The sensory cortex and tactile sensitivity

On the basis of electrophysiological studies, the cortical projection areas of the dorsal column-medial lemniscus pathway (somatosensory areas I and II) have been held to be essential for fine, discriminative sensation. One aspect of this hypothesis was tested by determining the effects of ablations of these areas on the tactile threshold in monkeys. Thresholds were measured with a graduated series of nylon filaments bent against the palm. Repeated determinations were made until a consistent level of performance was achieved both before and after the cortical removals. Monkeys subjected to removal of the SI hand area contralateral to the tested palm showed only transient or no deficits; others with complete ablations of SI and SII either in the contralateral or in both hemispheres showed at most only slight residual losses. These results, indicating that the lemniscal projection areas are not necessary to sustain a high level of tactile sensitivity, are contrasted with the severe deficits in gross somatosensory discriminations that have recently been found to follow removal of the SI hand area.