Phantoms in the Brain: Probing the Mysteries of the Human Mind (42 page)

Why don't these faint, internally generated images (the cat under the couch, the monkey in the chair) or beliefs, for that matter, have strong qualia? Imagine how confusing the world would be if they did. Actual perceptions need to have vivid, subjective qualia because they are driving decisions and you cannot afford to hesitate. Beliefs and internal images, on the other hand, should not be qualia−laden because they need to be tentative and revocable. So you believe—and you can imagine—that under the table there is a cat because you see a tail sticking out. But there
could
be a pig under the table with a transplanted cat's tail. You must be willing to entertain that hypothesis, however implausible, because every now and then you might be surprised.

What is the functional or computational advantage to making qualia irrevocable? One answer is stability. If you constantly changed your mind about qualia, the number of potential outcomes (or "outputs") would be infinite; nothing would constrain your behavior. At some point you need to say "this is it" and plant a flag on it, and it's the planting of the flag that we call qualia. The perceptual system follows a rationale something like this: Given the available information, it is 90 percent certain that what you are seeing is yellow (or dog or pain or whatever). Therefore, for the sake of argument, I'll assume that it
is
yellow and act accordingly, because if I keep saying, "Maybe it's not yellow," I won't be able to take the next step of choosing an appropriate course of action or thought. In other words, if I treated perceptions as beliefs, I would be blind (as well as being paralyzed with indecision). Qualia are irrevocable
in order to eliminate hesitation
and to confer certainty
to decisions.9 And this, in turn, may depend on which particular neurons are firing, 164

how strongly they're firing and what structures they project to.

•

When I see the cat's tail sticking out from under the table, I "guess" or "know" there is a cat under the table, presumably attached to the tail. But I don't literally see the cat, even though I literally see the tail. And this raises another fascinating question: Are seeing and knowing— the qualitative distinction between perception and conception—completely different, mediated by different types of brain circuitry perhaps, or is there a gray area in between? Let's go back to the region corresponding to the blind spot in my eye, where I can't see anything. As we saw in the Chapter 5 discussion on Charles Bonnet syndrome, there is another kind of blind spot—the enormous region behind my head— where I also can't see anything (although people don't generally use the term "blind spot" for this region). Of course, ordinarily you don't walk around experiencing a huge gap behind your head, and therefore you might be tempted to jump to the conclusion that you are in some sense filling in the gap in the same way that you fill in the blind spot. But you don't. You can't. There is no visual neural representation in the brain corresponding to this area behind your head. You fill it in only in the trivial sense that if you are standing in a bathroom with wallpaper in front of you, you assume that the wallpaper continues behind your head. But even though you assume that there is wallpaper behind your head, you don't literally see it. In other words, this sort of "filling in" is purely metaphorical and does not fulfill our criterion of being irrevocable. In the case of the "real" blind spot, as we saw earlier, you can't change your mind about the area that has been filled in. But regarding the region behind your head, you are free to think, "In all likelihood there is wallpaper there, but who knows, maybe there is an elephant there."

Filling in of the blind spot is therefore fundamentally different from your failure to notice the gap behind your head. But the question remains, Is the distinction between what is going on behind your head and the blind spot qualitative or quantitative? Is the dividing line between "filling in" (of the kind seen in the blind spot) and mere guesswork (for things that might be behind your head) completely arbitrary? To answer this, consider another thought experiment. Imagine we continue evolving in such a way that our eyes migrate toward the sides of our heads,

while preserving the binocular visual field. The fields of view of the two eyes encroach farther and farther behind our heads until they are almost touching. At that point let's assume you have a blind spot behind your head (between your eyes) that is identical in size to the blind spot that is in front of you. The question then arises, Would the completion of objects across the blind spot behind your head be true filling in of qualia, as with the real blind spot, or would it still be conceptual, revocable imagery or guesswork of the kind that you and I experience behind our heads? I think that there will be a definite point when the images become irrevocable, and when robust perceptual representations are created, perhaps even re−created and fed back to the early visual areas. At that point the blind region behind your head becomes functionally equivalent to the normal blind spot in front of you. The brain will then suddenly switch to a completely novel mode of representing the information; it will use neurons in the sensory areas to signal the events behind your head irrevocably (instead of neurons in the thinking areas to make educated but tentative guesses as to what might be lurking there).

Thus even though blind−spot completion and completion behind your head can be logically regarded as two ends of a continuum, evolution has seen fit to separate them. In the case of your eye's blind spot, the chance that something significant is lurking there is small enough that it pays simply to treat the chance as zero. In the case of the blind area behind your head, however, the odds of something important being there (like a burglar holding a gun) are high enough that it would be dangerous to fill in this area irrevocably with wallpaper or whatever pattern is in front of your eyes.

So far we have talked about three laws of qualia—three logical criteria for determining whether a system is conscious or not—and we have considered examples from the blind spot and from neurological patients. But 165

you may ask, How general is this principle? Can we apply it to other specific instances when there is a debate or doubt about whether consciousness is involved? Here are some examples: It's known that bees engage in very elaborate forms of communication including the so−called bee waggle dance. A scout bee, having located a source of pollen, will travel back to the hive and perform an elaborate dance to designate the location of the pollen to the rest of the hive. The question arises, Is the bee conscious when it's doing this?10 Since the bee's behavior, once set in motion, is irrevocable and since the bee is obviously acting on some short−term memory representation of the pollen's location, at least two of the three criteria for consciousness are met. You might then jump to the conclusion that the bee is conscious when it engages in this elaborate communication ritual. But since the bee lacks the third criterion—flexible output—I would argue that it is a zombie. In other words, even though the information is very elaborate, is irrevocable and held in short−term memory, the bee can only do one thing with that information; only one output is possible—the waggle dance. This argument is important, for it implies that mere complexity or elaborateness of information processing is no guarantee that there is consciousness involved.

