Use of target drift in heading judgments.

The change in direction of a target object relative to a translating observer (or a point fixed relative to the observer), "target drift," provides information about the observer's direction of self-movement (i.e., heading) with respect to the target. Relative drift rate (normalized with cues to motion-in-depth) provides information about the observer's absolute direction of heading relative to the surrounding scene. We investigated the utility of target drift by comparing heading judgments with target drift and "extra-drift" cues (the cues available in the changing optic array except target drift) in isolation and together during simulated forward translation. Across four experiments, we found that with the target drift cue alone, participants were able to make precise judgments of both nominal and absolute heading (≤1.53°). Judgments were at least as precise with the target drift cue alone as with extra-drift cues alone. The addition of extra-drift cues to the drift cue did not improve precision, and the pattern of reaction times suggests that the two cues are processed independently. We conclude that target drift can be an effective and powerful cue for heading judgments.