Coding of shape features in the macaque anterior intraparietal area.

The exquisite ability of primates to grasp and manipulate objects relies on the transformation of visual information into motor commands. To this end, the visual system extracts object affordances that can be used to program and execute the appropriate grip. The macaque anterior intraparietal (AIP) area has been implicated in the extraction of affordances for the purpose of grasping. Neurons in the AIP area respond during visually guided grasping and to the visual presentation of objects. A subset of AIP neurons is also activated by two-dimensional images of objects and even by outline contours defining the object shape, but it is unknown how AIP neurons actually represent object shape. In this study, we used a stimulus reduction approach to determine the minimum effective shape feature evoking AIP responses. AIP neurons responding to outline shapes also responded selectively to very small fragment stimuli measuring only 1-2°. This fragment selectivity could not be explained by differences in eye movements or simple orientation selectivity, but proved to be highly dependent on the relative position of the stimulus in the receptive field. Our findings challenge the current understanding of the AIP area as a critical stage in the dorsal stream for the extraction of object affordances.