Comparison of neuronal firing rates in somatosensory and posterior parietal cortex during prehension.
To evaluate their functional roles during prehension, single-unit recordings were made in the hand area of primary somatosensory areas 3b, 1 and 2 (S-I) and posterior parietal areas 5 and 7 (PPC) of the same animal. Response profiles of mean firing rate during performance of a multistage reach, grasp, and lift task were analyzed to determine the period(s) of peak firing and to measure statistically significant rises or falls in rate compared with baseline. We used the peak firing stage(s) to subdivide the population into classes tuned to single actions or two successive stages, or into multiaction groups that had sustained facilitation (BT) or inhibition (GI) during hand-object interactions. Four times as many neurons fired at peak rates during acquisition stages (approach, contact, grasp) than upon release, and their firing rates were higher. Grasping evoked the strongest responses, as grasp-tuned neurons had the highest peak rates in the population; BT, contact-grasp, and grasp-lift cells also fired maximally in the grasp stage. Grasping also coincided with maximal inhibition of GI cells, as well as of neurons tuned to approach or relaxation of grasp. Holding evoked the lowest mean rates, and had the fewest tuned cells. S-I and PPC showed significant differences in behaviors evoking peak firing as well as facilitation and inhibition; these correlated with input modalities in each area. Hand contact with the object and positioning of the fingers for grasp was the most strongly represented behavior in anterior S-I, where 61% received tactile inputs from glabrous skin. Nearly 60% were facilitated at contact, 38% fired at peak rates, and 10% were inhibited; release of grasp evoked peak firing in only 5% of 3b-1 neurons. In posterior S-I, where proportions of tactile and deep inputs were similar, positioning and grasping elicited peak responses in 38% and 31%, respectively; 80% were facilitated or inhibited during grasping. During lift and hold, inhibition rose to 43%, while excitation declined under 10%. PPC had the highest proportions firing at peak rates during hand preshaping before contact (28%) and had the most facilitated responses (38%) in this stage. Only 10% fired at peak rates during grasping. During later manipulatory actions, proportions of facilitated and inhibited responses in PPC were similar to those in posterior S-I. The data support models in which PPC plans hand movements during prehension rather than guiding their execution. Sensory monitoring of hand-object interaction occurs in S-I, where cells sense specific hand behaviors, signal stage completion, enable error correction, and may update grasp programs formulated in PPC. The results are discussed in relation to those obtained from lesion studies in humans.