Selectivity for the shape, size, and orientation of objects for grasping in neurons of monkey parietal area AIP

In this study, we mainly investigated the visual selectivity of hand-manipu lation-related neurons in the anterior intraparietal area (area AIP) while the animal was grasping or fixating on three-dimensional (3D) objects of di fferent geometric shapes, sizes, and orientations. We studied the activity of 132 task-related neurons during the hand-manipulation tasks in the light and in the dark, as well as during object fixation. Seventy-seven percent (101/132) of the hand-manipulation-related neurons were visually responsive , showing either lesser activity during manipulation in the dark than durin g that in the light (visual-motor neurons) or no activation in the dark (vi sual-dominant neurons). Of these visually responsive neurons, more than hal f (n = 66) responded during the object-fixation task (object-type). Among t hese, 55 were tested for their shape selectivity during the object-fixation task, and many (n = 25) were highly selective, preferring one particular s hape of the six different shapes presented (ring, cube, cylinder, cone, sph ere, and square plate). For 28 moderately selective object-type neurons, we performed multidimensional scaling (MDS) to examine how the neurons encode the similarity of objects. The results suggest that some moderately select ive neurons responded preferentially to common geometric features shared by similar objects (flat, round, elongated, etc.). Moderately selective nonob ject-type visually responsive neurons, which did nor respond during object fixation, were found by MDS to be more closely related to the handgrip than to the object shape. We found a similar selectivity for handgrip in motor- dominant neurons that did not show any visual response. With regard to the size of the objects, 16 of 26 object-type neurons tested were selective for both size and shape, whereas 9 object-type neurons were selective for shap e but not for size. Seven of 12 nonobject-type and all (8/8) of the motor-d ominant neurons examined were selective for size, and almost all of them we re also selective for objects. Many hand-manipulation-related neurons that preferred the plate and/or ring were selective for the orientation of the o bjects (17/20). These results suggest that the visual responses of object-t ype neurons represent the shape, size, and/or orientation of 3D objects, wh ereas those of the nonobject-type neurons probably represent the shape of t he handgrip, grip size, or hand-orientation. The activity of motor-dominant neurons was also, in part, likely to represent these parameters of hand mo vement. This suggests that the dorsal visual pathway is concerned with the aspect of form, orientation, and/or size perception that is relevant for th e visual control of movements.