Abstract
During sideways movement, optic flow parsing - in which an object’s speed is extracted from a scene viewed by a moving observer - has been shown to be incomplete, resulting in an overestimation of object speed when target and observer move in opposite directions (Jörges & Harris, 2022, AP&P 84: 25-46). Here we assess the efficiency of optic flow parsing for an object moving in depth while the observer is also moving towards or away from the object. Participants were immersed in a 3D virtual environment and asked to compare the speed of a sphere moving towards or away from them at 2, 3, and 10m/s relative to a ball moving sideways either while they were stationary or during visually simulated self-motion (either forwards or backwards at 6 or 10 m/s) evoking a range of retinal speeds. The speed of the sideways-moving ball was adjusted using an adaptive staircase to match the perceived speed of the sphere. Overall, flow parsing was incomplete. When the observer and sphere moved in opposite directions, the perceived speed of the sphere was greater than when the observer was static. Results were mixed when the observer and the sphere were moving in the same direction. The perceived direction of the sphere’s movement depended on its retinal motion. We conclude that movement-in-depth flow parsing is incomplete. Our results are relevant to perceptual processing in various real-world settings, such as driving or crossing the road.