Phantoms in the Brain: Probing the Mysteries of the Human Mind (20 page)

I was not sure. What does he mean by "more real than real"? There is a school of art called superrealism in which the paintings of things like

Campbell's soup cans are created with the kind of fine detail you only get through a magnifying glass. These objects are strange to look at, but maybe that was how Larry saw images in his scotoma.

"Does this bother you, Larry?"

"Well, it kind of does because it makes me curious about why I experience them, but it really doesn't get in my way. I'm much more worried about the fact that I'm blind than about the fact that I see hallucinations. In fact, sometimes they are fun to watch because I never know what I'm going to see next."

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"Are the images you see, like this monkey in my lap, things you've seen before in your life or can the hallucinations be completely new?"

Larry thought a moment and said, "I think they can be completely new images, but how can that be? I always thought that hallucinations were limited to things you've already seen elsewhere in your life. But then lots of times the images are rather ordinary. Sometimes, when I'm looking for my shoes in the morning, the whole floor suddenly is covered with shoes. It's hard to find my own shoes! More often the visions come and go, as if they have a life of their own, even though they are unconnected to what I'm doing or thinking about at the time."

Not long after my conversations with Larry, I met another Charles Bonnet patient, whose world was stranger yet. She was plagued by cartoons! Nancy was a nurse from Colorado who had an arteriovenous malformation or AVM—basically a cluster of swollen and fused arteries and veins in the back of her brain. If it were to rupture, she could die from a brain hemorrhage, so her doctors zapped the AVM with a laser to reduce it in size and "seal it off." In so doing they left scar tissue on parts of her visual cortex. Like Josh, she had a small scotoma and hers was immediately to the left of where she was looking, covering about ten degrees of space.

(If she stretched her arm out in front of her and looked at her hand, the scotoma would be about twice the size of her palm.)

"Well, the most extraordinary thing is that I see images inside this scotoma," Nancy said, sitting in the same chair that Larry had occupied earlier. "I see them dozens of times a day, not continuously, but at different times lasting several seconds each time."

"What do you see?"

"Cartoons."

"What?"

"Cartoons."

"What do you mean by cartoons? You mean Mickey Mouse?"

"On some occasions I see Disney cartoons. But most commonly not. Mostly what I see is just people and animals and objects. But these are always line drawings, filled in with uniform color like comic books. It's most amusing. They remind me of Roy Lichtenstein drawings."

"What else can you tell me? Do they move?"

"No. They are absolutely stationary. The other thing is that my cartoons have no depth, no shading, no curvature."

So that's what she meant when she said they were like comic books. "Are they familiar people or are they people you've never seen?" I asked.

"They can be either," Nancy said. "I never know what's coming next."

Here is a woman whose brain creates Walt Disney cartoons in defiance of copyright. What is going on? And how could any sane person see a monkey on my lap and accept it as normal?

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To understand these bizarre symptoms, we are going to have to revise our models of how the visual system and perception operate from day to day. In the not too distant past, physiologists drew diagrams of visual areas with arrows pointing up. An image would be processed at one level, sent on up to the next level and so on, until the "gestalt" eventually emerged in some mysterious manner. This is the so−called bottom−up view of vision, championed by artificial intelligence researchers over the last three decades, even though many anatomists have long emphasized that there are massive feedback pathways projecting from the so−called higher areas to lower visual areas. To pacify these anatomists, textbook diagrams usually also included arrows pointing backward, but, by and large, the notion of back projections was given more lip service than functional meaning.

A newer view of perception—championed by Dr. Gerald Edelman of the Neurosciences Institute in La Jolla, California—suggests that the brain's information flow resembles the images in a funhouse full of mirrors, continually reflected back and forth, and continually changed by the process of reflection.12 Like separate light beams in a funhouse, visual information can take many different paths, sometimes diverging, sometimes reinforcing itself, sometimes traveling in opposite directions.

If this sounds confusing, let's return to the distinction I made earlier between seeing a cat and imagining a cat.

