Split-Brain Functioning

The oldest approach to gathering information about asymmetry between the two hemispheres of the brain is observation of behavioral changes or impairments in individuals with a brain injury that is clearly confined to one hemisphere of the brain. Lesions or areas of injury can now be identified using various techniques that allow visualization of the living brain. These brain-imaging techniques allow visualization of various properties of the brain such as cerebral blood flow patterns, and glucose utilization by different parts of the brain, as well as damage and unusual tissue masses. These brain-imaging techniques include computed tomography, magnetic resonance imaging, and x rays. These techniques aid in making inferences about the role of particular areas of the brain in contributing to certain behaviors.

The effects of brain injury must be interpreted with care as the brain tends to adapt to damage, and thus alter how it operates. Observed changes in behavior may more accurately reflect compensation of the remaining unimpaired tissue. Unimpaired tissue may also have a negative reaction, functioning worse than it did previous to the injury and increasing observed behavioral impairment. In sum, behavioral impairment due to injury to a particular area of the brain does not necessarily indicate that the injured area had controlled the impaired behavior. Finally, naturally occurring damage or lesions to the brain may occur across a number of different brain areas, and their effects on an observed behavior may be complex and unspecifiable.

Split-brain surgery is another method used to study lateralized functions of the brain and it has yielded a great amount of information. In split-brain surgery, the corpus callosum is severed. This procedure is used to stop the spread of seizure activity between the hemispheres in those with severe epilepsy. Patients who have had their corpus callosum severed show no changes in most of their daily behaviors. Their intellectual functioning and overall personality seem unaffected.

But Roger Sperry and colleagues, by presenting sensory material to only one hemisphere, allowed observation of the functioning of the two hemispheres in isolation. Presentation to only one hemisphere can be accomplished by presenting stimuli to only one side of a sensory system, such as one eye, ear, or hand, while preventing perception by the sensory organ on the other side of the body. For example, a stimulus might be shown to only one eye, or an object might be put in a patient's hand, making sure the patient could not see it. The patient is then asked about various aspects of the stimulus to assess how information is transferred between the hemispheres, and which aspects of information are available to the particular hemisphere being assessed. It should be noted that even when the corpus callosum is severed, the two hemispheres of the brain do communicate, albeit in a more limited fashion, through other connecting nerve fibers.

Injecting sodium amytal into the carotid artery on one side of the neck is another technique that allows observation of the lateralized functioning of the two hemispheres. The sodium amytal acts to temporarily anesthetize the brain hemisphere on the side into which it was injected. A patient then would be asked to perform certain tasks, and those tasks which the patient cannot perform or shows impaired performance in are then thought to be controlled to some extent by the anesthetized hemisphere.

Examinations of brains during autopsy have been used to locate brain injuries that are associated with specific behavioral impairment observed while the individual was alive. Researchers may also electrically stimulate certain areas of the brain to see which behavioral functions are affected. This does not hurt the patient as the brain does not have pain receptors. Various brain activities such as metabolic rate and blood flow may also be measured during behaviors associated with sensory processes.

Research on hemispheric lateralization in neurologically normal individuals has also been carried out primarily by studying visual field asymmetries and by using a dichotic listening procedure wherein subjects are presented with two different verbal messages, one to each ear, at the same time. Because the corpus callosum is intact in these individuals, researchers have had to tailor their stimulus presentation methods, for instance, by only very briefly flashing visual stimuli, in order to compare the abilities of the hemispheres in these individuals. These modifications seem to successfully tap into hemispheric differences, and results from this body of research generally support findings of hemispheric specialization in split-brain patients and in those with other neurological impairment. This research is important in that generalizing findings from split-brain patients and those with neurological impairment to those who are neurologically unimpaired is problematic.