Stereoscopic bias and sensitivity during asynchronous binocular presentation

Temporal offsets in binocular presentation can give rise to a perception of depth much like that provided by spatial disparities (the Pulfrich effect). One explanation for this phenomenon is that the brain is sensitive to temporal as well as spatial disparities between corresponding points in the world. Here we investigated the interaction between spatial disparities and temporal offsets between presentations to the two eyes that could provide a temporal disparity cue. Specifically, we used human psychophysics to investigate (a) how asynchronous presentation affects stereoscopic acuity and (b) whether the temporal order with which information reaches the two eyes provides a temporal disparity signal that biases depth perception. Observers were required to judge the sign of a test line's (45×5′) relative horizontal disparity. Presentation was either binocularly continuous for 120ms or the test line was shown alternately to each eye (50, 25, 17 or 8 Hz) for 120ms. Psychophysical data (method of constant stimuli) were fit to obtain estimates of the slope (sensitivity) and 50 threshold (bias). Stereoscopic acuity was reduced from the continuous presentation baseline under conditions of stimulus alternation, with the effect most pronounced at 8 Hz. Above 8 Hz, however, binocular temporal offsets were not the critical factor: acuity was the same as that measured when binocular presentation was simultaneous but flickering on and off. Finally, the temporal order of presentation did not introduce systematic bias in the observers' depth judgments. These results suggest, at best, a very weak interaction between temporal offsets between presentation to the two eyes and spatial disparity signals under conditions of continuous temporal alteration