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. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.