22:50,882 --> 00:22:53,090 Now, that's the kind of thing you guys would probably 421 00:22:53,090 --> 00:22:55,360 enjoy doing yourselves-- look at various illusions 422 00:22:55,360 --> 00:22:57,110 and start playing around with them. 423 00:22:57,110 --> 00:22:58,120 This is an example. 424 00:22:58,120 --> 00:23:00,620 This is a basic wallpaper illusion. 425 00:23:00,620 --> 00:23:03,640 And now you turn this into an equal checkerboard, OK? 426 00:23:03,640 --> 00:23:07,280 And if you keep the lines in between, 427 00:23:07,280 --> 00:23:08,720 and you get no effect whatsoever. 428 00:23:09,830 --> 00:23:13,080 And then here we eliminate those gray bars, 429 00:23:13,080 --> 00:23:15,760 and the illusory effect is greatly reduced. 430 00:23:15,760 --> 00:23:18,610 So consequently, the illusory effect 431 00:23:18,610 --> 00:23:21,150 has to do with both the fact that you 432 00:23:21,150 --> 00:23:25,470 have space between the alternating black and white 433 00:23:25,470 --> 00:23:29,110 bars, meaning these gray lines here, 434 00:23:29,110 --> 00:23:34,390 and that you have them offset. 435 00:23:34,390 --> 00:23:36,605 So when it's not offset, you don't get it. 436 00:23:36,605 --> 00:23:38,730 And when you don't have the bars, you don't get it. 437 00:23:38,730 --> 00:23:42,590 So these two factors play an important role 438 00:23:42,590 --> 00:23:46,340 in giving rise to the illusory effect. 439 00:23:46,340 --> 00:23:51,370 Now, what we also can examine is what happens at isoluminance. 440 00:23:52,590 --> 00:23:55,030 And in this case, once again, what happens 441 00:23:55,030 --> 00:23:57,470 is that the illusory effect is still there, 442 00:23:57,470 --> 00:24:00,960 but it is reduced under isoluminant conditions. 443 00:24:00,960 --> 00:24:03,230 And this is not the isoluminant actually 444 00:24:03,230 --> 00:24:04,730 because of the projector. 445 00:24:04,730 --> 00:24:06,640 But when it's perfectly isoluminant, 446 00:24:06,640 --> 00:24:09,500 then the illusory effect is largely lost. 447 00:24:10,780 --> 00:24:11,280 All right. 448 00:24:11,280 --> 00:24:15,510 So now we have a variant of this illusion which is fascinating. 449 00:24:15,510 --> 00:24:22,300 And this one I will show you on the next page, OK? 450 00:24:22,300 --> 00:24:25,350 And if you look at just this display, 451 00:24:25,350 --> 00:24:29,200 and you are convinced that we have lines that are alternating 452 00:24:29,200 --> 00:24:32,000 that are filtered back and forth, right? 453 00:24:33,770 --> 00:24:35,760 Well, is that really true? 454 00:24:35,760 --> 00:24:41,320 Well, it turns out if you take a bunch of red lines here 455 00:24:41,320 --> 00:24:43,820 that are perfectly parallel, OK, and then you 456 00:24:43,820 --> 00:24:47,720 superimpose those on the display, 457 00:24:47,720 --> 00:24:51,220 you can see that actually the display is parallel. 458 00:24:51,220 --> 00:24:52,210 OK? 459 00:24:52,210 --> 00:24:54,650 The reason it doesn't look parallel 460 00:24:54,650 --> 00:24:59,795 is due to the way these individual little pieces 461 00:24:59,795 --> 00:25:03,270 that you're not even conscious of are arranged. 462 00:25:03,270 --> 00:25:04,945 So let's look at that in more detail. 463 00:25:06,520 --> 00:25:08,400 Let me add here one more thing. 464 00:25:08,400 --> 00:25:11,300 At isoluminance, you get practically no effect. 465 00:25:11,300 --> 00:25:13,790 So these illusions, this one and the previous one, 466 00:25:13,790 --> 00:25:17,200 heavily depend on contrast differences. 467 00:25:17,200 --> 00:25:17,700 OK. 468 00:25:17,700 --> 00:25:18,780 So here we go. 469 00:25:18,780 --> 00:25:20,710 Here's the basic illusory effect. 470 00:25:20,710 --> 00:25:24,820 And the way this was constructed is shown enlarged on top. 471 00:25:24,820 --> 00:25:31,460 Alternate lines have 1, 2, 1, 2, 1, 2, 1. 472 00:25:31,460 --> 00:25:32,060 OK? 473 00:25:32,060 --> 00:25:36,810 What you have in the top line, and the third, and so on, 474 00:25:36,810 --> 00:25:38,320 you have it in this orientation. 475 00:25:38,320 --> 00:25:40,220 Here you have them reversed. 476 00:25:40,220 --> 00:25:44,560 And that vague cue is what somehow 477 00:25:44,560 --> 00:25:47,080 provides the illusory effect for you. 478 00:25:47,080 --> 00:25:51,090 Now, if you make all the lines similar, 479 00:25:51,090 --> 00:25:54,610 OK, in other words don't reverse the alternate lines, 480 00:25:54,610 --> 00:25:58,750 then what you see here is actually 481 00:25:58,750 --> 00:26:01,190 the bars are all tilted slightly upward. 482 00:26:01,190 --> 00:26:01,970 OK? 483 00:26:01,970 --> 00:26:03,690 And because of that, the illusory effect 484 00:26:03,690 --> 00:26:06,830 is not as dramatic as this one, but you still have it. 485 00:26:06,830 --> 00:26:07,590 All right? 486 00:26:07,590 --> 00:26:14,090 So what then you can do with this 487 00:26:14,090 --> 00:26:19,090 is to enlarge on this effect as shown here. 488 00:26:19,090 --> 00:26:23,515 It looks like that you have somehow a very unequal set 489 00:26:23,515 --> 00:26:30,860 of lines, as a result of which you do not 490 00:26:30,860 --> 00:26:33,140 have a bunch of clear-cut squares. 491 00:26:33,140 --> 00:26:37,380 And if you, again, superimpose true parallel lines through it, 492 00:26:37,380 --> 00:26:40,010 you see that they are actually parallel. 493 00:26:40,010 --> 00:26:40,510 All right. 494 00:26:40,510 --> 00:26:46,060 So that then explains that illusory effect 495 00:26:46,060 --> 00:26:51,760 in terms of how we somehow integrate information of shape 496 00:26:51,760 --> 00:26:55,790 that we are not even aware of to have 497 00:26:55,790 --> 00:26:57,985 a perception of a whole thing. 498 00:26:57,985 --> 00:26:59,610 Now, that's not much of an explanation. 499 00:26:59,610 --> 00:27:02,194 It doesn't tell you what neurons are involved 500 00:27:02,194 --> 00:27:03,110 or anything like that. 501 00:27:03,110 --> 00:27:05,350 But it gives you a general concept 502 00:27:05,350 --> 00:27:07,730 of how that illusory effect is created. 503 00:27:12,020 --> 00:27:17,800 Now we come to yet another fascinating illusory effect, 504 00:27:17,800 --> 00:27:23,660 and that one is in the next few pages in the handout. 505 00:27:23,660 --> 00:27:28,250 So if you take the backside of page 3, 506 00:27:28,250 --> 00:27:32,010 you will see this illusory effect here. 507 00:27:32,010 --> 00:27:33,470 Does everybody see that? 508 00:27:33,470 --> 00:27:38,100 Now, this illusory effect is one which gives you a sense 509 00:27:38,100 --> 00:27:39,950 that you have a whole bunch of swirls. 510 00:27:39,950 --> 00:27:40,942 All right? 511 00:27:40,942 --> 00:27:42,275 Does everybody see those swirls? 512 00:27:44,310 --> 00:27:49,780 Instead of seeing 1, 2, 3, 4 circles, you see some swirls. 513 00:27:49,780 --> 00:27:54,530 And you say what on earth is giving us that sensation? 514 00:27:54,530 --> 00:27:58,670 So what you want to do then is to look at it analytically. 515 00:27:58,670 --> 00:28:00,780 And what you see here is you have 516 00:28:00,780 --> 00:28:04,870 alternating black and white squares, first of all. 517 00:28:04,870 --> 00:28:06,880 And you say, well, is alternating 518 00:28:06,880 --> 00:28:10,170 black and white squares the central point? 519 00:28:10,170 --> 00:28:12,550 But then if you look at it more closely, 520 00:28:12,550 --> 00:28:14,810 you can see this is-- I don't know how they came up 521 00:28:14,810 --> 00:28:17,950 with this one-- what you see here, if you look at each 522 00:28:17,950 --> 00:28:20,440 of these, that there's a gradual shift as you go 523 00:28:20,440 --> 00:28:25,440 around the circle in the orientation of the square. 524 00:28:25,440 --> 00:28:27,550 So this square becomes progressively, 525 00:28:27,550 --> 00:28:31,420 very slowly, more rotated. 526 00:28:31,420 --> 00:28:32,050 OK? 527 00:28:32,050 --> 00:28:33,799 And that goes around the clock, and that's 528 00:28:33,799 --> 00:28:35,715 true for every one of those circles. 529 00:28:35,715 --> 00:28:42,740 And you can say, well, is it due to the black and white squares? 530 00:28:42,740 --> 00:28:46,310 Is it due to the change in the orientation of the squares 531 00:28:46,310 --> 00:28:48,230 that you're not even aware of? 532 00:28:48,230 --> 00:28:49,050 Or what? 533 00:28:49,050 --> 00:28:50,880 So what kind of experiment do you 534 00:28:50,880 --> 00:28:54,430 do to check this out, to come up with some understanding? 535 00:28:54,430 --> 00:28:56,820 What kind of game can you play? 536 00:28:56,820 --> 00:28:58,970 Well, the first game you can play, 537 00:28:58,970 --> 00:29:00,840 so you can say, well, let's eliminate 538 00:29:00,840 --> 00:29:04,130 these slight changes in orientation. 539 00:29:05,280 --> 00:29:05,910 OK? 540 00:29:05,910 --> 00:29:10,150 And so what you can do instead, as shown on the next page, 541 00:29:10,150 --> 00:29:15,070 is that you have a bunch of circles, 542 00:29:15,070 --> 00:29:18,290 black and white circles, which are all the same. 543 00:29:18,290 --> 00:29:21,110 And as you can see here, there's no swirls. 544 00:29:21,110 --> 00:29:26,000 You have four circles, very clear. 545 00:29:26,000 --> 00:29:29,670 So therefore, obviously, the squares, 546 00:29:29,670 --> 00:29:32,670 changing their orientation in a subtle fashion going around 547 00:29:32,670 --> 00:29:36,120 in the clock, is very important in introducing 548 00:29:36,120 --> 00:29:37,600 this illusory effect. 