Thursday, April 01, 1999

The Interpreter Within: The Glue of Conscious Experience

By: Michael S. Gazzaniga Ph.D.

A generation of brilliant scientists is racing to understand consciousness. The answer, warns one of America’s leading cognitive neuroscientists, may leave us less than satisfied—unless we realize that our sense of a unified self is just another of our brain’s survival systems. Professor Gazzaniga introduces your brain’s “interpreter.”

The problem of consciousness has been called (among other things) the Ultimate Question, the Great Problem, and the Holy Grail of neuroscience. The puzzle seems to get more intriguing as the brain is understood down to the molecular level. How, in this world of ionized sodium channels and neurotransmitters, does awareness emerge and become self-conscious?

Michael S. Gazzaniga, Ph.D., editor in chief of The Journal of Cognitive Neuroscience, suggests a surprising answer to the mystery of consciousness. Perhaps it just isn’t that complicated. We should stop seeking a single, integrated consciousness in our multifunctional brains, and ask not “How do we become a conscious self?” but “What makes possible our experience of a single, continuous, conscious self?”

The room overflowed with hyper-excited people; the Great Man himself was going to hold forth. Nobel laureate Sir Francis Crick, codiscoverer with James Watson of the structure of DNA, had come to the Society for Neuroscience meeting to tell us his views on the nature of conscious experience. For the past 20 years since he was a mere 60 years old, Crick had turned his attention away from molecular biology and toward this supreme problem of neuroscience. He amused us with a tape recording of a lively exchange several years ago between Sir John Eccles and himself. Eccles remarked that he wanted “something more” than the brain to be involved in consciousness; Crick would consider only the neuroscience of conscious experience. When the tape ended, Crick made a plea to journalists: “Stop writing about the topic because there is really nothing new to say”—but then went on to present at length his ideas about the role of the visual cortex and consciousness.

At the heart of the matter is our deep-seated belief that there is not only a neuroscience of consciousness, but also a neuroscience of human consciousness. It is as if something terribly complex happens as the brain enlarges to its human form, something that triggers our capacity for self-reflection, for ennui, for lingering moments, for—well, understanding Francis Crick’s thoughts.

Chairs scoot forward, novitiates seek clues to follow up, sycophants ready their compliments—if not for the talk’s substance, at least for the privilege of hearing Crick’s words. Despite Sir Francis’s own warning against blind adulation for the topic (let alone the major players), adulation continues unabated.

Ironically, across the road from the Salk Institute for Biological Sciences, Crick’s bully pulpit, is a group of smart neuroscientists who believe that there is much more to say about consciousness. Giulio Tononi and Gerald Edelman of the Scripps Research Institute have weighed in with a proposal that challenges Crick’s and Christof Koch’s idea of determining which brain areas participate in conscious experience by identifying which ones connect to the frontal lobes. Tononi and Edelman appeal instead to what they call the “dynamic core hypothesis,” the idea that a dynamic system moving throughout the brain activates specific circuits that produce conscious awareness. That is, instead of first determining the structures involved in conscious experience, they would look at its dynamic capacities, the ever-changing neural activity that constitutes a functioning nervous system.

As these gladiators joust, their creative proposals offer many details that could yield to further scientific analysis. The dynamic core of the brain, for example, should be less active during sleep (even though it is known that neural activity is more active during REM than during non-REM sleep). Crick and Koch rule out the primary visual cortex (V1) as active in conscious experience but must then figure out why mental imagery appears to involve V1.

The field is crowded not only with neuroscientists but also philosophers, physicians, physicists, and others. Two leading philosophers, Daniel Dennett of Tufts University and John Searle of the University of California, Berkeley, have a standing argument over how to view the problem.

In his book Consciousness Explained, Dennett takes the functionalist view that direct conscious experience will disappear as a “problem” once we understand basic mental functions such as color perception, language, and attention. An explanation of consciousness will come in the form of understanding patterns of neural functioning in space and time. What we call conscious experience will turn out to be a complex map of such patterns: patterns theoretically programmable in silicon, conferring conscious qualities on the computer chip (or other silicon artifact).

