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: re ach, grasp, manipulation, and release. To determine how such actions are re presented in primary somatosensory (S-l) 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 pe rformed 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 ne urons innervated by deep receptors that sensed extension movements of the f ingers, or by tactile receptors in hairy skin sensing stretch. Grasp-inhibi ted responses were the most common type. Tonic firing rates of these cells dropped significantly during approach as the hand was preshaped for graspin g, or at contact when grasp was initiated, and persisted until hand motion ceased or as the grip relaxed. Maximum suppression of firing occurred at gr asp completion. Their lack of specificity for particular hand behaviors for med 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. Fir Firing rates were significantly depressed during grasping and manipulation when the hand interacted directly with the object, but were enhanced prior to contact, w hen the hand was preshaped (approach-tuned), or upon relaxation of grasp an d release of the object from the hand (lower- or relax-tuned). Grasp-inhibi ted responses occurred primarily in S-I, whereas biphasic inhibitory activi ty was recorded mainly in PPC. Suppression of activity within these populat ions may thereby increase the saliency of excitatory responses to acquisiti on 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 respon ses to active and passive extension movements suggests that the inhibitory responses may provide important postural and motor information about the ha nd kinematics when performing skilled tasks.