549 00:29:37,600 --> 00:29:42,050 Well, then you can say, OK, but is that all of it? 550 00:29:42,050 --> 00:29:45,130 And so what we can do here, as shown on the next page, 551 00:29:45,130 --> 00:29:48,300 is we can maintain the orientation shift, 552 00:29:48,300 --> 00:29:55,820 but we can make them all the same contrast, as we have here. 553 00:29:55,820 --> 00:29:58,700 And when you do that, you have have a rather weak illusory 554 00:29:58,700 --> 00:29:59,200 effect. 555 00:29:59,200 --> 00:30:02,720 There's still a little bit left maybe, but it's kind of weak. 556 00:30:02,720 --> 00:30:08,360 And then if you instead make it isoluminant, 557 00:30:08,360 --> 00:30:10,890 you get some in-between effect. 558 00:30:10,890 --> 00:30:16,170 So on the basis of that, we can conclude 559 00:30:16,170 --> 00:30:19,920 that very important in this display 560 00:30:19,920 --> 00:30:24,420 is that you have a gradual shift in the orientations 561 00:30:24,420 --> 00:30:26,220 of these squares. 562 00:30:26,220 --> 00:30:30,160 And that, in turn, suggests that somehow the way these squares 563 00:30:30,160 --> 00:30:36,230 activate orientation selective cells in V1 564 00:30:36,230 --> 00:30:38,670 plays an important role in giving rise 565 00:30:38,670 --> 00:30:40,775 to this strange illusory effect. 566 00:30:42,700 --> 00:30:43,200 OK. 567 00:30:43,200 --> 00:30:45,000 So now what we are going to do is 568 00:30:45,000 --> 00:30:48,090 to look at yet another illusory effect, which 569 00:30:48,090 --> 00:30:50,690 has a different source, and that one 570 00:30:50,690 --> 00:30:53,390 is the famous Muller-Lyer illusion. 571 00:30:53,390 --> 00:30:56,260 The fact is that these two bars here 572 00:30:56,260 --> 00:31:00,260 are identical in size-- do I have the picture-- like that. 573 00:31:00,260 --> 00:31:01,370 OK? 574 00:31:01,370 --> 00:31:05,420 But when you put these inducing elements, as we will call them, 575 00:31:05,420 --> 00:31:10,000 OK, on there, you'll swear that this one is longer 576 00:31:10,000 --> 00:31:11,110 than this one is. 577 00:31:11,110 --> 00:31:12,020 OK? 578 00:31:12,020 --> 00:31:16,040 So now we can ask the question, what kind of manipulations 579 00:31:16,040 --> 00:31:21,480 can we undertake to see what that is all about? 580 00:31:21,480 --> 00:31:24,060 And let me come back to that in a minute. 581 00:31:24,060 --> 00:31:26,090 So what you can do here is to make 582 00:31:26,090 --> 00:31:30,970 the inducing elements and the bars different in color 583 00:31:30,970 --> 00:31:35,510 or in contrast, and you still get a strong illusory effect. 584 00:31:35,510 --> 00:31:36,010 OK? 585 00:31:38,100 --> 00:31:42,350 So making them isoluminant or making them 586 00:31:42,350 --> 00:31:46,430 opposite in contrast does not eliminate this illusion. 587 00:31:46,430 --> 00:31:49,140 So the theory that somebody has come up with 588 00:31:49,140 --> 00:31:53,020 is shown here, which I'm not sure I buy, 589 00:31:53,020 --> 00:31:56,170 but that's nevertheless a theory, 590 00:31:56,170 --> 00:31:59,550 it claims that the brain interprets 591 00:31:59,550 --> 00:32:03,120 these two displays as differences in depth, 592 00:32:03,120 --> 00:32:08,910 as if here we had a protruding edge in a building inside 593 00:32:08,910 --> 00:32:12,210 or a receding edge as shown here. 594 00:32:12,210 --> 00:32:16,220 And that difference that we almost unconsciously, 595 00:32:16,220 --> 00:32:19,440 if you will, interpret as being in different depths 596 00:32:19,440 --> 00:32:22,200 is what gives rise to the illusion 597 00:32:22,200 --> 00:32:23,690 according to this theory. 598 00:32:24,980 --> 00:32:27,090 Of course, there's a simpler theory simply saying 599 00:32:27,090 --> 00:32:30,510 that we just integrate these two bits of information. 600 00:32:30,510 --> 00:32:31,010 All right. 601 00:32:31,010 --> 00:32:36,020 So now we can ask the question, if you have illusory effects 602 00:32:36,020 --> 00:32:40,070 of this sort, is this something that really takes place higher 603 00:32:40,070 --> 00:32:44,040 up in the brain, or does it take place in the retina where we 604 00:32:44,040 --> 00:32:49,910 cannot separate these inducing and testing elements? 605 00:32:49,910 --> 00:32:51,945 Well, the way you can do this-- I'm 606 00:32:51,945 --> 00:32:54,330 going to show you another illusion which 607 00:32:54,330 --> 00:32:55,450 is similar to this. 608 00:32:56,810 --> 00:32:59,520 Here we have the so-called Ebbinghaus illusion. 609 00:33:00,570 --> 00:33:02,800 This is a very similar kind of illusion. 610 00:33:02,800 --> 00:33:05,450 This is a test element and an inducing element. 611 00:33:05,450 --> 00:33:09,594 The test element here are these two circles, 612 00:33:09,594 --> 00:33:11,260 and you swear that this one is a smaller 613 00:33:11,260 --> 00:33:13,580 circle than that one is, right? 614 00:33:13,580 --> 00:33:15,590 And we have a bunch of small circles, 615 00:33:15,590 --> 00:33:17,630 and here we have bunch of large circles. 616 00:33:17,630 --> 00:33:19,920 So now we can ask, well, is this something 617 00:33:19,920 --> 00:33:22,280 that takes place in the retina or what? 618 00:33:22,280 --> 00:33:25,055 So what kind of experiment can you do? 619 00:33:25,055 --> 00:33:27,480 Well, the experiment that you can do 620 00:33:27,480 --> 00:33:30,045 is to present these displays interocularly. 621 00:33:31,870 --> 00:33:32,975 What does that mean? 622 00:33:32,975 --> 00:33:35,780 What that means is that you present this 623 00:33:35,780 --> 00:33:38,470 to the left eye and this to the right eye. 624 00:33:38,470 --> 00:33:41,430 And you present this device that's 625 00:33:41,430 --> 00:33:43,630 used by psychologists for this kind of thing. 626 00:33:43,630 --> 00:33:45,190 It's called a tachistoscope. 627 00:33:45,190 --> 00:33:46,120 You look through it. 628 00:33:46,120 --> 00:33:49,130 It can present things separately through the two eyes, 629 00:33:49,130 --> 00:33:51,190 and you can flash them on briefly 630 00:33:51,190 --> 00:33:54,120 so you don't get any binocular rivalry. 631 00:33:54,120 --> 00:33:56,660 And when you do that, then you can systematically 632 00:33:56,660 --> 00:34:01,080 vary the size of these to make sure 633 00:34:01,080 --> 00:34:04,150 that you can get a quantitative measure of it. 634 00:34:04,150 --> 00:34:06,860 And when you do that, you find that the illusory effect 635 00:34:06,860 --> 00:34:10,620 is just as good under interocular conditions 636 00:34:10,620 --> 00:34:13,430 as it is under binocular viewing conditions. 637 00:34:14,659 --> 00:34:16,469 Because of this then-- this has been 638 00:34:16,469 --> 00:34:18,790 done with many kinds of illusions 639 00:34:18,790 --> 00:34:20,580 including the Muller-Lyer one. 640 00:34:20,580 --> 00:34:24,580 And when you do that, you show, in essence, 641 00:34:24,580 --> 00:34:27,260 that this is an effect that takes place 642 00:34:27,260 --> 00:34:29,270 most likely in the cortex, certainly 643 00:34:29,270 --> 00:34:32,350 not in the retina or the lateral geniculate nucleus. 644 00:34:32,350 --> 00:34:34,639 It takes place someplace where the images 645 00:34:34,639 --> 00:34:36,210 from the left and two eyes converge. 646 00:34:37,290 --> 00:34:37,790 All right. 647 00:34:37,790 --> 00:34:43,139 So now yet another set of illusions 648 00:34:43,139 --> 00:34:45,679 that I want to show you. 649 00:34:47,230 --> 00:34:51,085 I think that these illusory effects are done. 650 00:34:52,280 --> 00:34:55,710 You can take this home and enjoy it or show it to your friends. 651 00:34:55,710 --> 00:34:59,200 Now I'm going to show you a set of just two 652 00:34:59,200 --> 00:35:01,140 kinds of motion illusions. 653 00:35:01,140 --> 00:35:02,590 OK, here's the first one. 654 00:35:03,910 --> 00:35:05,930 I want you to look at the screen, 655 00:35:05,930 --> 00:35:08,410 and I'm going to flash something on briefly, 656 00:35:08,410 --> 00:35:09,850 and I'm going to do it repeatedly. 657 00:35:09,850 --> 00:35:10,350 Ready? 658 00:35:14,420 --> 00:35:15,644 What do you see? 659 00:35:15,644 --> 00:35:16,810 Do you see something moving? 660 00:35:21,150 --> 00:35:24,370 A very brief sense of motion that you see. 661 00:35:24,370 --> 00:35:26,600 This twirls a little bit. 662 00:35:26,600 --> 00:35:31,480 And this is called the snake illusion, 663 00:35:31,480 --> 00:35:32,805 which is really curious. 664 00:35:34,540 --> 00:35:39,270 This has, again, appeared in quite a number of textbooks, 665 00:35:39,270 --> 00:35:41,900 and we do not have a really good explanation for it. 666 00:35:41,900 --> 00:35:45,720 But we have another one, which I think is related. 667 00:35:45,720 --> 00:35:46,220 OK. 668 00:35:46,220 --> 00:35:48,330 So what I'm going to do here, this 669 00:35:48,330 --> 00:35:51,150 is called the waterfall illusion. 670 00:35:51,150 --> 00:35:54,360 So what we are going to do here is 671 00:35:54,360 --> 00:35:56,230 I'm going to set this in motion. 672 00:35:56,230 --> 00:35:59,680 In this case, I don't want you to fixate. 673 00:35:59,680 --> 00:36:01,240 What I want you to do is just keep 674 00:36:01,240 --> 00:36:02,736 looking at it in various places. 675 00:36:02,736 --> 00:36:04,860 You can look here, here, here, and here, and so on. 676 00:36:04,860 --> 00:36:05,630 OK? 677 00:36:05,630 --> 00:36:07,740 And I'm going to let it run for a while, 678 00:36:07,740 --> 00:36:09,240 and then I'm going to stop it. 679 00:36:09,240 --> 00:36:09,740 OK? 680 00:36:09,740 --> 00:36:13,220 And I want you to tell me what is your aftereffect. 681 00:36:13,220 --> 00:36:13,720 Ready? 682 00:36:22,550 --> 00:36:23,050 OK. 683 00:36:23,050 --> 00:36:25,020 I'm about to stop it. 684 00:36:25,020 --> 00:36:25,754 Are you ready? 