Analysis of a Split-Brain Drawing Task with Both Right and Left Hands

by j.w.gibson, ms

A procedure to help alleviate seizures associated with severe epilepsy is to surgically sever the corpus callosum. The corpus callosum is a large bundle of nerves that connects corresponding parts of the brain to each other (Carlson, 2004). It allows for the brain to send messages back and forth between the right and left hemispheres. Patients suffering from severe epilepsy may suffer from seizures that occur numerous times in a day. Researchers have found that severing the corpus callosum drastically decreases the frequency and intensity of epileptic seizures since the different hemispheres of the brain are no longer able to transfer excessive electrical energy.

Lateralization of brain functioning has been known for quite some time. In 1826, a French doctor named Marc Dax noted that more than 40 of his patients suffering from loss of speech consistently had damage to the left side of the brain (Sternberg, 2003). It is now well supported that the left and right hemispheres are indeed specialized for different types of neural processing, and indeed in some respects can be considered as separate brains (Sternberg, 2003). Both hemispheres of the brain receive sensory information from the opposite side of the body. They share this information via the corpus callosum so that each knows what the other is "perceiving and doing" (Carlson, 2004). Extensive research of Sperry (Sperry, 1964) and Gazzaniga (Gazzaniga, 1995), among others has lead to a strong argument for hemispheric specialization.

The left hemisphere contains language processing while the right hemisphere seems to be dominant for spatial visualization (Green, Clark, Hickey, Hutsler, & Gazzaniga, 1999; Kalat, 2001; Rosenzweig, Leiman, & Breedlove, 1999; Sternberg, 2003; Wills, DuHamel, & Vaccaro, 1995). In addition to language processing, the left hemisphere is also important for smooth skilled movement (Gazzaniga, 1995). Researchers have found that while the right is far superior in terms of spatial processing, it does have some limited ability to comprehend verbal instructions; however, it is wholly incapable of producing speech (Carlson, 2004; Eliassen, Baynes, & Gazzaniga, 2000).

Cutting the corpus callosum leads to some interesting behaviors. Because the two hemispheres are incapable of communicating with each other, split-brain patients have noted that their left-hand seems to act on its own. For instance, "patients may find themselves putting down a book held in the left hand, even if they have been reading it with great interest. This conflict occurs because the right hemisphere, which controls the left hand, cannot read and therefore fids the book boring" (Carlson, 2004). The left-hemisphere receives sensory information from the right side, and vice versa. Because sensory information is processed on the opposite site, split-brain patients are not able to access certain types of information when asked to recall. An exception to this rule of crossed representation is olfaction.
Olfaction occurs on the same side of the brain that the nostril resides. Therefore, a scent detected in the right nostril is processed on the right side of the brain. When split-brain patients are asked to identify the odor of something presented to their left-hemisphere, there are able to name it. However, when the odor is presented to the right side they are unable to find the word, but they are able to physically find an object that represents the odor (Kalat, 2001; Rosenzweig, Leiman, & Breedlove, 1999).

When split-brain patients try to replicate drawings from pairs of words presented to different hemispheres there is no integration of the concepts. For instance, Kingstone and Gazzaniga conducted an experiment where they flashed two words "Bow" and "Arrow" to different hemispheres. They then asked the participant to draw what they had seen and surprisingly the participant drew a bow and arrow, leading the researchers to believe that the concepts had been integrated (Gazzaniga, 2002). However, upon further tasks with different word pairs (sky, scraper) the participants obviously did not integrate the concept into skyscraper, but instead drew a "comb-like scraper" with a sky above (Gazzaniga, 2002). Thus split-brain patients are not capable of integrating both hemispheres knowledge about visual information to make a unified concept or representation. Instead, it is as if the two hemispheres are unaware of each other's processing.

Split-brain patients cannot access the lexiconal information of words that reside in the left-hemisphere when information requiring this type of information is presented to the right-hemisphere. While the right side is capable of some language processing such as matching words to pictures, performing spelling and rhyming tasks, and categorizing objects, it is incapable of syntactical meaning, and indeed most people's right hemispheres "cannot handle even the most rudimentary language" (Gazzaniga, 2002).

If a patient was presented with a three-dimensional object to draw they would be successful with the left hand yet not the right. This is because of the contralateral wiring of the brain. Visual-spatial processing resides in the right-hemisphere, which controls the left hand. Without the corpus callosum intact, there exists virtually no communication or transfer of information from one hemisphere to the other. The right hand, which is controlled by the left-hemisphere, has no knowledge of the 3-dimensional object therefore it cannot replicate it. If instead the object was translated into a word such as "cube" the left side could process this language information and instruct the right hand to represent the three-dimensional object.

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