Abstract
Observers can perceive heading over a ground plane with an accuracy of 1.2° using radial patterns of optic flow (Warren et al., 1988). Can observers successfully use information other than optic flow to extract heading? An alternative idea is that the perceived flow of visible elements can be used even when optic flow is absent. To investigate this question, we determined the accuracy of heading perception using two stimuli: a luminance defined dioptic stimulus containing smooth optic flow and a scintillating random dot cinematogram (SRDC) stimulus (Julesz, 1971), devoid of any optic flow relating to the task. The SRDC stimulus consists of a sequence of random-dot stereograms with single-frame lifetimes, which to each eye appears as a scintillating display of uniform dot density. Thus, there is no relevant optic flow signal in the SRDC stimulus. We then employed a discrimination task to assess translational heading thresholds for each condition using the method of constant stimuli. Subjects viewed simulated self-motion parallel to a ground plane covered with randomly placed objects through a head mounted display. In the last frame of each trial, motion ceased and a vertical target line appeared at the horizon, remaining visible until a response was made. Observers were required to judge whether they were moving to the left or to the right of the target. Bearing angle between the heading direction and the target varied randomly between ±0.5° and 6.0°. Environmental features were approximately matched for visibility in the two conditions. Data were collapsed across heading direction and positive-negative bearing angles. Mean thresholds of 75% correct for 4 observers were less than ∼1° for the dioptic condition and less than ∼2° for the SRDC condition. Thus, observers can perceive heading only slightly less accurately using the SRDC stimuli than using a stimulus with optic flow.