The effects of familiar size and object trajectories on time-to-contact judgements

Many interceptive actions involve interactions with objects that are familiar to the observer and have known sizes. Two experiments investigated how known size influences observers’ perception of time-to-contact (T _c). Participants made T _c judgements of objects that were either ambiguously sized, standard-size in identity/familiarity, or off-size in identity/familiarity, and simulated as approaching on linear trajectories (Experiment 1), or linear versus parabolic trajectories (Experiment 2). In Experiment 1, T _c judgements were influenced by the size of the object in the three object identity/familiarity conditions; the greatest size effect occurred in the off-size condition compared to the ambiguous size and standard-size conditions. The results of Experiment 2 replicated these results and found that size effects were not reduced with displays simulating parabolic trajectories, that is, displays simulating ecologically valid free-falling objects. Taken together, the finding that T _c judgements are influenced by object identity/familiarity does not provide support for the tau hypothesis, nor the hypothesis that T _c judgements are based solely on optic expansion rates. However, the results do provide support for the proposition that T _c judgements are based on a combination of rate of retinal image expansion and object identity/familiarity information, the latter information requiring observers to have prior experience with, or knowledge about, the objects.