From Greg Williams (30119)
The following excerpts are from Richard A. Schmidt, MOTOR CONTROL AND
LEARNING: A BEHAVIORAL EMPHASIS, Human Kinetics Publishers, Champaign,
Illinois, 1982 (dedicated to "former mentors" Jack A. Adams and Franklin M.
Henry).
[diagram on page 204 shows "closed-loop model for movement control" with
(surprise!) the reference signal and comparator inside the person]
205 - "The idea is that the system [diagrammed on page 204] can 'compute' the
expected nature of... sensations in the form of a reference and can compare
the feedback it receives on a particular trial with the feedback it expects to
receive.... Closed-loop models... are thought of in essentially two ways.
First, they provide a basis for knowing if a movement produced is correct or
not.... A second way... concerns the control of ongoing movements..."
206 - "The closed-loop model... has been very effective and useful for certain
kinds of responses.... the model seems to have considerable appeal for
movements in which something is regulated at some constant value... These
movements are called TRACKING responses... The most important generalization
from... [tracking] research is that if the models are used in computer and/or
mechanical SIMULATIONS of the human in which the computer or mechanical device
is controlled in ways analogous to those in... [the diagram on page 204],
these nonliving devices 'come alive' to behave in ways nearly
indistinguishable from their human counterparts. By proper adjustment of
certain mathematical or electronic elements in the devices (called
parameters), the system can be made to show many human characteristics, and
they track with essentially the same levels of error.... The large body of
experimental literature suggests that because the human can be mimicked so
well by computers that use closed-loop mechanisms such as shown in... [the
diagram on page 204], the human in these [tracking] situations can be regarded
as a closed-loop control system, responding essentially by analyzing the
feedback produced against the reference of correctness and issuing
corrections. The evidence does not prove that humans actually track this way,
but the agreement between theoretical predictions and data is very strong, and
alternative theories cannot boast of similar success."
210 - "Engineers can design robots and other machines to behave... using what
they call POINT-TO-POINT COMPUTATION methods. The position of the limb at each
point in space and at each time in the movement is represented by a reference
for correctness, and the system can be made to track this set of positions
across time to produce an action with a particular form. But the system must
be very 'smart,' and it must process information very rapidly, even for the
simplest of movements. All of these references for correctness must be stored
somewhere, and each of the points will be different if the movement begins
from a slightly different place or if it is to take a slightly different
pathway through space.
Engineers have generally found that these methods are very inefficient for
machine (robot) control, which has led many motor behavior researchers (see
Kelso, Holt, Kugler, & Turvey, [in G.E. Stelmach & J. Requin, eds., TUTORIALS
IN MOTOR BEHAVIOR,] 1980; Kugler, Kelso, & Turvey, [in same,] 1980) away from
these kinds of control processes to explain human skills....
A compromise position is that only certain positions in the movements are
represented by references of correctness. One viewpoint is that feedback from
the movement when it is at its endpoint is checked against a reference for
correctness and that subsequent corrections are initiated to move the limb to
the proper position. These views of motor control hold that the limb is more
or less 'thrown' in the direction of the endpoint by some kind of open-loop
control and that then the limb 'homes in on' the target by closed-loop
control."
211 - "... the information-processing mechanisms, which lie at the very heart
of the closed-loop system shown... [on page 204], require a great deal of TIME
AND ATTENTION for stimuli to be processed to yield a response.... with rapid
actions sufficient time is not available for the system to (a) generate an
error, (b) detect the error, (c) determine the correction, (d) initiate the
correction, and (e) correct the movement before a rapid movement is completed.
Muhammad Ali's left jab is a good example. The movement itself is about 40
msec; yet, according to our estimates [made earlier in the book on the basis
of movement-correction experiments], detecting an aiming error and correcting
it during the same response should require about 150 to 200 msec -- the time
necessary to complete the activities of the stages of information
processing.... Another problem is that the time between two responses that are
not grouped (see... [M.C. Smith, in W.G. Koster, ed., ATTENTION AND
PERFORMANCE II, 1969]) was found to be essentially 190 msec. Therefore, the
time between successive corrections (because corrections are responses) should
be similarly separated. This information has serious consequences for models
of human limb control that demand a large number of attention-based
corrections in a very short period of time. Point-to-point computation models
have this basic problem when human performance is considered."
212 - "What about the possibility that the central nervous system contains
closed-loop mechanisms that do not require any attention?
215 - "... experiments like Dewhurst's ([IEEE TRANS. BIOMED. ENGR. 14, 167-
171] 1967) show that corrections for suddenly presented changes in position
can be initiated far more rapidly than the earlier 200-msec estimates, with
correction latencies being from 30 to 80 msec in the various investigations
that have been done. This kind of result suggests that the information-
processing stages are not involved in these corrections, as the stages require
too much time for processing."
243 - "On strict experimental grounds [deafferentation studies], the evidence
does not really show that the [open-loop] response-chaining hypothesis is
incorrect, although it strongly suggests it. A more reasonable hypothesis
would be that feedback is not essential for the production of at least some
movements, although it is likely that feedback provides increased flexibility
and improved fine movement control. The possibility remains that under some
conditions or for certain kinds of skills the response-chaining hypothesis
might be correct, but it seems fair to say that it is not correct in general."
249 - "Henry and Harrison (PERCEPT. MOT. SKILLS 13, 351-354] 1961) asked
subjects to begin with a finger on a key located by their hip and at a 'go'
signal to move forward-upward to trip a string in front of their right
shoulder. They were to do this as quickly as possible. The simple RT in this
situation was 214 msec on the average, and the movement time was almost the
same, at 199 msec. Sometimes a second light would come on indicating that the
subject should avoid tripping the string or at least begin to slow the limb as
quickly as possible. The 'stop' signal could come on at one of four points:
110, 190, 270, and 350 msec after the 'go' signal....
250 - Henry and Harrison measured the time to begin to decelerate the limb
after a 'stop' signal.
Their results are simple. Only when the 'stop' signal was given at the 110-
msec location was there a tendency for the subjects to start to slow the
movement before it had been completed. But the more interesting feature of
these data is the subject's response in the 190-msec condition. Notice here
that the 'stop' signal came on 24 msec BEFORE the movement even started, and
yet the movement was carried out without interruption....
If the information-processing stages are too slow to be involved in details
of a particular action..., then the question is: What does produce these
patterns of action? The best theory to have been proposed at this point is
that these patterns are PREPROGRAMMED, structured in advance, and run off as a
unit without much possibility of modification from events in the environment.
To me, this is one of the strongest lines of evidence available that movements
are controlled by motor programs."
There is more, but I'm tired of typing for now. If this is useful, let me know
and I'll probably post another installment.
As ever,
Greg