Abstract
The visual area V6A is a crucial node of the dorso-medial visual stream. It is located on the medial parieto-occipital cortex and contains neurons modulated during reaching movements (Fattori et al. 2001, Eur J Neurosci 13, 2309–2313). Here we report that V6A neurons are modulated also during hand preshaping and finger closure, both necessary for grasping visual objects. Extracellular recordings were carried out in 2 awake Macaca fascicularis, performing reach-to-point and reach-to-grasp tasks with the contralateral arm and with the head restrained. Neural activity was also recorded during natural prehension movements performed by monkeys to reach for and grasp pieces of food. Natural prehension movements were recorded by a digital camcorder: arm movements were recorded on the video band (25 frames/s) and neuronal activity on the audio band (44 KHz) of digital tapes. Digital editing software allowed the frame by frame analysis of digitized videoclips, and the comparison of video frames with firing patterns (Gardner et al. 1999, Exp Brain Res 127, 329–354). From 93 single neurons recorded in V6A, 48% were modulated during the reach-to-point task and 71% during the reach-to-grasp task. Grip formation modulated 63% of cells. During natural prehension, 44% of V6A cells (N=59) were modulated during the last phase of prehension, when, having the hand already reached the target, fingers close around the object. Present data suggest that, in addition to the anterior intra-parietal cortex, also the medial parieto-occipital cortex of macaques is involved in the visuomotor guidance of hand grasping. This is the first evidence of the involvement of the dorso-medial visual stream (to whom the medial parieto-occipital cortex belongs) both in reaching and in grasping objects.