Cortical Representation

Mapping of cortical areas involved in sensation has been carried out in experimental animals and during neurosurgical procedures in humans, but it has also been carried out more recently in intact humans by techniques such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). These techniques, which are described in and referenced in the , have led to major advances not only in sensory physiology but also in all aspects of cortical function in normal humans.

From the specific sensory nuclei of the thalamus, neurons carrying sensory information project in a highly specific way to the two somatic sensory areas of the cortex: somatic sensory area I (SI) in the postcentral gyrus and somatic sensory area II (SII) in the wall of the sylvian fissure. In addition, SI projects to SII. SI corresponds to Brodmann's areas 1, 2, and 3. Brodmann was a histologist who painstakingly divided the cerebral cortex into numbered areas based on their histologic characteristics.

The arrangement of the thalamic fibers in SI is such that the parts of the body are represented in order along the postcentral gyrus, with the legs on top and the head at the foot of the gyrus (. Not only is there detailed localization of the fibers from the various parts of the body in the postcentral gyrus, but also the size of the cortical receiving area for impulses from a particular part of the body is proportionate to the number of receptors in the part. The relative sizes of the cortical receiving areas are shown dramatically in , in which the proportions of the homunculus have been distorted to correspond to the size of the cortical receiving areas for each. Note that the cortical areas for sensation from the trunk and back are small, whereas very large areas are concerned with impulses from the hand and the parts of the mouth concerned with speech.

Studies of the sensory receiving area emphasize the very discrete nature of the point-for-point localization of peripheral areas in the cortex and provide further evidence for the general validity of the doctrine of specific nerve energies . Stimulation of the various parts of the postcentral gyrus gives rise to sensations projected to appropriate parts of the body. The sensations produced are usually numbness, tingling, or a sense of movement, but with fine enough electrodes it has been possible to produce relatively pure sensations of touch, warmth, and cold. The cells in the postcentral gyrus are organized in vertical columns, like cells in the visual cortex . The cells in a given column are all activated by afferents from a given part of the body, and all respond to the same sensory modality.

SII is located in the superior wall of the sylvian fissure, the fissure that separates the temporal from the frontal and parietal lobes. The head is represented at the inferior end of the postcentral gyrus, and the feet at the bottom of the sylvian fissure. The representation of the body parts is not as complete or detailed as it is in the postcentral gyrus.