Afferent input and cortical organisation: a study with magnetic stimulation

Previously, we had described a technique for investigating probable GABAerg ic cortical inhibitory circuits in conscious man using transcranial magneti c stimulation. This type of inhibition has been termed intracortical inhibi tion. During voluntary contraction, activity in the circuits responsible fo r this inhibition is reduced. The mechanism by which this reduction in acti vity is brought about is unknown. However, evidence exists to suggest that afferent input may be, at least in part, responsible for the reduction in i nhibition. The experiments described here were designed to investigate this possibility further. The results of these experiments showed that afferent input, produced by electrical peripheral-nerve stimulation, reduced the le vel of intracortical inhibition. Also, motor imagery, which activates simil ar brain regions as overt movement, but does not result in afferent input, failed to produce significant changes in intracortical inhibition. We concl ude from these results that afferent input is capable of altering activity in cortical inhibitory circuits. The relevance of these findings to the mec hanisms involved in cortical reorganisation is discussed.