Motor Representation
‘From the outset the grasping movement is magically at its completion; it can begin only by anticipating its end, since to disallow taking hold is sufficient to inhibit the action.’
(Merleau-Ponty, 2012, p. 119)
motor representation(/schema)
Let me go back and start with some almost uncontroversial facts about motor representations and
their action-coordinating role.
Why postulate motor representations at all? [Dependence of present actions on future actions
is one reason for doing so.]
A goal represented motorically triggers a process which, via computations of things
like end states, starting states and smoothness, eventually results in joint
displacements; and when things go well, these joint displacements together with the
resulting bodily configurations bring about, or constitute, the occurrence of the goal.
But of course this is a simplification. Motor representations can trigger processes
which result in further goals being represented, as for example when a motor
representation of the transporting of an object triggers representations of reaching,
grasping, placing and releasing.
The processes linking motor representations are planning like in two respects:
(i) means-end ...
... and (ii) relational constraints
(Rizzolatti & Sinigaglia, 2008, p. figure 1.1)
‘The posterior section of the frontal lobe contains the motor areas,’ (p.~4)
Now you know as much about the brain as I do.
Mention primary and supplementary motor areas : we use the term ‘motor’ loosely
(compare ‘visual’, which also has narrower and broader uses in
neuroscience).
Fogassi et al. (2005, p. figure 1b)
grasp to eat or grasp to place.
placing in the shoulder container minimizes kinematic differences
(This is not yet relevant; but will matter later for representing
outcomes vs kinematics as this could be used to illustrate how changing
the outcome changes the (marker of) representation)
Fogassi et al. (2005, p. figure 5)
It is now a familiar, if still interestingly controversial idea, that motor representation
leads a DOUBLE LIFE.
For it is involved not only in coordinating the performance of small-scale purposive
actions like reaching, grasping, placing and transporting but also in action observation.
If you were to observe someone phi-ing,
then motor representations would occur in you
much like those that would occur in you if it were you, not her, who was phi-ing.
This is not our focus,
it’s just a handy fact that simplifies testing.
We know this in large thanks to the discovery of mirror neurons and their consequences.
‘(A) Congruence between the visual and the motor response of a mirror neuron. Unit 169 has a stronger
discharge during grasping to eat than during grasping to place, both when the action is executed and
when it is observed. Conventions as in Fig. 1. (B) Population-averaged responses during motor and
visual tasks (12).’
Motor representations concerning a particular type of action are
involved not only in performing an action of that type but also sometimes in observing one. That is,
if you were to observe Ayesha reach for, grasp, transport and then place a pen, motor representations
would occur in you much like those that would also occur in you if it were you---not Ayesha---who was
doing this.
Converging evidence for this assertion comes from a variety of methods and measures;
but I won’t mention alll of that here. Except one TMS experiment ...
D’Ausilio et al. (2009, p. figure 1)
For a quick illustration of how we know about the double life of motor representation, consider
this experiment ....
‘Double TMS pulses were applied just prior to stimuli presentation to selectively prime the cortical
activity specifically in the lip (LipM1) or tongue (TongueM1) area’
(D’Ausilio et al., 2009, p. 381)
‘We hypothesized that focal stimulation would facilitate the perception of
the concordant phonemes ([d] and [t] with TMS to TongueM1), but that
there would be inhibition of perception of the discordant items
([b] and [p] in this case). Behavioral effects were measured via reaction
times (RTs) and error rates.’ (D’Ausilio et al., 2009, p. 382)
D’Ausilio et al. (2009, p. figure 2)
Prediction confirmed: TMS to lip area facilitates identification of labial
phonetic gestures.
Labial = relating to the lips
Villiger, Chandrasekharan, & Welsh (2010, p. figure 1AB)
TMS to measure MEP
Read the frames left-to-right although the action direction is to the left (confusing)
Villiger et al. (2010, p. figure 2)
Incidentally, ‘the observed direction of the modulation was not consistent with previous TMS
literature. Specifically, MEP amplitudes were significantly lower in the Object-Present than in the
Object-Absent conditions (Fig. 2), suggesting that there was an inhibitory effect of object
manipulation on the activity of M1 during action observation.’
Umiltà et al. (2008, p. figure 1)
Umiltà et al, 2008 : single cell recordings in monkeys
MEPs (TMS amplified) in humans
Cattaneo, Sandrini, & Schwarzbach (2010, p. figure 1)
TMS MEP, humans.
Shown video, then a static picture.
Is this the same goal as you saw in the video?
Press one of two keys.
‘They were explicitly told to ignore the effector and make a judgment on the type of act only.’
Cattaneo et al. (2010, p. figure 3)
Key finding: TMS to both ventral premotor cortex (PMv) and left supramarginal gyrus (SMG)
increases RTs regardless of whether it’s the same effector or a different effector.
(You can’t see same/different effector in this figure.)
KEY: superior temporal sulcus (STS), and a parietofrontal system consisting of the intraparietal
sulcus (IPS) and inferior parietal lobule (IPL) plus the ventral premotor cortex (PMv) and caudal
part of inferior frontal gyrus (IFG). In some instances also, the superior parietal lobule (SPL)
Cattaneo et al. (2010, p. figure 4)
By contrast, TMS to superior temporal sulcus (STS) increased RT only for judgements
where the video effector was the same as the photo effector.
Markers of motor representation
The experiments providing such evidence typically involve a marker of motor representation,
such as a pattern of neuronal firings, a motor evoked potential or a behavioural performance
profile, which, in controlled settings, allows sameness or difference of motor representation
to be distinguished. Such markers can be exploited to show that the sameness and difference
of motor representation is linked to the sameness and difference of an outcome such as the
grasping of a particular object.
(Pioneering uses of this method include G. Rizzolatti et al., 1988; Giacomo Rizzolatti, Fogassi, & Gallese, 2001;
it has since been developed in many ways: see, for example,
Hamilton & Grafton (2008); Cattaneo, Caruana, Jezzini, & Rizzolatti (2009); Cattaneo et al. (2010); Rochat et al. (2010); Bonini et al. (2010); Koch et al. (2010).)
To illustrate, consider a sequence of actions which might be involved in shoplifting an apple: you have to secure the apple, transport it, and position it in your pocket.
Each of these outcomes can be represented motorically.
Motor processes are planning-like in that they involve computing means from ends.
Thus a representation of an end like securing it [the apple] can trigger a process
that results in the representation of outcomes that are means to this end.
Motor processes are also planning-like in that which means are selected in preparing an
action that will occur early in the sequence may affect needs that will arise only later
in a later part of the actions.
For instance, how the apple is grasped at an early point in the sequence may be determined
in part by what would be a more comfortable way for the other hand to grasp it later.
So motor representations of outcomes guide planning-like processes.
This is why I think it’s not just that they carry information about outcomes
like grasping an apple, but that they also represent such outcomes.
some
motor representations (/schema) represent outcomes
‘a given motor act may change ... as a function of what motor act will follow it—a sign of planning’
Cohen & Rosenbaum 2004, p. 294
In conclusion, two ideas about the control of action.
First, I follow Jeannerod (2006) and others in rejecting the view that all motor representations specify only bodily configurations, joint displacements and end states.
Instead some motor representations specify outcomes to which actions are directed,
such as the grasping of a particular handle or the transporting of a given object.
Second, some motor processes involve computing means from ends and generating sensory expectations concerning the effects of actions (Wolpert, Doya, & Kawato, 2003, p. e.g.).