What Kind of Thing is an Information Variable? The (Annoying) Case of Tau

The central contribution of the ecological approach is the idea of ecological information. Information variables are higher order relations that remain invariant over time as the elements of the pattern change. These relations are the kind of thing that can specify a dynamical property of the environment and support direct perception of that environment. 

This is all a little abstract. One nice, simple example is the variable tau which specifies time-to-contact (TTC). TTC is an important property of objects approaching you that, if perceived, would support you intercepting or avoiding the object. Tau is one variable that specifies TTC and therefore might get used by organisms to perceive TTC.

This post will use tau as an example of information because it's straight forward and has lots of the relevant key features. However, tau is a pain in the ass because organisms typically don't actually use it - it's too limited in its scope to be the best information. People discussing this fact sometimes says it reveals a weakness in the ecological approach. It doesn't; it just reveals a weakness in tau (and the error ecological psychologists made getting as excited as they did about it as an exemplar). It highlights a lot of useful issues, though, so I thought it was still worth the post.