This chapter focuses on the perception side of the perception-action system we will end up with for behaviour. It covers the objects of perception (affordances), the means of perception (law-based information, informationL), and the notion of prospective control (vs. anticipating or predicting the future).
The chapter title is 'how to get about among things'. This stands in contrast to how things are normally framed, in terms of distance or space perception. Perception is for action, it is about enabling safe, skilled activity in a cluttered world. Because of this, the objects of perception (what perception is about) must be action related - affordances. For Turvey, affordances are properties of the environment (not relations between environment and organism) and they are perceived if and only if there is informationL about that affordance present and detected.
Turvey then introduces the idea that perception and action are duals that define a duality (rather than the beginning and end of a linear chain of causation). Note this is a duality, and not a dualism - the two things are not different in kind, but are parts of a greater whole that are intimately and lawfully connected to one another. This implies a relationship between what information specifies, and the energy required to move with respect to what it specifies. The main idea here though is that it is not perception then action, it is only perception-action.
Turvey then spends time on the optical basis for getting about among things; ecological optics. Ecological optics is not physical optics (which is about things like wavelengths), nor is it physiological optics (which is about the anatomy and physiology of the eye). Ecological optics is about what is available in the light prior to any interaction with an organism, and therefore what is available to stimulate the retina, etc. The key is optic flow: structured light distributions incident to illuminated surfaces.
The optical structure available at a single point is specific to the stationary surface layout that gave rise to that structure (this is why the Ames Room works, for example). Optic flow is the general case; optical structure in which the transformations are specific to the surface layout and the relative motions of the observer and the environment. Turvey works through the example of tau in some detail, to illustrate how movement generates an optical flow field and how that field is structured specific to the motion. He then briefly reviews more complicated work on other geometrical features of optical arrays (transformations such as divergence, curl, and deformation; Koenderink, 1986). These are examples of the kinds of maths required, and the target of that maths (the structure of the optic array). Turvey also nods to Stoffregan's notion of a global array (although I will note that this hasn't really gone anywhere yet and it's not clear what the medium of this array is).
Finally, Turvey reviews how optical information can support prospective (future oriented) control. In general there are two approaches to dealing with the future. We have encountered the idea of prediction/anticipation, and the various problems with that. In short, it is a form of induction and it suffers from all the problems of that. Instead, informationL will allow prospective control, via specification of what will occur, if present conditions continue (Bootsma calls this 'the current future'). Turvey of course looks to the outfielder problem as the key example here.
(I will note here that this topic shows up in the modern literature in the form of information-based control (Bill Warren) vs affordance-based control (Brett Fajen). Turvey nods to Fajen and acknowledges that the information used for control should be about affordances; but the debate is not engaged with here.)
Finally, as a prelude to the next chapter, Turvey notes again the idea that all this information is dual to/reciprocal with the action side of the equation.
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