Searle heatedly disagrees. While consciousness in silicon might be remotely possible, it is far more likely that consciousness is uniquely tied to the workings of biological systems. Something about biologic stuff gives rise to personal conscious experience.

These bright people simply cannot put aside the topic of consciousness and how the brain generates it. New devotees are world-class scientists who rush in where angels have feared to tread (or have already crashed and burned). They are treating the problem as no different from, say, perception. Consciousness is there; the brain does it; therefore, let us begin to break it down and study it.

There is no stopping things now, I guess. Look around at the periodicals bulging with articles on the topic. Slip “consciousness” into a scientific database and watch thousands upon thousands of references scroll up. Go to the library, where shelves devoted to books on consciousness are groaning under their weight. The volumes from two major conferences in Tucson that addressed it are themselves encyclopedic. Our species wants to know more, understand more. Yet the topic still generates deep anxiety for most scientists. This anxiety is fueled in part by quips such as the one currently making the rounds: according to the official 1989 entry on consciousness in The International Dictionary of Psychology, “nothing worth reading has yet been written about it.” Also, if the answer can (and I assume someday will) be represented by simple equations, it will not be personally fulfilling. 

We have to shed our expectation that a scientific understanding of consciousness will sweep away our sense of strangeness, like finding out how ships get in bottles.


Why? Why is the hunt for the neuronal side of the story so palpably distant from a true sense of personal understanding of conscious experience? The answer, I believe, is that consciousness is an instinct—a built-in property of brains. Like all instincts, it is just there. You do not learn to be conscious, and you cannot unlearn the reality of conscious experience. Someday we will achieve a more mechanistic understanding of its operation, but I warn you now: That won’t be especially fulfilling on a personal level. We have to shed our expectation that a scientific understanding of consciousness will sweep away our sense of strangeness, like finding out how ships get in bottles.

Take our reproductive instinct. Does it help our sense of desire to understand the role of testosterone when we see a shapely figure across the room? Or take the human instinct for language. Does it help us to enjoy language if we understand that grammar is a universal built-in reflex but that our lexicon is learned? Understanding the problem of consciousness, however, may be essential to our ultimate ability to deal with some mental disorders. Disorders of conscious experience, whether autism or schizophrenia or dementia, will be illuminated by a mechanistic understanding of personal conscious experience.


My own thinking on this topic started early, in Roger Sperry’s laboratory at the California Institute of Technology on an afternoon almost 40 years ago, when I first tested a split-brain patient. It seemed that, whatever consciousness was, you could have two of them after the surgical severing of the corpus callosum connecting the two cerebral hemispheres. Mind Left did not appear to know about Mind Right, and vice versa. Those first impressions, which still endure, nevertheless left much to be desired as a sophisticated perspective on the question of consciousness. My plight as a researcher echoed Tom Wolfe’s admonition to practice writing for 20 years before you seek a publisher.

Classic split-brain research highlighted how the left brain and the right brain serve distinctive functions and led us to believe that the brain is a collection of modules. The left brain (or hemisphere) is specialized not only for language and speech but also for intelligent behavior. After the human cerebral hemispheres are disconnected, the patient’s verbal IQ remains intact, and his problem-solving capacity (as observed in hypothesis formation tasks) remains unchanged for the left hemisphere. Indeed, that hemisphere seems to remain unchanged from its presurgical capacity. Yet the largely disconnected right hemisphere, which is the same size as the left, becomes seriously impoverished for many cognitive tasks. While it remains superior to the left hemisphere in certain activities (in recognizing upright faces, having better skills in paying attention, and perhaps in expressing emotions), it is poorer after separation at problem solving and many other mental activities.

Apparently the left brain has modules specialized for higher cognitive functions, while the right has modules specialized for other functions. Visuo-spatial function, for example, is generally more acute in the right hemisphere, but left-hemisphere integration may be needed to perform higher-order tasks. The use of tactile information to build spatial representations of abstract shapes appears to be better developed in the right hemisphere, but tasks such as the Block Design test, which are typically associated with the right parietal lobe, appear to require integration between the hemispheres in some patients. Furthermore, even though the right hemisphere is better able to analyze unfamiliar facial information than is the left hemisphere, and the left is better able to generate voluntary facial expressions, both hemispheres can generate facial expression when spontaneous emotions are expressed.

