Four questions of time to contact: a critical examination of research on interceptive timing.

Four questions concerning the use and perception of time to contact, tc, are identified. (i) Is tc information used in the timing of interceptive actions? (ii) If so, what control strategies are used? (iii) What are the perceptual sources of tc information and which of them do people use? (iv) How is the information extracted by the perceptual systems? Research relevant to these questions is reviewed and analysed. In connection with question (i), theoretical work on the special case of catching a moving object is analysed. It is concluded that treatments of catching which involve the use of tc information provide the best account of timing. In connection with question (ii), two types of control strategy suggested in the literature are identified: an intermittent strategy and a continuous strategy. Evidence for a continuous strategy is reconsidered and shown to be at least as well if not better accounted for by an intermittent strategy. Other empirical evidence for intermittent control is also discussed. In connection with question (iii) a simple unifying method is outlined with which all tc information so far presented in the literature can be derived, and examples are given. The viability of various types of information as sources of tc is examined by considering the errors which would result from their use. Finally, in connection with question (iv) the role of 'looming detectors' in the extraction of tc information is considered. These are frequently proposed as mechanisms for extracting the tc information provided by Lee's optic variable, tau. The analysis provided indicates that, despite the existence of a well-known and popular theory, due mainly to Lee, about how interceptive actions are timed, very little is actually known about perceptual timing. It is not yet certain whether tc information is used in interceptive timing tasks, what kinds of control strategies are involved, what sources of information people use to time their actions, or what perceptual processing is involved in the extraction of tc information.