agenesis of the corpus callosum A chronic condition in which the corpus callosum fails to develop. The condition has been increasingly recognized due to the widespread use of imaging techniques that reveal the distinctive ventricular pattern that occurs when this major fiber tract fails to develop. Persons with this condition may be cognitively normal but show increased interhemi-spheric transfer times on a variety of tests.

alien (anarchic) hand sign A condition in which one hand performs complex motor acts outside of the person's conscious control. It can occur in either the dominant or nondominant hand. One form of anarchic hand is thought to occur following a callosal lesion. The other type occurs after disruption of the motor system due to medial frontal lobe damage. Both types are usually intermittent and transitory.

anterior commissure A fiber bundle that connects the two hemispheres. It is inferior to the corpus callosum, near the rostrum. It is thought to connect the anterior temporal lobes, but in humans it may carry fibers from more widely distributed areas.

body of the corpus callosum The central fibers of the corpus callosum. The fibers lie between the genu and the isthmus.

callosotomy The surgical section of the corpus callosum. It is usually accomplished in two stages. The anterior two-thirds of the callosum is sectioned first, followed by the section of the splenium if satisfactory seizure control has not been achieved. In humans, this procedure is only used in the treatment of intractable epilepsy.

commissurotomy The surgical section of the corpus callosum, the anterior commissure, and the hippocampal commissure. This more radical procedure was introduced when the first callosotomies appeared to be ineffective. It remains an alternative to the callosotomy.

contralateral This term refers to the hand, visual field, etc. that is on the opposite side from the structure under discussion. The left hemisphere controls the right or contralateral hand.

fiber tract A group of axons that follows the same path through the brain.

genu The anterior fibers of the corpus callosum that appear to bend like a knee in sagittal section before becoming the more horizontal body of the corpus callosum.

hippocampal commissure One of the fiber tracts inferior and dorsal to the callosum that is usually cut during the posterior section of the callosum. It carries fibers from the hippocampus, a structure important in forming new memories.

homonomous hemianopsia Cortical blindness in one complete visual field. It commonly results from destruction of the right or left occipital lobe, causing blindness in the contralateral visual field.

ipsilateral This term refers to the hand, visual field, etc. that is on the same side as the structure under discussion. The left hemisphere does not control the left or ipsilateral fingers well, although it can participate in ipsilateral limb movement.

isthmus The portion of the corpus callosum just anterior to the splenium.

laterality This term refers to the tendency for one hemisphere to perform a particular cognitive or motor task better than the other.

rostrum The most anterior portion of the corpus callosum.

splenium The most posterior portion of the corpus callosum. It conveys visual information between the hemispheres.

split-brain A term used to refer to a person who has undergone either commissurotomy or callosotomy for the treatment of intractable epilepsy.

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