Abstract: PURPOSE: During self-motion, the visual system can perceive the direction of self-motion (heading) and identify scene-relative object motion from optic flow (flow parsing). However, little is known about the temporal dynamics of heading perception and flow parsing. Here we addressed this question by examining how the accuracy of heading perception and flow parsing changes with exposure time to optic flow.
METHODS: A stereo display simulated forward translation at 0.3m/s through a cloud of 58 red wireframe objects (depth: 0.69-1.03m) placed on one side of the image plane (56ºx33º). Five display durations (100ms, 200ms, 400ms, 700ms, & 1000ms) were tested. For heading perception, on each trial, heading was randomly chosen from -10° (left) to 10° (right). Participants were asked to indicate perceived heading at the end of the trial using a mouse-controlled probe. For flow parsing, on each trial, heading was fixed at 0° and a yellow dot probe (diameter: 0.25º; depth: 0.86m) moved vertically for 100ms in the scene. Objects were placed on the opposite side of the probe in the image plane to remove local motion cues around the probe. The speed (2º/s) and eccentricity (4º) at the midpoint of the probe's motion were equated across display durations. A nulling motion component was added to the probe's motion using an adaptive staircase to determine when the probe was perceived to move vertically in the scene. This nulling motion component was used to compute the accuracy of flow parsing.
RESULTS: Across 12 participants, while the accuracy of heading perception increased with exposure time, the accuracy of flow parsing decreased with exposure time to optic flow.
CONCLUSIONS: The opposite trend of temporal dynamics of heading perception and flow parsing suggests that although these two processes both rely on optic flow, they involve separate neural substrates and compete for the same limited attention resource.

Key words: self-motion, heading, optic flow, object motion, locomotion