Time to contact (TTC) is specified optically by tau, and studies suggest that observers are sensitive to this information. However, TTC judgements also are influenced by other sources of information, including pictorial depth cues. Therefore, it is useful to identify these sources of information and to determine whether and how their effects combine when multiple sources are available. We evaluated the effect of five depth cues on TTC judgements. Results indicate that relative size, height in field, occlusion, and motion parallax influence TTC judgements. When multiple cues are available, an integration (rather than selection) strategy is used. Finally, the combined effects of multiple cues are not always consistent with a strict additive model and may be task dependent.

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Estimates of the time to contact (TTC) between 2 objects with simulations of no self-motion (SM) or SM at various speeds and directions were measured. The latter violates assumptions of prior analyses of optical TTC information. In some cases, SM can affect TTC estimates, but in other cases, observers may compensate for effects of SM on optic flow. Under some conditions, estimates were comparable for no SM and forward or backward SM, even when SM nullified the constriction of the optical gap between the 2 objects. Furthermore, background texture can affect TTC estimates. Results suggest that observers can judge the TTC between 2 objects during SM but performance obtained during stationary viewing may not generalize to SM. Factors other than optical TTC information must be considered in models of perceived collision.Information in the optic array specifies when an object will hit or pass the observation point and when it will hit another designated object (e.g.,

Results indicate that under some conditions the Sander parallelogram illusion can affect time-to-contact (TTC) estimation in a prediction-motion (PM) task and in an interceptive action (IA). The illusion also affected mimed manual prehension. The implication is that the timing of responses in the PM and IA tasks may involve an estimate of TTC that is based on the perceived dimensions of the environment. Further research is warranted in the development of models of perceived collision and of visually guided actions.

To move through the environment safely, people must make effective judgments about collisions. It has been asserted that most studies of time-to-collision judgments are limited due to a lack of visual realism (Manser & Hancock, 1996). Studies that compared performance among displays which differed in realism provided mixed results. We measured judgments about whether, and when, two objects would have collided with each other. Results from simulations of scenes with colored, textured surfaces and a moving observer were mostly comparable to earlier results from simulations of black-and-white, line-drawn objects and a stationary observer (DeLucia, 1995; DeLucia & Meyer, 1999). Texture and self-motion affected performance in a restricted set of conditions and did not eliminate errors due to misleading depth cues. Increases in realism, which incur more costs and computational time, may not always be justifiable from a performance standpoint. Results have design implications for simulators and virtual reality systems.