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.