Transcranial magnetic stimulation can be used to test connections to primary motor areas from frontal and medial cortex in humans

Surface EMG responses (MEPs) were recorded from the relaxed first dorsal in terosseous (FDI) of 16 normal subjects following transcranial magnetic stim ulation (TMS) over the hand area of the primary motor cortex. These test re sponses were conditioned by a subthreshold stimulus applied 2-15 ms beforeh and over a range of anterior or medial sites. Stimuli applied 3-5 cm anteri or to the hand motor area (site A) or 6 cm anterior to the vertex on the na sion-inion line (site B) inhibited the test responses at short latency. The largest effect was seen when the interstimulus interval was 6 ms and the i ntensity of the conditioning stimulus was equal to 0.9x active motor thresh old (AMT) at the hand area. Increasing the intensity to 1.2x AMT produced f acilitation. Suppression of surface EMG responses was mirrored in the behav ior of single motor units. Conditioning stimuli had no effect on responses evoked in the active FDI muscle by transcranial electric stimulation of mot or cortex nor on forearm flexor H reflexes even though XMPs in the same mus cle were suppressed at appropriate interstimulus intervals. We conclude tha t low-intensity TMS over presumed premotor areas of frontal cortex can enga ge corticocortical connections to the primary motor hand area. (C) 2001 Aca demic Press.