On the basis of a critical review of studies that examined the use of temporal information in the regulation of movement, J. P. Wann (1996) concluded that there is little evidence in favor of the use of tau. Although more experimental work is certainly needed, progress can only be made if (a) the conceptual confusion emanating from a lack of distinction between specification (i.e., information) and what is specified (i.e., relevant property of the environmentactor system) is resolved, and (b) the way in which information is used in the regulation of movement is reconsidered. It is argued that continuous control models incorporating firstorder time-to-contact related information not only explain the results obtained but also allow testable accounts of the principles involved in kinematic trajectory formation.A change in the relative distance between an observer and an object (or a surface) gives rise to a change in the optic angle subtended at the point of observation by the object. Thus, a change in the optic angle may be used by an observer to detect the existence of relative motion. Moreover, the pattern of change of the optic angle contains information about certain characteristics of the relative motion, signaling, for instance, whether the relative distance increases or decreases and, in the latter case, whether a collision is imminent (Schiff, 1965). Lee (1974Lee ( ,1976Lee ( ,1980 demonstrated that besides these qualitative aspects, the pattern of change of the optic angle also contains quantitative temporal information, signaling the time remaining until contact if the velocity of approach were to remain constant.The identification of such quantitative temporal information in the optic flow pattern opened the door for the development of a theory of the (temporal) regulation of movement, and since then a large number of studies have been reported that attempted to test and/or further develop such a theory. Today, the optical flow pattern descriptor identified by Lee (1974Lee ( , 1976Lee ( , 1980)-which he coined T (tau)-figures prominently in most textbooks on perception or movement. However, this by no means implies that the debate on the manner in which, if at all, such an optical variable is used in the regulation of movement is closed, as is exemplified by the critical article of Wann (1996). On Distinguishing Specification FromWhat Is SpecifiedBefore entering into the argument about the use of T-like variables in the timing of action, a persistent and pervasive conceptual confusion needs to be resolved first. This confusion is generated by the fact that, more often than not, students of perception and movement fail to distinguish between the property of the Environment-Actor System (EAS) that is thought to be relevant for the regulation of movement and its optical (or acoustical, mechanical, chemical, etc.) representation, that is, the distinction between what is specified (the property of the EAS) and its specification (the informative flow pattern that can be detected). In the introductory example...

In the light of the intensity coupling effects reported in the literature, subjects' capacities for independently controlling the absolute velocity of their movement at the point of interception was evaluated by asking them to learn to propel orthogonally approaching balls, varying in their speed of approach, into target boxes placed at various distances from the interception point. As required for successful accomplishment of the task, movement velocity was found to be adapted to the distance to be covered, with the near target giving rise to lower velocities and the far target giving rise to higher velocities even when the conditions were presented in random order. Nevertheless, even though target distance accounted for a substantial proportion of the total variance, a small but significant effect of ball approach speed on movement velocity was found, suggesting that intensity coupling is a persistent but modulable phenomenon in interception tasks.

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