685 00:36:25,754 --> 00:36:27,170 Pay close attention to what you're 686 00:36:27,170 --> 00:36:28,545 going to perceive once it stops. 687 00:36:32,100 --> 00:36:33,630 Anybody see what happened? 688 00:36:33,630 --> 00:36:34,692 What did you see? 689 00:36:34,692 --> 00:36:36,080 AUDIENCE: [INAUDIBLE]. 690 00:36:36,080 --> 00:36:39,050 PROFESSOR: You see the water running up here, right? 691 00:36:39,050 --> 00:36:39,550 OK. 692 00:36:39,550 --> 00:36:42,550 I'll do it once more so you can appreciate it. 693 00:36:54,410 --> 00:36:55,831 Pretty nice, huh? 694 00:36:55,831 --> 00:36:56,330 OK. 695 00:36:56,330 --> 00:37:04,480 So this illusory effect is one that also occurs interocularly. 696 00:37:06,570 --> 00:37:14,210 And it has been shown that when you record from area MT, 697 00:37:14,210 --> 00:37:19,720 we talked about the fact that area MT is a region where cells 698 00:37:19,720 --> 00:37:24,360 respond selectively to direction of motion. 699 00:37:24,360 --> 00:37:26,690 Well, people have shown that if you, 700 00:37:26,690 --> 00:37:31,310 for an extended period of time, a minute maybe or whatever, 701 00:37:31,310 --> 00:37:39,590 you stimulate a cell in MT with this kind of a display, 702 00:37:39,590 --> 00:37:45,450 then after you stop it, OK, the cells with the opposite sense 703 00:37:45,450 --> 00:37:50,750 of motion will discharge some, which 704 00:37:50,750 --> 00:37:55,710 then is attributable to the upward sensation that you have. 705 00:37:55,710 --> 00:37:58,040 So it's clear enough that this is something 706 00:37:58,040 --> 00:38:01,690 that does take place predominantly by virtue 707 00:38:01,690 --> 00:38:06,140 of direction-selective cells and, probably, 708 00:38:06,140 --> 00:38:09,060 by direction-selective cells in area MT. 709 00:38:10,480 --> 00:38:10,980 All right. 710 00:38:10,980 --> 00:38:13,550 So then what we are going to do here 711 00:38:13,550 --> 00:38:15,750 is we're going to look at some other limitations 712 00:38:15,750 --> 00:38:19,080 and ambiguities in perception. 713 00:38:19,080 --> 00:38:21,190 And in particular, we're going to examine 714 00:38:21,190 --> 00:38:23,000 some figure/ground relationships. 715 00:38:24,310 --> 00:38:27,030 The first one I'm going to show you is this one. 716 00:38:27,030 --> 00:38:32,400 If you look at these two diagonals, OK, a and b, 717 00:38:32,400 --> 00:38:35,510 they certainly look very, very different, don't they? 718 00:38:35,510 --> 00:38:36,080 OK? 719 00:38:36,080 --> 00:38:38,260 One looks shorter and a different angle. 720 00:38:38,260 --> 00:38:41,857 But now if I eliminate the trapezoid in the surround, 721 00:38:41,857 --> 00:38:43,690 you can see that they're actually identical. 722 00:38:44,700 --> 00:38:45,200 OK? 723 00:38:45,200 --> 00:38:46,283 I'll show it to you again. 724 00:38:50,340 --> 00:38:51,660 Pretty dramatic. 725 00:38:51,660 --> 00:38:53,525 So that's an interesting illusory effect. 726 00:38:53,525 --> 00:38:55,380 And it's clear enough that this is something 727 00:38:55,380 --> 00:38:58,560 that takes place in higher cortical areas. 728 00:38:58,560 --> 00:39:02,430 Yet another one here that I'm going to show you also 729 00:39:02,430 --> 00:39:05,810 has something to do with figure/ground relationships, 730 00:39:05,810 --> 00:39:07,630 which has been extensively studied, 731 00:39:07,630 --> 00:39:09,640 especially by Gestalt psychologists. 732 00:39:09,640 --> 00:39:12,770 And I spoke about this a little bit the last time. 733 00:39:12,770 --> 00:39:16,890 What you see here is a bunch of vases, so to speak, right? 734 00:39:19,049 --> 00:39:19,840 That's good enough. 735 00:39:19,840 --> 00:39:21,170 So what I'm going to do now, I'm going 736 00:39:21,170 --> 00:39:22,544 to show you another set of vases. 737 00:39:22,544 --> 00:39:23,950 Are you ready? 738 00:39:23,950 --> 00:39:25,630 There is another set of vases. 739 00:39:25,630 --> 00:39:27,371 Everybody see the vases? 740 00:39:27,371 --> 00:39:28,870 Now, what if I tell you that there's 741 00:39:28,870 --> 00:39:30,380 much more to this than just vases? 742 00:39:31,440 --> 00:39:33,800 So if you look really closely, you're 743 00:39:33,800 --> 00:39:35,560 going to see something else there. 744 00:39:35,560 --> 00:39:37,017 What else do you see there? 745 00:39:37,017 --> 00:39:38,420 AUDIENCE: Faces. 746 00:39:38,420 --> 00:39:41,540 PROFESSOR: Oh, OK, two faces, two guys facing each other. 747 00:39:41,540 --> 00:39:47,520 Now, if I change the relative number of elements in here, 748 00:39:47,520 --> 00:39:50,130 which changes the figure/ground relationships, 749 00:39:50,130 --> 00:39:52,230 you get a different effect. 750 00:39:52,230 --> 00:39:54,650 Here you have-- predominantly you 751 00:39:54,650 --> 00:39:56,520 see the faces rather than the vases. 752 00:39:57,810 --> 00:39:58,620 OK? 753 00:39:58,620 --> 00:40:01,090 And that's a cute way of putting it. 754 00:40:01,090 --> 00:40:03,080 I see the faces, not the vases. 755 00:40:03,080 --> 00:40:04,580 And then you switch it, and you say, 756 00:40:04,580 --> 00:40:06,930 I see the vases, not the faces. 757 00:40:06,930 --> 00:40:07,530 All right? 758 00:40:07,530 --> 00:40:10,970 So that then is a way you can play around, again, 759 00:40:10,970 --> 00:40:14,050 with various illusory effects by fiddling around 760 00:40:14,050 --> 00:40:16,750 on your computer and creating things like that. 761 00:40:16,750 --> 00:40:17,510 OK. 762 00:40:17,510 --> 00:40:22,590 Now, yet another strange effect is 763 00:40:22,590 --> 00:40:25,330 where you can have difficulty telling what's 764 00:40:25,330 --> 00:40:27,250 what is this kind of a display. 765 00:40:27,250 --> 00:40:29,525 You say, well, what's happening here? 766 00:40:29,525 --> 00:40:31,310 Are arrows pointing to the right, 767 00:40:31,310 --> 00:40:33,370 or are they pointing to the left? 768 00:40:33,370 --> 00:40:37,570 And that reminds one of a fellow called Yogi Berra. 769 00:40:37,570 --> 00:40:40,140 How many of you have ever heard of Yogi Berra? 770 00:40:40,140 --> 00:40:41,840 Oh, my goodness, very good. 771 00:40:41,840 --> 00:40:42,340 OK. 772 00:40:42,340 --> 00:40:49,130 So he was a famous catcher for the New York Yankees, right? 773 00:40:49,130 --> 00:40:52,060 Well, he was a very interesting guy, 774 00:40:52,060 --> 00:40:54,170 had all kinds of interesting statements. 775 00:40:54,170 --> 00:40:58,010 And one of those many curious statements, one of them 776 00:40:58,010 --> 00:40:59,360 is-- it doesn't relate to this. 777 00:40:59,360 --> 00:41:00,680 It just comes into my head. 778 00:41:00,680 --> 00:41:02,810 "It isn't over until it's over." 779 00:41:02,810 --> 00:41:04,260 That's one of his, OK? 780 00:41:04,260 --> 00:41:06,080 This one here that's relevant is, 781 00:41:06,080 --> 00:41:09,163 "When you get to a fork in the road, take it." 782 00:41:09,163 --> 00:41:10,150 Yeah? 783 00:41:10,150 --> 00:41:11,810 And that's a Yogi Berra quote. 784 00:41:11,810 --> 00:41:14,290 And so what we have here is a sign that you 785 00:41:14,290 --> 00:41:17,629 come to a fork in the road, and you have to decide to take it. 786 00:41:17,629 --> 00:41:18,670 What are you going to do? 787 00:41:18,670 --> 00:41:21,253 Are you going to go this way, or are you going to go that way? 788 00:41:21,253 --> 00:41:23,090 So that's a kind of game you can play 789 00:41:23,090 --> 00:41:27,120 with this kind of strange effect that you can create 790 00:41:27,120 --> 00:41:31,490 that is confusing your ability to organize 791 00:41:31,490 --> 00:41:32,420 your visual percepts. 792 00:41:33,920 --> 00:41:37,350 Another famous one, maybe not that famous, actually. 793 00:41:37,350 --> 00:41:39,330 Maybe some of you have seen this. 794 00:41:39,330 --> 00:41:43,475 This, again, has appeared many, many years ago in The New. 795 00:41:43,475 --> 00:41:43,975 Yorker. 796 00:41:45,050 --> 00:41:47,030 It says, "I'm turning into my mother." 797 00:41:47,030 --> 00:41:48,580 Now, keep looking at it and tell me, 798 00:41:48,580 --> 00:41:51,980 why do you think that that comment is being made? 799 00:41:56,210 --> 00:42:00,730 What we have here, a young woman looking at another young woman, 800 00:42:00,730 --> 00:42:01,650 right? 801 00:42:01,650 --> 00:42:05,140 But now if you keep looking at this display, look. 802 00:42:05,140 --> 00:42:07,470 This is a nose here, and this is a mouth. 803 00:42:09,170 --> 00:42:10,450 You see an old lady now? 804 00:42:11,411 --> 00:42:11,910 OK. 805 00:42:13,190 --> 00:42:16,660 You played around with this, it gives you a confusing percept. 806 00:42:16,660 --> 00:42:18,820 And ultimately, you can see the young person 807 00:42:18,820 --> 00:42:22,840 looking that way or an old lady looking the other way. 808 00:42:22,840 --> 00:42:26,800 I mean looking not backwards but across. 809 00:42:27,942 --> 00:42:29,400 And that's why this statement says, 810 00:42:29,400 --> 00:42:33,350 "I'm turning into my mother." 811 00:42:33,350 --> 00:42:38,880 So the essence here is that playing around 812 00:42:38,880 --> 00:42:41,780 with these kinds of illusory effect is a lot of fun, 813 00:42:41,780 --> 00:42:44,940 and we all very much enjoy looking at illusions. 814 00:42:44,940 --> 00:42:47,340 And some of us like to play around with them 815 00:42:47,340 --> 00:42:52,875 to try to figure out what their existence is for the reasons. 816 00:42:52,875 --> 00:42:55,120 Now, I want to show you one more, 817 00:42:55,120 --> 00:42:57,720 which I think is very appropriate at this time. 818 00:42:57,720 --> 00:43:00,270 This is the famous Greek key motif. 