Phantoms in the Brain: Probing the Mysteries of the Human Mind (19 page)

It has been known for over two decades now that what we call the visual system is actually several systems; that there are multiple specialized cortical areas concerned with different visual attributes such as motion, color and other dimensions. Does filling in occur separately in each of these areas, or does it occur all at once in just one single area? To find out, we asked Josh to look at the center of a blank screen on the computer monitor, and then we suddenly switched on a pattern of twinkling black dots on a red background.

Josh whistled, apparently taking as much delight in all this as I was. "My God, doctor," he said, "I can actually see my scotoma for the first time." He yanked a felt pen from my hand and much to my dismay proceeded to 74

start drawing on the monitor, producing what appeared to be an outline of the irregular margins of the scotoma (Josh's ophthalmologist, Dr. Lilian Levinson, had earlier mapped out his scotoma using a sophisticated technique called perimetry and I could therefore compare his drawing with hers; they were identical).

"But Josh, what do you see inside the scotoma?" I asked.

"Well, it's very strange, doctor. For the first few seconds, I saw only the red color bleeding into this part of the screen, but the twinkling black dots did not fill in. Then after a few seconds, the dots filled in, but they weren't twinkling. And last, the actual twinkle—the motion sensation—filled in as well." He turned around, rubbed his eye, looked at me and said, "What does all this mean?"11

The answer is that filling in seems to occur at different speeds for different perceptual attributes like color, motion (twinkle) and texture. Motion takes longer to fill in than color, and so on. Indeed, such differential filling in provides additional evidence that such specialized areas do exist in the human brain. For if perception were just one process happening in a single location in the brain, it should happen all at once, not in stages.

Finally, we tested Josh's ability to fill in more sophisticated shapes, like the corners of squares. Remember when you tried aiming your blind spot on a corner, it was chopped off—your brain apparently couldn't fill it in. When we tried the same experiment on Josh, we got the opposite result. He had no difficulty in seeing the missing corner, proving that very sophisticated types of completion were taking place in his brain.

By now, Josh was feeling tired, but we had succeeded in making him as intensely curious about the filling−in process as we were. Having heard the King Charles story from me, he decided to try to aim his scotoma at my graduate student's head. Would his brain prefer to complete her head (contrary to what happened in your blind spot) to prevent such a horrendous spectacle? The answer is no. Josh always saw this person with a head missing. Thus he could fill in parts of simple geometric shapes but not complex objects like faces or things of that nature. This experiment again shows that filling in is not simply a matter of guesswork, for there is no reason Josh shouldn't have been able to "guess" that my student's head was still there.

An important distinction must be made between perceptual and conceptual completion. To understand the difference, just think of the space behind your head now as you are sitting on your chair reading this book.

You can let your mind wander, thinking about the kinds of objects that might be behind your head or body. Is there a window? A Martian? A gaggle of geese? With your imagination, you can "fill in" this missing space with just about anything, but since you can change your mind about the content, I call this process conceptual filling in.

Perceptual filling in is very different. When you fill in your blind spot with a carpet design, you don't have such choices about what fills that spot; you can't change your mind about it. Perceptual filling in is carried out by visual neurons. Their decisions, once made, are irreversible: Once they signal to higher brain centers "Yes, this is a repetitive texture" or "yes, this is a straight line," what you perceive is irrevocable. We will return to this distinction between perceptual and conceptual filling in, which philosophers are very interested in, later when we talk about consciousness and whether Martians see red in Chapter 12. For now, it suffices to emphasize that we're dealing with true perceptual completion across the scotomas, not just guesswork or deduction.

This phenomenon is far more important that one might imagine from the parlor games I've just described.

Decapitating department chairmen is amusing, but why should the brain engage in perceptual completion?

