Research Note 86-67

CONTROLLING THE TEMPORAL STRUCTURE OF LIMB MOVEMENTS:

A RESPONSE

I Richard A. Schmidt

* University of California, Los Angeles

0) C

DTIC ELEC'TEI

Contracting Officer's Representative

J. Orasanu

BASIC RESEARCH

Milton Katz, Director

U.S. Army

Research Institute for the Behavioral and Social Sciences

May 1986

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ARI Research Note 86-67

4. TITLE (andSubtitle) S. TYPE OF REPORT A PEP!OD COVERED

Controlling the Temporal Structure of Limb Interim June, 1985

Movements: A Response 6. PERFORMING ORG REIORT NUMBER

7. AUTHOR(*) 6 CONTRACT OR GRANT NUMBER(eJ

Richard A. Schmidt MDA 903-85-K-0225

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Motor Control Laboratory

Department of Kinesiology 2QI61102B74F

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U.S. Army Research Institute for the Behavioral May 1986

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II SUPPLEMENTARY NOTES

COR: J. Orasanu

15 KEY WORDS (Continue an reveree side If necosaary aid identify by block number)

mass-spring control

motor-programs,

kinematics

limb movefn-imt

20 AMTI TACT Mmmwje am rverse &it& .1 It coorvy m den tty by block n~Lber

This report is a response to a recent article by Berkinblit, et.al.,

which suggested that motor-program models, in which a centrally programmed

temporal structure was a key ingredient, seemed incorrect. In particular,

Schmidt takes issue with their discussion of Fel'dman's (1974) mass-spring

model which denies a central program for limb movement timing and amplitudes.

Schmidt argues that evidence for unidirectional actions and sequential

movements limits the utility of the mass-spring model.

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1

The Berkinblit et al. (1985) target article presents so

many new and exciting ideas about movement control that I feel

frustrated in not having more space here to agree, amplify, or

criticize. But I found particularly fascinating the

discussion of "mass-spring control," and here I direct my

remarks at the role of centrally programmed timing in such models.

An important event was Polit and Bizzi's (1978) finding

that deafferented monkeys could move reasonably accurately to

unseen targets, even when perturbed briefly. Such findings,

when put together with a number of others (e.g., Kelso & Holt,

1980; Schmidt & McGown, 1980), suggested that motor-program

models, in which a centrally programmed temporal structure was

a key ingredient, seemed incorrect (Schmidt, 1980, 1982).

These findings supported theories in which only an equilibrium

point was specified, with the limb moving to the endpoint as a

mechanical system. Turvey's (1977) popularization of

Fel'dman's (1966, 1974) original model encouraged a "dynamical"

viewpoint (e.g., Kelso, Holt, Kugler, & Turvey, 1980; Kelso,

Holt, Rubin, & Kugler, 1981), stressing that the central

control of timing was not necessary. Berkinblit et al. seem to

take this perspective also, but they convey a mixed message, as

I will try to point out here.

Other perspectives are possible, but I view the mass1 [

spring models as being important because they eliminated the

need for central timing in movement. They were elegantly

simple, such that specifying a minimal number of parameters

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caused the temporal structure of the action (i.e., the

kinematics) to emerg as a consequence of the dynamics

involved. One apparent problem (Wallace, 1981) was their

seeming inability to account for the phasic EMG activities seen

in rapid limb movements. But in the target article, the

authors review Fel'dman's (1974) arguments about how even the

temporal EMG structure can emerge from the dynamics (see Figure

4F). Indeed, the authors conclude that "It is unnecessary to

suggest that there is a 'central program' for (the EMG bursts')

timing and amplitudes..." (p. 19). There are two major reasons

why I feel that this conclusion is not warranted, at least in

general terms.