In addition to the skills named above, our big human brains have hundreds if not thousands more individual capacities. Our uniquely human skills may well be produced by minute, circumscribed neuronal networks, sometimes referred to as “modules,” but our highly modularized brain generates a feeling in all of us that we are integrated and unified. If we are merely a collection of specialized modules, how does that powerful, almost self-evident feeling come about?


The answer appears to be that we have a specialized left-hemisphere system that my colleagues and I call the “interpreter.” This Interpreter is a device (or system or mechanism) that seeks explanations for why events occur. The advantage of having such a system is obvious. By going beyond simply observing contiguous events to asking why they happened, a brain can cope with such events more effectively should they happen again.

We revealed the Interpreter in an experiment using a “simultaneous concept test.” The split-brain patient is shown two pictures, one presented exclusively to his left hemisphere, one exclusively to his right. He is then asked to choose from an array of pictures the ones he associates with the pictures that were presented (or “lateralized”) to his left brain and his right brain. In one example of this, a picture of a chicken claw was flashed to the left hemisphere and a picture of a snow scene to the right. Of the array of pictures then placed in front of the subject, the obviously correct association was a chicken for the chicken claw and a shovel for the snow scene. Split-brain subject Case One did respond by choosing the shovel with his left hand and the chicken with his right. Thus each hemisphere picked the correct answer. Now the experimenter asked the left-speaking hemisphere why those objects were picked. (Remember, it would only know why the left hemisphere had picked the shovel; it would not know why the disconnected right brain had picked the shovel.) His left hemisphere replied, “Oh, that’s simple. The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” In other words, the left brain, observing the left hand’s response, interprets the response in a context consistent with its own sphere of knowledge—one that does not include information about the snow scene presented to the other side of the brain.

One can influence the left-brain interpreter in many ways. As I mentioned, we wanted to know if the emotional response to stimuli presented to half of the brain would influence the emotional tone of the other half. Using an optical-computer system that detects the slightest eye movement, we projected an emotion-laden movie to the right hemisphere. (If the patient tried to cheat and move the eye toward the movie, it was electronically shut off.)

When we did this experiment with Case Two, the movie that her right hemisphere saw was about a vicious man pushing another off  a balcony and then throwing a firebomb on top of him. The movie then showed other men trying to put the fire out. When the subject was first tested on this problem, she could not access speech from her right hemisphere. She was able to speak only out of her left brain. When asked what she had seen, her left brain (the half brain that had not actually seen the movie) replied, “I don’t really know what I saw. I think just a white flash.”  When I asked, “Were there people in it?”  Case Two replied, “I don’t think so. Maybe just some trees, red trees like in the fall.” I asked, “Did it make you feel any emotion?” and she answered, “I don’t really know why, but I’m kind of scared. I feel jumpy. I think maybe I don’t like this room, or maybe it’s you; you’re getting me nervous.” She turned to one of the research assistants and said, “I know I like Dr. Gazzaniga, but right now I’m scared of him for some reason.” 

Because the alteration in brain physiology is general, the Interpreter is able to note the mood and immediately attributes some cause to it. This is a powerful mechanism; once clearly seen, it makes one wonder how often we are victims of spurious emotional–cognitive correlations.

This kind of effect is common to all of us. A mental system that is operating outside the conscious realm of the left hemisphere’s Interpreter generates a mood that alters the general physiology of the brain. Because the alteration in brain physiology is general, the Interpreter is able to note the mood and immediately attributes some cause to it. This is a powerful mechanism; once clearly seen, it makes one wonder how often we are victims of spurious emotional–cognitive correlations.

