Two eyes more sensitive than one: Monocular speed discrimination is better across eyes than within an eye

Abstract

When an object moves directly towards or away from an observer, it projects opposite horizontal directions of motion upon the two retinae. Binocular 3D motion perception relies on this cue, the inter-ocular velocity difference (IOVD). However, many animals have little or no binocular overlap, but are known to depend upon inter-ocular velocity comparisons for navigation (Srinivasan et al., 1996). Likewise, even humans have substantial portions of monocular visual field. We asked whether human subjects use similar inter-ocular velocity comparisons across the monocular portions of the two eyes’ views, and assessed whether these monocular IOVDs are processed similarly to motions in other parts of the monocular and binocular visual fields. Subjects performed a 2AFC speed discrimination on a pair of drifting gratings (~10 deg radius) as a function of baseline speed (10-40 deg/s), across a wide range of binocular and monocular portions of the visual fields. “Intra-monocular velocity discrimination” involved comparing both gratings within one eye’s monocular field, while “inter-monocular velocity discrimination” required comparison across the 2 monocular fields. We also performed a reference condition involving more conventional dichoptic viewing within the central binocular visual field. Speed discrimination thresholds were significantly lower for inter-monocular viewing (i.e., one grating in each eye’s monocular field) compared to intra-monocular (i.e., both gratings in one eye’s monocular field). Control experiments confirmed the intra-monocular advantage was not due to inter-monocular crowding or interference. Performance in the central binocular control conditions was better overall, but did not depend on whether stimuli were presented within or across the 2 eyes. IOVDs may be a privileged visual computation, both within the central (binocular) visual field and across the far temporal (monocular) fields. These results suggest that both binocular and monocular IOVDs may reflect the same basic mechanism.

Meeting abstract presented at VSS 2015