Judging the direction of object motion-in-depth during simulated self-motion

There are many situations in which the direction of a moving object must be judged when both the observer and the object are moving (e.g. when running to catch a football). Previous research has focused on the influence of object motion on judgements of the direction of self-motion (i.e. heading), but the converse relationship has not yet been explored. We examined the effect of simulated self-motion on discrimination of the direction of object motion-in-depth (MID) by superimposing a simulated approaching object on a large-angle radial flow field. The location of the focus of expansion (FOE) and the speed of the flow pattern were varied from trial to trial, and the simulated object never overlapped the background flow elements. The location of the FOE had a systematic effect on perceived direction of object MID. When the location of the FOE was offset by 7 degrees to the right of the approaching object, observers perceived the object's trajectory to be shifted 4-5 degrees rightward. Simulated forward self-motion (i.e. central FOE) introduced a 1.5-3 degree bias towards the center, in which observers perceived the object's trajectory as coming more towards their head. These effects occurred even though the flow field did not alter the motion-in-depth ratio (of the rate of expansion to the rate of lateral motion) for the approaching object. These findings are consistent with judgements of time to collision during simulated self-motion (Gray & Regan, 2000).