Studies in Perception and Action V M. A. Grealy & J. A. Thomson (Eds.) © 1999 Lawrence Erlbaum Associates, Inc.
Cathy M. Craig & Reinoud J. Bootsma
Movement & Perception, University of the Mediterranean, Marseille, France
In every day life we are confronted with situations where we have to adapt our movement behaviour relative to the movement of other people or objects. These adaptations are based on our ability to detect the information specifying the relevant characteristics of our interaction with our environment. In the domain of optical specification of temporal relations, Tresilian ( 1991) proposed to distinguish between two types of informational variables, namely local tau variables that specify temporal information with respect to objects in head-on approaches (i.e. Time to Contact as previously defined by Lee, 1976) and global tau variables specifying temporal information with respect to the Time to Passage. Kaiser and Mowafy ( 1993) suggested that observers would rely on the latter type of information when engaging in global navigation, where objects are passed as opposed to being collided with.
Given, however, the evolutionary cost of having a system that would rely on one informational variable to specify time for one situation and another informational variable for a different situation, we propose the use of a single optical variable, namely a composite tau ( Bootsma, 1988; Bootsma & Oudejans, 1993), that combines lateral displacement (angle χ) and expansional (angle ϕ) information so that the time remaining until contact or to passage is accurately specified at any given moment in time (see Fig. 1).