Unidirectional actions. First, there is evidence that

contrdicts the Fel'dman (1974) model, and the model

presented in the target article. Wadman, Denier van der Gon,

Geuze, and Mel (1979) and Shapiro and Walter (1985) have used

which

quick (150-ms) positioning movements,/ typically show a phasic

EMG pattern: the agonist is activated, the agonist is abruptly

terminated as an antagonist burst occurs, and then the agonist

burst appears again at the end of the movement, presumably for

stabilization. An important finding comes from a paradigm in

which the limb is occasionally unexpectedly blocked

mechanically, so that no movement can occur on that trial. On

such trials, the temporal structuring of the EMG activities is

remarkably similar to that seen in the "normal" unblocked

trials, especially during the first 100-120 ms, with some

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(probably) reflex-based modifications later in the pattern.

The question is, What (or who?) is responsible for the temporal

structure seen in the *normal' and blocked trials? To me, the

structure cannot have simply emerged from the dynamics as

Fel'dman (1974) would have it, as the dynamics were massively

disrupted by the mechanical block; yet the EMG timing was

nearly unaffected. A better hypothesis, in my view, is that

this temporal structure is centrally organized, and *run off"

as the movement unfolds. I take these findings as evidence

against Fel'dman's (1974) view in particular, and the dynamical

viewpoint (Kelso et al., 1980, 1981) in general.

I am confused slightly by the authors' position about

the temporal structure of single positioning actions. After

they argue that such responses are not timed centrally, they

say that "movement speed may depend on the speed of shift of

the equilibrium point...0 (p. 17; see also p. 38). Now, if we

have the equilibrium point shifting from one value to another

at different speeds, is this not tantamount to an underlying

temporal structure for the action? I see this solution --

together with that of Bizzi, Accornero, Chapple, and Hogan

(1982), that the equilibrium point 'sweeps' continuously

(presumably under central control) -- as a necessary, but

disappointing, retreat from the elegant mass-spring view that

had no temporal structure at all. And, in support of the

sweeping equilibrium point, Berkinblit et al. cite a model

(Adamovich & Fel'dman, 1984) and supporting data (Adamovich,

Burlachkova, & Fel'dman, 1984) in which a centrally regulated

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4

temporal structure is a critical, dominant feature. In fact,

there are as many as thre separate timing structures in this

model, and more if more movements are *superimposed," as they

term it.

Sgiie.tial m Second, the mass-spring model

does not hold very well for sqetial movements. Using rapid

reversal movements (Schmidt, McGown, Quinn, & Hawkins, 1985),

the subject moves a lever to a target, reverses direction, and

-: follows through beyond the starting position, all with a goal

* " movement time of approximately 300 ms. Unexpectedly loading

the limb inertially causes the reversal point to be shorter

than on normally unloaded trials, which contradicts the notion

(Kelso et al., 1981) that the limb is behaving as an

oscillating mass-spring system. This result can be understood

in terms of an underlying, relatively unchangeable, temporal

structure, where a correction for the added load could not be

made before the previously timed antagonist burst 'arrived' to

reverse the limb's direction. These results also bring into

question the generality of the equifinality notion (see also

Schmidt, 1980; Schmidt & McGown, 1980; Schmidt et al., 1985).

But Berkinblit et al. have extended the mass-spring

modeling to long movement sequences in a clever way, suggesting

that the limbs achieve a series of equilibrium points which

together make up a sequential action. But in doing so, the

role of central timing in movement production now becomes a

dominant part of their model. They note (p. 39) that the

7
5

temporal structure of the frog's actions appears to be sped-up

and slowed-down as a unit, which suggests "the existence of a

common sweep generator (whose rate of sweep) is a controllable

parameter." As far as I can tell, this is just the idea that

has been used in the concept of a generalized motor program

(Schmidt, 1975, 1982) which they have denied earlier. The

distinction seems to be between (a) the timing of the sequence

of shifts in equilibrium point, which is controlled centrally,

and (b) the timing involved in the achievement of each of them,

which they argue is not timed centrally, and with which I

disagree.

