[Avery Andrews 930117.1426]
Gary Cziko 930117.0125
My wild guess about trills would go as follows. They are too fast to
be produced by a feedback system tracking kinesthetic reference
levels (e.g. for a feedback system to solve (or better, evade) the
dynamics problem). So something else has to happen. A suggestion is
this. alpha-gamma efferents can (putatively) be driven not only
by the outputs of ordinary comparators, but by what I'll call
`fixed pattern generators' (FPG). Which is a box with an output
wire over which comes a waveform, and an input over which comes
an error signal. But what the error signal is is an instruction to
reorganize--e.g. produce a different pattern.
So the waveform goes down the efferent line, and stuff happens on a
(locally) open-loop basis. But, globally, if the results don't sound
good, the FPG is told to reconfigure, until, hopefully, the output
matches what is desired. Maybe this could be called `second order'
feedback: the reference level (or `specification') is a collection
of auditory properties (fast but even tempo, nice tone or whatever it
is that musicians manage to produce), the `second order' comparator
assesses the extent to which the heard output of the instrument
has these properties, and drives reorganization of the FPG accordingly.
FPG's can be made more intelligent by giving them parameters, such as
the stiffness of the action of the piano being played on at the moment,
but they are rigid in structure and *hard* to build: the musician will
presumably be in trouble with an instrument where some keys are sticky
and others aren't, since her FPG's probably won't have been constructed
with parameters for the properties of individual keys.
Avery.Andrews@anu.edu.au