The difference in differences ...
Here is the argument. Consider two sequences of sensory encounters: (a) a sequence of sensory
encounters with two phonetic events that do not differ with respect to category (both are
realisations of /d/, say), and (b) a sequence of sensory encounters with two phonetic events that do
so differ (one is a realisation of /d/ the other of /g/, say).11 Let the events encountered in the
first sequence differ from each other acoustically in the same way and by the same amount as the
events encountered in the second sequence differ from each other. (That it is possible to find two
such pairs of events follows from the fact that we enjoy categorical perception of speech.) The two
sequences are depicted in Fig. 3. Now:
Repp & Knoblich 2009, figure 3
Repp and Knoblich (\citeyear{repp:2007_action}) asked expert and non-expert pianists to press two keys in sequence, where the first key was sometimes to the left, and sometimes to the right, of the second key. The key presses produced an \emph{ambiguous tone pair}, that is, a pair with the property that the first tone is sometimes perceived as lower in pitch than the second whereas at other times it is perceived as higher in pitch \citep{deutsch:1987_tritone}.
The tones always occurred in the same order regardless of which key was pressed first. By asking subjects to report how they perceived the relative pitches of the tones, Repp and Knoblich found that, for the expert pianists, the direction of the key presses influenced the perceived direction of the change in pitch. Could what influences experience in this case be not a motor representation but merely the occurrence of a movement, or perhaps even the perception of a movement of the subject's own fingers? Against these possibilities, note that the effect was not observed in non-expert pianists: for them the direction of movement did not measurably influence the perceived pitches. Since the direction of movement was the same for both groups, if the influence were due to movement only we would expect it to occur irrespective of piano-playing expertise. Instead it seems likely that differences in expertise between the two groups of subjects affected how the movements they performed were represented motorically, and that these differences in motor representation are in turn what explains their perceptions of relative pitches.
It is not only in performing action that motor representation can influence experience: the same can occur in observing action. Thus in another experiment, Repp and Knoblich (\citeyear{repp:2009_performed}) compared observing someone else perform a sequence of key presses with performing the same sequence oneself. They found the same effect on experiences of an ambiguous tone pair in expert pianists regardless of whether they were observing or performing the action.
Sometimes, which judgement about pitch an experience provides a reason for depends on what is represented motorically; and this dependence is systematic, of course, for it reflects how pianos work.
These studies, and others like them,%
\footnote{
See also \citet{zwickel:2010_interference} who investigate effects of action on visual experience of motion, and \citet{schutz-bosbach:2007_perceptual} for a review.
}
provide relatively direct evidence that motor representation can shape experience.