Phantoms in the Brain: Probing the Mysteries of the Human Mind (21 page)

"You mean she's blind on the left side?"

"No, not blind. She just doesn't pay attention to what's on her left. That's why we call it neglect."

The next day I was able to demonstrate this to Sam's satisfaction by doing a simple clinical test on Ellen. I sat directly in front of her and said, "Fixate steadily on my nose and try not to move your eyes." When her gaze was fixed, I held my index finger up near her face, just to the left of her nose, and wiggled it vigorously.

"Ellen, what do you see?"

"I see a finger wiggling," she replied.

"Okay," I said. "Keep your eyes fixed on the same spot on my nose." Then, very slowly and casually, I raised the same finger to the same position, just left of her nose. But this time I was careful not to move it abruptly.

"Now what do you see?"

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Ellen looked blank. Without having her attention drawn to the finger—via motion or other strong cues—she was oblivious. Sam began to understand the nature of his mother's problem, the important distinction between blindness and neglect. His mother would ignore him completely if he stood on her left side and did nothing.

But if he jumped up and down and waved his arms, she would sometimes turn around and look.

For the same reason, Ellen fails to notice the left side of her face in a mirror, forgets to apply makeup on the left side of her face, and doesn't comb her hair or brush her teeth on that side. And, not surprisingly, she even ignores all the food on the left side of her plate. But when her son points to things in the neglected area, forcing her to pay attention, Ellen might say, "Ah, how nice. Fresh−squeezed orange juice!" or "How embarrassing. My lipstick is crooked and my hair unkempt."

Sam was baffled. Would he have to assist Ellen for the rest of her life with simple day−to−day chores like applying makeup? Would his mother remain like this forever, or could I do something to help her?

I assured Sam that I'd try to help. Neglect is a fairly common problem1 and I've always been intrigued by it.

Beyond its immediate relevance to a patient's ability to care for herself, it has profound implications for understanding how the brain creates a spatial representation of the world, how it deals with left and right and how we are able—at a moment's notice—to pay attention to different portions of the visual scene. The great German philosopher Immanuel Kant became so obsessed with our "innate" concepts of space and time that he spent thirty years pacing up and down his veranda thinking about this problem. (Some of his ideas later inspired Mach and Einstein.) If we could somehow transport Ellen back in a time machine to visit him, I'm sure he'd be just as fascinated by her symptoms as you or I and would wonder whether we modern scientists had any inkling of what causes this strange condition.

When you glance at any visual scene, the image excites receptors in your retina and sets in motion a complex cascade of events that culminate in your perception of the world. As we noted in earlier chapters, the message from the eye is first mapped onto an area in the back of brain called the primary visual cortex. From there it is relayed along two pathways, the how pathway to the parietal lobe and the what pathway to the temporal lobe (see Figure 4.5, Chapter 4). The temporal lobes are concerned with recognizing and naming individual objects and responding to them with the appropriate emotions. The parietal lobes, on the other hand, are concerned with discerning the spatial layout of the external world, allowing you to navigate through space, reach out for objects, dodge missiles and otherwise know where you are. This division of labor between temporal and parietal lobes can explain almost all of the peculiar constellation of symptoms one sees in neglect patients in whom one parietal lobe—especially the right—is damaged, as is the case with Ellen. If you let her wander around by herself, she will not pay attention to the left side of space and anything that happens in it. She will even bump

into objects on her left side or stub her left toe on a raised pavement. (I'll later explain why this doesn't happen with left parietal damage.) However, because Ellen's temporal lobes are still intact, she has no difficulty recognizing objects and events as long as her attention is drawn to them.

But "attention" is a loaded word, and we know even less about it than we do about neglect. So the statement that the neglect arises from a "failure to pay attention" doesn't really tell us very much unless we have a clear notion of what the underlying neural mechanisms might be. (It's a bit like saying that illness results from a failure of health.) In particular, one would like to know how a normal person—you or I—is able to attend selectively to a single sensory input, whether you are trying to listen to a single voice amid the background din of voices at a cocktail party or just trying to spot a familiar face in a baseball stadium. Why do we have this vivid sense of having an internal searchlight, one that we can direct at different objects and events around us?2

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We now know that even so basic a skill as attention requires the participation of many far−flung regions of the brain. We've already talked about the visual, auditory and somatosensory systems, but other special brain regions carry out equally important tasks. The reticular activating system—a tangle of neurons in the brain stem that projects widely to vast regions of the brain—activates the entire cerebral cortex, leading to arousal and wakefulness, or—when needed—a small portion of the cortex, leading to selective attention. The limbic system is concerned with emotional behavior and evaluation of the emotional significance and potential value of events in the external world. The frontal lobes are concerned with more abstract processes like judgment, foresight and planning. All of these areas are interconnected in a positive feedback loop—a recursive, echolike reverberation—that takes a stimulus from the outside world, extracts its salient features and then bounces it from region to region, before eventually figuring out what it is and how to respond to it.3 Should I fight, flee, eat or kiss? The simultaneous deployment of all these mechanisms culminates in perception.

When a large, threatening stimulus—say, an image of a menacing figure, perhaps a mugger looming toward me on the street in Boston— first comes into my brain, I haven't the slightest idea of what it is. Before I can determine, aha, perhaps that's a dangerous person, the visual information is evaluated by both the frontal lobes and the limbic system for relevance and sent on to a small portion of the parietal cortex, which, in conjunction with appropriate neural connections in the reticular for−

mation, enables me to direct my attention to the looming figure. It forces my brain to swivel my eyeballs toward something important out there in the visual scene, pay selective attention to it and say, "Aha!"

But imagine what would happen if any part of this positive feedback loop were interrupted so that the whole process was compromised. You would then no longer notice what was happening on one side of the world.