One advantage my scheme has over other theories of consciousness is that it allows us unambiguously to answer such questions as, Is a bee conscious when it performs a waggle dance? Is a sleepwalker conscious? Is the spinal cord of a paraplegic conscious—does it have its own sexual qualia—when he (it) has an erection? Is an ant conscious when it detects pheromones? In each of these cases, instead of the vague assertion that one is dealing with various degrees of consciousness—which is the standard answer—one should simply apply the three criteria specified. For example, can a sleepwalker (while he's sleepwalking) take the "Pepsi test"—that is, choose between a Pepsi Cola and a Coca Cola? Does he have short−term memory? If you showed him the Pepsi, put it in a box, switched off the room lights for thirty seconds and then switched them on again, would he reach for the Pepsi (or utterly fail like the zombie in Denise)? Does a partially comatose patient with akinetic mutism (seemingly awake and able to follow you with his eyes but unable to move or talk) have short−term memory? We can now answer these questions and avoid endless semantic quibbles over the exact meaning of the word "consciousness."

Now you might ask, "Does any of this yield clues as to where in the brain qualia might be?" It is surprising that many people think that the seat of consciousness is the frontal lobes, because nothing dramatic happens to qualia and consciousness per se if you damage the frontal lobes— even though the patient's personality can be profoundly altered (and he may have difficulty switching attention). I would suggest instead that most of the action is in the temporal lobes because lesions and hyperactivity in these structures are what most often produce striking disturbances in consciousness. For instance, you need the amygdala and other parts of the temporal lobes for seeing the significance of things, and surely this is a vital part of conscious experience. Without this structure you are a zombie (like the fellow in the famous Chinese room thought experiment proposed by the philosopher John Searle11) capable only of giving a single correct output in response to a demand, but with no ability to sense the meaning of what you are doing or saying.

Everyone would agree that qualia and consciousness are not associated with the early stages of perceptual processing as at the level of the retina. Nor are they associated with the final stages of planning motor acts when behavior is actually carried out. They are associated, instead, with the intermediate stages of processing12—a stage where stable perceptual representations are created (yellow, dog, monkey) and that have meaning (the infinite implications and possibilities for action from which you can choose the best one). This happens mainly in the temporal lobe and associated limbic structures, and, in this sense, the temporal lobes are the interface between perception and action.

The evidence for this comes from neurology; brain lesions that produce the most profound disturbances in consciousness are those that generate temporal lobe seizures, whereas lesions in other parts of the brain only 166

produce minor disturbances in consciousness. When surgeons electrically stimulate the temporal lobes of epileptics, the patients have vivid conscious experiences. Stimulating the amygdala is the surest way to

"replay" a full experience, such as an autobiographical memory or a vivid hallucination. Temporal lobe seizures are often associated not only with alterations in consciousness in the sense of personal identity, personal destiny and personality, but also with vivid qualia—hallucinations such as smells and sounds. If these are mere memories, as some claim, why would the person say, "I literally feel like I'm reliving it"?

These seizures are characterized by the vividness of the qualia they produce. The smells, pains, tastes and emotional feelings—all generated in the temporal lobes—suggest that this brain region is intimately involved in qualia and conscious awareness.

Another reason for choosing the temporal lobes—especially the left one—is that this is where much of language is represented. If I see an apple, temporal lobe activity allows me to apprehend all its implications almost simultaneously. Recognition of it as a fruit of a certain type occurs in the inferotemporal cortex, the amygdala gauges the apple's significance for my well−being and Wernicke's and other areas alert me to all the nuances of meaning that the mental image—including the word "apple"—evokes; I can eat the apple, I can smell it; I can bake a pie, remove

its pith, plant its seeds; use it to "keep the doctor away," tempt Eve and on and on. If one enumerates all of the attributes that we usually associate with the words "consciousness" and "awareness," each of them, you will notice, has a correlate in temporal lobe seizures, including vivid visual and auditory hallucinations, "out of body" experiences and an absolute sense of omnipotence or omniscience.13 Any one of this long list of disturbances in conscious experience can occur individually when other parts of the brain are damaged (for instance, disturbances of body image and attention in parietal lobe syndrome), but it's only when the temporal lobes are involved that they occur simultaneously or in different combinations; that again suggests that these structures play a central role in human consciousness.

Until now we have discussed what philosophers call the "qualia" problem—the essential privacy and noncommunicability of mental states—and I've tried to transform it from a philosophical problem into a scientific one. But in addition to qualia (the "raw feel" of sensations), we also have to consider the self—the

"I" inside you who actually experiences these qualia. Qualia and self are really two sides of the same coin; obviously there is no such thing as free−floating qualia not experienced by anyone and it's hard to imagine a self devoid of all qualia.

But what exactly is the self? Unfortunately, the word "self" is like the word "happiness" or "love"; we all know what it is and know that it's real, but it's very hard to define it or even to pinpoint its characteristics. As with quicksilver, the more you try to grasp it the more it tends to slip away. When you think of the word

"self," what pops into your mind? When I think about "myself," it seems to be something that unites all my diverse sensory impressions and memories (unity), claims to be "in charge" of my life, makes choices (has free will) and seems to endure as a single entity in space and time. It also sees itself as embedded in a social context, balancing its checkbook and maybe even planning its own funeral. Actually we can make a list of all the characteristics of the "self—just as we can for happiness—and then look for brain structures that are involved in each of these aspects. Doing this will someday enable us to develop a clearer understanding of self and consciousness—although I doubt that there will be a single, grand, climactic "solution" to the problem of the self in the way that DNA is the solution to the riddle of heredity.