Motor representations are involved in performing and preparing actions. Not all representations represent patterns of joint displacements and bodily configurations: some represent outcomes such as the grasping of an object, which may be done in different ways in different contexts.
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What Are Motor Representations?
Consider very small scale actions, such as playing a chord, dipping a brush into a can of paint, placing a book on a shelf or cracking an egg. Often enough, the early part of such an action carries information about how the action will unfold. For example, in grasping a book (or tall cylinder) you would probably hold its middle, which makes lifting it less effortful. But if you are about to place the book on a high shelf, you are more likely to grasp the book at one end, which makes lifting it more awkward now but will later make placing it easier (Cohen & Rosenbaum, 2004; Meyer, Wel, & Hunnius, 2013). For another illustration, imagine you are a cook who needs to take an egg from its box, crack it and put it (except for the shell) into a bowl ready for beating into a carbonara sauce. How tightly you now need to grip the egg depends, among other things, on the forces to which you will later subject the egg in lifting it. It turns out that people reliably grip objects such as eggs just tightly enough across a range of conditions in which the optimal tightness of grip varies. How tightly you initially grip the egg indicates your anticipated future hand and arm movements (compare Kawato, 1999).
This anticipatory control of grasp, like several other features of action performance, is not plausibly a consequence of mindless physiology. It indicates that control of action involves representations concerning how actions will unfold in the future. These and other representations which characteristically play a role in coordinating very small scale actions are labelled ‘motor representations’.
What Do Motor Representations Represent?
An initially tempting view would be that they represent sequences of bodily configurations and joint displacements only. However there is a significant body of evidence for the opposing view that some motor representations represent outcomes to which purposive actions are directed, such as the placing of a book or the breaking of an egg. These are outcomes which might, on different occasions, involve very different bodily configurations and joint displacements (see Rizzolatti & Sinigaglia, 2010 for a selective review). The experiments providing such evidence typically involve a marker—such as a pattern of neuronal firings, a motor evoked potential or a behavioural performance profile—which allows sameness or difference of motor representation to be distinguished. Such markers can be exploited to show that the sameness and difference of motor representations is linked to the sameness and difference of outcomes such as the grasping of a particular object.
This supports the view that some motor representations represent outcomes such as the placing of an object (so not only sequences of bodily configurations and joint displacements).
Why Consider Them to Be Motoric?
If some motor representations do indeed represent such outcomes, why consider them to be motoric at all? Part of the answer concerns their role in preparing and performing actions. Motor representations can trigger processes which are like planning in some respects. These processes are planning-like in that they involve starting with representations of relatively distal outcomes and gradually filling in details, resulting in motor representations whose contents can be hierarchically arranged by the means--end relation (Grafton & Hamilton, 2007). Some processes triggered by motor representations are also planning-like in that they involve meeting constraints on the selection of means by which to bring about one outcome that arise from the need to select means by which, later, to bring about another outcome (Rosenbaum, Chapman, Weigelt, Weiss, & Wel, 2012). So motor processes are planning-like both in that they involve computation of means--ends relations and in that they involve satisfying relational constraints on the selection of means.
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Much more could be said about what motor representations are and why they are necessary; key sources include Rosenbaum (2010), Prinz (1990), Wolpert, Ghahramani, & Jordan (1995), Jeannerod (1988) and Giacomo Rizzolatti & Sinigaglia (2008). Related theoretical considerations have also been identified by philosophers, notably by Bach (1978) on ‘executive representations’. ↩︎
Pioneering uses of this method include G. Rizzolatti et al. (1988); Giacomo Rizzolatti et al. (2001); it has since been developed in many ways: see, for example, Hamilton & Grafton (2008); Cattaneo et al. (2009); Cattaneo et al. (2010); Rochat et al. (2010); Bonini et al. (2010); Koch et al. (2010). ↩︎
Another part of the answer concerns the role of motor representation of outcomes in reducing the number of kinematic parameters to be computed, which facilitates planning and control of action (see, for example, Santello, Flanders, & Soechting, 2002; Tessitore, Sinigaglia, & Prevete, 2013). ↩︎