Motor Abundance & the Affordances for Reaching-to-Grasp

Movements are never the same twice, even when you are trying to do that same thing over and over. Variability is an inescapable fact of trying to organise and run a complex system such as a human body. But there is more than one source of variability in movement; there's noise, and then there's redundancy, and these are not the same thing. 

Our movement systems are redundant; specifically, they always have more degrees of freedom available than are ever required to perform a given task. This means that there is always more than one way to perform any given task, and this can range from slight variations to complete reorganisations. 

Redundancy is a feature, not a bug. It means that we can reliably achieve a task goal in the face of perturbations that range from trial-to-trial fluctuations in execution up to surprises like tripping or the sudden appearance of an obstacle. However, it poses two related control problems. First, a problem of action selection: given that there are many functional organisations of degrees of freedom that could solve that task, which do we choose, and why? Second, a problem of action control: once we have our degrees of freedom organised, we still have some left over that need to be actively controlled; how do we do this, and why do we control them the way we do?

The Bliss of Motor Abundance

The fundamental question in psychology boils down to "why did that person do that thing they just did?". Welcome to my new favourite example.

Figure 1. I am pretty sure this move totally makes sense

I saw this on Twitter the other day and I couldn't stop watching it. What the ever-loving hell is that cat doing? Then I realised I had a hypothesis, and that it was a fun example of an ecological affordance analysis. So here we are, at last, with a cat-themed blog post. I'll review this manouver, claim that it's all about effecting affordances, and then chat a little about complex actions like this.

Affordance-based control (Fajen 2005, 2007)

The most commonly studied tasks in the ecological approach involve the perceptual control of actions such as interception and steering. These models all involve perceiving some variable and moving so as to null the discrepancy between a current value and an ideal value. However, none of these approaches involve the perception of affordances; specifically, none of them address how people work to keep the required corrections possible, given their action capabilities. Fajen (2005, 2007) proposes affordance-based control, an ecological research framework that brings these questions to the fore and leads to the discovery of new, affordance based control strategies that account well for the data and solve the problems of simple information-based control models. 

My current sense is that Fajen is absolutely correct in his assessment of the problems and has done sterling work developing an ecological solution. What follows is a brief description of the problems and his solutions; in the future I will blog some thoughts as I work to align my throwing affordance work with this framework.