Empirical and theoretical issues in the perception of time to contact

Four questions concerning the perceptual source of information about time to contact (tc) are addressed: (a) What conditions are required for the optic variable tau to play a role in the perception of tc? (b) When these conditions are met, does tau alone provide sufficient information for accurate timing of interceptive actions? (c) Does a distance divided by velocity account of tc perception provide a convincing alternative to an account that is based on tau? (d) Is there any empirical evidence that distinguishes the two accounts? A "global" type of tau variable and a "local" type of tau variable are distinguished, each with different limitations. The discussion is largely concerned with local tau variables, 2 versions of which are identified. It is concluded that tau alone cannot provide sufficient information for skilled timing. An extended tau-based account presented in an earlier article (Tresilan, 1990) is discussed. It is argued that no extant empirical data can distinguish the extended account from the distance divided by velocity account.