819 00:43:00,270 --> 00:43:02,450 Everybody's seen this, I presume. 820 00:43:02,450 --> 00:43:05,895 Does everybody know where this is displayed, actually, 821 00:43:05,895 --> 00:43:07,265 at several places? 822 00:43:08,860 --> 00:43:10,041 In the Senate. 823 00:43:10,041 --> 00:43:10,540 OK? 824 00:43:10,540 --> 00:43:15,400 Whenever you have a picture of the Senate Building, 825 00:43:15,400 --> 00:43:16,660 you will see this. 826 00:43:16,660 --> 00:43:17,720 All right? 827 00:43:17,720 --> 00:43:20,190 Now, that's a little bit amusing, actually, 828 00:43:20,190 --> 00:43:21,350 if you think about it. 829 00:43:25,110 --> 00:43:27,460 So this has become symbolic of democracy. 830 00:43:28,930 --> 00:43:31,640 And considering just what happened most recently, 831 00:43:31,640 --> 00:43:34,210 you could almost say that, perhaps, 832 00:43:34,210 --> 00:43:39,240 what's happening here is that you cannot tell whether we see 833 00:43:39,240 --> 00:43:42,140 this guy or this guy here going around. 834 00:43:42,140 --> 00:43:42,640 OK? 835 00:43:43,457 --> 00:43:45,915 And one of them you could say, oh, these are the Democrats. 836 00:43:45,915 --> 00:43:47,730 These are the Republicans. 837 00:43:47,730 --> 00:43:51,066 And you have sort of a conflict as to which ones you see. 838 00:43:51,066 --> 00:43:51,800 All right? 839 00:43:51,800 --> 00:43:54,420 And that is, in a way, symbolic of what's going on today. 840 00:43:56,580 --> 00:43:59,120 But to educate you a little bit, which 841 00:43:59,120 --> 00:44:03,500 I suspect probably most of you know already anyway, 842 00:44:03,500 --> 00:44:09,930 this Greek key motif actually has its source 843 00:44:09,930 --> 00:44:13,080 in the so-called myth of the Labyrinth 844 00:44:13,080 --> 00:44:15,260 that had imprisoned the Minotaur. 845 00:44:16,810 --> 00:44:19,360 Who knows who the Minotaur is? 846 00:44:19,360 --> 00:44:20,240 Oh, very good. 847 00:44:20,240 --> 00:44:21,180 Many of you do. 848 00:44:21,180 --> 00:44:21,680 All right. 849 00:44:21,680 --> 00:44:28,190 So the Minotaur was a creature that 850 00:44:28,190 --> 00:44:31,410 was half bull and half human. 851 00:44:31,410 --> 00:44:32,140 All right? 852 00:44:32,140 --> 00:44:34,980 And what happened, according to this myth-- and let me tell you 853 00:44:34,980 --> 00:44:38,310 it's a myth, and it's a bit of risque thing. 854 00:44:38,310 --> 00:44:41,700 There was a king called King Minos 855 00:44:41,700 --> 00:44:46,580 whose wife Pasiphae-- Pasiphae, I'm not sure 856 00:44:46,580 --> 00:44:51,650 how you pronounce it properly-- is the one who had intercourse 857 00:44:51,650 --> 00:44:56,790 with a bull and gave birth-- sorry, 858 00:44:56,790 --> 00:44:59,700 had intercourse with a bull. 859 00:44:59,700 --> 00:45:02,270 And as a result of that, gave birth 860 00:45:02,270 --> 00:45:07,570 to the Minotaur, which was a half-bull, half-human monster, 861 00:45:07,570 --> 00:45:08,740 if you will. 862 00:45:08,740 --> 00:45:09,750 All right? 863 00:45:09,750 --> 00:45:13,510 And that became so threatening and so irritating, 864 00:45:13,510 --> 00:45:16,550 I guess, to King Minos that he asked 865 00:45:16,550 --> 00:45:24,610 one of his subjugates, Daedalus, to create a maze, a huge maze, 866 00:45:24,610 --> 00:45:28,300 and he put the Minotaur in it. 867 00:45:28,300 --> 00:45:30,900 So it was such a maze that the Minotaur just 868 00:45:30,900 --> 00:45:31,910 couldn't get out of it. 869 00:45:31,910 --> 00:45:32,880 All right? 870 00:45:32,880 --> 00:45:35,310 But then things got progressively worse, 871 00:45:35,310 --> 00:45:38,360 and King Minos decided that he had better 872 00:45:38,360 --> 00:45:42,220 get this Minotaur killed. 873 00:45:42,220 --> 00:45:50,979 And so he asked a person whose name is Theseus, OK, 874 00:45:50,979 --> 00:45:56,820 to go into the maze and kill the Minotaur. 875 00:45:58,420 --> 00:46:03,730 Now, the king also had another offspring 876 00:46:03,730 --> 00:46:06,450 whose name is Ariadne-- nice name, Ariadne. 877 00:46:08,740 --> 00:46:14,700 And she said to Theseus that if you take a ball of thread 878 00:46:14,700 --> 00:46:19,870 and you walk through the maze, once you 879 00:46:19,870 --> 00:46:24,070 do what you have to do, you can follow the thread back, 880 00:46:24,070 --> 00:46:26,470 and that way you can get out of the maze. 881 00:46:26,470 --> 00:46:29,120 So that's what Theseus did, and then he 882 00:46:29,120 --> 00:46:34,060 went on and killed the Minotaur. 883 00:46:35,890 --> 00:46:39,010 Now, the story didn't end there because somehow, 884 00:46:39,010 --> 00:46:43,890 for reasons I'm not sure I know the details anymore, 885 00:46:43,890 --> 00:46:47,420 King Minos had a fall out with Daedalus, 886 00:46:47,420 --> 00:46:51,760 and he jailed Daedalus and his son Icarus. 887 00:46:53,220 --> 00:46:54,620 And while they were in jail, they 888 00:46:54,620 --> 00:46:57,050 tried to figure out how they could escape. 889 00:46:57,050 --> 00:47:05,250 And so Daedalus fashioned a pair of wings 890 00:47:05,250 --> 00:47:11,170 made out of bird's feathers and wax, and then put it on Icarus. 891 00:47:11,170 --> 00:47:14,940 And then Icarus, by doing this, he could fly. 892 00:47:14,940 --> 00:47:15,780 OK? 893 00:47:15,780 --> 00:47:18,530 And then he flew and escaped from the prison. 894 00:47:18,530 --> 00:47:23,470 But unfortunately, he flew fairly high up 895 00:47:23,470 --> 00:47:25,180 where the sun was very hot. 896 00:47:25,180 --> 00:47:32,070 That melted the wax, and Icarus, sadly, fell to his death 897 00:47:32,070 --> 00:47:33,460 as a result. 898 00:47:33,460 --> 00:47:35,530 So that's quite the story. 899 00:47:35,530 --> 00:47:38,160 And that's something that obviously some of you 900 00:47:38,160 --> 00:47:38,940 already know. 901 00:47:38,940 --> 00:47:41,930 And those of you who don't, you will probably 902 00:47:41,930 --> 00:47:46,590 remember, as so many Greek myths are just fascinating, 903 00:47:46,590 --> 00:47:48,180 ancient, old stories. 904 00:47:48,180 --> 00:47:48,680 All right. 905 00:47:48,680 --> 00:47:53,000 So then here is actually a reconstruction 906 00:47:53,000 --> 00:47:59,760 of the kill of the Minotaur by Theseus. 907 00:47:59,760 --> 00:48:06,140 And this is actually a sculpture that exists in Greece. 908 00:48:08,410 --> 00:48:09,350 Pretty nice, huh? 909 00:48:11,630 --> 00:48:12,130 OK. 910 00:48:12,130 --> 00:48:17,960 Now, the last confusing image that I wanted to show you 911 00:48:17,960 --> 00:48:23,280 is this one that had appeared-- I think this appeared also 912 00:48:23,280 --> 00:48:28,160 in The New Yorker, but I'm not 100% sure. 913 00:48:28,160 --> 00:48:31,440 What you have here is a confusion-- 914 00:48:31,440 --> 00:48:34,660 do we have three or two prongs here? 915 00:48:34,660 --> 00:48:37,960 And the reason I show this sometimes in class 916 00:48:37,960 --> 00:48:42,960 is because in our department when it was originally 917 00:48:42,960 --> 00:48:45,930 formed by Hans-Lukas Teuber, he created 918 00:48:45,930 --> 00:48:47,930 what is called three prongs. 919 00:48:47,930 --> 00:48:48,990 OK? 920 00:48:48,990 --> 00:48:51,930 And then it became sort of an issue as to, well, 921 00:48:51,930 --> 00:48:54,020 what are the three prongs? 922 00:48:54,020 --> 00:48:55,020 Are there two prongs? 923 00:48:55,020 --> 00:48:56,120 Are there three prongs? 924 00:48:56,120 --> 00:48:57,810 Who belongs into which prong? 925 00:48:57,810 --> 00:49:00,430 And so somebody came up with this picture 926 00:49:00,430 --> 00:49:04,950 that was put up as a joke in the department when 927 00:49:04,950 --> 00:49:07,670 the department was still in one of the old buildings that 928 00:49:07,670 --> 00:49:09,771 doesn't even exist anymore. 929 00:49:09,771 --> 00:49:10,270 All right. 930 00:49:10,270 --> 00:49:17,500 So that then brings me to the end of the illusory effects 931 00:49:17,500 --> 00:49:20,280 that I thought we'd talk about briefly. 932 00:49:20,280 --> 00:49:23,645 And I will now move on and talk about visual prosthesis. 933 00:49:28,591 --> 00:49:29,090 All right. 934 00:49:29,090 --> 00:49:38,670 Now, as far as prosthetics are concerned, 935 00:49:38,670 --> 00:49:41,920 you're going to hear quite a bit about that from Chris Brown 936 00:49:41,920 --> 00:49:49,580 because in audition, an auditory prosthetic device, the cochlear 937 00:49:49,580 --> 00:49:52,400 implant, has become an incredible success. 938 00:49:53,420 --> 00:49:57,730 There are well over 50,000 people in the country now 939 00:49:57,730 --> 00:50:00,470 who have a prosthetic device like that 940 00:50:00,470 --> 00:50:03,940 that enables them to hear, to converse. 941 00:50:03,940 --> 00:50:05,340 It's really incredible. 942 00:50:05,340 --> 00:50:10,270 Unfortunately, when it comes to visual prostheses, 943 00:50:10,270 --> 00:50:14,180 we have nothing comparable at this stage. 944 00:50:14,180 --> 00:50:20,320 It's a much needed device that will take many, many, many more 945 00:50:20,320 --> 00:50:22,970 years to create. 946 00:50:22,970 --> 00:50:25,780 But I thought I'd give you sort of a crude sense of how 947 00:50:25,780 --> 00:50:28,690 that kind of work progresses because there are 948 00:50:28,690 --> 00:50:31,430 many laboratories that are trying to come up 949 00:50:31,430 --> 00:50:37,230 with something that will provide an ability for people to see. 950 00:50:37,230 --> 00:50:42,610 Now, I can tell you that in the world 951 00:50:42,610 --> 00:50:46,320 there are more than 40 million people who are blind. 