Detecting object movement during self-movement: The importance of local motion contrast, position change and optic flow

Abstract

We have shown that observers can identify object movement within the complex pattern of retinal motion that arises during movement of the observer. We hypothesised (Rushton & Warren, 2005) that this process exploits the brain's well-documented sensitivity to optic flow (global patterns of motion that are characteristic of self-movement) and data collected in previous studies is compatible with this hypothesis. Here we report strong direct tests of alternative solutions. We first remove the retinal disparities within the scene that are necessary for object movement to be identified by local (within disparity plane) motion contrast; we do this by presenting the scene to a single eye, whilst presenting the probe object (which varies in depth) to both eyes. We find the observers still demonstrate behaviour compatible with the identification of object movement. In a second study, we investigate the effect of removing motion (flow) from the display. We do this by presenting either a natural continuous change in (simulated) observer viewpoint, or an unnatural step change. We find that in the latter case, despite local motion and position change information being preserved, performance is significantly impaired. Taken together these results point to a central role for optic flow processing in the identification of scene-relative object movement.