You would be a neglect patient.

But we still have to explain why neglect occurs primarily after injury to the right parietal lobe and not to the left. Why the asymmetry? Though the real reason continues to elude us, Marcel Mesulam of Harvard University has proposed an ingenious theory. We know that the left hemisphere is specialized for many aspects of language and the right hemisphere for emotions and "global" or holistic aspects of sensory processing. But Mesulam suggests there is another fundamental difference. Given its role in holistic aspects of vision, the right hemisphere has a broad "searchlight" of attention that encompasses both the entire left and entire right visual fields. The left hemisphere, on the other hand, has a much smaller searchlight, which is confined entirely to the right side of the world (perhaps because it is so busy with other things, such as language). As a result of this rather odd arrangement, if the left hemisphere is damaged, it loses its searchlight, but the right can compensate because it casts a searchlight on the entire world. When the right hemisphere is damaged, on the other hand, the global searchlight is gone but the left hemisphere cannot fully compensate for the loss because its searchlight is confined only to the right side. This would explain why neglect is only seen in patients whose right hemisphere is damaged.

So neglect is not blindness, but rather a general indifference to objects and events on the left. But how profound is this indifference? After all, even you and I, when driving home from work ignoring familiar terrain, will perk up immediately if we see an accident. This suggests that at some level the unattended visual information from the road must have been getting through. Is Ellen's indifference an extreme version of the same phenomenon? Is it possible that even though she doesn't notice things consciously, some of the information "leaks" through? Do these patients at some level "see" what they don't see? This is not an easy question to answer, but in 1988 two Oxford researchers, Peter Haligan and John Marshall,4 took up the challenge. They devised a clever way to demonstrate that neglect patients are subconsciously aware of some of the things that are going on on their left side, even though they appear not to be. They showed patients drawings of two houses, one below the other, that

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were completely identical except for one salient feature—the house on the top had flames and smoke spewing from windows on the left. They then asked the patient whether the houses looked the same or different. The first neglect patient whom they studied said, not surprisingly, that the houses looked identical, since he did not pay attention to the left side of either drawing. But when forced to choose—"Come on, now, which house would you rather live in?"—he picked the bottom house, the one not on fire. For reasons he could not express, he said that he "preferred" that house. A form of blindsight, perhaps? Could it be that even though he is not paying attention to the left side of the house, some of the information about the flames and smoke leaks through to his right hemisphere through some alternate pathway and alerts him to danger? The experiment implies once again that there is no blindness in the left visual field, for if there were, how could he process this level of detail about the left side of the house under any circumstances?

Neglect stories are very popular with medical students. Oliver Sacks5 tells the strange tale of a woman who, like many left hemineglect patients, ate food only from the right side of her plate. But she knew what was up and realized that if she wanted all her dinner, she had to shift her head, so as to see the food on the left. But given her general indifference to the left and reluctance even to look to the left she adopted a comically ingenious solution. She rolled her wheelchair in a huge circle to the right, traveling 340 degrees or so until finally her eyes would fall on the uneaten food. That consumed, she'd make another rotation, to eat the remaining half of the food on her plate, and so on, round and round, until it was gone. It never occurred to her that she could just turn left because—for her—the left simply didn't exist.

One morning not long ago while I was fixing the sprinkler system in our yard, my wife brought me an interesting−looking letter. I receive many letters each week, but this one was postmarked from Panama and had an exotic stamp and curious lettering. I wiped my hands on a towel and started to read a rather eloquent description of what it feels like to suffer from hemineglect.

"When I came to, other than having a severe headache, I perceived absolutely no adverse effects of my mishap," wrote Steve, a former Navy captain who had heard about my interest in neglect and wanted to see me in San Diego for a consultation. "In fact, other than a headache, I felt good. Not wanting to worry my wife—knowing full well I'd had a

heart attack and that the head pain was subsiding—I told her that she should not worry; I was fine.

"She responded, 'No, you're not, Steve. You've had a stroke!'

"A stroke? This statement left me both surprised and slightly amused. I'd seen stroke victims on television and in real life, people who either stared into nothingness or showed clear signs of paralysis in a limb or in the face. Since I perceived none of these symptoms, I could not believe my wife was anywhere near correct.

"Actually, I was completely paralyzed on the left side of my body. Both my left arm and left leg were affected as well as my face. Thus began my odyssey into a strange warped world.

"To my mind, I was fully aware of all parts of my body on the right side. The left side simply did not exist!

You might feel I'm exaggerating. Someone looking at me would see a person with limbs that, though paralyzed, obviously exist and are just as obviously connected to my body.

"When I shaved, I neglected the left side of my face. When I dressed, I would incessantly leave the left arm outside its sleeve. I would incor−recdy button the right button side of my clothing to the left buttonholes, even though I had to complete this operation with my right hand.

"There is no way," Steve concluded, "that you can have any idea of what happens in Wonderland unless a denizen describes it to you."

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Neglect is clinically important for two reasons. First, although a majority of patients recover completely after a few weeks, there is a subset in whom the disorder can persist indefinitely. For them, neglect remains a genuine nuisance even though it may not be a life−threatening disorder. Second, even those patients who seem to recover from neglect quickly can be seriously handicapped because their indifference to the left during the first few days hinders rehabilitation. When a physical therapist urges them to exercise the left arm, they don't see the point in doing so because they don't notice that it is not performing well. This is a problem because in stroke rehabilitation most recovery from paralysis occurs in the first few weeks and after this

"window of plasticity," the left hand tends not to regain function. Physicians, therefore, do their utmost to coax people into using their left hands and legs in the first few weeks—a task frustrated by the neglect syndrome.