When we see a cat, its shape, color, texture and other visible attributes will impinge upon our retina and travel through the thalamus (a relay station in the middle of the brain) and up into the primary visual cortex for processing into two streams or pathways. As discussed in the previous chapter, one pathway goes to regions dealing with depth and motion—allowing you to grab or dodge objects and to move around the world—and the other to regions dealing with shape, color and object recognition (these are the how and what vision pathways). Eventually, all the information is combined to tell us that this is a cat—say, Felix—and to enable us to recall everything we've ever learned or felt about cats in general and Felix in particular. Or at least that's what the textbooks tell us.

Now think of what's going on in your brain when you imagine a cat.13 There's good evidence to suggest that we are actually running our visual machinery in reverse! Our memories of all cats and of this particular cat flow from top to bottom—from higher regions to the primary visual cortex—and the combined activities of all these areas lead to the perception of an imaginary cat by the mind's eye. Indeed, the activity in the primary visual cortex may be almost as strong as if you really did see a cat, but in fact the cat is not there. This means that the primary visual cortex, far from being a mere sorting office for information coming in from the retina, is more like a war room where information is constantly being sent back from scouts, enacting all sorts of scenarios, and then information is sent back up again to those same higher areas where the scouts are working.

There's a dynamic interplay between the brain's so−called early visual areas and the higher visual centers, culminating in a sort of virtual reality simulation of the cat. (All this was discovered mainly from animal experiments and neuroimaging studies in humans.)

It's not yet clear exactly how this "interplay" occurs or what its function might be. But it may explain what is happening in the Charles Bonnet patients like Larry and Nancy or the senior citizens sitting in a darkened room at the nursing home. I suggest that they are filling in missing information in much the same way that Josh did except that they are using high−level stored memories.14 So, in Bonnet syndrome, the images are based on a sort of "conceptual completion" rather than perceptual completion; the images being "filled in" are coming from memory (top down)—not from the outside (bottom up). Clowns, water lilies, monkeys and cartoons populate the blind region rather than just the information immediately surrounding the scotoma such as lines and small "x's." Of course, when Larry sees a monkey in my lap he isn't duped; he knows perfectly well it's not real because he realizes it's highly improbable that there should be a monkey in my office.

But if this argument is correct—if the early visual areas are activated each time you imagine something—then why don't you and I hallucinate all the time or at least occasionally confuse our internally generated im−

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ages with real objects? Why don't you see a monkey in the chair when you simply think of one? The reason is that even if you close your eyes, cells in your retina and in early sensory pathways are constantly active—

producing a flat, baseline signal. This baseline signal informs your higher visual centers that there is no object (monkey) hitting the retina— thereby vetoing the activity evoked by top−down imagery. But if the early visual pathways are damaged, this baseline signal is removed and so you hallucinate.15

It makes good evolutionary sense that even though your internal images can be very realistic, they can never actually substitute for the real thing. You cannot, as Shakespeare said, "cloy the hungry edge of appetite by bare imagination of a feast." A good thing, too, because if you could satisfy your hunger by thinking about a feast, you wouldn't bother to eat and would quickly become extinct. Likewise, any creature that could imagine orgasms is unlikely to transmit its genes to the next generation. (Of course, we can do so to a limited extent as when our hearts pound when imagining an amorous encounter—the basis of what is sometimes called visualization therapy.)

Additional support for this interaction between top−down imagery and bottom−up sensory signals in perception comes from what we saw in phantom limb patients who have vivid impressions of clenching their nonexistent fingers and digging imaginary fingernails into their phantom palms, generating unbearable pain.

Why do these patients actually
feel
clenching, "nails digging" and pain, whereas you or I can imagine the same finger position but feel nothing? The answer is that you and I have real input coming in from our hands telling us that there is no pain, even though we have memory traces in our brain linking the act of clenching with nails digging (especially if you don't often cut your nails). But in an amputee, these fleeting associations and preexisting pain memories can now emerge without contradiction from ongoing sensory input. The same sort of thing might be happening in Charles Bonnet syndrome.

