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The Interface Problem: Motor Representation vs Intention

For a single action, which outcomes it is directed to may be multiply determined by an intention and, seemingly independently, by a motor representation. Unless such intentions and motor representations are to pull an agent in incompatible directions, which would typically impair action execution, there are requirements concerning how the outcomes they represent must be related to each other. This is the interface problem: explain how any such requirements could be non-accidentally met.

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This part of the lecture was given by Johan Heemskerk:


We have seen arguments for three claims about motor representation:

Some motor representations represent outcomes rather than, say, only joint displacements and bodily configurations (see Motor Representation).

There are actions whose directedness to an outcome is grounded in motor representation (see Motor Representations Ground the Directedness of Actions to Goals).

Motor representation differs from intention with respect to representational format (see Motor Representations Aren’t Intentions).

A consequence of these claims is that a single instrumental action may involve representations of the outcomes to which it is directed in at least two different representational formats, motor and propositional. This leads to what we will call the interface problem, which this section introduces.

The Interface Problem

Realising it is rapidly going cold, you form an intention to drink the tea. Your hand expertly secures the mug and moves it to your mouth exactly as it opens. Nothing is spilled in these exquisitely coordinated movements.

As this illustrates, there are cases in which a particular action is guided both by one or more intentions and by one or more motor representations. In at least some such cases, the outcomes specified by the intentions match the outcomes specified by the motor representations. Furthermore, this match is not always accidental.

How do non-accidental matches between intention and motor representation come about? (This question is The Interface Problem)

This question is a problem because two natural routes to answering the question are unavailable. Appealing to common causes of intentions and motor representations is a non-starter; and appealing to content-respecting causal processes despite a lack of inferential integration between intentions and motor representations amounts to no more than a stab in the dark.

Background: Anarchic Hand Syndrome

Marchetti & Della Sala (1998, p. 196) characterise this syndrome as involving:

‘the occurrence of complex movements of an upper limb which are clearly goal-directed and well executed but unintended (Della Sala et al., 1994). These unwanted movements cannot be voluntarily interrupted and might interfere with the desired actions carried out by the other (healthy) hand. The patients are aware of the bizarre and potentially hazardous behaviour of their hand but cannot inhibit it. They often refer to the feeling that one of their hands behaves as if it has a will of its own, but never deny that this capricious hand is part of their own body. The bewilderment comes from the surprising and unwanted actions, not from a sensation of lack of belonging of the hand.’

For further details, see Young (2013, p. 58ff) (who also quote the above).

Background: Action Slips

action slips are actions that run contrary to intentions (Norman, 1981). For instance:

‘I was at the end of a salad bar line, sprinkling raisins on my heaping salad, and reached into my left pocket to get a five-dollar bill. The raisins knocked a couple of croutons from the salad to the tray. I reached and picked them up, intending to pop them into my mouth. My hands came up with their respective loads simultaneously, and I rested the hand with the croutons on the tray and put the bill in my mouth, actually tasting it before I stopped myself.’ (Norman, 1981, p. 10)

For a philosophers’ perspective on action slips, see Mylopoulos (2022) (who also introduces many excellent scientific sources).

Further Reading

Recent work on the Interface Problem about intention and motor representation includes:

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action slip : ‘A slip is a form of human error defined to be the performance of an action that was not what was intended’ (Norman, 1981, p. 1). Examples include saying canpakes for pancakes or pouring coffee on to cereal.
inferential integration : For states to be inferentially integrated means that: (a) they can come to be nonaccidentally related in ways that are approximately rational thanks to processes of inference and practical reasoning; and (b) in the absence of obstacles such as time pressure, distraction, motivations to be irrational, self-deception or exhaustion, approximately rational harmony will characteristically emerge, eventually, among those states.
instrumental action : An action is instrumental if it happens in order to bring about an outcome, as when you press a lever in order to obtain food. (In this case, obtaining food is the outcome, lever pressing is the action, and the action is instrumental because it occurs in order to bring it about that you obtain food.)
You may encounter variations on this definition of instrumental in the literature. For instance, Dickinson (2016, p. 177) characterises instrumental actions differently: in place of the teleological ‘in order to bring about an outcome’, he stipulates that an instrumental action is one that is ‘controlled by the contingency between’ the action and an outcome. And de Wit & Dickinson (2009, p. 464) stipulate that ‘instrumental actions are learned’.
match : [of outcomes] Two collections of outcomes, A and B, match in a particular context just if, in that context, either the occurrence of the A-outcomes would normally constitute or cause, at least partially, the occurrence of the B-outcomes or vice versa.
To illustrate, one way of matching is for the B-outcomes to be the A-outcomes. Another way of matching is for the B-outcomes to stand to the A-outcomes as elements of a more detailed plan stand to those of a less detailed one.
[of plan-like structures] In the simplest case, plan-like hierarchies of motor representations match if they are identical. More generally, plan-like hierarchies match if the differences between them do not matter in the following sense. For a plan-like hierarchy in an agent, let the self part be those motor representations concerning the agent's own actions and let the other part be the other motor representations. First consider what would happen if, for a particular agent, the other part of her plan-like hierarchy were as nearly identical to the self part (or parts) of the other's plan-like hierarchy (or others' plan-like hierarchies) as psychologically possible. Would the agent's self part be different? If not, let us say that any differences between her plan-like hierarchy and the other's (or others') are not relevant for her. Finally, if for some agents' plan-like hierarchies of motor representations the differences between them are not relevant for any of the agents, then let us say that the differences do not matter.
[of motivational states] Two motivational states match in a particular context just if, in that context, the actions one would cause and the actions the other would cause are all proper ways of fulfilling both motivational states.
motor representation : The kind of representation characteristically involved in preparing, performing and monitoring sequences of small-scale actions such as grasping, transporting and placing an object. They represent actual, possible, imagined or observed actions and their effects.
problem : a question that is difficult to answer.


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Della Sala, S., Marchetti, C., & Spinnler, H. (1991). Right-sided anarchic (alien) hand: A longitudinal study. Neuropsychologia, 29(11), 1113–1127.
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