The Left and Right Brain Hemispheres: Independent Centers of Consciousness?
"I'm of two minds on the matter." "I can't make up my mind." "I'm having an internal argument." Our language is full of idioms that make it sound as if there were two disagreeing voices inside our heads. Often, that is indeed how it feels. But is that sensation physiologically supported? Can a brain fight with itself? Can there be multiple independent centers
of consciousness in a single head? Until the 1960s, there was no way for us to test this feeling of internal disagreement. But when a surgery aimed at alleviating epileptic seizures also isolated the two hemispheres of the patient's brain, science was surprisingly afforded that opportunity.
The left and right hemispheres of the brain are connected by a dense bundle of neurons called the corpus callosum. This bundle is primarily responsible for communication of information between the two hemispheres, connecting them with approximately 200 million callosal axons (in humans.) (1) In some cases of multifocal epilepsy, the electrical discharges that cause seizures can start in one hemisphere and spread to the other by way of the corpus callosum, greatly increasing the severity of the fit. Sometimes this condition is unresponsive to medication, at which point the spasms can only be controlled with more drastic measures.(2)
In 1961, Dr. Michael Gazzaniga performed an operation which had been pioneered on animals by Drs. Ronald Meyers and Roger Sperry, but which had never before been tested on human patients. In this procedure, called a commissurotomy, the surgeon opens the skull, lays back the brain coverings with a cerebral retractor, and cuts through the corpus callosum. While this prevents a seizure from spreading, it also prevents information from being passed between hemispheres. Thanks to Dr. P. J. Vogel, we now know that severing the anterior ¾ of the corpus callosum can effectively stop the spread of a seizure, while allowing full communication between the hemispheres to remain. (3) However, the behavior of full-commissurotomy patients has been extensively documented, and provides fascinating insight into the specialization of the hemispheres, the nature of the brain, and the nature of consciousness itself.
To understand these behaviors, one must first remember that neurological wiring of the body is, for the most part, contralateral. Signals travel from the left side of the body to the right hemisphere of the brain and back, and vice versa. For example, the left hemisphere "sees" out of the right eye, and moves the right hand.
Patients who have undergone a commissurotomy, "split brain patients," experience strange, acute post-operational symptoms: many have trouble speaking, or are completely mute; often they experience inter-manual conflict, where their hands cannot cooperate; when speech is possible, many remark that their left hand is behaving in a "foreign," "alien" manner, and they express surprise that it is acting so "purposefully." These symptoms fade over time. The long-term symptoms are much more difficult to distinguish in an everyday setting. Split brain patients function normally in social settings, except for slight memory problems. Pianists can still play the piano, artists can still paint. Once in an experimental setting however, more phenomena can be observed which point to the dramatic impact of full commissurotomy on cerebral function. (2)
Commissurotomy patients are usually tested with a tachistoscope, a device that presents to the patient a screen with a focal point in the center. The tachistoscope then flashes a word, picture, or scene in the non-overlapping visual field of one eye. This is done faster than the eye can move, to ensure that only one hemisphere of the brain receives the input.(4) When a key and a ring are flashed simultaneously in the left and right fields respectively, a normal viewer reports seeing "keyring". A split brain patient reports seeing only "ring". Even more notably, if the patient is told to use his left hand to pull the object he saw out of a bag, he pulls out a key. When asked what it is he pulled out, (without looking) the patient says, "A ring." (5)
Why the apparent duality?
The explanation requires a rethinking of the brain. Think of the hemispheres as separate, independent entities – the left hemisphere is commonly the center of language, while the right hemisphere is mute. (2) When the patient was asked what he saw, the "speaking" hemisphere replied truthfully: it saw a ring. It received no information from the right hemisphere (left eye) because of the commissurotomy, and so had no way of knowing about the key. When the patient was asked to use his left hand to pull out what he saw, the right hemisphere, which saw a key, told the left hand to pull out a key. Subtle tactile sensations are contralaterally wired, so the left hemisphere could not feel the key; thinking all that was shown was a ring, it assumed that the left hand pulled out a ring.
