CORTICOCORTICAL INHIBITION IN HUMAN MOTOR CORTEX

1. In ten normal volunteers, a transcranial magnetic or electric stimu lus that was subthreshold for evoking an EMG response in relaxed muscl es was used to condition responses evoked by a later, suprathreshold m agnetic or electric test shock. In most experiments the test stimulus was given to the lateral part of the motor strip in order to evoke EMG responses in the first dorsal interosseous muscle (FDI). 2. A magneti c conditioning stimulus over the hand area of cortex could suppress re sponses produced in the relaxed FDI by a suprathreshold magnetic test stimulus at interstimulus intervals of 1-6 ms. At interstimulus interv als of 10 and 15 ms, the test response was facilitated. 3. Using a foc al magnetic stimulus we explored the effects of moving the conditionin g stimulus to different scalp locations while maintaining the magnetic test coil at one site. If the conditioning coil was moved anterior or posterior to the motor strip there was less suppression of test respo nses in the FDI. In contrast, stimulation at the vertex could suppress FDI responses by an amount comparable to that seen with stimulation o ver the hand area. With the positions of the two coils reversed, condi tioning stimuli over the hand area suppressed responses evoked in leg muscles by vertex test shocks. 4. The intensity of both conditioning a nd test shocks influenced the amount of suppression. Small test respon ses were more readily suppressed than large responses. The best suppre ssion was seen with small conditioning stimuli (0.7-0.9 times motor th reshold in relaxed muscle); increasing the intensity to motor threshol d or above resulted in less suppression or even facilitation. 5. Two e xperiments suggested that the suppression was produced by an action on cortical, rather than spinal excitability. First, a magnetic conditio ning stimulus over the hand area failed to produce any suppression of responses evoked in active hand muscles by a small (approximately 200 V, 50 mus time constant) anodal electric test shock. Second, a vertex conditioning shock had no effect on forearm flexor H reflexes even tho ugh responses in the same muscles produced by magnetic cortical test s hocks were readily suppressed at appropriate interstimulus intervals. 6. Small anodal electric conditioning stimuli were much less effective in suppressing magnetic test responses than either magnetic or cathod al electric conditioning shocks. This suggests that neither refractori ness of corticospinal axons nor activity in pyramidal recurrent collat erals was important in producing suppression. 7. The results are consi stent with the idea that a weak magnetic conditioning stimulus over th e motor cortex can engage intracortical inhibitory circuits. The possi ble relationship to previously described inhibitory effects from motor cortex stimulation in man and GABAergic inhibitory mechanisms in anim als is discussed.