Our senses limit our actions, and this is a good thing

It can be so helpful sometimes to revisit older texts that were part of your intellectual trail, but which haven’t been retread for a while. Today, I met with my PhD student Alannah to discuss a book chapter by Karl Newell, ‘Constraints on the Development of Coordination‘. The last time I thought about this paper properly was when Johann Issartel and I set out to write a critique of it 10 years ago (this has yet to materialise, but may happen yet), and I haven’t looked at it since then. Alannah’s project is about motor development in children with visual-impairment, and so it seemed like a relevant source of theoretical ideas for her thesis, and something that would be worth discussing. I’m very glad we did.

The paper sets out a theory of the development of coordination, essentially the principles by which children come to acquire skilful control of their movements. A central idea is as follows. There are too many ways to move. All the possible ways of rotating joints, contracting or relaxing muscles, and shifting limb parts through space, means that there is a huge mathematical problem for the developing brain to solve: how to reduce these possibilities from an infinite set to a workable set for controlling intentional behaviour (this is a crude summary of Bernstein’s Degrees of Freedom problem).

Part of the answer to this problem lies in the concept of ‘constraints‘. Constraints are limits on how physical things can move. Gravity. Limb mass. The material springiness of connections between muscles, ligaments, tendons, and bones. Boundaries of frequencies of signals to and from the central nervous system. The properties of structures, objects and events in the immediate environment. All these things reduce the degrees of freedom available. Thus, coordinated behaviour emerges from how different constraints force organisation of the component parts involved. As a somewhat removed illustration, a murmuration of starlings emerges from the combined constraints of gravity, air-flow, wing shape, and a few simple (though yet undiscovered) rules governing how each bird responds to motions of other birds in their visual field. Rather than thousands of birds all flying around at random, these constraints limit their possible paths of motion to a smaller subset of codependent trajectories. The results is a beautiful, coordinated complex system (see video below). The idea is that human (and other animal) movement obeys similar natural laws, whatever they may turn out to be. Thus, the concept of constraints on coordination provides a starting point to a solution to the Degrees of Freedom problem. This idea is summarised nicely in a line quoted from another paper by Kugler, Kelso and Turvey: “it is not that actions are caused by constraints, it is rather that some actions are excluded by them”.

Importantly, information picked up through our senses can also constrain movement. That is, when functioning to guide action, vision/audition/proprioception/etc., all limit the range movements that can/should be made. We tested (informally) this idea today, by having me close my eyes and draw a figure-of-eight in the air. When I made the same movements with eyes open, the pattern was more accurate, and consistent. The set of finger movement possibilities was reduced by the visual constraint of how my limb moved in relation to the intended pattern. Perception limits action. This brings me to a ‘Eureka’ moment I had when re-reading the paper, and which Alannah and I discussed in earnest today.

Visual-impairment is not a constraint on coordination, but rather a reduction in constraints. Having limited or no visual access to one’s own limbs, or objects/structures/events in the environment, does not limit movement but rather removes a limit on movement. Thus, movement development is affected by having fewer informational stabilisers and contours to follow.  Of course, other modalities (audition, proprioception, etc.) can and do impose constraints on movement, and optimal patterns of coordination may be discovered by someone with visual-impairment through these limiters. The goal now becomes identifying the best ways to organise task and environmental constraints to help the children uncover these solutions, rather than trying to replace visual ‘input’ through other channels. As a result, thinking about vision and other senses as limitations on movement will really shift the way Alannah and I have been viewing perceptual motor development in children with visual-impairment.

(Re-)reading older papers is a good idea!


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