Since the existence and role of central timed

structures are perhaps the most critical issue distinguishing

motor program theories (e.g., Schmidt, 1982) from dynamical

theories (Kelso et al., 1980, 1981) for movement control, I am

disappointed that the authors are ambiguous about the role of

timing in their model. The evidence and arguments here point

me strongly toward the necessary existence of such structures

in theories of movement control.

NOTES

1. The authors correctly point out that there are two (or

perhaps even more) mass-spring models -- the OL model

from Polit & Bizzi (1978), and the A, model discussed

here.
6

REFERENCES

Adamovich, S.L., Burlachkova, N.I., & Fel'dman, A.G. (1984).

Wave nature of the central process of formation of the

trajectories of change in the joint angle of man.

Biophysics, 29, 130-134.

Adamovich, S.V., & Fel'dman, A.G. (1984). Model of the central

regulation of the parameters of motor trajectories.

BiophysiQ, 2a, 338-342.

Berkinblit, M.B., Feldman, A.G., & Fukson, 0.1. (in press).

Adaptability of innate motor patterns and motor control

mechanisms. Thg ehaviorgl 4qnEd Brain Sciences.

Bizzi, E., Accornero, N., Chapple, N., & Hogan, N. (1982). Arm

trajectory formation in monkeys. Experimental Brain

Research, 46, 139-143.

Fel'dman, A.G. (1966). Functional tuning of the nervous system

with control of movement or maintenance of a steady

posture. II. Controllable parameters of the muscles.

Biophysics, 11, 565-578.

Fel'dman, A.G. (1974). Control of muscle length. Biophysics,

19, 766-771.

Kelso, J.A.S., & Holt, K.G. (1980). Exploring a vibratory

systems analysis of human movement production. Journal

21 Neurophysiology, 43, 1183-1196.

Kelso, J.A.S., Holt, K.G., Kugler, P.N., & Turvey, M.T.

(1980). On the concept of coordinative structures as

a - * . . - * -. * * * * . * -*. . . . -. . - . a .
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dissipative structures: II. Empirical lines of

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Kelso, J.A.S., Holt, K.G., Rubin, P., & Kugler, P.N. (1981).

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processes: Theory and data. Journal Qf Motor

Beavior, 11, 226-261.

Polit, A., & Bizzi, E. (1978). Processes controlling arm

movements in monkeys. e , 201, 1235-1237.

Schmidt, R.A. (1975). A schema theory of discrete motor skill

learning. Psychological Review, 8Z, 225-260.

Schmidt, R.A (1982). Motor cotrol AC& l1nng : A

mbioral Champaign, IL: Human Kinetics Press.

Schmidt, PLA. (1980). On the theoretical status of time in

motor-program representations. In G.E. Stelmach & J.

Requin (Eds.), T in m bavior. Amsterdam:

North-Holland.

Schmidt, L.A., & McGown, C. (1980). Terminal accuracy of

unexpectedly loaded rapid movements: Evidence for a

mass-spring mechanisms in programming. Journal DI

Motor Bhai, 12, 149-161.

Schmidt, R.A., McGown, C., Quinn, J.T., & Hawkins, B. (1985).

Unexpected inertial loading in rapid reversal

movements: Violations of equifinality. Manuscript

submitted for publication, UCLA.
- - - .~T - " . -P ~--

a 8

Shapiro, D.C., & Walter, C.B. (1982). Control of rapid bimanual

aiming movements: The effect of a mechanical block.

So yfo Neuroscience A r , a(2), 733. (Abstract).

Turvey, M.T. (1977). Preliminaries to a theory of action with

reference to vision. In R. Shaw & J. Bransford (Eds.),

rceiving, acting, d knowing. Hillsdale, NJ:

Erlbaum.

Wadman, W.J., Denier van der Gon, J.J., Geuze, R.H., & Mol,

C.R. (1979). Control of fast goal-directed arm

movements. Journal ofj Hum Movement Studies, J, 3-17.

Wallace, S.A. (1981). An impulse-timing theory for reciprocal

control of muscular activity in rapid, discrete

movements. Journal of M Bhavio, 13 , 144-160.

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