THE EFFECT OF VOLUNTARY CONTRACTION ON CORTICOCORTICAL INHIBITION IN HUMAN MOTOR CORTEX

1. It has been previously shown that in a relaxed target muscle, at sh ort interstimulus intervals (ISIs) (up to 6 ms) a conditioning subthre shold transcranial magnetic stimulus can cause suppression of the EMG response evoked by a magnetic test stimulus. At longer ISIs (7-15 ms) facilitation of the test response is seen. This type of inhibition has been termed ipsilateral cortico-cortical inhibition. 2. The effect of a minimal tonic contraction on ipsilateral cortico-cortical inhibitio n has been investigated in the first dorsal interrosseous (FDI). 3. At short ISIs there was significantly less inhibition of the test respon se during the maintenance of minimal voluntary tonic contraction of th e target muscle (FDI). 4. At longer ISIs (7-15 ms) there was significa ntly less facilitation of the test response during a tonic contraction than during relaxation. 5. Minimal activation of an ipsilateral proxi mal muscle (biceps) had no significant effect on the degree of inhibit ion seen in the relaxed target muscle (FDI). 6. We suggest that volunt ary drive reduces the excitability of inhibitory circuits in cortical areas that project to the active muscle.