These startling findings are one example of a long series of experiments conducted to pinpoint the nature of the dual-hemisphere brain. Researchers over the years have examined everything from language capability and pitch acuity to academic strengths and compensation mechanisms. Among other things, they have found that the left brain is better at language, math, and analytical processing, while the right brain
is superior in visuospatial processing, music, and form. Both hemispheres can control shoulder and upper arm movement, but only contralateral hemispheres can control fine hand motion and visual/audio processing. (2)
Over time, the hemispheres develop compensatory "clueing" mechanisms. For example, a patient may begin speaking the names of objects out loud to get her left hand to retrieve them. If she is trying to figure out what she is holding in her left hand without looking, the left hand may dig the point of a pencil into its palm, shooting ipsilaterally-wired pain signals to both hemispheres and causing the left hemisphere to realize that it's holding something sharp.(6)
The hemispheres seem to have different opinions, too – a split brain patient named Paul awoke from surgery to show an unheard-of amount of language skill in his right hemisphere. Though it could not speak, there was not only word association, but also complete sentence comprehension. Researchers were able to ask each hemisphere the same questions and compare the written answers. When asked, "Who are you?" both hemispheres answered, "Paul." But when asked, "What do you want to be?" the right hemisphere answered, "Automobile racer," while the left replied, "Draftsman." (1)
So here we have a picture of a dual consciousness, a picture so powerful we refer to each hemisphere as a separate person. How accurate is this image? Do we really have two separate centers of identity, which work together until they are split? Is this a real-life Jekyll and Hyde condition, with two minds in the same head? How separate is split hemispheric consciousness in reality? Gazzaniga comes to an interesting conclusion:
After many years of fascinating research on the split brain, it appears that the inventive and interpreting left hemisphere has a conscious experience very different from that of the truthful, literal right brain. Although both hemispheres can be viewed as conscious, the left brain's consciousness far surpasses that of the right. Which raises another set of questions that should keep us busy for the next 30 years or so. (7)
Not only does he believe that each hemisphere has its own consciousness, he clearly finds the left hemisphere to be superior to the right. In contrast, Dennett, author of Consciousness Explained, states:
When Dr. Jekyll changes into Mr. Hyde, that is a strange a mysterious thing. Are they two people taking turns in a single body? But here is something stranger: Dr. Juggle and Mr. Boggle [standing for the left and right cerebral hemispheres of a single body] too, take turns in one body. But they are as like as identical twins! Whyt hen say that they have changed into one another? Well, why not: if Dr. Jekyll can change into a man as different as Hyde, surely it must be all the easier for Juggle to change into Boggle, who is exactly like him. We need conflict or strong difference to shake our natural assumption that to one body there corresponds at most one agent (6)
His argument here is that, since there are no significant differences in personality between the two hemispheres (as there are with Jekyll and Hyde) there are not two separate consciousnesses.
In his book, Dennett seeks to discredit a theory he calls the Cartesian Theater of the mind. In the Cartesian Theater theory, the mind consists of a sum of parts that feed into a central ethereal "theater," a site of synthesis, a nucleus of consciousness with input from everywhere in the body.(8) That theory has interesting parallels to the brain's being the nucleus of the nervous system, interpreting inputs from all over; but in this theory, there is a central locale where this synthesis is projected. Where is that in a split brain patient?
To apply that theory to the brain, we would see a single "consciousness," independent of individual hemispheres, yet viewing their information in a sort of mental simulcast. Without the corpus callosum, this simulcast would have the handicap of receiving not a single synthesized feed of information from an integrated brain, but two distinct and sometimes contradictory lines of input. The mental "picture" is split, and confusing for a while, but in the end it is comprehendible by the "consciousness." This would account for the disorienting acute post-operational symptoms, and the subsequent return to normal social function. In this model, it is not the consciousness that is split, simply the picture presented to the consciousness.
So which is it? Does splitting the brain split consciousness into two distinct instantiations? Or does splitting the brain merely disjoint the "theater projection" presented to the single immaterial consciousness of an individual? That is a question open to debate. Those who believe in a transcendent human soul may subscribe more easily to the Cartesian Theater application, while devoted followers of the brain=behavior philosophy might more readily agree with Gazzaniga. Perhaps the contradictions evidenced by split brain behavior point to a dramatically strange Jekyll-Hyde situation, with a foreign and mute brain-half in control of the left side of the body. Or maybe, when their hands fight each other, commissurotomy patients are simply experiencing what we all do: they can't make up their minds.
(1) Split Brain Consciousness An extensive educational site about the brain and split brain studies, on Macalester College's site.
(2) The Split Brain A paper by J. Bogen et al. Bogen was one of the first extensive researchers of this phenomenon. This paper on Caltech's site is an excellent comprehensive resource.
(3) Splitting the Human Brain Article by Paul Pietsch PhD on the ShuffleBrain website, a resource a la Serendip.
(4) Neuroscience for Kids A good, simple, but detailed description provided by the faculty of the University of Washington.
(5) Two Brains or One? The Split Brain Studies A history of Sperry and the early studies.
(6) Dennett on the Split Brain A philosopher's review of Dennett's book, on the Psycoloquy website.
(7) Human Neurobiology: Split Brain Research An overview on the Educational Cyber-Playground website.
(8) Stage Effects in the Cartesian Theater: A review of Daniel Dennett's Consciousness Explained Another philosophical review, on the Psyche website.