Phantoms in the Brain: Probing the Mysteries of the Human Mind (18 page)

spokes (Figure 5.7). Notice that when you do this, unlike what you observed with the corner of the square, you do not see a gap or smudge. You do indeed "complete" the gap—you actually see the spokes converging into a vortex at the center of your blind spot.

So it appears that there are some things you can complete across the blind spot and other things you cannot, and it's relatively easy to discover these principles by simply experimenting with your own blind spot or a friend's.

Some years ago, Jonathan Piel, the former editor of
Scientific American,
invited me to write an article on the blind spot for that journal.

Figure 5.6
Move the page toward you until the hatched disk falls on the blind spot. Does the corner of the
square get completed'? The answer is that most people see the corner "missing" or "smudged"; it does not get
filled in. This simple demonstration shows that filling in is not based on guesswork; it is not a high−level
cognitive process.

Soon after the article appeared, I received hundreds of letters from readers who tried the various experiments I had described or had devised new ones of their own. These letters made me realize how intensely curious people are about the inner workings of their visual pathways. One chap even embarked on a whole new style of art and had a show of his own paintings at an art gallery. He had created various complex geometric designs, which you have to view with one eye, aiming your blind spot at a specific section of the painting.

Like James Thurber, he had used his blind spot creatively to inspire his art.

I hope these examples have given you a feel for what it is like to "fill in" missing portions of the visual field.

You have to bear in mind, though, that you have had a blind spot all your life and you might be especially skilled at this process. But what if you lost a patch of visual cortex as a result of disease or accident? What if a much larger hole in your visual field—a scotoma—suddenly appeared? Such patients do exist 70

Figure 5.7
Amazingly, when the blind spot is aimed at the center of a bicycle wheel, no gap is seen. People
usually report that the spokes converge toward a vortex.

and they present a valuable opportunity to study how far the brain can go in supplying the "missing information" when needed. Migraine patients have transient scotomas, but I decided it would be best to study someone who had a large permanent blind spot in his visual field, and that is how I met Josh.9

Josh was a large man with Brezhnev−like eyebrows, a barrel chest and meaty hands. Yet he exuded a natural twinkle and sense of humor that infused what would otherwise be a rather menacing body type with the burly sweetness of a teddy bear. Whenever Josh laughed, everyone in the room chuckled with him. Now in his early thirties, some years earlier he had suffered an industrial accident in which a steel rod penetrated the back of his skull, punching a hole in his right occipital pole in the primary visual cortex. When Josh looks straight ahead, he has a blind spot about the size of my palm to the left of where he's looking. No other part of his brain was damaged. When Josh came to see me, he said that he was well aware that he had a large blind spot.

"How do you know?" I asked.

"Well, one problem is that I often walk into the women's room."

"Why is that?"

"Because when I look at the sign women straight on, I don't see the 'w' and the 'o' to the left. I just see 'men.' "

Josh insisted, however, that other than these occasional hints that something was wrong, his vision seemed surprisingly normal. In fact, given his deficit, he was surprised by the unitary nature of his visual world.

"When I look at you," he said, "I don't see anything missing. No pieces are left out." He paused, knitted his eyebrows, studied my face and then broke into a huge smile. "If I pay careful attention, Dr. Ramachandran, I notice that one of your eyes and an ear are missing! Are you feeling okay?"

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Unless he scrutinized his visual field, Josh seemed to fill in the missing information with no trouble. Although researchers have known for a long time that patients like Josh exist (and live quite normally except when frightening women in ladies' rooms), many psychologists and physicians have remained skeptical of the filling−in phenomenon. For example, the Canadian psychologist Justine Sergent claimed that patients like Josh are confabulating or engaging in a kind of unconscious guesswork when they say they can see normally.

(He guesses that there is wallpaper in his scotoma because there is wallpaper everywhere else.) This type of guesswork, she said, would be very different from the types of true perceptual completion that you experienced when you had a line passing through your blind spot.10 But I realized that Josh gave us the opportunity to find out what is really going on inside a scotoma. Why try to second−guess the mechanisms of vision from scratch when we could ask Josh?

Josh swept into the laboratory one drizzly, cold afternoon, propped an umbrella in one corner and lit up the room with his cheerfulness. He was dressed in a plaid shirt, loose jeans and beat−up running shoes, damp with mud from the walk into our building. We were going to have some fun today. Our strategy was simply to repeat on Josh all the experiments you just did on your own blind spot. First, we decided to see what would happen if we ran a line through his scotoma, where a big piece of the visual field was missing. Would he see the line as having a gap, or would he fill it in?

But before we did the experiment, we realized we had a minor technical problem. If we gave Josh an actual line, asked him to look straight

ahead and tell us whether he saw a complete line or piece missing, he might "cheat" inadvertently. He might accidentally move his eyes a tiny amount, and the slight motion would bring the line into his normal visual field and would tell him that the line is complete. We wanted to avoid that so we simply presented Josh with two half lines on either side of his scotoma and asked him what he saw. Would he see a continuous line or two half lines? Recall that when you tried this little experiment using your own blind spot, you saw the lines as complete.

He considered for a moment and said, "Well, I see two lines, one above, one below and there's a big gap in the middle."

"Okay," I said. This was not going anywhere.

"Wait!" said Josh, squinting. "Wait a minute. You know what? They're growing toward each other."

"What?"