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The answer lies in a Darwinian explanation of how the visual system evolved. One of the most important principles in vision is that it tries to get away with as little processing as it can to get the job done. To economize on visual processing, the brain takes advantage of statistical regularities in the world—such as the fact that contours are generally

continuous or that table surfaces are uniform—and these regularities are captured and wired into the machinery of the visual pathways early in visual processing. When you look at your desk, for instance, it seems likely that the visual system extracts information about its edges and creates a mental representation that resembles a cartoon sketch of the table (again, this initial extraction of edges occurs because your brain is mainly interested in regions of change, of abrupt discontinuity, at the edge of desk, which is where the information is). The visual system might then apply surface interpolation to "fill in" the color and texture of the table, saying in effect, "Well, there's this grainy stuff here; it must be the same grainy stuff all over." This act of interpolation saves an enormous amount of computation; your brain can avoid the burden of scrutinizing every little section of the desk and can simply employ loose guesswork instead (bearing in mind the distinction between conceptual guesswork and perceptual guesswork).

What has all of this got to do with James Thurber and other patients with Charles Bonnet syndrome? Might the findings that we have discussed so far about the brain's capacity for "filling in" blind spots and scotomas also help us understand the extraordinary visual hallucinations they experience?

Medical syndromes are named after their discoverers, not the patients who suffer from them, and this one was named after a Swiss naturalist, Charles Bonnet, who lived from 1720 to 1773. Even though he suffered from precarious health and was always on the brink of losing his own eyesight and hearing, Bonnet was a shrewd observer of the natural world. He was the first person to observe parthenogenesis—the production of offspring by an unfertilized female—and that led him to propose an absurd theory known as preformationism, the idea that each egg carried by a female must contain an entire preformed individual, presumably with miniature eggs of its own, each of which in turn contains even tinier individuals with eggs, and so on, ad infinitum. As luck would have it, many physicians remember Charles Bonnet as the gullible chap who hallucinated little people in eggs and not as the insightful biologist who discovered parthenogenesis.

Fortunately, Bonnet was more perceptive when he observed and reported on an unusual medical situation in his own family. His maternal grandfather, Charles Lullin, had successfully undergone what in those days was dangerous and traumatic surgery—the removal of cataracts at

age seventy−seven. Eleven years after the operation, the grandfather began suffering vivid hallucinations.

People and objects would appear and disappear without warning, grow in size and then recede. When he stared at the tapestries in his apartment, he saw bizarre transformations involving people with strange gazes and animals that were, he realized, flowing from his brain and not the weaver's loom.

This phenomenon, as I mentioned earlier, is fairly common in elderly people with visual handicaps like macular degeneration, diabetic retinopathy, corneal damage and cataracts. A recent study in the
Lancet,
a British medical journal, reported that many older men and women with poor vision hide the fact that they "see things which aren't really there." Out of five hundred visually handicapped people, sixty admitted that they hallucinated, sometimes only once or twice a year, but others experienced visual fantasies at least twice a day.

For the most part the content of their imaginary world is mundane, perhaps involving an unfamiliar person, a bottle or a hat, but the hallucinations can also be quite funny. One woman saw two miniature policemen guiding a midget villain to a tiny prison van. Others saw ghostly translucent figures floating in the hallway, dragons, people wearing flowers on their heads and even beautiful shining angels, little circus animals, clowns and elves. A surprising number of them report seeing children. Peter Halligan, John Marshall and I once saw a patient at Oxford who not only "saw" children in her left visual field but could actually hear their laughter, only to turn her head and realize no one was there. The images can be in black and white or color, stationary 76

or in motion, and just as clear as, less clear than or more clear than reality. At times the objects blend into actual surroundings so that an imaginary person sits in a real chair, ready to speak. The images are rarely threatening—no slavering monsters or scenes of brutal carnage.

Patients were always easily corrected by others while hallucinating. A woman said that she once sat at her window watching cows in a neighboring meadow. It was actually very cold and the middle of winter, and she complained to her maid about the cruelty of the farmer. The astonished maid looked, saw no cows and said,

"What are you talking about? What cows?" The woman flushed with embarrassment. "My eyes are tricking me. I can't trust them anymore."