952 00:50:46,320 --> 00:50:50,140 In the United States, there are more than a million individuals 953 00:50:50,140 --> 00:50:51,630 who are blind. 954 00:50:51,630 --> 00:50:54,706 So because of that-- that's a huge number-- 955 00:50:54,706 --> 00:50:57,920 it, of course, is extremely desirable to come up 956 00:50:57,920 --> 00:51:00,730 with some device to do this for you. 957 00:51:00,730 --> 00:51:04,040 Now, this is a very, very long history, 958 00:51:04,040 --> 00:51:05,815 and people have tried all sorts of things. 959 00:51:07,210 --> 00:51:13,250 One of those is to try to induce the ability to see something 960 00:51:13,250 --> 00:51:18,550 in the world that you normally see with your eye 961 00:51:18,550 --> 00:51:21,090 by putting it through some other sense. 962 00:51:21,090 --> 00:51:23,791 Now, a simple example of that is Braille. 963 00:51:23,791 --> 00:51:24,290 All right? 964 00:51:24,290 --> 00:51:26,270 Everybody knows what Braille is, right? 965 00:51:26,270 --> 00:51:32,260 That's you can use your hand to feel something that 966 00:51:32,260 --> 00:51:34,250 has protruding elements in it that 967 00:51:34,250 --> 00:51:36,290 can be shown in terms of letters. 968 00:51:36,290 --> 00:51:38,740 And so as you get into an elevator and you can't see, 969 00:51:38,740 --> 00:51:41,860 you can feel it, and then you can push the right button 970 00:51:41,860 --> 00:51:43,490 to get to the right floor. 971 00:51:43,490 --> 00:51:44,350 OK? 972 00:51:44,350 --> 00:51:46,430 Now, that's one way it has been done. 973 00:51:46,430 --> 00:51:51,480 Another approach had been to present stimulation 974 00:51:51,480 --> 00:51:55,020 to your somatosensory system, either by putting something 975 00:51:55,020 --> 00:51:58,220 on your back or putting something on your fingertip, 976 00:51:58,220 --> 00:52:03,220 that an image would be converted into actually activating 977 00:52:03,220 --> 00:52:06,070 your somatosensory system that would 978 00:52:06,070 --> 00:52:12,320 be equivalent to something that you would perceive if you could 979 00:52:12,320 --> 00:52:13,180 see. 980 00:52:13,180 --> 00:52:16,420 So that's the basic thing. 981 00:52:16,420 --> 00:52:19,650 And a tremendous amount of effort 982 00:52:19,650 --> 00:52:27,200 has been going on to try to come up with something to create 983 00:52:27,200 --> 00:52:30,140 a workable prosthetic device for the blind. 984 00:52:30,140 --> 00:52:31,590 Now, there's a lot of disagreement 985 00:52:31,590 --> 00:52:38,730 as to where such a device should be placed in the body 986 00:52:38,730 --> 00:52:42,940 and also a lot of debate of what kind of procedures 987 00:52:42,940 --> 00:52:43,750 should be used. 988 00:52:43,750 --> 00:52:46,920 Now, some people advocate that it 989 00:52:46,920 --> 00:52:49,200 should be done on the retina. 990 00:52:49,200 --> 00:52:51,140 Some people advocate it should be 991 00:52:51,140 --> 00:52:55,890 done in the lateral geniculate nucleus. 992 00:52:55,890 --> 00:52:57,780 Actually, some work is being done 993 00:52:57,780 --> 00:53:01,200 at Harvard trying to come up with something 994 00:53:01,200 --> 00:53:02,860 for the lateral geniculate nucleus. 995 00:53:02,860 --> 00:53:05,355 And there's some people who think 996 00:53:05,355 --> 00:53:08,940 it would be best to do it in the visual cortex. 997 00:53:08,940 --> 00:53:12,290 So let me sort of go into this in a bit more detail. 998 00:53:12,290 --> 00:53:15,920 And let me, first of all, raise the question of, 999 00:53:15,920 --> 00:53:19,630 what kinds of devices, and what kinds of necessities 1000 00:53:19,630 --> 00:53:23,920 do we have for converting vision into something 1001 00:53:23,920 --> 00:53:27,080 that enables blind people to see? 1002 00:53:27,080 --> 00:53:29,840 Well, first of all, it's very important 1003 00:53:29,840 --> 00:53:33,210 to be able to see some basic patterns, very important to see 1004 00:53:33,210 --> 00:53:36,380 motion, and very important, often ignored, 1005 00:53:36,380 --> 00:53:38,230 is that you got to be able to see something 1006 00:53:38,230 --> 00:53:41,300 in the third dimension, because a person who is blind, 1007 00:53:41,300 --> 00:53:43,630 he should be able to walk in the world. 1008 00:53:43,630 --> 00:53:45,330 And if you're going to walk around, 1009 00:53:45,330 --> 00:53:48,740 you should be able to see where things are in depth. 1010 00:53:48,740 --> 00:53:51,060 So those are the basic requirements. 1011 00:53:51,060 --> 00:53:55,600 You're certainly not required to process color information. 1012 00:53:55,600 --> 00:53:56,601 So that's not important. 1013 00:53:56,601 --> 00:53:59,016 And there are several other things that are not important. 1014 00:53:59,016 --> 00:54:00,880 But these are the three most important ones 1015 00:54:00,880 --> 00:54:03,570 that you would like to have if you are going 1016 00:54:03,570 --> 00:54:05,940 to create a prosthetics device for the blind. 1017 00:54:05,940 --> 00:54:08,215 Now, then the next question is, how are we 1018 00:54:08,215 --> 00:54:09,840 going to deal with this, and what kinds 1019 00:54:09,840 --> 00:54:12,080 of issues and problems are we going to deal with? 1020 00:54:12,080 --> 00:54:16,520 First of all, the big issue is what kind of prosthetic device 1021 00:54:16,520 --> 00:54:17,255 should we use. 1022 00:54:18,810 --> 00:54:21,040 One of those that I've already mentioned 1023 00:54:21,040 --> 00:54:26,670 is to try to convert visual impressions 1024 00:54:26,670 --> 00:54:31,580 into other modalities such as your somatosensory system. 1025 00:54:31,580 --> 00:54:32,320 All right? 1026 00:54:32,320 --> 00:54:39,260 The other is to create some sort of stimulating device, 1027 00:54:39,260 --> 00:54:42,280 which there are various kinds. 1028 00:54:42,280 --> 00:54:44,130 But one of those would be, for example, 1029 00:54:44,130 --> 00:54:47,500 to put a bunch of electrodes into the brain, 1030 00:54:47,500 --> 00:54:52,210 or into the retina, or whatever, and then selectively stimulate 1031 00:54:52,210 --> 00:54:53,950 through this series of electrodes 1032 00:54:53,950 --> 00:54:56,780 to mimic the visual field. 1033 00:54:56,780 --> 00:54:59,550 Now, the other one, and it is a big issue, 1034 00:54:59,550 --> 00:55:03,500 what brain areas should be considered? 1035 00:55:03,500 --> 00:55:06,110 And because we're at an early stage in this, 1036 00:55:06,110 --> 00:55:11,000 I think it is desirable for people 1037 00:55:11,000 --> 00:55:17,390 to pursue just about any area they think might work. 1038 00:55:18,400 --> 00:55:23,859 Now, the problem with a retinal implant-- 1039 00:55:23,859 --> 00:55:25,150 there are two problems with it. 1040 00:55:25,150 --> 00:55:29,360 One is the retina is extremely small, and very confined, 1041 00:55:29,360 --> 00:55:36,630 and has these millions of receptors and ganglion cells, 1042 00:55:36,630 --> 00:55:39,180 and how can you selectively activate them? 1043 00:55:39,180 --> 00:55:41,000 Now, the biggest problem, though, 1044 00:55:41,000 --> 00:55:46,060 is that in most blind people, the retina 1045 00:55:46,060 --> 00:55:48,490 becomes non-functional. 1046 00:55:48,490 --> 00:55:53,380 And even worse, it degenerates in a relatively short time 1047 00:55:53,380 --> 00:55:58,225 so that it no longer is useful for electrical stimulation 1048 00:55:58,225 --> 00:56:00,540 or for whatever kind of stimulation 1049 00:56:00,540 --> 00:56:02,340 you wish to engage in. 1050 00:56:02,340 --> 00:56:05,549 So because of that, some people have 1051 00:56:05,549 --> 00:56:07,340 moved on to the lateral geniculate nucleus. 1052 00:56:08,590 --> 00:56:12,440 That area has some problems because it's deep in the brain, 1053 00:56:12,440 --> 00:56:13,465 and it's quite small. 1054 00:56:14,930 --> 00:56:15,920 OK? 1055 00:56:15,920 --> 00:56:19,370 And also, it tends to degenerate over time, 1056 00:56:19,370 --> 00:56:24,200 although it takes it a lot longer than the degeneration 1057 00:56:24,200 --> 00:56:25,460 of the retina. 1058 00:56:25,460 --> 00:56:31,305 Thirdly, people have considered stimulating the visual cortex, 1059 00:56:31,305 --> 00:56:32,680 and that's what I'm going to talk 1060 00:56:32,680 --> 00:56:35,110 about in a bit more detail just now. 1061 00:56:36,470 --> 00:56:39,680 But the last thing that's very important here-- I think it's 1062 00:56:39,680 --> 00:56:40,340 the last thing. 1063 00:56:40,340 --> 00:56:41,798 Yeah, just let me go back. 1064 00:56:46,658 --> 00:56:47,610 Ah, sorry. 1065 00:56:52,880 --> 00:56:55,510 The third big issue is, can you create 1066 00:56:55,510 --> 00:56:57,275 a device that has longevity? 1067 00:56:58,440 --> 00:57:00,350 As soon as you put something into the brain 1068 00:57:00,350 --> 00:57:03,350 and into the eye, there's going to be 1069 00:57:03,350 --> 00:57:05,520 some sort of adverse reaction that 1070 00:57:05,520 --> 00:57:07,830 may kill the cells at the tip of the electrodes 1071 00:57:07,830 --> 00:57:08,950 or something like that. 1072 00:57:08,950 --> 00:57:12,450 So all kinds of approaches have been 1073 00:57:12,450 --> 00:57:19,400 created to try to increase the longevity of the device. 1074 00:57:19,400 --> 00:57:21,770 Now, I'm going to next look at what 1075 00:57:21,770 --> 00:57:24,990 happens in the visual cortex, which 1076 00:57:24,990 --> 00:57:29,750 is a promising, at least I think it is, a promising site 1077 00:57:29,750 --> 00:57:33,810 where you could electrically stimulate and use 1078 00:57:33,810 --> 00:57:37,310 that to create a visual impression. 1079 00:57:37,310 --> 00:57:37,810 All right. 1080 00:57:37,810 --> 00:57:40,240 If you do that-- here is a monkey brain. 