Part of Withagen's critique of specification and whether it's necessary to underpin direct perception is a brief review of some empirical literature that shows people using non-specifying variables. I want to spend a few posts reviewing these, because all good potentially sensible ideas need data to confirm whether they're right or not.
First up, the perception of relative mass after a collision. Events in the world are dynamic, that is, they involve motion caused by a pattern of underlying forces. Perceptual systems want access to the underlying dynamics of events, because this is the level at which the event is defined (Wilson & Bingham, 2001). However, perceptual systems can only detect kinematics, that is, motion - this is the perceptual bottleneck (Bingham, 1988 and this note on dynamics and kinematics). We can only perceive the underlying dynamics of an event, according to the ecological approach, if we can detect motion that is specific to that dynamic. Runeson coined the phrase kinematic specification of dynamics (Runeson & Frykholm, 1983) and investigated whether there were such kinematic patterns and whether we can detect them. Working with Claire Michaels and David Jacobs, he has also investigated the use of non-specifying variables.
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