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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Notes

This part of the lecture was given by Johan Heemskerk:

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Recap

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.

Equally, however, intentions and motor representations do not always match. Examples of mismatch can be found in Anarchic Hand Syndrome, action slips and apraxia (see below).

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). More carefully:

an action ‘slip is a form of human error defined to be the performance of an action that was not intended’ (Norman, 1981, p. 1)

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).

Background: apraxia

Apraxia provides a further illustration that intentions and motor representations do not always match.

Apraxia is a higher order motor disorder ‘observed in patients who, in spite of having no problem in executing simple actions (e.g. grasping an object), fail in actions involving more complex, and perhaps more conceptual, representations’ (Jeannerod, 2006, p. 12).^[1]

Why we care about apraxia: those who exhibit apraxia can perform simple actions but cannot perform more complex actions which are nothing but structures of those simple actions. For instance, one patient could use tools like spoons and forks, razors and toothbruhes
effectively but at breakfast cereal with a fork and put toothpaste onto a razor (Mayer, Reed, Schwartz, Montgomery, & Palmer, 1990, p. table 11–1, 262).

In cases of ideomotor apraxia, patients have difficulty performing actions in response to instructions but can nevertheless perform the same actions when presented with a visual stimulus:

‘Patients with ideomotor apraxia may exhibit differing degrees of impairment depending on testing conditions. For instance, patients typically have greatest difficulty performing gestures elicited by verbal command. They tend to have less difficulty imitating a gesture or acting in response to a visually presented object. They may be least impaired when asked to use the object itself’ (Gross & Grossman, 2008, p. 490)

Mayer et al. (1990, p. 263) suggest that apraxia is continuous with action slips:

‘our observations suggest that there is a continuum of executive impairment, such that at the most severe end, patients experience difficulties with planning and organizing in even the most routine (ADL) tasks and are incapable of engaging in higher-level, nonroutine activities. With recovery, or less severe impairment, the patient may carry out ADL tasks normally but break down where more extensive planning and organization is required (e.g., managing family budgets).’ (Mayer et al., 1990, p. 263).

Taken together, then, observations of action slips and cases of apraxia strongly support the view that intentions and motor representations do not always match.

Further Reading

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

• Mylopoulos & Pacherie (2016);
• Fridland (2016);
• Shepherd (2019);
• Christensen (2021);
• Ferretti & Caiani (2019); and
• Ferretti & Zipoli Caiani (2021).

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