Submitted on May 29, 2007
Accepted on October 12, 2007
Neurophysiology of prehension: III. Representation of Object Features in Posterior Parietal Cortex of the Macaque Monkey
Esther P. Gardner1*, K. Srinivasa Babu2, Soumya Ghosh3, Adam Sherwood1, and Jessie Chen1
1 Department of Physiology and Neuroscience, New York University School of Medicine, New York, New York, United States
2 Department of Physiology and Neuroscience, New York University School of Medicine, New York, New York, United States; Department of Neurological Sciences, Christian Medical College, Vellore, Tamilnadu, India
3 Department of Physiology and Neuroscience, New York University School of Medicine, New York, New York, United States; Centre for Neuromuscular and Neurological Disorders, University of Western Australia, Nedlands, Perth, Western Australia, Australia
* To whom correspondence should be addressed. E-mail: gardne01{at}endeavor.med.nyu.edu.
Neurons in posterior parietal cortex may serve both proprioceptive and exteroceptive functions during prehension, signaling hand actions and object properties. To assess these roles, we used digital video recordings to analyze responses of 83 hand-manipulation neurons in area 5 as monkeys grasped and lifted objects that differed in shape (round and rectangular), size (large and small spheres), and location (identical rectangular blocks placed lateral and medial to the shoulder). The task contained 7 stages -- approach, contact, grasp, lift, hold, lower, relax -- plus a pretrial interval. The four test objects evoked similar spike trains and mean rate profiles that rose significantly above baseline from approach through lift, with peak activity at contact. Although representation by the spike train of specific hand actions was stronger than distinctions between grasped objects, 34% of these neurons showed statistically significant effects of object properties or hand postures on firing rates. Somatosensory input from the hand played an important role, as firing rates diverged most prominently upon contact, as grasp was secured. The small sphere -- grasped with the most flexed hand posture -- evoked the highest firing rates in 43% of the population. 21% distinguished spheres that differed in size and weight, and 14% discriminated spheres from rectangular blocks. Location in the workspace modulated response amplitude, as objects placed across the midline evoked higher firing rates than positions lateral to the shoulder. We conclude that area 5 neurons, like those in area AIP, integrate object features, hand actions, and grasp postures during prehension.
