In an earlier article in this journal, B. Shaw, McGowan, and Turvey (1991) resented an acoustic variable they supposed would specify the time to contact with a sound-emitting source moving rectilinearly toward an observer at constant speed. Their formulation, t = 2l(dI/dt), follows Lee's (1976) main ideas in his derivation of optical tau for small visual angles. In this article I compare the functions of vision and hearing more generally and consider what information vision and hearing would use in normal circumstances. In this context, one may then ask a more specific question, Which kind of auditory information might be used about impending collisions? I make the following points: (a) The specification analysis of Shaw et al. (1991) is a substantial contribution to the growing field of ecological acoustics, (b) the idea that a more general attack on the acoustic guidance of action (Shaw et al., 1991, p. 254) starts with a proposal for an uncommon case without considering the general functions of seeing and hearing first seems inconsistent, (c) an auditory variable specifying time to turn or time to jump is needed, and (d) I do not believe that the auditory system can use the kind of information proposed by Shaw et al. (1991) in estimating time to contact.
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