Egocentric distance estimation

Egocentric distance estimation requires eye-head position signals

Gunnar Blohm¹, J. Douglas Crawford^1,2
¹Centre for Vision Research, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada and Canadian Institutes of Health Research, Group for Action and Perception
²Departments of Psychology, Biology and Kinesiology & Health Sciences, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada

In order to successfully reach to an object presented in the visual field, the brain must reconstruct the egocentric spatial location of this object from available retinal and extraretinal information. The retinal images from both eyes are merged to provide a unique (cyclopean) representation of object direction. Retinal disparity between eyes provides information about the object’s distance from the cyclopean eye. The complex geometry of eye-in-head and head-on-body rotations suggests that retinal disparity information of a reach target may not be invariant with regards to gaze (cyclopean eye-in-space) direction.

Here, we developed a 3-dimensional (3D) binocular model that incorporates the complete geometry of eye and head rotational positions. We show that different eye-head orientations produce distinct retinal disparities so that, given a target viewed at fixed retinal disparity and cyclopean retinal location, the brain cannot reconstruct target distance without knowledge of eye and head positions. Thus, extraretinal eye and head positions are needed, in addition to retinal disparity and vergence signals, to compute an estimate of distance.

This represents the first theoretical evidence showing that the depth component of a desired reach can be accurately computed only if the brain takes into account the linkage geometry of the eye and head. This calculation thus requires a complete 3D visuo-motor reference frame transformation.