Depression of neuronal firing rates in somatosensory and posterior parietal cortex during object acquisition in a prehension task

Prehension is an object-oriented behavior consisting of four components: reach, grasp, manipulation, and release. To determine how such actions are represented in primary somatosensory (S-I) and posterior parietal cortex (PPC), we used digital video to synchronize spike trains of neurons recorded in Brodmann's areas 3b, 1, 2, 5, and 7 with the hand kinematics as monkeys performed a prehension task. Statistical analyses indicated that one-third of task-modulated neurons showed significantly depressed firing rates during object acquisition and/or manipulation. This population was dominated by neurons innervated by deep receptors that sensed extension movements of the fingers, or by tactile receptors in hairy skin sensing stretch. Grasp-inhibited responses were the most common type. Tonic firing rates of these cells dropped significantly during approach as the hand was preshaped for grasping, or at contact when grasp was initiated, and persisted until hand motion ceased or as the grip relaxed. Maximum suppression of firing occurred at grasp completion. Their lack of specificity for particular hand behaviors formed the inhibitory counterpart of broadly tuned cells that fired prolonged bursts during grasp and manipulatory stages of prehension. The remainder of the task-inhibited population showed biphasic responses. Firing rates were significantly depressed during grasping and manipulation when the hand interacted directly with the object, but were enhanced prior to contact, when the hand was preshaped (approach-tuned), or upon relaxation of grasp and release of the object from the hand (loweror relax-tuned). Grasp-inhibited responses occurred primarily in S-I, whereas biphasic inhibitory activity was recorded mainly in PPC. Suppression of activity within these populations may thereby increase the saliency of excitatory responses to acquisition and manipulation of objects. Reduction of firing during prehension might also signal the flexed postures used to retain objects in the hand, rather than a generalized gating of sensory information. The similarity of responses to active and passive extension movements suggests that the inhibitory responses may provide important postural and motor information about the hand kinematics when performing skilled tasks.