In Barry Levinson's 1988 movie Rain Man, narcissistic yuppie Charlie Babbitt (played by Tom Cruise) learns that he has an autistic, savant brother (played by Dustin Hoffman) during a trip to Cincinnati, Ohio aimed at settling the deceased father's estate. Dissatisfied with his allotted share of the Babbitt estate Charlie decides to take his brother Raymond back to L.A. to meet with his lawyers. Raymond, however, has intense fear of flying, and the two venture on a cross-country car trip, which involves a detour to Las Vegas to win money at the casinos by using Raymond's extreme memory abilities to count cards.

The movie was inspired by the real Rain Man Kim Peek, who sadly passed away in 2011. Like Raymond, Kim Peek had extreme memory abilities. He remembered almost everything he had ever read and could 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.

Most people with savant syndrome suffer from autism. However, in spite of lending inspiration to a movie character with autism, Kim Peek did not himself suffer from autism. A brain imaging study (MRI) of Peek's brain revealed an absence of the corpus callosum, the anterior commissure and the hippocampal commissure, the three main parts of the neurological system that transfer information between the right and the left hemispheres of the brain. It was presumably this separation of the hemispheres that allowed Kim Peek to simultaneously read both pages of an open book and retain the information.

Kim Peek's condition, which is also known as "agenesis of the corpus callosum" is among the most common human brain malformations, occurring in at least 1 in 4000 individuals. A rudimentary form of the corpus callosum appears during the 12th week of pregnancy, and the interhemispheric brain structure continues to develop during pregnancy and early childhood. In fact, this important nerve bundle is still maturing until early adulthood. There are numerous factors that can cause an absence or a malformation of the corpus callosum, including genetics, infections and intoxication during pregnancy or early childhood.

Individuals without an intact corpus callosum sometimes experience linguistic and social impairments. The main linguistic and social problems stem from difficulties understanding non-literal language, including idioms, proverbs, irony, sarcasm, subtle jokes and conversational implicatures. Most adult English speakers will understand that "all good things come to he who waits" is advocating patience and is not to be understood literally. But for a person without a fully developed connection between the two hemispheres, the proverb may stand out as a plainly false statement saying that if only you wait, you will get everything that is good. A connection between the hemispheres apparently is required to be able to override the literal meaning of language and rely on context for interpretation.

The lack of a connection between the two hemispheres also seems to promote a greater focus on, and interest in, details and local patterns and a need for rigorous routines and repetition. Some of these features have also been observed in people with autism spectrum disorder (ASD). On one theory of ASD, autistic traits arise because of limited connections between the two hemispheres, which would explain the similar behaviors of some autistic people and individuals without an intact corpus callosum.

This theory of autism, also known as the "hypoconnectivity theory," has not yet been confirmed. However, new research shows that the lack of a corpus callosum may lead to increased brain connectivity in each of the two hemispheres. The new results come from a metaanalysis of the brain imaging data from seven individuals with agenesis of the corpus callosum, using a mathematical tool known as "connectomic analysis." The metaanalysis revealed that while little information got transferred among hemispheres in the patients, there was greater communication between local areas of the brain. This increased local connectivity may explain both the tendency to be obsessed with details and the increased intellectual abilities of people like Kim Peek. Though decreased holistic brain interaction is likely to negatively influence how we understand contextual cues, the increased local connectivity could lead to more thorough and faster processing of factual information, which in turn could increase memory abilities.

Kim Peek was born without connected hemispheres. The neurological bridges between the hemispheres simply never developed. In the late 1960s, split-brain surgery, or corpus calloscotomy, was introduced as treatment for intractible epileptic seizures, the occurrence of sporadic electrical storms in the brain. The surgical procedure involves severing the corpus callosum. After a split-brain surgery the two hemispheres do not exchange information as efficiently as before, leading to a disconnection 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 a disconnection syndrome. In patients with a disconnection 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.

One of Gazzaniga and Sperry's 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.

While surgical severing of callosal and other commissures normally results in a disconnection syndrome, patients with callosotomy in childhood and individuals born without a corpus callosum do not suffer these consequences, suggesting that the plasticity of the brain in the fetus and small child allows for information transfer that is relevant to agency and action via alternative neural pathways.

About the Authors

Kristian Marlow, M.S.

Kristian Marlow is a graduate student at the University of Missouri-St. Louis and a member of the St. Louis Synesthesia Lab.

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