Our recent investigations have looked further at the properties of the Interpreter and how it influences mental skills. For example, there are hemisphere-specific changes in the accuracy of memory processes. Specifically, the predilection of the left hemisphere to interpret events has an impact on the accuracy of memory. When subjects are presented with pictures representing common events (e.g., getting up in the morning or making cookies) and several hours later asked to say if pictures in another series appeared in the first, both hemispheres are equally accurate in recognizing the previously viewed pictures and rejecting the unrelated ones. Only the right hemisphere, however, correctly rejects pictures in the second set that were not previously viewed but were related to pictures previously viewed. The left hemisphere incorrectly “recalls” significantly more of these related pictures as having occurred in the first set, presumably because they fit into the schema it has constructed. This finding is consistent with the hypothesis that a left-hemisphere “Interpreter” constructs theories to assimilate perceived information into a comprehensible whole. In doing so, however, the process of elaborating (story making) has a deleterious effect on the accuracy of perceptual recognition. This result has been shown with verbal as well as visual material.

A more recent example of the Interpreter can be found in studies of Case Three, a split-brain patient who can speak out of his right hemisphere as well as his left. His naming of stimuli in the left field seems to be increasing at a rapid rate. Although there is no convincing evidence of any genuine visual transfer between the hemispheres, during trials when the patient was certain of the name of the stimulus, he maintained that he saw it well. On trials when he was not certain of the name of the stimulus, he maintained that he did not see it well. This is consistent with the view that the left hemisphere’s Interpreter actively constructs a mental portrait of past experience, even though that experience did not directly occur in that hemisphere. This experience was probably caused by the left hemisphere’s Interpreter giving meaning to right hemisphere spoken responses, possibly by activating the left hemisphere mental imagery systems.

Our right hemisphere behaves more like the rat’s. It does not try to interpret its experience to find the deeper meaning; it lives only in the thin moment of the present. But when the left brain is asked to explain why it is attempting to psych out the whole sequence, it always comes up with a theory, however spurious.

The left hemisphere’s capacity for continual interpretation may mean that it is always looking for order and reason, even where there are none. This came out dramatically in a study by George Wolford and me. On a simple test that requires one to guess if a light is going to appear on the top or the bottom of a computer screen, we humans perform in an inventive way. The experiment manipulates the stimulus to appear on the top 80 percent of the time. While it quickly becomes evident that the top button is being illuminated more often, we keep trying to figure out the whole sequence—and deeply believe that we can. We persist even if, by adopting this strategy, we are rewarded only 68 percent of the time (whereas if we guessed “top” repeatedly, by rote, we would be rewarded 80 percent of the time). Rats and other animals are more likely to learn to maximize their score by pressing only the top button. Our right hemisphere behaves more like the rat’s. It does not try to interpret its experience to find the deeper meaning; it lives only in the thin moment of the present. But when the left brain is asked to explain why it is attempting to psych out the whole sequence, it always comes up with a theory, however spurious.


Neurology yields weird examples of how the Interpreter can work, and understanding the Interpreter increases our insight into some bizarre syndromes. Take, for example, a malady called “anosagnosia,” in which a person denies awareness of a problem he has. People who suffer from right parietal lesions that render them hemiplegic and blind on their left side frequently deny that they have any problem. The left half of their body, they insist, is simply not theirs. They see their paralyzed left hand but maintain that it has nothing to do with them. How could this be?

Consider what may happen as a result of a lesion in a person’s optic tract. If the lesion is in a nerve that carries information about vision to the visual cortex, the damaged nerve ceases to carry that information; the patient complains that he is blind in part of his visual field. For example, such a patient might have a huge blind spot to the left of the center of his visual field. He rightly complains.

If another patient, however, has a lesion not in the optic tract but the visual cortex, creating a blind spot of the same size and in the same place, he does not complain at all. The reason is that the cortical lesion is in the place in his brain that represents that exact part of the visual world, the place that ordinarily would ask, “What is going on to the left of visual center?” In the case of the lesion on the optic nerve, this brain area was functioning; when it could not get any information from the nerve, it put up a squawk—something is wrong. When that same brain area is itself lesioned, the patient’s brain no longer cares about what is going on in that part of the visual field; there is no squawk at all. The patient with the central lesion does not have a complaint because the part of the brain that might complain has been incapacitated, and no other can take over.

