Topography of the inhibitory and excitatory responses to transcranial magnetic stimulation in a hand muscle.

We studied the excitatory motor evoked potentials (MEPs) and the inhibitory (silent period) responses to focal transcranial magnetic stimulation (TMS) in the abductor pollicis brevis (APB) of 5 normal subjects to learn whether the scalp topography of the two responses differed. At the scalp location where stimulation produced the highest-amplitude MEP in the voluntarily activated APB, stimulus intensities below the MEP threshold produced silent periods with little or no preceding facilitation. The silent periods had a mean duration of 26.8 +/- 6.8 msec and a mean onset latency of 27.6 +/- 3.6 msec, which was 7.2 +/- 2.3 msec longer than the latency of MEPs produced in the APB by higher stimulus intensities. A period of excitation, with an onset latency of 50-80 msec, often followed the silent period. On averaged trials, a stimulus intensity just above the threshold of the MEP at its optimal position produced MEPs followed by silent periods at a cluster of scalp locations 1 cm apart on the central scalp (medial area) and silent periods with very slight or no preceding facilitation in 3-9 locations lateral to the MEP area (lateral area). This finding was confirmed in 3 subjects with maps constructed from statistical analysis of multiple trials. These maps also showed that MEPs produced from the medial area occurred 4-6 msec earlier than those produced from the lateral area. The integral of the silent period tended to be larger in the lateral area. The motor representation of APB, as defined by TMS, is not homogeneous but rather contains at least two components that differ physiologically and topographically.