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 interosseous (FDI) of 16 normal subjects following transcranial magnetic stimulation (TMS) over the hand area of the primary motor cortex. These test responses were conditioned by a subthreshold stimulus applied 2-15 ms beforehand over a range of anterior or medial sites. Stimuli applied 3-5 cm anterior to the hand motor area (site A) or 6 cm anterior to the vertex on the nasion-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 intensity of the conditioning stimulus was equal to 0.9x active motor threshold (AMT) at the hand area. Increasing the intensity to 1.2x AMT produced facilitation. Suppression of surface EMG responses was mirrored in the behavior of single motor units. Conditioning stimuli had no effect on responses evoked in the active FDI muscle by transcranial electric stimulation of motor cortex nor on forearm flexor H reflexes even though MEPs in the same muscle were suppressed at appropriate interstimulus intervals. We conclude that low-intensity TMS over presumed premotor areas of frontal cortex can engage corticocortical connections to the primary motor hand area. Copyright 2001 Academic Press.