Research Article: The Role of Binocular Disparity in Stereoscopic Images of Objects in the Macaque Anterior Intraparietal Area

Date Published: February 7, 2013

Publisher: Public Library of Science

Author(s): Maria C. Romero, Ilse C. L. Van Dromme, Peter Janssen, Samuel G. Solomon.


Neurons in the macaque Anterior Intraparietal area (AIP) encode depth structure in random-dot stimuli defined by gradients of binocular disparity, but the importance of binocular disparity in real-world objects for AIP neurons is unknown. We investigated the effect of binocular disparity on the responses of AIP neurons to images of real-world objects during passive fixation. We presented stereoscopic images of natural and man-made objects in which the disparity information was congruent or incongruent with disparity gradients present in the real-world objects, and images of the same objects where such gradients were absent. Although more than half of the AIP neurons were significantly affected by binocular disparity, the great majority of AIP neurons remained image selective even in the absence of binocular disparity. AIP neurons tended to prefer stimuli in which the depth information derived from binocular disparity was congruent with the depth information signaled by monocular depth cues, indicating that these monocular depth cues have an influence upon AIP neurons. Finally, in contrast to neurons in the inferior temporal cortex, AIP neurons do not represent images of objects in terms of categories such as animate-inanimate, but utilize representations based upon simple shape features including aspect ratio.

Partial Text

The analysis of visual information about objects takes place in the primate brain both in the ventral and in the dorsal visual stream [1], [2]. The Anterior Intraparietal area (AIP) is a key end-stage area of the dorsal stream that is crucial for object grasping [3]. Many AIP neurons respond to real-world objects [4]–[6] during object grasping and during passive fixation. Furthermore, AIP neurons encode the three-dimensional (3D) structure of shapes defined by binocular disparity [7], [8]. Recently, we also showed that a subpopulation of AIP neurons may encode two-dimensional shape features across a limited number of spatial positions and sizes [9].

We investigated the role of depth information, derived from binocular disparity and static monocular depth cues, on neural selectivity for images of objects in AIP. Most AIP neurons did not require the presence of binocular disparity in the images. However, stimuli in which disparity information was congruent with the depth information in the monocular depth cues, elicited stronger responses, indicating an influence arising from monocular depth cues. This indicates that, in the representational space of AIP, object images are represented on the basis of simple shape features but not on intuitive category membership.