Afferent input and cortical organisation: a study with magnetic stimulation

Previously, we had described a technique for investigating probable GABAergic cortical inhibitory circuits in conscious man using transcranial magnetic stimulation. This type of inhibition has been termed intracortical inhibition. During voluntary contraction, activity in the circuits responsible for this inhibition is reduced. The mechanism by which this reduction in activity is brought about is unknown. However, evidence exists to suggest that afferent input may be, at least in part, responsible for the reduction in inhibition. 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 level of intracortical inhibition. Also, motor imagery, which activates similar brain regions as overt movement, but does not result in afferent input, failed to produce significant changes in intracortical inhibition. We conclude from these results that afferent input is capable of altering activity in cortical inhibitory circuits. The relevance of these findings to the mechanisms involved in cortical reorganisation is discussed.