Another woman said, "In my dreams I experience things which affect me, which are related to my life. These hallucinations, however, have nothing to do with me." Others are not so sure. An elderly childless man was intrigued by recurrent hallucinations of a little girl and boy and wondered whether these hallucinations reflected his unfulfilled wish to

become a father. There's even a report of a woman who saw her recently deceased husband three times a week.

Given how common this syndrome is, I am tempted to wonder whether the occasional reports of "true"

sightings of ghosts, UFOs and angels by otherwise sane intelligent people may merely be examples of Charles Bonnet hallucinations. Is it any surprise that roughly one third of Americans claim to have seen angels? I'm not asserting that angels don't exist (I have no idea whether they do or not) but simply that many of the sightings may be due to ocular pathology.

Poor lighting and the changing tones at dusk favor such hallucinations. If the patients blink, nod their heads or turn on a light, the visions often cease. Nevertheless, they have no voluntary control over the apparitions, which usually appear without warning. Most of us can imagine the scenes these people describe—a miniature police van with miniature criminals running about—but we exert conscious control over such imaginations.

With Charles Bonnet syndrome, on the other hand, the images appear completely unbidden as if they are real objects.

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This sudden appearance of intrusive images was apparent in the case of Larry MacDonald, a twenty−seven−year−old agronomist who suffered a terrible automobile accident. Larry's head smashed into the windshield, fracturing the frontal bones above his eyes and the orbital plates that protected his optic nerves. Comatose for two weeks, he could neither walk nor talk when he regained consciousness. But that wasn't the worst of his problems. As Larry recalls, "The world was filled with hallucinations, both visual and auditory. I couldn't distinguish what was real from what was fake. Doctors and nurses standing next to my bed were surrounded by football players and Hawaiian dancers. Voices came at me from everywhere and I couldn't tell who was talking." Larry felt panic and confusion.

Gradually, however, his condition improved as his brain struggled to repair itself after the trauma. He regained control over his bodily functions and learned to walk. He could talk, with difficulty, and learned to distinguish real voices from imagined ones—a feat that helped him suppress the auditory hallucinations.

I met Larry five years after his accident because he had heard about my interest in visual hallucinations. He talked slowly, with effort, but was otherwise intelligent and perceptive. His life was normal except for one astonishing problem. His visual hallucinations, which used to occur anywhere and everywhere in his visual field with brilliant colors and spinning motions, had retreated into the lower half of his field of vision, where 77

he was completely blind. That is, he would only see imaginary objects below a center line extending from his nose outward. Everything above the line was completely normal; he would always see what was really out there. Below the line, he had intermittent recurrent hallucinations.

"Back in the hospital, colors used to be a lot more vivid," Larry said.

"What did you see?" I asked.

"I saw animals and cars and boats, you know. I saw dogs and elephants and all kinds of things."

"You can still see them?"

"Oh, yeah, I see them right now here in the room."

"You are seeing them now as we speak?"

"Oh, yeah!" said Larry.

I was intrigued. "Larry, you said that when you see them ordinarily, they tend to cover other objects in the room. But right now you're looking straight at me. It's not like you see something covering me right now, right?"

"As I look at you, there is a monkey sitting on your lap," Larry announced.

"A monkey?"

"Yes, right there on your lap."

I thought he was joking. "Tell me how you know you're hallucinat−ing."

"I don't know. But it's unlikely there would be a professor here with a monkey sitting in his lap so I think there probably isn't one." He smiled cheerfully. "But it looks extremely vivid and real." I must have looked shocked, for Larry continued, "For one thing they fade after a few seconds or minutes, so I know they're not real. And even though the image sometimes blends quite well into the rest of the scene around it, like the monkey on your lap," he continued, "I realize that it is highly improbable and usually don't tell people about it." Speechless, I glanced down at my lap while Larry just smiled. "Also, there is something odd about the images—they often look too good to be true. The colors are vibrant, extraordinarily vivid, and the images actually look more real than real objects, if you see what I mean."