Absence of responses to microstimulation at the hand-face border in baboon primary motor cortex

Intracortical microstimulation (ICMS) was used to map the primary motor cortex of four adult female baboons, anesthetized with a mixture of halothane and nitrous oxide and supplemented with sodium pentobarbital. The sequence of observed muscle contractions in response to ICMS provided evidence of an orderly mototopic representation of the tongue, face, hand, forearm and upper body. A zone of cortex unresponsive to microstimulation was consistently observed at the border between the face and hand representation of the mototopic map. This zone was observed in all four animals and was consistent over time. Repeated confirmations of the unresponsive nature of these regions were obtained both early and late in the same experiment. No motor-unit responses or muscle contractions were detected by electromyographic (EMG) recording during stimulation of the unresponsive zones. The absence of both visually observed and EMG-recorded contractions and the fact that muscle contractions could be elicited from adjacent regions of cortex with ICMS as low as 1-5 microA provide compelling evidence that the finding reflects a true physiological condition rather than an experimental artifact.