Effectivities for Reach-to-Grasp Actions

I just reviewed the affordance properties that produce the spatial structure in reach-to-grasp actions, and there's an unquestioned assumption lurking in that analysis. Luckily, Mon-Williams & Bingham (2011) actually did question it, so I wanted to cover that part of the paper here.

The assumption in the analyses I described last time is that the unit of action is the aperture between the finger and thumb, and not the fingers themselves. Bingham refers to this as an opposition vector (Iberall, Bingham, & Arbib, 1986; van Bergen et al, 2007). In some ways, this is a weird idea; the action system working to control a space between limbs, and not the limbs! Smeets & Brenner (1999) proposed that grasping is actually about the two limbs. Mon-Williams & Bingham tested these hypotheses and found evidence in favour of the opposition vector.

I want to walk through this in a little detail, though, as of course identifying the relevant elemental variables is part of an UCM analysis, and affordance research helps here too. The task analysis that reveals affordance property options also points to effectivity property options (at least it should - these are complementary after all!). But another part of the UCM approach is that it can, in principle, test hypotheses about elemental and performance variables, so I want to lay this out as well.

Is Direct Perception Plausible? Ecological Information

We're in the home stretch of working through how direct perception is, at least, an option, and how the ecological approach in particular attempts to make it work. We've talked about what direct vs indirect means, the kinds of properties direct perception needs to be out there in order to work, and affordances/effectivities-as-dispositions as the specific properties ecological psychology claims are out there and fit the bill. 

I ended that last post by highlighting ecological psychology had one last thing to do in order to be plausible, and that is to have a way to bring affordances and effectivities together into a kind of contact that allows them to work together. That contact can't be mechanical, or simply physical proximity, because almost all of the things we perceive and act with respect to are not in that kind of contact. The ecological solution is informational contact, and so this post will build on the pieces I've assembled to identify what kind of thing ecological information has to be in order to work. 

Is Direct Perception Plausible? The Case for Affordances-as-Dispositions

The first post in this series laid out the rules for what makes a theory of perception direct or indirect. In order to avoid having to require organisms to somehow figure out behaviourally relevant properties (indirect perception), direct perception requires that such properties are out there already, ready and able to be detected. Such properties are a bit weird - by definition, they must be properties of objects that include some reference to the organism doing the detecting (specifically, they are impredicative properties). 

The second post laid out some reasons to think that such circularly defined properties are legitimate options for the physical world. Properties like this are required by the mathematics of the quantum scale; and so, while a bit weird, are possible. Organisms don't work with quantum properties, though, so if they only happen at that scale, this doesn't help a theory of direct perception. The key to making impredicative properties work is measurement. At the quantum scale, all properties are uncertain until measurement collapses the uncertainty of the thing measured. At the ecological scale, properties aren't uncertain in this way, but which properties are 'primary' still depends on the measurement device; to a polar planimeter, for example, the higher order property 'area' is simple and lower order properties such as 'length' must be figured out. The reverse is true of a ruler. 

We have a set of pieces now. Impredicativity requires measurement to affect which properties are immediately available to the measurer, this is possible at the ecological scale, and with the right measurement device higher-order properties can be immediately available. This establishes an in-principle case in favour of direct perception as an option. Next, if we can identify the higher-order form of behaviourally relevant properties, we will identify what kind of measurement device is required for these to be directly available. 

The main ecological hypothesis is that higher-order behaviourally relevant properties, which we call affordances, take the form of dispositional properties. This post will walk through how this works and how it fulfils what's required to support direct perception.

(As many of you know, not everyone is happy with the dispositional ontology, and propose instead that affordances are relations. I am not going to lay out the parallel case for how this ontology fulfils the requirements I have been laying out, for the simple reason that I do not think it can do any such thing and I have yet to see any successful attempt to make it work. I talk about this in this paper, currently still living in review limbo.)