Cues to viewing distance for stereoscopic depth constancy

A veridical estimate of viewing distance is required in order to determine the metric structure of objects from binocular stereopsis. One example of a judgment of metric structure, which we used in our experiment, is the appa rently circular cylinder task (E B Johnston, 1991 Vision Research 31 1351-1 360). Most studies report underconstancy in this task when the stimulus is defined purely by binocular disparities. We examined the effect of two factors on performance: (i) the richness of t he cues to viewing distance (using either a naturalistic setting with many cues to viewing distance or a condition in which the room and the monitors were obscured from view), and (ii) the range of stimulus disparities (cylin der depths) presented during an experimental run. We tested both experience d subjects (who had performed the task many times before under full-cue con ditions) and naive subjects. Depth constancy was reduced for the naive subjects (from 62% to 46%) when t he position of the monitors was obscured. Under similar conditions, the exp erienced subjects showed no reduction in constancy. In a second experiment, using a forced-choice method of constant stimuli, we found that depth cons tancy was reduced from 64% to 23% in naive subjects and from 77% to 55% in experienced subjects when the same set of images was presented at all viewi ng distances rather than using a set of stimulus disparities proportional t o the correct setting. One possible explanation of these results is that, under reduced-cue condit ions, the range of disparities presented is used by the visual system as a cue to viewing distance.