Two representations of the hand in area 4 of a primate. II. Somatosensory input organization.

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Abstract
There are 2 spatially separate motor representations of the hand (digits and wrist) in area 4 of the squirrel monkey. The pattern of somatosensory afferent input to the 2 motor representations is examined. Multiple microelectrode penetrations were made into area 4 and neighboring area 3a. The adequate stimulus and receptive field were determined for each neuron that could be activated by natural stimulation of somatosensory afferents. Intracortical stimulation was used to determine the motor representation in each track. Neurons in area 4 of the squirrel monkey received substantial input from cutaneous as well as deep afferents. Neurons that responded to cutaneous input were segregated from those that responded to deep input. Neurons receiving cutaneous input were concentrated in the caudal hand motor representation. Those receiving deep input were concentrated in the rostral hand motor representation and in area 3a. The receptive fields of 90% of the 260 neurons activated by cutaneous input were located on the volar surface of the hand. These neurons were recorded in both digit and wrist regions of the caudal zone. There was no correlation between the location of cutaneous receptive fields and the type of movements evoked by intracortical stimulation. The receptive fields of units receiving deep input and the movement evoked by stimulation at the same site usually were confined to the same body part. There was no fixed relationship between the direction of passive movement that excited neurons and the direction of movement evoked by intracortical stimulation. Motor cortex region containing the representation of the distal forelimb can no longer be considered to be a homogeneous structure that receives predominately kinesthetic input. This region contains 2 spatially separate motor representations that can be defined on the basis of their somatosensory afferent input. The concentration of cutaneous input in the caudal hand motor representation suggests that it may be preferentially involved in movements that use tactile feedback for their execution and guidance. The concentration of deep input in the rostral motor representation suggests that it might be preferentially involved in movements that use kinesthetic feedback for their execution and guidance.