What is Motor Map Topography?

Motor map topography describes how parts of the brain's motor cortex are related to certain parts of the body. This information is found by stimulating different parts of the motor cortex and recording physical responses made by the body. When a site on the cortex is stimulated, a specific muscle group responds by moving. This indicates which area of the brain is responsible for the movement of those muscles. Taken together, a number of responses provides a map of the motor cortex.

The motor cortex is a thin strip of the cerebral cortex at the back of the frontal lobes that runs from ear to ear over the brain. Through motor map topography, each part of the motor cortex can be associated with the part of the body it is responsible for controlling. Using electrodes to stimulate the surface of the brain, researchers have been able to make movement maps of the cortex. Motor map topography has revealed that some body parts capable of precise movements take up a disproportionate amount of motor cortex space when compared to their size.

Illustrations of the motor cortex show the greater representation of the more precise muscle groups. For instance, while the fingers are small, the part of the cortex responsible for their movement is usually larger than that of the arms. The thumb alone stands out for the amount of motor cortex that is associated with it. Motor map topography similarly reveals that, in humans, the face, lips, and tongue, which are necessary for speech and expression, are also better represented than some larger body parts. If the human body were drawn according to the map of the motor cortex, it would have big lips, hands, and feet, but a thin trunk and very skinny arms and legs.

The development of motor map topography played an important role in the discovery that the brain can change throughout adult life. This was first discovered in animal experiments, where animals were encouraged to form specific tasks with the hands, for instance. Later tests revealed that long-term habits changed the amount of motor cortex devoted to the the hands in this type of experiment.

This discovery has had important consequences for the treatment of brain injuries in humans. For example, a stroke can cause a part of the motor cortex to lose its ability to function, leaving patients unable to effectively move some parts of the body. Specific training exercises have been developed to help recruit other parts of the brain to take over, allowing the patient to regain normal functioning.