Second consideration which complicates the interface problem: scale.
This shows that we can’t think of the interface problem merely as a way of intentions
setting problems to be solved by motor representations: there may be multiple intentions
at different scales, and in some cases an intention may operate at a smaller scale than
a motor representation.
Suppose you have an intention to tap in time with a metronome.
Maintaining synchrony will involve two kinds of correction: phase and period shifts.
These appear appear to be made by mechanisms acting independently, so that correcting errors involves a distinctive pattern of overadjustment.
Adjustments involving phase shifts are largely automatic, adjustments involving changes in period are to some extent controlled.
How are period shifts controlled?
Importantly this is not currently known.
One possibility is that period adjustments can be made intentionally \citep[as][p.~2599 hint]{fairhurst:2013_being};
another is that there are a small number of ‘coordinative strategies’ \citep{repp:2008_sensorimotor} between which agents with sufficient skill can intentionally switch in something like the way in which they can intentionally switch from walking to running.
But either way, there can be two intentions: a larger-scale one to tap in time with a metronome
and a smaller-scale one to adjust the tapping which results in a period shift.
EP: Skilled piano playing means being able to have intentions with respect to larger units than a
novice could manage. But in playing a 3-voice fugue you may need to pay attention to a particular
nger in order to keep the voices separate. So you need to be able to attend to both ‘large chunks’
(e.g. chords) of action and ‘small chunks’ (e.g. keypresses) simultaneously.
BACKGROUND:
Because no one can perform two actions without introducing some tiny variation between them, entrainment of any kind depends on continuous monitoring and ongoing adjustments \citep[p.~976]{repp:2005_sensorimotor}.
% \textcite[p.~976]{repp:2005_sensorimotor}: ‘A fundamental point about SMS is that it cannot be sustained without error correction, even if tapping starts without any asynchrony and continues at exactly the right mean tempo. Without error correction, the variability inherent in any periodic motor activity would accumulate from tap to tap, and the probability of large asynchronies would increase steadily (Hary & Moore, 1987a; Voillaume, 1971; Vorberg & Wing, 1996). The inability of even musically trained participants to stay in phase with a virtual metronome (i.e., with silent beats extrapolated from a metronome) can be demonstrated easily in the synchronization–continuation paradigm by computing virtual asynchronies for the continuation taps. These asynchronies usually get quite large within a few taps, although occasionally, virtual synchrony may be maintained for a while by chance.’
% \citet[p.~407]{repp:2013_sensorimotor}: ‘Error correction is essential to SMS, even in tapping with an isochronous, unperturbed metronome.’
One kind of adjustment is a phase shift, which occurs when one action in a sequence is delayed or brought forwards in time.
Another kind of adjustment is a period shift; that is, an increase or reduction in the speed with which all future actions are performed, or in the delay between all future adjacent pairs of actions.
These two kinds of adjustment,
phase shifts and period shifts,
appear to be made by mechanisms acting independently, so that correcting errors involves a distinctive pattern of overadjustment.%
\footnote{%
See \citet[pp.~474–6]{schulze:2005_keeping}. \citet{keller:2014_rhythm} suggest, further, that the two kinds of adjustment involve different brain networks.
Note that this view is currently controversial: \citet{loehr:2011_temporal} could be interpreted as providing evidence for a different account of how entrainment is maintained.
}
\citet[p.~987]{repp:2005_sensorimotor} argues, further, that while adjustments involving phase shifts are largely automatic, adjustments involving changes in period are to some extent controlled.
% (‘two error correction processes, one being largely automatic and operating via phase resetting, and the other being mostly under cognitive control and, presumably, operating via a modulation of the period of an internal timekeeper’ \citep[p.~987{repp:2005_sensorimotor})
One possibility is that period adjustments can be made intentionally \citep[as][p.~2599 hint]{fairhurst:2013_being};
another is that there are a small number of ‘coordinative strategies’ \citep{repp:2008_sensorimotor} between which agents with sufficient skill can intentionally switch in something like the way in which they can intentionally switch from walking to running.
