Michael S. Gazzaniga, Ph.D.
Professor of Psychology
Center for Cognitive Neuroscience
Dartmouth College
Hanover, New Hampshire
March 15, 1999

Consciousness and the Brain


My thinking about the question of consciousness started early since the afternoon in Roger Sperry's lab at Caltech almost 40 years ago when I tested the first split-brain patient. Right off the bat it seemed that whatever consciousness was, one could have two of them after the neurons which connected the two cerebral hemispheres were surgically separated. Mind left did not appear to know about mind right, and vice versa. Those first impressions, while factually enduring, left much to be desired as a sophisticated perspective on the question of consciousness. My situation echoed Tom Wolfe's admonition: practice writing for 20 years before you seek a publisher.

Classic split-brain work has highlighted how the left and right brain each possesses its own particular functions. The studies would lead one to believe that the brain is a collection of modules. Thus, the left hemisphere is specialized for not only language and speech, but also for intelligent behavior. After human cerebral hemispheres are disconnected, a patient's verbal IQ remains intact, and his problem-solving capacity, as observed in hypothesis formation tasks, remains unchanged for the left hemisphere. The left hemisphere thus remains unchanged from its pre-surgical capacity; yet the largely disconnected, same-size right hemisphere becomes seriously impoverished in many cognitive tasks. While the largely isolated right hemisphere remains superior to the isolated left hemisphere in certain activities such as recognizing upright faces, having better attentional skills, and perhaps also expressing more emotions it is poor at problem solving and numerous other mental activities. Thus, the left has modules specialized for higher cognitive function 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; however, tasks such as the Block Design test from the WAIS, 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 are capable of facial expression when spontaneous emotions are expressed.

The uniquely human skills we possess may well be produced by minute and circumscribed neuronal networks sometimes referred to as "modules." And yet our highly modularized brain generates a feeling in all of us that we are integrated and unified. How does that feeling come about, even though we are a collection of specialized modules? The answer appears to be that there is a specialized left hemisphere system we have designated as the "interpreter," a device that seeks explanations for why events occur. The advantage of having such a system is obvious. By going beyond observing contiguous events and asking why they happened, a brain can cope with these same events more effectively, should they happen again. Recent investigations of ours have extended research on the properties of the interpreter and how its presence influences other mental skills. There are, for example, hemisphere-specific changes in the accuracy of memory processes. The predilection of the left hemisphere to interpret events has an impact on the accuracy of memory. When subjects were presented with pictures that represented common events (i.e., getting up in the morning or making cookies) and then several hours later were asked to identify whether pictures in another series appeared in the first, both hemispheres were equally accurate in recognizing the previously viewed pictures and rejecting the unrelated ones. Only the right hemisphere, though, correctly rejected pictures in the second set that were not previously viewed but were related to pictures from the first. The left hemisphere incorrectly "recalled" significantly more of these pictures as having occurred in the first set, presumably because they fit into the schema it had constructed regarding the event. This finding is consistent with the view of a left hemisphere "interpreter" that constructs theories to assimilate perceived information into a comprehensible whole. In doing so, however, the elaborative processing has a deleterious effect on the accuracy of perceptual recognition. This result has been extended to include verbal material.

By emphasizing specialized circuits that arise from natural selection, we see that the brain is not a unified neural net that supports a general problem solving device. If we accept this notion, we can concentrate on the possibility that smaller, more manageable circuits produce awareness of a species' capacities. Holding fast to the notion of a unified neural net means we can understand human conscious experience only by figuring out the interactions of billions of neurons. That task is hopeless. My scheme is not.

The same split-brain research that exposed shocking differences between the two hemispheres also revealed that the human left hemisphere has the interpreter. The left brain interpreter's job is to interpret our behavior and responses, whether cognitive or emotional, to environmental challenges. The interpreter constantly establishes a running narrative of our actions, emotions, thoughts, and dreams. It is the glue that keeps our story unified and creates our sense of being a coherent, rational agent. It brings to our bag of individual instincts the illusion that we are something other than what we are. It builds our theories about our own life, and these narratives of our past behavior seep into our awareness.

The problem of consciousness, then, is tractable. We do not have to find the code of one huge, interacting neural network. Instead, we must find the common and perhaps simple neural circuit(s) that enables vertebrates to be aware of their species-specific capacities, and the problem is solved. The same enabling circuit(s) in the rat is most likely present in the human brain, and understanding that point makes the problem scientifically tractable. What makes us so grand is that the circuit has so much more to work with in the human brain.

Our brains are automatic because physical tissue carries out what we do. How could it be any other way? That means they do it before our conceptual self knows about it. But the conceptual self grows and grows and reaches proportions that find the biologic fact that our brain does things before we are consciosuly aware of them of interest but not disheartening. The interpretation of things past has liberated us from a sense of being tied to the demands of the environment, and 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 our centrality. Because of that we can drive our automatic brains to greater accomplishment and enjoyment of life.