Interaction between intracortical inhibition and facilitation in human motor cortex.

1. In seven normal subjects, subthreshold transcranial magnetic conditioning stimuli (using a figure-of-eight coil) were applied over the motor cortex in order to evoke activity in intracortical neuronal circuits. The net effect on cortical excitability was evaluated by measuring the effect on the size of EMG responses elicited in the abductor digiti minimi (ADM) muscle by a subsequent suprathreshold test stimulus. 2. A single conditioning stimulus suppressed the size of the test response at interstimulus intervals (ISIs) of 1-4 ms whereas the response was facilitated at ISIs of 6-20 ms. The facilitation could be augmented if pairs of conditioning stimuli were given. 3. Inhibition and facilitation appeared to have separate mechanisms. The threshold for inhibition (0.7 active motor threshold) was slightly lower than that for facilitation (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 posterior-anterior currents and minimal with lateromedial current. 4. Direct corticospinal effects were probably not responsible for the results since facilitation of cortical test responses could be produced by conditioning stimuli which had no effect on the amplitude of H reflexes elicited in active ADM muscle. 5. Inhibition and facilitation appeared to interact in a roughly linear manner, consistent with separate inputs to a common neurone. 6. We suggest that subthreshold transcranial magnetic stimulation is capable of activating separate populations of excitatory and inhibitory interneurones in the motor cortex.