But why did Nancy always see cartoons in her scotoma? One possibility is that in her brain the feedback comes mainly from the what pathway in the temporal lobe, which, you will recall, has cells specialized for color and shapes but not for motion and depth, which are handled by the how pathway. Therefore, her scotoma is filled with images that lack depth and motion, having only outlines and shapes, as do cartoons.

If I'm right, all these bizarre visual hallucinations are simply an exaggerated version of the processes that occur in your brain and mine every time we let our imagination run free. Somewhere in the confused welter of interconnecting forward and backward pathways is the interface between vision and imagination We don't have clear ideas yet about where this interface is or how it works (or even whether there is a single interface), but these patients provide some tantalizing clues about what might be going on. The evidence from them suggests that what we call perception is really the end result of a dynamic interplay between sensory signals and high−level stored information about visual images from the past. Each time any one of us encounters an object, the visual system begins a constant questioning process. Fragmentary evidence comes in and the higher centers say, "Hmmmmm, maybe this is an animal." Our brains then pose a series of visual questions: as in a twenty−questions game. Is it a mammal? A cat? What kind of cat? Tame? Wild? Big?

Small? Black or white or tabby? The higher visual centers then project partial "best fit" answers back to lower visual areas including the primary visual cortex. In this manner, the impoverished image is progressively worked on and refined (with bits "filled in," when appropriate). I think that these massive feed forward and feedback projections are in the business of conducting successive iterations that enable us to home in on the closest approximation to the truth.16 To overstate the argument deliberately, perhaps we are hallucinating all the time and what we call perception is arrived at by simply determining which hallucination best conforms to the current sensory input. But if, as happens in Charles Bonnet syndrome, the brain does not receive confirming visual stimuli, it is free simply to make up its own reality. And, as James Thurber was well aware, there is apparently no limit to its creativity.

Through the Looking Glass

The world is not only queerer than we imagine; it is queerer than we ean imagine.

—
J.B.S. Haldane

Who was this rolling out of the bedroom in a wheelchair? Sam couldn't believe his eyes. His mother, Ellen, had just returned home the night before, having spent two weeks at the Kaiser Permanente hospital recuperating from a stroke. Mom had always been fastidious about her looks. Clothes and makeup were Martha Stewart perfect, with beautifully coiffed hair and fingernails painted in tasteful shades of pink or red.

But today something was seriously wrong. The naturally curly hair on the left side of Ellen's head was uncombed, so that it stuck out in little nestlike clumps, whereas the rest of her hair was neatly styled. Her green shawl was hanging entirely over her right shoulder and dragging on the floor. She had applied rather bright red lipstick to her upper right and lower right lips, leaving the rest of her mouth bare. Likewise, there was a trace of eyeliner and mascara on her right eye but the left eye was unadorned. The final touch was a spot of rouge on her right cheek—

very carefully applied so as not to appear as if she were trying to hide her ill health but enough to demonstrate that she still cared about her looks. It was almost as though someone had used a wet towel to erase all the makeup on the left side of his mother's face!

"Good grief!" cried Sam. "What did you do to your makeup?"

Ellen raised her eyebrow in surprise. What was her son talking about? She had spent half an hour getting ready this morning and felt she looked as good as she possibly could, given the circumstances.

Ten minutes later, as they sat eating breakfast, Ellen ignored all the food on the left side of her plate, including the fresh−squeezed orange juice she so loved.

Sam raced for the phone and called me, as one of the physicians who had spent time with his mother at the hospital. Sam and I had gotten to know one another while I had been seeing a stroke patient who shared a room with his mother. "It's all right," I said, "don't be alarmed. Your mother is suffering from a common neurological syndrome called hemi−neglect, a condition that often follows strokes in the right brain, especially in the right parietal lobe. Neglect patients are profoundly indifferent to objects and events in the left side of the world, sometimes including the left side of their own bodies."