We discuss the following three-step approach to testing the hypothesis that a given task of collision avoidance or achievement is carried out by predicting the location of the approaching object at some time in the future: (1) Derive theoretical equations that embody candidate hypothesis; (2) find whether the human visual system contains a mechanism that processes the designated retinal image variable independently of other retinal image variables; (3) carry out field studies to find whether individuals base performance on the designated retinal image variable when performing the task in question. We review equations that relate monocular and binocular retinal information to the direction of an object's motion in three-dimensional space and to its time to collision or passage. We discuss a psychophysical technique for finding whether the human visual system is selectively sensitive to the information in question and can unconfound co-varying retinal image variables. We present a quantitative psychophysical model of the early processing of changing-size, changing-disparity, motion in depth and time-to-contact (TTC) that takes dynamic characteristics into account and that accounts for a wide range of psychophysical data including the common case of an object whose retinal image changes shape as the object grows closer to the observing eye.
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