He held up his right index finger vertically, pointing upward, to mimic the bottom line and his left index finger pointing downward to mimic the top line. At first the two fingertips were two inches apart, and then Josh started moving them toward each other. "Okay," he said excitedly. "They're growing, growing, growing, growing together, and now there's one complete line." As he said this, his index fingers touched.

Not only is Josh filling in, but the filling in is happening in real time. He could watch it and describe it, contrary to claims that the phenomenon doesn't exist in people with scotomas.

Clearly some nerve circuits in Josh's brain were taking two half lines, lying on either side of the scotoma, as sufficient evidence that there is a complete line there, and these circuits are sending this message to higher centers in Josh's brain. So his brain could complete information across the huge, gaping hole right near his center of gaze in much the same way that you did across your natural blind spot.

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Next we wondered what would happen when we deliberately misaligned the two lines. Would he complete it with a diagonal line? Or would his visual system simply give up? Presented with this display, Josh said, "No dice. They're not completed. I see a gap. Sorry."

"I know that; just tell me what happens."

A couple of seconds later Josh exclaimed, "Oh, my God, look what's happening!"

"What?"

"Hey, they started like this and now they're moving toward each other like this." He again held up his fingers to show the two lines moving sideways. "Now they're completely lined up, and now they're filling in like that. Okay, now it's complete." The whole process lasted five seconds, an eternity as far as the visual system is concerned. We repeated the experiment several times with identical results.

So it seemed fairly clear we are dealing with genuine perceptual completion here, for why else would it take so many seconds? If Josh were guessing, he should guess immediately. But how far could we push this? How sophisticated is the visual system's capacity to "insert" the missing information? What if we used a vertical column of "XV instead of a plain line? Would he actually hallucinate the missing "X's"? What if we used a column of smiling faces? Would he fill in the scotoma with smiling faces?

So we put the vertical column "X's" on the computer screen and asked Josh to look to the immediate right of this column so that the middle three "X's" fell on the scotoma.

"What do you see?" I asked.

"I see 'X's' on top, 'X's' on the bottom, and there's a big gap in the middle."

I told him to keep looking at it since we had already established that filling in takes time.

"Look, doctor, I'm staring at it and I know you want me to see an 'X' there, but I don't see it. No 'X's.' Sorry."

He stared at it for three minutes, four minutes, five minutes, and then we both gave up.

Then I tried a long vertical row of tiny 'x's,' one set above and one below the scotoma. "Now what do you see?"

"Oh, yeah, it's a continuous column of 'x's,' little 'x's.' " Josh turned to me and said, "I know you're really tricking me. There are no 'x's' really there. Are there?"

"I'm not going to tell you. But I want to know one more thing. Do the 'x's' on the left side of where you're looking (which I knew were in his scotoma) appear any different from the ones above and below?"

Josh replied, "It looks like a continuous column of 'x's.' I don't see any difference."

Josh was filling in the little "x's" but not the big "X's." This difference is important for two reasons. First it rules out the possibility of confabulation. Often in neurology tests, patients will make up a story, putting on a show for the physician's benefit. Knowing there were "x's" above and below, Josh could have guessed that he

"saw" them in between without really doing so. But why would he only engage in such guesswork for the little "x's" and not the big ones? Since he did not fill in the missing large "X's," we can assume that in the case of the little

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"x's" we're dealing with a genuine perceptual completion process, not with guesswork or confabulation.

Why did the genuine perceptual completion occur only for the little "x's" and not the large ones? Perhaps the brain treats the tiny "x's" as forming a continuous texture and therefore completes it, but when confronted with large "x's" it switches to a different mode of operation and "sees" that some of the "X's" were missing. My hunch is that the tiny letters activated a different part of Josh's visual pathway, one that deals with continuity of textures and surfaces, whereas the large letters would be processed in the pathway in his temporal lobes that is concerned with objects (discussed in the last chapter) rather than surfaces. It makes sense that the brain should be especially skilled at completing gaps when dealing with continuous surface textures and colors but not when dealing with objects. The reason is that surfaces in the real world are usually composed of uniform

"stuff" or surface texture—like a block of grainy wood or a sandstone cliff—but there is no such thing as a natural surface made up of large alphabetical letters or faces. (Of course man−made surfaces like wallpaper can be made of smiling faces, but the brain didn't originally evolve in a man−made world.) To test the notion that completion of textures and "stuff" across a gap can occur much more easily than completion of objects or letters, I was tempted to try something a bit outlandish. I put up the numerals 1, 2 and 3 above the scotoma and 7, 8 and 9 below. Would Josh perceptually complete the sequence? What would he see in the middle? Of course, I used tiny numerals to ensure that the brain would treat them as a "texture."

"Hmmm," said Josh, "I see a continuous column of numbers, vertically aligned numbers."

"Can you see a gap in the middle?"

"No."

"Can you read them out loud for me?"

"Urn, one, two, three, urn, seven, eight, nine. Hey, that's very strange. I can see the numbers in the middle, but I can't read them. They look like numbers, but I don't know what they are."

"Do they look blurred?"

"No, they don't look blurred. They kind of look strange. I can't tell what they are—like hieroglyphics or something."

We had induced a curious form of temporary dyslexia in Josh. Those middle numbers did not exist, were not flashed before his eyes, yet his brain was making up the textural attributes of the number string and completing it. This is another striking demonstration of division of labor in the visual pathways. The system in his brain that deals with surfaces and edges is saying, "There is numberlike stuff in this region—that's what you should see in the middle," but since there are no actual numbers, his object pathway remains silent and the net result is illegible "hieroglyphics"!