Visual Processing of Impending Collision: Differential Processing of Object Motion and Self-motion

摘要

As an object approaches an observer's eye, the optical variable tau, defined as the inverse relative expansion rate of the object's image on the retina (D. N. Lee, 1976), approximates the time to collision (TTC). Many studies including our own (Yan, et al, JoV, 2011) have provided support for the tau strategy. The initial proposal for the use of tau in visual motor control assumed that the type of visual motion was irrelevant (i.e., whether the object is moving towards an observer or an observer is moving towards the object). In the present study, we investigated the use of tau during self-motion and compare it to optically comparable situations of object motion. When approaching a stationary object during self-motion, the retinal image of the object will expand on the retina while at the same time the retinal image of the background behind the object will also expand, although at a lesser rate. i.e., the expansion of the object image in relative term compared to an expanding background is smaller than the expansion of the same object against a stationary background. Through three experiments, we demonstrated that that during forward self-motion, observers overestimated TTC compared to situations of equivalent object motion and such overestimations were likely contributed by the use of relative expansion of the target against the expanding background. The overestimation was seen in relative TTC judgement task (discriminating TTCs of two movements) but not on absolute TTC judgment task (estimating the magnitude of TTC of one movement). The importance of relative 3D motion signal found here is consistent with the well-known phenomenon in which relative motion in a 2-dimensional plane between the object and background is known to be processed at neural and behavioural levels. The results provide important insight into the critical visual features in target directed movement.