Sensorimotor cortical control of isometric force in the monkey.

Recordings from single neurones in the primary somatosensory (SI) and motor (MI) cortex of monkeys trained to precisely regulate force between thumb and index finger have disclosed the following contrasting properties between neurones in these two cortical regions: (1) the existence of neurones with similar discharge patterns within MI and SI but, between these regions, significantly different distributions of the classes of discharge patterns; (2) a late onset of activity change in SI neurones in relation to force increase as compared to significantly earlier changes in MI neurones; (3) linear relations between firing rate and isometric force for SI and MI neurones, however with a larger range of rate-force slopes in SI as compared to MI; (4) infrequent motor reactions to intracortical microstimulation in SI but frequent reactions in MI; (5) a majority of SI neurones with cutaneous afferent input in contrast to predominant input from deep tissues to MI neurones; and (6) context independent visually evoked activity observed exclusively in MI neurones. These major differences suggest that SI neuronal activity most likely reflects the input from peripheral receptors rather than, as postulated for MI neurones, the participation in movement initiation and the control of muscular contractions.