Cerebral Lateralization

By way of motor fibers originating in the precentral gyrus, each cerebral cortex controls movements of the contralateral (opposite) side of the body. At the same time, somatesthetic sensation from each side of the body projects to the contralateral postcentral gyrus as a result of decussation (crossing over) of fibers. In a similar manner, images falling in the left half of each retina project to the right occipital lobe, and images in the right half of each retina project to the left occipital lobe. Each cerebral hemisphere, however, receives information from both sides of the body because the two hemispheres communicate with each other via the corpus callosum, a large tract composed of about 200 million fibers.

Motor cerebral cortex

Basal _ nuclei

Claustrum

Corpus _ striatum

Motor cerebral cortex

Claustrum

Corpus _ striatum

Basal Nuclei

Cerebellum

Spinal cord

Cerebellum

Spinal cord

■ Figure 8.11 The basal nuclei. These are structures of the cerebrum containing neurons involved in the control of skeletal muscles (higher motor neurons). The thalamus is a relay center between the motor cerebral cortex and other brain areas.

The corpus callosum has been surgically cut in some people with severe epilepsy as a way of alleviating their symptoms. These split-brain procedures isolate each hemisphere from the other, but, surprisingly, to a casual observer split-brain patients do not show evidence of disability as a result of the surgery. However, in specially designed experiments in which each hemisphere is separately presented with sensory images and the patient is asked to perform tasks (speech or writing or drawing with the contralateral hand), it has been learned that each hemisphere is good at certain categories of tasks and poor at others (fig. 8.12).

In a typical experiment, the image of an object may be presented to either the right or left hemisphere (by presenting it to either the left or right visual field only) and the person may be asked to name the object. Findings indicate that, in most people, the task can be performed successfully by the left hemisphere but not by the right. Similar experiments have shown that the left hemisphere is generally the one in which most of the language and analytical abilities reside.

Olfaction m v

V"

Olfaction

Speech, writing Left ear

Main language center

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  • laura
    What is cerebral lateralization?
    8 years ago

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