Sensory input to the motor fields of the agranular frontal cortex: a comparison of the precentral, supplementary motor and premotor cortex.

Arm displacements applied to the passive, but awake monkey are powerful stimuli for activating neurones in somatotopically appropriate areas of the precentral cortex. We have found that neurones in medial area 6 (SMA) and in lateral area 6 (PMC) may likewise be activated by such kinesthetic stimuli, at latencies which are only slightly longer than in area 4. Confirming previous findings, PMC neurones were sometimes also responsive to visual stimuli. The 'somatosensory' cells in the SMA were found in the microexcitable zone of the more posterior part of the SMA from where motor effects were elicited in arm and trunk muscles. These sensory neurones tended to be clustered together and they were only exceptionally excited antidromically by peduncular stimulation. Thus, somatosensory signals have access to both the PMC and SMA suggesting that both areas may be implicated in sensory-guided or sensory-triggered movements.