On the focal nature of inhibition and facilitation in the human motor cortex

Objective: To find the area of the human cortex from which inhibition and facilitation of corticospinal neurons could be obtained. Methods: A patient with seizures had an array of 64 electrodes placed over the left fronto-temporal cortex. The motor evoked potential (MEP) elicited by stimulating through one pair of electrodes was conditioned by stimuli that were subthreshold for a MEP given through adjacent pairs of electrodes. Results: The MEP recorded over the right abductor pollicis brevis produced by stimulating over the hand area of the left cortex could be inhibited (at intervals less than 5 ms) and facilitated (at intervals greater than 5 ms) by subthreshold conditioning stimuli delivered through neighbouring pairs of electrodes. The inhibition and facilitation were only obtained when the conditioning stimuli were delivered within 1–2 cm of the test site. The sites producing inhibition and facilitation were not identical. Conditioning stimuli over the face area did not inhibit the MEP produced by stimulating the hand area or vice versa. Conclusion: The inhibition and facilitation of corticospinal neurons projecting to a given muscle arise from small areas close to those corticospinal neurons.