Integration of retinal disparity and fixation-distance related signals toward an egocentric coding of distance in the posterior parietal cortex of primates.

For those movements that are directed toward objects located in extrapersonal space, it is necessary that visual inputs are first remapped from a retinal coordinate system to a body-centered one. The posterior parietal cortex (PPC) most likely integrates retinal and extraretinal information to determine the egocentric distance of an object located in three-dimensional (3-D) space. This determination requires both a retinal disparity signal and a parallel estimate of the fixation distance. We recorded from the lateral intraparietal area (LIP) to see if single neurons respond to both vergence angle and retinal disparity and if these two signals are integrated to encode egocentric distance. Monkeys were trained to make saccades to real targets in 3-D space. When both fixation distance and disparity of visual stimuli were varied, the disparity tuning of individual neurons display a fixation-distance modulation. We propose that the observed modulation contributes to a spatial coding domain intermediate between retinal and egocentric because the disparity tuning shifts in a systematic way with changes in fixation distance.