Responses to two-dimensional shapes in the macaque anterior intraparietal area.

Neurons in the macaque dorsal visual stream respond to the visual presentation of objects in the context of a grasping task and to three-dimensional (3D) surfaces defined by binocular disparity, but little is known about the neural representation of two-dimensional (2D) shape in the dorsal stream. We recorded the activity of single neurons in the macaque anterior intraparietal area (AIP), which is known to be crucial for grasping, during the presentation of images of objects and silhouette, outline and line-drawing versions of these images (contour stimuli). The vast majority of AIP neurons responding selectively to 2D images were also selective for at least one of the contour stimuli with the same boundary shape, suggesting that the boundary is sufficient for the image selectivity of most AIP neurons. Furthermore, a subset of these neurons with foveal receptive fields generally preserved the shape preference across positions, whereas for more than half of the AIP population the center of the receptive field was at a parafoveal location with less tolerance to changes in stimulus position. AIP neurons frequently exhibited shape selectivity across different stimulus sizes. These results demonstrate that AIP neurons encode not only 3D but also 2D shape features.