Comparing binocular, biocular and monocular cues for time-to-contact

Abstract

Judgements of time-to-contact (TTC) have been shown to be influenced by both monocular cues, such as looming, and binocular cues, such as changing disparity (Gray & Regan, 1998, Vision Research, 38: 499–512). We investigated the contribution of binocular motion information when presented in conjunction with monocular motion cues in a TTC task. We compared TTC judgements in three conditions, a) combined cue - consistent looming and changing disparity, b) monocular - looming showed to one eye only, c) biocular - both eyes presented with a looming stimulus but no change in disparity. Observers were shown a circular patch of dots which appeared to move directly toward them for 588ms at a constant speed, simulating one of three TTCs (1.8s, 2.5s or 3.2s). The stimulus was then replaced with a blank screen for a variable interval before the screen flashed. Observers were asked whether the stimulus would have contacted them before or after the flash, had it continued in its trajectory. This data was used to determine whether observers could perform a TTC judgement in the present task. Analysis suggested that four of eight observers' responses did not in fact reflect TTC judgements, but were based on other stimulus parameters. For those who based their responses on TTC information, we derived approximations of TTC estimates for each condition. Three of these four observers gave more accurate TTC estimates under the combined cue condition than monocular and biocular. No observer's responses differed between monocular and biocular conditions, suggesting that judgements in these conditions were based on monocular information only. The results lead us to conclude that under present conditions, binocular cues can aid TTC estimation when present and consistent with monocular cues, but no influence occurs when binocular information indicating zero motion is presented alongside monocular motion cues.