OPTICAL SPECIFICATION OF TIME-TO-CONTACT FOR A SPECTATOR

In this article, we provide an account of the informational basis unde rlying judgments of time-to-contact (TTC) under the conditions examine d in Kim, Effken, and Carello (1998), in which observers watched colli sions between 2 objects from various vantage points, over varying coll ision paths, and with additional observer movement. First, we examine the informational candidates proposed by Bootsma and his colleagues (e .g., Bootsma & Oudejans, 1993) and by Tresilian (1990). Simulation res ults showed that these models do not: accurately describe TTC for such varying conditions. We present an alternative model, emphasizing the advantages of body scaling. Our model applied this scaling to the vert ical gap between edges of the opposing surfaces of each object. Becaus e all model terms are expressed in eyeheight units, distortions that o ccurred in the other models due to polar projection are eliminated. Th e proposed model provided veridical estimates of TTC for straight-line collision cases over a variety of viewing angles and with the additio n of observer movement, but not for collisions along circular paths, i n which the model only approximated TTC values.