The human brain is made up of the left and right hemispheres. These halves are almost identical in structure and are connected by the corpus callosum. Many studies have been done on how these cortical hemispheres work together and separately. Various methods of research involve behavioral observation of damaged and split brains, as well as brain mapping with various imaging and recording techniques.
Before modern research techniques and equipment were available, the only way to study cerebral lateralization was to study people with damaged brains. The damage might occur due to injury or disease. In the 1860s, researcher Pierre Broca found that damage in the left hemisphere caused the inability to form words, a condition called nonfluent aphasia.
Karl Wernicke later observed that if a different area in the left hemisphere was injured, a person could form words well but his speech was incomprehensible, a condition known as fluent aphasia. These observations resulted in the theory that language is produced and analyzed in the left hemisphere. Later research shows that language, especially in women, is sometimes governed by both hemispheres.
The corpus callosum is a thick network of nerves that connects the left and right hemisphere of the brain. This link helps one side of the body know what the other side is doing. Sometimes the corpus callosum is surgically separated, usually as a last effort to control epilepsy. This separation allows researchers to study how each hemisphere operates independently. One interesting finding is that split-brain patients sometimes find one hand acting on its own, for example, buttoning a shirt with one hand while the other unbuttons it.
Cerebral lateralization is often studied with the assistance of functional magnetic resonance imaging. The study participant lies in the MRI capsule while sounds or images are presented. The MRI scans the activity occurring in each hemisphere in reaction to the stimuli.
MRIs and other imaging techniques are difficult for the research subject. He has to lie perfectly still and remain for long periods in the confined space of the imaging machine. A new, noninvasive technique uses Functional Transcranial Doppler Ultrasound technology. The researcher attaches external electrodes near the participant's ears and asks him questions about a video. The electrodes convey information about which side of the brain is functioning during the viewing and question-answering periods.
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