As we move farther into the brain’s processing centers, we see the same pattern, but now the problem is with the interpretive function. The parietal cortex is where the brain represents how an arm is functioning, constantly seeking information on the arm’s whereabouts, its position in three-dimensional space. The parietal cortex monitors the arm’s existence in relation to everything else. If there is a lesion to sensory nerves that bring information to the brain about where the arm is, what is in its hand, or whether it is in pain or feels hot or cold, the brain communicates that something is wrong: “I am not getting input.” But if the lesion is in the parietal cortex, that monitoring function is gone and no squawk is raised, though the squawker is damaged.

Now let us consider our case of anosagnosia, and the disowned left hand. A patient with a right parietal lesion suffers damage to the area that represents the left half of the body. The brain area cannot feel the state of the left hand. When a neurologist holds a patient’s left hand up to the patient’s face, the patient gives a reasonable response:  “That’s not my hand, pal.” The Interpreter, which is intact and working, can’t get news from the parietal lobe, since the flow of information has been disrupted by the lesion. For the Interpreter, the left hand simply does not exist anymore, just as seeing behind the head is not something the Interpreter is supposed to worry about. It is true, then, that the hand held in front of him cannot be his. What is the mystery?

An even more fascinating syndrome is called “reduplicative paramnesia.” I once studied a patient with this syndrome. She was a lady who, although being examined in my office at New York Hospital, claimed we were in her home in Freeport, Maine. The standard interpretation of this syndrome is that the patient has made a duplicate copy of a place (or person) and insists that there are two.

This woman was intelligent; before the interview she was biding her time reading the New York Times. I started with the “So, where are you?” question.  “I am in Freeport, Maine. I know you don’t believe it. Dr. Posner told me this morning when he came to see me that I was in Memorial Sloan-Kettering Hospital and that when the residents come on rounds to say that to them. Well, that is fine, but I know I am in my house on Main Street in Freeport, Maine!”  I asked, “Well, if you are in Freeport and in your house, how come there are elevators outside the door here?”  The grand lady peered at me and calmly responded, “Doctor, do you know how much it cost me to have those put in?”

This patient has a perfectly fine Interpreter working away trying to make sense of what she knows and feels and does. Because of her lesion, the part of the brain that represents locality is overactive and sending out an erroneous message about her location. The Interpreter is only as good as the information it receives, and in this instance it is getting a wacky piece of information. Yet the Interpreter still has to field questions and make sense of other incoming information—information that to the Interpreter is self-evident. The result? It creates a lot of imaginative stories.


The Interpreter’s talents can be viewed on a larger canvas. I began this article by observing our deep belief that we can attain not only a neuroscience of consciousness but also a neuroscience of human consciousness. It is as if something wonderfully new and complex happens as the brain enlarges to its full human form. Whatever happens (and I think it is the emergence of the Interpreter module), it triggers our capacity for self-reflection and all that goes with it. How do we account for this?

I would like to make a simple, three-step suggestion. First, focus on what we mean when we talk about “conscious experience.” I believe this is merely the awareness we have of our capacities as a species— awareness not of the capacities themselves but of our experience of exercising them and our feelings about them. The brain is clearly not a general-purpose computing device; it is a collection of circuits devoted to these specific capacities. This is true for all brains, but what is amazing about the human brain is the sheer number of capacities. We have more than the chimp, which has more than the monkey, which has more than the cat, which runs circles around the rat. Because we have so many specialized systems, and because they may sometimes operate in ways that are difficult to assign to a given system or group of them, it may seem as though our brains have a single, general computing device. But they do not. Step one is to recognize that we are a collection of adaptive brain systems and, further, to recognize the distinction between a species’ capacities and how it experiences them.

Now consider step two. Can there be any doubt that a rat at the moment of copulation is as sensorially fulfilled as a human? Of course it is. Do you think a cat does not enjoy a good piece of cod? Of course it does. Or a monkey does not enjoy a spectacular swing? Again, it has to be true. Each species is aware of its special capacities. So what is human consciousness? It is awareness of the very same kind, except that we can be aware of so much more, so many wonderful things. A circuit, perhaps a single system or one duplicated again and again, is associated with each brain capacity. The more systems a brain possesses, the greater our awareness of capacities.

