1) Functional
definition, not anatomical. The mass-spring model given just
above is
misleading
since it implies that the CS is defined by a particular joint and its
controlling
muscles. Actually, although a CS should behave roughly like the muscle
group above, the muscles are organized on a temporary basis for the
achievement
of a particular skilled activity. That is, the same muscles play a role
in a very large number of different CS's, each specialized to achieve a
particular kind of act and called upon when needed. Thus, we may
imagine
the muscles of the vocal tract to be organized into one system of CS's
when chewing, another when drinking a fluid, another set when
pronouncing and English [p], and yet others when producing a French [p]
(assuming
competence in both languages).
2)
Multiple muscles, multiple joints. The above
model
is also misleading since it only deals with a single joint, but most
coordinative
structures must adjust several component gestures that may have trading
relationships with each other. Thus to achieve lip closure one needs a
``sufficient combination'' of upper lip lowering, lower lip raising and
jaw raising. This is much more complex than the combination of
muscles
to set the position of one joint, but it is the more general case. To
combine
joint motions correctly on the fly, the CS employs sensory information
fed back from, eg, muscle spindles, in the involved muscles and
joints
so that appropriate combinations can be achieved. Notice that
this
kind of feedback is (a) unambiguous about what correction is needed (at
least after you have trained up the system with much practice) and (b)
very fast relative to visual or auditory feedback that require cortical
processing (eg, 5-15 ms vs. 200-300 ms).
One consequence of these trading relationships is that individual
muscles
can trade-off. That is, in successive repetitions of a skilled gesture,
one may find that the work is being done by varying combinations of
muscle
activity. Notice that this `motor equivalence' implies
that
when one looks at motor activity below the level of the CS, the
complexity
and variability increase. So the simplest description of the skilled
gesture
may turn out to be that provided by the `goals' of the CS.
Looking
for more detail on a finer time scale should result in a description of
increasing complexity.
3) Cyclic
structure. Another general property of such a control
system
is its inherent tendency to behave periodically. It seems that cyclic
events
are a natural form for control. One primary reason, perhaps, is that a
spring-like system is a simple way to have dynamics intrinsic to the
system.
Another reason may be that periodic gestures can easily be nested
within
each other into hierarchical structures - a well-known property of
linguistic
structures. Thus we may imagine speech motor behavior very
schematically
as a sequence of oscillator-like breath-group cycles within which
are a string of oscillator-like syllable cycles within which are highly
damped half- cycles for consonant gestures.
4) Topological
invariance is
the fourth property of the CS. It refers to the apparent tendency of a
CS to specify the relative time within a gesture cycle at which another
event occurs. That is, each CS may be thought of as producing a damped
cyclic
event
occurring within the context of another periodic event at the next
higher
(that is, temporally longer) level. Thus, in bipedal locomotion we may
ask when the time of onset of leg raising, R, occurs relative to the
duration
of a complete step cycle, the interval AB. The notion of topological
invariance
means that it will occur at a fixed fraction of the duration of the
whole
step. Thus the interval AR/AB
(that is, the phase angle of whole
step cycle at which R occurs)
will be constant despite major changes in
the overall duration of AB,
that is, despite changes in the pace of
locomotion.
In speech, the counterpart of a step cycle, AB, is believed to be the
vowel
gesture (or syllable gesture) and the analogue of foot raising might be
the initiation of a syllable-final consonant gesture (Fowler, 1983).
Such
a structural property allows combinations of CSs to be invariant over
changes
in the overall rate of the act.
How
might such organizational units be brought to bear on the problem of
speech?
And why should linguists care about such structures if they are mere
low-level
motor strategies? Is there any reason to suppose that language itself
is
constrained by this style of control? I think there is good reason to
think
so. If we imagine speech as controlled by such a system of
decentralized
subsytems rather than by a single central executive that issues
"commands
to muscles," then we can begin to deal directly with the role of
linguistics
in speech production. After all, the CSs for speech will have much in
common
with phonetic features (but perhaps not the more abstract phonological
features). Without
taking responsibility for the skills involved in speech production,
linguistics
must continually foist off the
motor problem onto someone else.