The brain's processing of information affected by hemispheric transfer.
Posted Nov 06, 2012
Split-brain surgery, or corpus calloscotomy, is a drastic way of alleviating epileptic seizures, the occurrence of sporadic electrical storms in the brain. The procedure involves severing the corpus callosum, the main bond between the brain’s left and right hemispheres.
After a split-brain surgery, the two hemispheres do not exchange information as efficiently as before. This impairment can result in split-brain syndrome, a condition where the separation of the hemispheres affects behavior and agency.
Michael Gazzaniga and Roger W. Sperry, the first to study split brains in humans, found that several patients who had undergone a complete calloscotomy suffered from split-brain syndrome. In patients with split-brain syndrome, the right hemisphere, which controls the left hand and foot, acts independently of the left hemisphere and the person’s ability to make rational decisions. This can give rise to a kind of split personality, in which the left hemisphere give orders that reflect the person’s rational goals, whereas the right hemisphere issues conflicting demands that reveal hidden desires.
Gazzaniga and Sperry's split-brain research is now legendary. One of their child participants, Paul S., had a fully functional language center in both hemispheres. This allowed the researchers to question each side of the brain. When they asked the right side what their patient wanted to be when he grew up, he replied, "an automobile racer." When they posed the same question to the left, however, he responded, "a draftsman." Another patient pulled down his pants with the left hand and back up with the right in a continuing struggle. On a different occasion, this same patient's left hand made an attempt to strike the unsuspecting wife as the right hand grabbed the villainous limp to stop it.
Split personality is a rare consequence of a split brain. In some cases, impaired interhemispheric communication leaves personality intact but still allows people to use the two hemispheres to complete independent intellectual tasks.
An MRI scan of the savant Kim Peek, who inspired the fictional character Raymond Babbitt (played by Dustin Hoffman) in the movie Rain Man, revealed an absence of the corpus callosum, the anterior commissure and the hippocampal commissure, the three cables for information transfer between hemispheres.
As a consequence of this complete split, Peek, who sadly died last year, was able to simultaneously read both pages of an open book and retain the information. He apparently had developed language areas in both hemispheres. Peek was a living encyclopedia. He spent every day with his dad in the library absorbing information. Among his most impressive feats was his ability to provide traveling directions between any two cities in the world.
Today, hemisphere interaction can be studied using devices that measure the electric or magnetic fields surrounding the skull. Unlike split-brain surgery, these techniques are non-invasive.
A team of researchers from UC Santa Barbara, led by Gazzaniga, recently tested information transfer using MEG. Language is processed in areas of the temporal lobe on the left side of the head. When you read with your left eye, the information first ends up in the right hemisphere and must be transferred to the left hemisphere via the corpus callosum to be processed.
To test the efficiency of the hemispheric transfer, the researchers showed a randomized list of words and nonsense words to the left or right eye of a number of research participants. They then measured how effectively the subjects would be able to distinguish words from nonsense words. The study showed that subjects were significantly more efficient in determining the nature of the string of letters when the information was fed directly to the left hemisphere via the right eye. Apparently, the brain has difficulties processing information that has had to travel long distances.
The researchers didn't compare both-eye exposure to single-eye exposure. At first glance, it may seem that it would be an advantage to get information from both eyes. However, one can also imagine that hemispheric transfer has a hampering effect on language processing. If this is true, you might want to wear a pirate eye patch covering your left eye when completing the verbal section of the GRE. At the very least, be careful not to shut your right eye while under time pressure.
K. W. Doron, D. S. Bassett, M. S. Gazzaniga. Inaugural Article: Dynamic network structure of interhemispheric coordination. Proceedings of the National Academy of Sciences, 2012; DOI: 10.1073/pnas.1216402109