INTERACTION BETWEEN INTRACORTICAL INHIBITION AND FACILITATION IN HUMAN MOTOR CORTEX

1. In seven normal subjects, subthreshold transcranial magnetic condit ioning stimuli (using a figure-of-eight coil) were applied over the mo tor cortex in order to evoke activity in intracortical neuronal circui ts. The net effect on cortical excitability was evaluated by measuring the effect on the size of EMG responses elicited in the abductor digi ti minimi (ADM) muscle by a subsequent suprathreshold test stimulus. 2 . A single conditioning stimulus suppressed the size of the test respo nse at interstimulus intervals (ISIs) of 1-4 ms whereas the response w as facilitated at ISIs of 6-20 ms. The facilitation could be augmented if pairs of conditioning stimuli were given. 3. Inhibition and facili tation appeared to have separate mechanisms. The threshold for inhibit ion (0.7 active motor threshold) was slightly lower than that for faci litation (0.8 active threshold). Similarly, the inhibitory effect was independent of the direction of current flow induced in the cortex by the conditioning shock, whereas facilitation was maximal with posterio r-anterior currents and minimal with lateromedial current. 4. Direct c orticospinal effects were probably not responsible for the results sin ce facilitation of cortical test responses could be produced by condit ioning stimuli which had no effect on the amplitude of H reflexes elic ited in active ADM muscle. 5. Inhibition and facilitation appeared to interact in a roughly linear manner, consistent with separate inputs t o a common neurone. 6. We suggest that subthreshold transcranial magne tic stimulation is capable of activating separate populations of excit atory and inhibitory interneurones in the motor cortex.