Neuronal responses in sensorimotor cortex to ramp displacements and maintained positions imposed on hindlimb of the unanesthetized monkey

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1. Neuronal responses to passively imposed displacements of the foot were examined in area 3a, the precentral motor cortex (MI), and the first somatic sensory cortex (SI) of unanesthetized monkeys. The foot displacements, in the form of "ramps" and maintained displacements, were applied at several different velocities and achieved a number of maintained positions. The response in relation to the ramp was termed a "dynamic" response, while that in association with the maintained displacement was termed a "static" response. 2. Units in the rostral part of MI (MI/r) and in area 3a usually responded in relation both the ramps and to maintained displacements. The activity of the majority of area 3a and MI/r units was strongly modulated only during ramps of one direction. Such response properties contrast with those observed in the caudal part of MI (MI/c) and areas 3b and 1 of SI, where most units responded in relation to the ramp phase of the displacement in both of two ramp directions and not to the maintained displacements. 3. Virtually all units, including those in MI, displayed a dynamic response. However, the dynamic response amplitude of area 3a neurons greatly exceeded that of MI units for equal velocities of displacement. 4. For area 3a units, peak neuronal activity was well correlated with ramp velocity over the range examined. Such units were a minority in areas 3b and 1 of SI (excluding area 3a) and MI. 5. Most area 3a units and almost half of MI/r units displayed, in addition to a dynamic response, a static response that reflected foot position. MI units were more sensitive than area 3a units to equal degrees of maintained displacement. 6. These results demonstrate that in the unanesthetized, intact animal, responses of most neurons in area 3a and MI fall into two categories: a) mixed dynamic-static responses resembling those of muscle spindle afferents, and b) purely dynamic responses. These data are consistent with the hypothesis that afferent input concerning the velocity and position of a segment of a limb may have a role in cortically mediated regulation of movement and posture, and that both areas 3a and the MI cortex may be directly involved in this regulation. 7. The cytoarchitectonic characteristics of area 3a, as defined in single-unit studies, are discussed. It is argued that the salient characteristic of area 3a in the primate is a thinned internal granular layer (layer IV) that contrasts with the heavily granular area 3b and the agranular area 4.