1081 00:57:40,240 --> 00:57:42,030 The monkey is very well suited for this 1082 00:57:42,030 --> 00:57:44,180 because, as I have mentioned before, the posterior 1083 00:57:44,180 --> 00:57:47,330 part of the cortex-- see, this is area V1-- is lissencephalic. 1084 00:57:48,500 --> 00:57:54,720 And so it's relatively easy to place accurately electrodes 1085 00:57:54,720 --> 00:57:55,690 into it. 1086 00:57:55,690 --> 00:57:56,340 OK? 1087 00:57:56,340 --> 00:58:00,110 So then if what you do here-- here, just a reminder, 1088 00:58:00,110 --> 00:58:01,510 is the central sulcus. 1089 00:58:01,510 --> 00:58:02,810 Here's the lunate sulcus. 1090 00:58:02,810 --> 00:58:04,860 And here, of course, is area V1. 1091 00:58:04,860 --> 00:58:07,730 If you now take the contralateral hemifield, 1092 00:58:07,730 --> 00:58:10,930 that projects into this region. 1093 00:58:10,930 --> 00:58:11,430 OK? 1094 00:58:11,430 --> 00:58:13,670 This is shown out to about five degrees. 1095 00:58:13,670 --> 00:58:16,220 That will go to around here like that. 1096 00:58:16,220 --> 00:58:17,510 OK? 1097 00:58:17,510 --> 00:58:21,050 So now the next big question that, initially, 1098 00:58:21,050 --> 00:58:23,080 had been done actually in humans, 1099 00:58:23,080 --> 00:58:30,570 what do you see when you insert a microelectrode 1100 00:58:30,570 --> 00:58:34,490 into the brain-- as I say, this was done on humans initially-- 1101 00:58:34,490 --> 00:58:37,210 and you electrically stimulate? 1102 00:58:37,210 --> 00:58:38,030 What do you see? 1103 00:58:38,030 --> 00:58:42,310 And this work was done by Brindley many years ago. 1104 00:58:42,310 --> 00:58:46,730 And he discovered that when you electrically 1105 00:58:46,730 --> 00:58:54,220 stimulate in the human primary visual cortex, what you get 1106 00:58:54,220 --> 00:58:58,090 is a small, star-like image. 1107 00:58:58,090 --> 00:58:59,280 All right? 1108 00:58:59,280 --> 00:59:00,700 So that's what he found. 1109 00:59:00,700 --> 00:59:06,150 And so that then triggered a great deal of research, 1110 00:59:06,150 --> 00:59:10,080 including work in monkeys, to determine 1111 00:59:10,080 --> 00:59:13,160 what could be done to create a prosthetic device that 1112 00:59:13,160 --> 00:59:14,610 might just work. 1113 00:59:14,610 --> 00:59:17,870 So the first step then was to try 1114 00:59:17,870 --> 00:59:19,880 to determine what happens when you 1115 00:59:19,880 --> 00:59:21,530 put a microelectrode in here. 1116 00:59:21,530 --> 00:59:26,560 This is, by the way, the layout of the visual field 1117 00:59:26,560 --> 00:59:29,750 in the posterior cortex with much more area devoted 1118 00:59:29,750 --> 00:59:33,550 to the fovea than into the periphery because, of course, 1119 00:59:33,550 --> 00:59:39,160 there's a much higher packing density of receptor cells 1120 00:59:39,160 --> 00:59:41,850 as well as retinal ganglion cells in the fovea 1121 00:59:41,850 --> 00:59:44,470 representation than further out. 1122 00:59:44,470 --> 00:59:47,860 In the cortex, the thickness of the gray matter is constant. 1123 00:59:47,860 --> 00:59:50,490 It's about roughly 2 millimeters thick. 1124 00:59:52,330 --> 00:59:57,120 And the frequency, spatial frequency of the cells, 1125 00:59:57,120 --> 00:59:59,100 is pretty constant as well. 1126 00:59:59,100 --> 01:00:03,330 So therefore, you need to allocate more space 1127 01:00:03,330 --> 01:00:09,220 to the incoming huge number of retinal ganglion 1128 01:00:09,220 --> 01:00:11,640 cells projected to the geniculate 1129 01:00:11,640 --> 01:00:14,405 and then to the cortex than for the periphery. 1130 01:00:14,405 --> 01:00:15,780 And that's why you have this kind 1131 01:00:15,780 --> 01:00:18,605 of a layout of much more space allocated 1132 01:00:18,605 --> 01:00:21,440 to the central than to peripheral vision. 1133 01:00:21,440 --> 01:00:24,030 So that's a very important factor 1134 01:00:24,030 --> 01:00:27,870 in considering how to create a prosthetic device. 1135 01:00:27,870 --> 01:00:32,495 So now the first step in doing this kind of stuff, 1136 01:00:32,495 --> 01:00:36,090 if you're going to do serious experiments on monkeys, 1137 01:00:36,090 --> 01:00:38,630 is to determine, what does the monkey 1138 01:00:38,630 --> 01:00:42,590 see when you electrically stimulate the visual cortex? 1139 01:00:42,590 --> 01:00:46,130 And so the experiment that's been done here 1140 01:00:46,130 --> 01:00:50,050 is one in which a monkey has been trained 1141 01:00:50,050 --> 01:00:58,040 to make an eye movement to the bigger of two visual stimuli 1142 01:00:58,040 --> 01:00:59,750 or the brighter of two stimuli. 1143 01:00:59,750 --> 01:01:02,540 In this case, we have two visual targets. 1144 01:01:02,540 --> 01:01:04,050 By the way, one of these is going 1145 01:01:04,050 --> 01:01:08,600 to appear where the receptive field is of the cells 1146 01:01:08,600 --> 01:01:10,884 that you're going to stimulate in the visual cortex. 1147 01:01:10,884 --> 01:01:13,300 So then you may sometimes make this one brighter, this one 1148 01:01:13,300 --> 01:01:13,990 brighter. 1149 01:01:13,990 --> 01:01:16,870 And the monkey makes a saccade to it, 1150 01:01:16,870 --> 01:01:19,000 and he gets a drop of apple juice for reward. 1151 01:01:19,000 --> 01:01:22,030 And then you can systematically vary the relative size 1152 01:01:22,030 --> 01:01:25,750 of the two stimuli to see what kind of a function you get. 1153 01:01:25,750 --> 01:01:29,090 And the same thing you can do for the relative brightness 1154 01:01:29,090 --> 01:01:29,740 of it. 1155 01:01:29,740 --> 01:01:33,510 And then on some trials interspersed with those trials, 1156 01:01:33,510 --> 01:01:35,870 you present [INAUDIBLE] stimulation 1157 01:01:35,870 --> 01:01:37,850 with the visual target and see which 1158 01:01:37,850 --> 01:01:39,410 one does the monkey go to. 1159 01:01:39,410 --> 01:01:41,210 And then you systematically vary, 1160 01:01:41,210 --> 01:01:45,140 as I said, the brightness or the size of those stimuli. 1161 01:01:45,140 --> 01:01:45,640 OK? 1162 01:01:47,140 --> 01:01:50,270 We vary the contrast in this case, and in this case, 1163 01:01:50,270 --> 01:01:51,510 we vary the size. 1164 01:01:51,510 --> 01:01:52,529 OK? 1165 01:01:52,529 --> 01:01:53,820 So that's the basic experiment. 1166 01:01:53,820 --> 01:01:55,880 And if you do that systematically, 1167 01:01:55,880 --> 01:02:02,600 you can tell exactly what the size and the contrast 1168 01:02:02,600 --> 01:02:07,070 is of the visual stimulus created 1169 01:02:07,070 --> 01:02:08,470 by electrical stimulation. 1170 01:02:08,470 --> 01:02:10,900 If you do that-- here's an example. 1171 01:02:10,900 --> 01:02:13,140 When you present two visual stimuli 1172 01:02:13,140 --> 01:02:18,500 and you vary the percent contrast difference, all right, 1173 01:02:18,500 --> 01:02:21,630 of the two stimuli, you can see that it crosses over, indeed, 1174 01:02:21,630 --> 01:02:22,850 when they're identical. 1175 01:02:22,850 --> 01:02:25,270 So that tells you that this system works. 1176 01:02:25,270 --> 01:02:27,120 It says these are identical. 1177 01:02:27,120 --> 01:02:29,155 And the same thing is true for the size series. 1178 01:02:30,320 --> 01:02:32,820 So this way then, if you do the same experiment 1179 01:02:32,820 --> 01:02:36,620 with electrical stimulation, you can find out 1180 01:02:36,620 --> 01:02:38,940 where this 50% crossover point is. 1181 01:02:38,940 --> 01:02:42,050 This is for contrast, and this is for size. 1182 01:02:42,050 --> 01:02:45,340 So then you can establish this with slightly different 1183 01:02:45,340 --> 01:02:47,270 locations in the visual field. 1184 01:02:47,270 --> 01:02:49,440 And if you do that overall, you come up 1185 01:02:49,440 --> 01:02:51,850 with a summary statement, which says 1186 01:02:51,850 --> 01:02:55,380 that when you use currents between 20 and 120 microamps, 1187 01:02:55,380 --> 01:02:59,130 OK, at eccentricities between 2 and 1/2 and 3 and 1/2 degrees, 1188 01:02:59,130 --> 01:03:02,190 the contrast of the visual percept created in monkeys 1189 01:03:02,190 --> 01:03:06,280 is 6% to 12%, and the size is between 15 and 20 minutes 1190 01:03:06,280 --> 01:03:07,880 of visual angle. 1191 01:03:07,880 --> 01:03:10,710 Now, that's quite small fortunately. 1192 01:03:10,710 --> 01:03:15,010 So it's a local, little spot, like a star-like image. 1193 01:03:15,010 --> 01:03:18,980 Now, what do we mean by 6% to 12% contrast? 1194 01:03:18,980 --> 01:03:20,712 So let me tell you this, that that's 1195 01:03:20,712 --> 01:03:22,170 a useful thing for you to memorize. 1196 01:03:23,200 --> 01:03:25,400 We talk about percent contrast. 1197 01:03:25,400 --> 01:03:28,560 What you do is you measure, for example, 1198 01:03:28,560 --> 01:03:31,490 in candelas per square meter, what the contrast is 1199 01:03:31,490 --> 01:03:33,800 of this and what is the contest of the background. 1200 01:03:33,800 --> 01:03:36,710 Then you subtract one from the other, divide it 1201 01:03:36,710 --> 01:03:39,580 by the sum of the two, and you multiply by 100. 1202 01:03:39,580 --> 01:03:42,490 So when you do that, this is roughly 8% contrast. 1203 01:03:42,490 --> 01:03:45,070 This is roughly 75% contrast. 1204 01:03:45,070 --> 01:03:48,060 So it gives you a sense of what contrast means. 1205 01:03:48,060 --> 01:03:48,640 All right? 1206 01:03:48,640 --> 01:03:52,350 So what that means, in essence, that the visual stimulus 1207 01:03:52,350 --> 01:03:54,065 created by the electrical stimulation 1208 01:03:54,065 --> 01:03:56,030 is fairly low contrast. 1209 01:03:56,030 --> 01:03:56,530 All right. 