Expansion of the cortical representation of a specific skin field in primary somatosensory cortex by intracortical microstimulation.

Intracortical microstimulation (ICMS) was applied to a single site in the middle cortical layers (III-IV) in the koniocortical somatosensory fields of sodium pentobarbital-anesthetized rats (Sml) and new world monkeys (area 3b). Low-threshold cutaneous receptive fields were defined in the cortical region surrounding the stimulation site prior to and following 2-6 hr of 5 microA ICMS stimulation. ICMS stimulation did not usually affect the receptive field location, size, or responsiveness to tactile stimulation of neurons at the stimulation site. However, the number of cortical neurons surrounding the stimulation site with a receptive field that overlapped with the ICMS-site receptive field increased in all studied animals, resulting in an enlarged cortical representation of a restricted skin region spanning several hundred microns. The mean size of receptive fields changed in some but not all cases. These results provide evidence that the responses of cortical neurons are subject to change by the introduction of locally coincident inputs into a single location, and demonstrate a capacity for representational plasticity in the neocortex in the absence of peripheral stimulation. These experimental observations are consistent with hypotheses that the cerebral cortex comprises radially oriented populations of neurons that share a common input, and that these inputs are shaped by coincident activity (see Edelman, 1978, 1987; Merzenich, 1987; Merzenich et al., 1990; von der Malsburg and Singer, 1988).