Think of the variations in capacity within our own species; they are not unlike the vast differences between species. Years of split-brain research have shown that the left hemisphere has many more mental capacities than the right. The left is capable of logical feats that the right cannot manage. Even with both our hemispheres, however, the limits to human capacity are everywhere in the population. No one need be offended to realize that some people with normal intelligence can understand Ohm’s law, while others, such as yours truly, are clueless about hundreds of mathematical concepts. I do not understand them and never will; the circuits that would enable me to understand them are not in my brain.

When we realize that specialized brain circuits arose through natural selection, we understand that the brain is not a unified neural net that supports a general problem-solving device. If we accept this, we can concentrate on the possibility that smaller, more manageable circuits produce awareness of a species’ capacities. By contrast, holding fast to the notion of a unified neural net forces us to try to understand human conscious experience by figuring out the interactions of billions of neurons. That task is hopeless. My scheme is not.

The Interpreter is the glue that keeps our story unified and creates our sense of being a coherent, rational agent. To our bag of individual instincts, it brings theories about our life. These narratives of our past behavior seep into our awareness; they give us an autobiography.

Hence step three. The same split-brain research that exposed startling differences between the two hemispheres revealed as well that the human left hemisphere harbors our Interpreter. Its job is to interpret our responses—cognitive or emotional—to what we encounter in our environment. The Interpreter sustains a running narrative of our actions, emotions, thoughts, and dreams. The Interpreter is the glue that keeps our story unified and creates our sense of being a coherent, rational agent. To our bag of individual instincts, it brings theories about our life. These narratives of our past behavior seep into our awareness; they give us an autobiography.

Insertion of an Interpreter into an otherwise functioning brain creates many by-products. A device that begins by asking how one thing relates to another, a device that asks about an infinite number of things, in fact, and that can get productive answers to its questions, cannot help giving birth to the concept of self. Surely one question the device would ask is “Who is solving all these problems?” “Let’s call it ‘me’”—and away it goes! A device with rules for figuring out how one thing relates to another will quickly be reinforced for having that capacity, just as an ant’s solving where to have its evening meal reinforces the ant’s food-seeking devices. In other words, once mutational events in the history of our species had brought the Interpreter into existence, there would be no getting rid of it.

Our brains are automatic because physical tissue carries out what we do. How could it be otherwise?  Our brains are operating before our conceptual self knows it. But the conceptual self emerges and grows until it is able to find interesting—but not disheartening—the biological fact that our brain does things before we are consciously aware of them. The interpretation of things that we encounter has liberated us from a sense of being determined by our environment; it has created the wonderful sense that our self is in charge of our destiny. All of our everyday success at reasoning through life’s data convinces us of this. And because of the Interpreter within us, we can drive our automatic brains to greater accomplishment and enjoyment of life.  

Portions of this essay have appeared in The Mind’s Past and elsewhere.



Crick F, Koch C. Consciousness and neuroscience. Cerebral Cortex. 1998; 8(2):97-107.

Dennett, D. Consciousness Explained. Boston: Little, Brown and Co.; 1991.

Gazzaniga, MS. The Mind’s Past. Berkeley: University of California Press; 1998.

Searle, J. Minds, brains, and science. The Reith Lectures, British Broadcasting; 1984.

Tononi, G and Edelman, GM. Consciousness and Complexity. Science 1998; 282; 1856-1851.

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Scientific Advisory Board
Joseph T. Coyle, M.D., Harvard Medical School
Kay Redfield Jamison, Ph.D., The Johns Hopkins University School of Medicine
Pierre J. Magistretti, M.D., Ph.D., University of Lausanne Medical School and Hospital
Robert Malenka, M.D., Ph.D., Stanford University School of Medicine
Bruce S. McEwen, Ph.D., The Rockefeller University
Donald Price, M.D., The Johns Hopkins University School of Medicine

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