1210 01:03:56,530 --> 01:04:00,780 So now the next important factor, 1211 01:04:00,780 --> 01:04:04,120 once one knows what the visual percept is 1212 01:04:04,120 --> 01:04:06,840 that is created by electrical stimulation in the monkey, 1213 01:04:06,840 --> 01:04:08,890 the next thing is to ask the question, well, 1214 01:04:08,890 --> 01:04:12,270 what kind of electrode array should one put together? 1215 01:04:18,210 --> 01:04:18,840 All right. 1216 01:04:18,840 --> 01:04:21,540 Well, the big issue here has to do 1217 01:04:21,540 --> 01:04:23,900 with the so-called magnification factor 1218 01:04:23,900 --> 01:04:25,220 that I just talked about. 1219 01:04:25,220 --> 01:04:26,816 So let's look at this analytically. 1220 01:04:26,816 --> 01:04:28,440 First of all, I already mentioned this. 1221 01:04:28,440 --> 01:04:32,750 We have a very high packing density of photoreceptors 1222 01:04:32,750 --> 01:04:35,720 in the fovea and much lower density further out. 1223 01:04:36,930 --> 01:04:41,660 And so what you have is that this plate 1224 01:04:41,660 --> 01:04:45,365 goes through the optic nerve up to the visual cortex 1225 01:04:45,365 --> 01:04:47,580 to the lateral geniculate nucleus. 1226 01:04:47,580 --> 01:04:49,710 So that's the basic arrangement, meaning 1227 01:04:49,710 --> 01:04:53,590 that there are many, many more retinal ganglion 1228 01:04:53,590 --> 01:04:58,220 cells per unit area that impinge on the visual cortex 1229 01:04:58,220 --> 01:05:00,490 near the fovea than in the periphery. 1230 01:05:01,550 --> 01:05:06,150 So then to examine this, here we have, again, another picture. 1231 01:05:06,150 --> 01:05:07,560 It's the geniculate again. 1232 01:05:07,560 --> 01:05:11,370 Here's the fact that the cortex is constant in thickness. 1233 01:05:11,370 --> 01:05:15,810 And here's the layout of the visual field on the posterior 1234 01:05:15,810 --> 01:05:20,210 cortex with more area allocated to the fovea 1235 01:05:20,210 --> 01:05:22,300 than to-- excuse me, than to periphery. 1236 01:05:22,300 --> 01:05:22,800 All right. 1237 01:05:22,800 --> 01:05:25,455 So now I've shown you a variant of this before. 1238 01:05:26,640 --> 01:05:31,730 If you present an arrow across the visual field here, 1239 01:05:31,730 --> 01:05:36,470 this is the impression made with much more activation 1240 01:05:36,470 --> 01:05:39,630 in central vision than in peripheral vision. 1241 01:05:39,630 --> 01:05:41,620 I mean, you don't have to worry about that. 1242 01:05:41,620 --> 01:05:45,650 That simply tells you, as we had discussed before, 1243 01:05:45,650 --> 01:05:49,250 that the images created depend heavily 1244 01:05:49,250 --> 01:05:52,700 on where you stimulate in the visual field. 1245 01:05:52,700 --> 01:05:53,210 OK. 1246 01:05:53,210 --> 01:05:56,800 So now to look at this quantitatively, 1247 01:05:56,800 --> 01:06:00,200 so we put a bunch of dots up in the visual field. 1248 01:06:00,200 --> 01:06:02,370 This is in the central four degrees, 1249 01:06:02,370 --> 01:06:05,340 and this is the actual activation 1250 01:06:05,340 --> 01:06:06,680 in the visual cortex. 1251 01:06:06,680 --> 01:06:07,180 OK? 1252 01:06:08,400 --> 01:06:12,940 Meaning that in the fovea, we have two elements here, 1253 01:06:12,940 --> 01:06:15,990 this one and this one, here and here, that they take up 1254 01:06:15,990 --> 01:06:18,720 a lot of space, whereas in the periphery, 1255 01:06:18,720 --> 01:06:20,480 OK, they're very close to each other. 1256 01:06:20,480 --> 01:06:23,810 That's due to the so-called magnification factor. 1257 01:06:23,810 --> 01:06:26,310 And so one of the basic rules is that we've 1258 01:06:26,310 --> 01:06:32,190 got to be able to stimulate the visual cortex taking 1259 01:06:32,190 --> 01:06:34,620 into account the magnification factor. 1260 01:06:34,620 --> 01:06:37,310 So if you have this kind of spatial arrangement, 1261 01:06:37,310 --> 01:06:39,360 if we then were to put a bunch of electrodes 1262 01:06:39,360 --> 01:06:43,290 in like this, which we will call proportional electrodes, 1263 01:06:43,290 --> 01:06:45,640 then if you stimulated all those, 1264 01:06:45,640 --> 01:06:49,950 you would get a similar square with one proviso, 1265 01:06:49,950 --> 01:06:53,276 namely that the little dots would be smaller in the fovea, 1266 01:06:53,276 --> 01:06:54,650 obviously, than in the periphery. 1267 01:06:55,740 --> 01:06:59,350 So then if you have this kind of arrangement, what you can do 1268 01:06:59,350 --> 01:07:06,410 is you can take a camera that has a fixed number of elements, 1269 01:07:06,410 --> 01:07:08,480 and each of those elements you hook up 1270 01:07:08,480 --> 01:07:09,960 to one of the electrodes. 1271 01:07:09,960 --> 01:07:12,390 If you put the word "fiat lux" there, 1272 01:07:12,390 --> 01:07:16,120 this is what you hypothetically would create, like that. 1273 01:07:16,120 --> 01:07:16,960 OK? 1274 01:07:16,960 --> 01:07:18,550 That's what it would look like. 1275 01:07:18,550 --> 01:07:22,620 So in contrast with this arrangement, 1276 01:07:22,620 --> 01:07:25,300 if we were to use an electrode array, 1277 01:07:25,300 --> 01:07:28,070 which you can readily buy-- you cannot buy proportional arrays. 1278 01:07:28,070 --> 01:07:30,100 You have to somehow create them yourself. 1279 01:07:30,100 --> 01:07:35,280 But if you buy a fixed array, which the elements are 1280 01:07:35,280 --> 01:07:37,740 equidistant, then if you stimulated all those 1281 01:07:37,740 --> 01:07:41,840 you would get a butterfly image, butterfly image. 1282 01:07:41,840 --> 01:07:43,920 I will show a movie of that as well. 1283 01:07:43,920 --> 01:07:47,370 And that means that if you do that same conversion as before, 1284 01:07:47,370 --> 01:07:50,860 this is what you would see in contrast to this. 1285 01:07:50,860 --> 01:07:54,170 So in other words, it's a greatly distorted vision 1286 01:07:54,170 --> 01:07:57,120 because you didn't take into account the magnification 1287 01:07:57,120 --> 01:07:58,061 factor. 1288 01:07:58,061 --> 01:07:58,560 All right. 1289 01:07:58,560 --> 01:08:01,390 So then what you can do, actually, you 1290 01:08:01,390 --> 01:08:05,603 can also increase the number of electrodes in the center here. 1291 01:08:05,603 --> 01:08:06,610 OK? 1292 01:08:06,610 --> 01:08:08,700 And that would mean-- in this case 1293 01:08:08,700 --> 01:08:13,360 we have 428 elements-- that you can recreate fiat lux pretty 1294 01:08:13,360 --> 01:08:17,029 well with this increased number of elements, 1295 01:08:17,029 --> 01:08:20,750 where with the original set you couldn't make anything out. 1296 01:08:20,750 --> 01:08:23,020 Now, the alternate to this would be 1297 01:08:23,020 --> 01:08:24,420 that you could take a whole bunch 1298 01:08:24,420 --> 01:08:28,069 of equally-spaced electrodes, which you can commercially buy, 1299 01:08:28,069 --> 01:08:29,140 and put them in. 1300 01:08:29,140 --> 01:08:33,140 But then to get the same acuity, you would need, in this case, 1301 01:08:33,140 --> 01:08:37,130 something like over 600 electrodes, 1302 01:08:37,130 --> 01:08:40,950 whereas in this case you would only need about 420. 1303 01:08:40,950 --> 01:08:43,270 So because of that, obviously we want 1304 01:08:43,270 --> 01:08:46,240 to minimize the number of things you put into the brain, 1305 01:08:46,240 --> 01:08:49,670 it's desirable to use a proportional device. 1306 01:08:49,670 --> 01:08:51,279 So that's the first big step. 1307 01:08:51,279 --> 01:08:54,689 Now, let's assume that you have created a proportional device. 1308 01:08:54,689 --> 01:08:58,810 The next thing would be to see, how do you stimulate these 1309 01:08:58,810 --> 01:09:02,930 through a camera? 1310 01:09:02,930 --> 01:09:08,460 So what you do then is an experiment 1311 01:09:08,460 --> 01:09:10,729 in which you take a human subject, 1312 01:09:10,729 --> 01:09:13,590 and you can put a camera on a helmet, 1313 01:09:13,590 --> 01:09:16,649 and also, the person looks at a monitor. 1314 01:09:16,649 --> 01:09:18,865 And otherwise, the rest of the world 1315 01:09:18,865 --> 01:09:22,020 is excluded by having this bezel here. 1316 01:09:22,020 --> 01:09:24,670 And then the camera looks at something, 1317 01:09:24,670 --> 01:09:28,020 and then it can display the image here. 1318 01:09:28,020 --> 01:09:31,460 Well, one thing it can do, it can display the image as it is. 1319 01:09:31,460 --> 01:09:33,590 But the other thing that's much more important, 1320 01:09:33,590 --> 01:09:38,740 it can display that image as if electrical stimulation 1321 01:09:38,740 --> 01:09:39,950 had taken place. 1322 01:09:39,950 --> 01:09:42,109 Now, this becomes complicated because you 1323 01:09:42,109 --> 01:09:53,490 have to do a lot of conversion of the input from the camera 1324 01:09:53,490 --> 01:09:58,780 to the computer that's going to then drive the electrodes. 1325 01:09:58,780 --> 01:10:01,990 Now, the first important point, I mentioned this before, 1326 01:10:01,990 --> 01:10:06,690 is that if you present a small spot-- this is actual real data 1327 01:10:06,690 --> 01:10:12,020 here-- you present a small spot in the center of a small array 1328 01:10:12,020 --> 01:10:16,900 of cells in the visual cortex, you get a vigorous response, 1329 01:10:16,900 --> 01:10:18,950 whether a light increment or light decrement. 1330 01:10:18,950 --> 01:10:22,120 But if you use a larger spot, you get no response, 1331 01:10:22,120 --> 01:10:26,140 meaning that the cortical cells look at local differences. 1332 01:10:26,140 --> 01:10:28,620 That has a lot to do with what we talked about 1333 01:10:28,620 --> 01:10:29,950 with the illusions. 