Sensory inputs to the agranular motor fields: a comparison between precentral, supplementary-motor and premotor areas in the monkey

Abstract

Kinesthetic responses of neurones in the motor cortex, including the primary motor (MI), the supplementary motor (SMA) and the postarcuate premotor (PMC) areas, were investigated in the awake, chronically prepared monkey. In all three subareas, neurones were recorded which responded to passive elbow flexions and extensions induced by a torque motor. In the SMA, such cells were restricted to its posterior portion where intracortical microstimulation produced limb and trunk movements. The majority of SMA cells responds to both displacement directions, a quarter to either flexion or extension. Although the total proportion of SMA neurones responding to arm displacements was low (15%), it was noted that in 'correct' somatotopic penetrations, the responsiveness could be prominent. The latency distribution of the kinesthetic responses was similar to that of MI neurones with slightly less response latencies shorter than 20 ms in the SMA. With manually applied stimuli, SMA neurones responded mostly to joint rotations, but not to light cutaneous stimuli. Only two SMA neurones with somatosensory responses were identified as descending projection neurones, and some neurones were found to be modulated also during active grasping. In the PMC, a higher proportion of neurones (27%) reacted to the standardized arm displacements, the majority again responding to both directions. The latency distribution of the kinesthetic responses was similar to that of SMA neurones. In contrast to SMA neurones, many PMC neurones responded to light cutaneous stimuli. It was found that some of the 'somatosensory' PMC neurones were sometimes driven also by moving visual and, rarely, by auditory stimuli. Although there are obvious differences in the nature and possibly also in the amount of sensory inputs to the three motor cortical areas, the present results indicate that all three subareas receive somatosensory feedback and that they might therefore all be implicated in the generation of sensory-driven motor output.