1334 01:10:29,950 --> 01:10:32,140 And so what you have to create is 1335 01:10:32,140 --> 01:10:35,050 an algorithm that mimics that. 1336 01:10:35,050 --> 01:10:36,910 And so the way you do that is you 1337 01:10:36,910 --> 01:10:40,770 have these individual elements for each of the electrodes. 1338 01:10:40,770 --> 01:10:48,250 And if that individual element in the computer that 1339 01:10:48,250 --> 01:10:52,650 gets the input from the camera, if the whole element gets 1340 01:10:52,650 --> 01:10:57,480 activated, there's no real activation of the electrode. 1341 01:10:57,480 --> 01:10:59,210 But if an edge appears like here, 1342 01:10:59,210 --> 01:11:02,570 here, or here, then the electrode is activated. 1343 01:11:02,570 --> 01:11:03,190 OK? 1344 01:11:03,190 --> 01:11:07,830 So you create local differences by this conversion system, 1345 01:11:07,830 --> 01:11:12,160 all right, by this algorithm that then, 1346 01:11:12,160 --> 01:11:13,900 when the camera looks at the world, 1347 01:11:13,900 --> 01:11:17,000 will respond whenever there's an edge, an illumination 1348 01:11:17,000 --> 01:11:17,751 difference. 1349 01:11:17,751 --> 01:11:18,250 OK. 1350 01:11:18,250 --> 01:11:22,160 So now the way this looks is like this. 1351 01:11:23,440 --> 01:11:26,295 Again, as I've told you, whatever is in the fovea, 1352 01:11:26,295 --> 01:11:29,320 the dots are smaller and further out, but that's OK. 1353 01:11:29,320 --> 01:11:31,820 So that's the basic activation system. 1354 01:11:31,820 --> 01:11:34,530 And so now what we are going to do, 1355 01:11:34,530 --> 01:11:39,960 having created such an algorithm, 1356 01:11:39,960 --> 01:11:44,660 we're going to examine what the person who 1357 01:11:44,660 --> 01:11:48,030 carries this camera on his head can actually see. 1358 01:11:48,030 --> 01:11:50,630 Now, the way this can be done, you can either physically 1359 01:11:50,630 --> 01:11:54,223 move the stimuli in the world, or the person looks at it 1360 01:11:54,223 --> 01:11:56,820 and can slowly move his head around. 1361 01:11:56,820 --> 01:11:57,750 OK? 1362 01:11:57,750 --> 01:12:10,170 So let's first look at what would 1363 01:12:10,170 --> 01:12:19,790 happen if, instead of a proportional display, 1364 01:12:19,790 --> 01:12:24,490 we used an equally-spaced display like that butterfly 1365 01:12:24,490 --> 01:12:25,040 display. 1366 01:12:25,040 --> 01:12:25,640 OK? 1367 01:12:25,640 --> 01:12:28,720 So what I'm going to show you here on the left 1368 01:12:28,720 --> 01:12:34,795 is what the camera is viewing. 1369 01:12:35,820 --> 01:12:38,560 And here I'm going to show you what is actually 1370 01:12:38,560 --> 01:12:43,510 perceived by this algorithm, by the person, where this image is 1371 01:12:43,510 --> 01:12:46,320 converted into what he's looking at through that bezel. 1372 01:12:46,320 --> 01:12:47,076 OK, you ready? 1373 01:12:57,300 --> 01:12:59,330 I mean, you don't see a square at all, right? 1374 01:12:59,330 --> 01:13:02,930 What you see is, because you're not taking into account 1375 01:13:02,930 --> 01:13:05,430 the magnification factor, you get a huge distortion. 1376 01:13:06,480 --> 01:13:10,290 So now let's go move on and ask the question, 1377 01:13:10,290 --> 01:13:17,540 well, what happens when you are using a proportional display? 1378 01:13:17,540 --> 01:13:18,040 All right? 1379 01:13:23,440 --> 01:13:25,845 And now we get a rather nice story. 1380 01:13:29,180 --> 01:13:30,920 Again, the visual display's on the left, 1381 01:13:30,920 --> 01:13:32,605 and the image created is on the right. 1382 01:13:32,605 --> 01:13:33,104 Ready? 1383 01:13:37,280 --> 01:13:41,750 So the rule, therefore, is that you 1384 01:13:41,750 --> 01:13:51,480 must use a proportional image, proportional display, that 1385 01:13:51,480 --> 01:13:54,790 can be done either by having it truly proportional as you put 1386 01:13:54,790 --> 01:14:00,030 it into the visual cortex or by having an equally-spaced array, 1387 01:14:00,030 --> 01:14:04,250 and the computer computes the proportional arrangement. 1388 01:14:04,250 --> 01:14:05,670 But the disadvantage of the latter 1389 01:14:05,670 --> 01:14:07,503 is that you need many, many more electrodes. 1390 01:14:08,120 --> 01:14:08,620 All right. 1391 01:14:08,620 --> 01:14:11,360 Now, what I'm going to do is I'm going 1392 01:14:11,360 --> 01:14:17,310 to show you whether you can read something 1393 01:14:17,310 --> 01:14:19,680 when you use a proportional display like that. 1394 01:14:19,680 --> 01:14:22,990 And in this case, we're going to have a bunch of big letters, 1395 01:14:22,990 --> 01:14:25,430 and either the letters are going to move slowly 1396 01:14:25,430 --> 01:14:28,030 across the visual field, or your head 1397 01:14:28,030 --> 01:14:30,200 is going to move slowly with the camera like that 1398 01:14:30,200 --> 01:14:32,890 to view that display that's up on the wall. 1399 01:14:32,890 --> 01:14:33,630 OK? 1400 01:14:33,630 --> 01:14:42,540 So if we do that-- are you ready? 1401 01:14:43,860 --> 01:14:45,126 See if you can read it. 1402 01:14:52,680 --> 01:14:53,180 OK. 1403 01:14:53,180 --> 01:14:54,513 Everybody able to read that one? 1404 01:14:55,850 --> 01:14:56,910 OK. 1405 01:14:56,910 --> 01:14:58,704 Who remembers what "fiat lux" means? 1406 01:15:02,620 --> 01:15:04,680 I've only shown it about four or five times. 1407 01:15:04,680 --> 01:15:05,810 Let there be light. 1408 01:15:06,821 --> 01:15:07,320 OK. 1409 01:15:07,320 --> 01:15:12,390 So now, just to be [INAUDIBLE] attractive about it, 1410 01:15:12,390 --> 01:15:15,170 I'm going to show you one more display. 1411 01:15:16,190 --> 01:15:21,670 And this one is a display, again, 1412 01:15:21,670 --> 01:15:27,720 a series of letters and words, is one 1413 01:15:27,720 --> 01:15:40,990 that John F. Kennedy has used in a very, very famous statement, 1414 01:15:40,990 --> 01:15:43,880 which I think most of you will remember. 1415 01:15:43,880 --> 01:15:45,690 And eventually, as you see the whole thing, 1416 01:15:45,690 --> 01:15:47,100 you will be able to put it together. 1417 01:15:47,100 --> 01:15:47,683 Are you ready? 1418 01:17:13,281 --> 01:17:13,780 OK. 1419 01:17:13,780 --> 01:17:15,190 Everybody's seen that, right? 1420 01:17:19,150 --> 01:17:21,310 People nowadays, because of what's going on, 1421 01:17:21,310 --> 01:17:24,370 are less inclined to take his advice. 1422 01:17:33,560 --> 01:17:34,060 All right. 1423 01:17:34,060 --> 01:17:36,680 So that is then the essence of what 1424 01:17:36,680 --> 01:17:38,135 I was going to cover today. 1425 01:17:39,140 --> 01:17:43,610 And so let me now provide you with a brief summary. 1426 01:17:45,430 --> 01:17:48,490 I will start with the prosthetics then. 1427 01:17:48,490 --> 01:17:49,210 OK? 1428 01:17:49,210 --> 01:17:52,410 Research on visual prosthetics is in its infancy, 1429 01:17:52,410 --> 01:17:53,650 as I've noted. 1430 01:17:53,650 --> 01:17:55,880 A great deal of basic research is 1431 01:17:55,880 --> 01:17:58,621 needed before such a device can become effective. 1432 01:17:58,621 --> 01:18:00,120 That's very unlike what you're going 1433 01:18:00,120 --> 01:18:03,620 to hear about the auditory system. 1434 01:18:03,620 --> 01:18:06,660 The brain area that holds considerable promise 1435 01:18:06,660 --> 01:18:11,350 for the prosthetic device based on electrical stimulation 1436 01:18:11,350 --> 01:18:13,340 is area V1. 1437 01:18:13,340 --> 01:18:16,470 Now, a prosthetic device for electrical stimulation of V1 1438 01:18:16,470 --> 01:18:19,270 must take into account the magnification factor. 1439 01:18:19,270 --> 01:18:21,350 That's absolutely essential, and too many 1440 01:18:21,350 --> 01:18:22,890 people are ignoring it. 1441 01:18:22,890 --> 01:18:24,962 Now, in a different color, we're going 1442 01:18:24,962 --> 01:18:26,810 to talk about the illusions. 1443 01:18:26,810 --> 01:18:29,290 There is no unitary explanation for 1444 01:18:29,290 --> 01:18:32,360 the great many visual illusions extant. 1445 01:18:32,360 --> 01:18:34,290 I pointed out to you that different illusions 1446 01:18:34,290 --> 01:18:37,540 have different sources and different explanations. 1447 01:18:37,540 --> 01:18:40,120 The most popular theory explaining the Hermann grid 1448 01:18:40,120 --> 01:18:42,700 illusion based on the center/surround organization 1449 01:18:42,700 --> 01:18:45,650 of retinal ganglion cells is incorrect. 1450 01:18:47,110 --> 01:18:50,050 That's a polite way of saying it's all wrong. 1451 01:18:50,050 --> 01:18:53,630 A more likely theory is the one that 1452 01:18:53,630 --> 01:18:57,285 assumes that area V1 cells are involved. 1453 01:18:58,330 --> 01:19:01,380 Retinal adaptation processes can explain illusions 1454 01:19:01,380 --> 01:19:03,580 based on aftereffects, and you've 1455 01:19:03,580 --> 01:19:05,320 seen quite a few of those. 1456 01:19:05,320 --> 01:19:09,700 And many illusions disappear under isoluminant conditions. 1457 01:19:09,700 --> 01:19:13,830 And lastly, there are no viable theories that explain illusions 1458 01:19:13,830 --> 01:19:16,670 based on figure/ground relationships. 1459 01:19:16,670 --> 01:19:18,440 We know what they are, but we don't 1460 01:19:18,440 --> 01:19:23,350 know how the brain resolves figure and ground effects. 1461 01:19:23,350 --> 01:19:28,120 So that then is the end of what I wanted to cover today. 1462 01:19:28,120 --> 01:19:32,525 And next time, we are going to talk about eye movement. 1463 01:19:33,780 --> 01:19:38,100 And I hope that you will enjoy that as well.