Journal of Experimental Psychology: Human Perception and Performance Copyright © 1977 by the American Psychological Association, Inc.

VOL. 3, No. 4

NOVEMBER 1977

Motor Control Mechanisms Underlying Human Movement Reproduction J. A. Scott Kelso Motor Behavior Laboratory, University of Iowa Reproduction accuracy in a blind finger-positioning task was examined under normal conditions and conditions in which subjects were deprived of proprioceptive feedback from joint and cutaneous sources through the use of a nerve block applied to the wrist. In Experiment 1, in which .subjects were allowed to define their own movements (preselected), no significant differences in reproduction error were found between normal and deprived feedback conditions. Also, subjects were unable to detect when a planned movement was unexpectedly obstructed—and they perceived that it had been executed as desired. These results were compared to a situation in which subjects moved to an experimenter-defined stop (constrained). When deprived of peripheral feedback, subjects were unable to detect the locus of the stop, a factor that led to large decrements in reproduction error relative to normal constrained performance. In Experiment 2 subjects reproduced selected movement amplitudes (distance) and end positions (location) under normal and nerve-block conditions. Distance and location information were manipulated by altering reproduction-movement starting positions. Although no significant differences in errors existed for normal movements, location reproduction was superior to distance reproduction under block conditions. Further, location accuracy was unaffected by the removal of proprioceptive inputs, and distance accuracy significantly deteriorated in all subjects. These results were interpreted in light of behavioral and neurophysiological data and indicated (a) preselected movements are not dependent on peripheral cues gaining access to central awareness while constrained movements are, and (b) the terminal location of preselected movements may be centrally determined. Recent work on the physiology of movement has indicated different levels of complexity in motor programming. Research on monkeys by Brooks and his colleagues (see Brooks, 1975 for a review) has revealed that continuous, well-learned movements seem relatively impervious to peripheral feedback manipulations, and discontinuous, exploratory-type movements are subject to modification by added visual or auditory information, Continuous movements therefore appear under predominantly central feed-forward control, whereas peripheral feedback is clearly used in the discontinuous mode.

529

A somewhat analogous situation occurs in studies of eye-head coordination by Bizzi and his colleagues (Bizzi, 1974). Under visually triggered conditions—that is, when the head and eyes follow the unexpected presentation of a visual stimulus—even saccadic movements (traditionally considered ballistic or preprogrammed) have been shown to be modulated by peripheral feedback from the vestibular apparatus. Other behavioral situations, in contrast, appear to result in an entirely different organization of motor output, If, for example, the monkey is trained to predict or anticipate the onset of a visual

530

J. A. SCOTT KELSO

stimulus, the timing of eye and head movements is quite different. With the predictive mode, the head begins to move well before (150-200 msec) the eye saccade, suggesting there is a distinctive "saccadic program" in use functioning independent of sensory information from vestibulo-receptors (Bizzi, Kalil, & Morasso, 1972). The element of anticipation in the foregoing mode of operation suggests that the saccadic program may take the form of central feed-forward signals containing information about the anticipated consequences of the motor act—in this case a head-eye movement. Whereas neurophysiological work has pointed to the existence of motor programs, the field of human motor behavior, in spite of the overtures of some (Keele, 1968; Lashley, 1951), has tended to emphasize closed-loop notions of motor control (e.g., Adams, 1971). Feedback from the periphery is viewed as establishing an internal memory representation that guides and controls movement. Supporting the closed-loop position is a group of studies showing that the end position of the limb is an important cue for the reproduction of movement (Keele & Ells, 1972; Laabs, 1973). Such findings have been obtained under conditions (termed constrained) in which the blindfolded subject The research reported here is a portion of a doctoral dissertation submitted to the University of Wisconsin—Madison under the supervision of George E. Stelmach, to whom the author extends his appreciation. Support for the research was provided by National Institute of Mental Health Grant MH 22081-01 and by National Institute of Education Grant NE-G-3-0009. The research on joint replacement referred to in Experiment 1 was suptported by Biomedical Research Support Grant FR-0703S to the author from the General Research Support Branch, Division of Research Resources, Bureau of Health Professions, Education, and Manpower Training, the National Institutes of Health. A brief discussion of Experiment 1 was presented at a symposium on "The Motor Program," sponsored by the North American Society for the Study of Sport and Physical Activity, in Austin, Texas, May 1976. Thanks are extended to two anonymous reviewers and Michael Posner for very helpful comments. Requests for reprints should be sent to J. A. Scott Kelso, Motor Behavior Laboratory, University of Iowa, Iowa City, Iowa 52242.

makes a movement to an experimenter-defined stop and then reproduces it—either immediately or after a brief period of time— from a different starting position. The subject is therefore performing in an exploratory mode (since the end-point of the movement is not known until he hits the stop) and hence is dependent on processing proprioceptive information presumably arising from joint receptors (Marteniuk & Roy, 1972). Recent data has shown, under voluntary, preselected conditions, in which the subject is allowed to define his or her own movement and then reproduce it, performance is considerably enhanced over constrained or passive conditions (see Kelso & Stelmach, 1976 for a review). This finding suggests that in the preselected or predictive mode, movements may be less dependent on peripheral feedback than closed-loop accounts would have us assume. As investigators in speech and eye-movement control have argued, the greater the ability of the central nervous system (CNS) to "predictively determine" a motor response, the less is the need for peripheral sensory feedback (Festinger & Canon, 1965; MacNeilage & MacNeilage, 1973). Of course, whether a similar argument can be applied to voluntary limb movements in humans is an open question. The most direct method of assessing whether preselected movements are dependent upon proprioceptive inputs for effective performance would be to eliminate such information and examine reproduction in its absence. Unfortunately techniques adopted thus far (Laszlo & Bairstow, 1971) have confounded sensory and motor impairment, owing to the nonselectivity of the nerve block employed (Glencross & Oldfield, 1975; Kelso, Stelmach, & Wanamaker, 1974). If published evidence is a guide, the "wrist cuff" technique adopted by Merton (1964) and others (Goodwin, McCloskey, & Matthews, 1972) overcomes this deficiency. The advantage of this procedure is joint and cutaneous receptors in the hand can be rendered insentient via the inflation of a child's sphygmomanometer cuff at the wrist, while the long muscles in the forearm largely respon-

MOVEMENT REPRODUCTION

sible for finger flexion and extension are unaffected. The present experiments therefore examine preselected movements under normal conditions and conditions in which joint and cutaneous afferents have been eliminated. If such inputs are unimportant in voluntary, preselected movements, no differences in reproduction error between normal and cuff conditions are predicted. In this case, accuracy of preselected movements has to be dependent upon a contribution from other sources of information. These could take the form of information about the movement generated within the CNS itself, information provided by muscle receptors, or some combination of both. By employing a "restrained" condition, in which the subject is asked to select and execute a movement but is prevented from doing so by the experimenter, it is theoretically possible to examine the role of these sources of information. According to Goodwin et al. (1972), information on the failure to execute a movement in the absence of joint and cutaneous inputs may be provided by a mismatch signal between returning muscle afferent signals and the information expected on the basis of proper execution of the planned movement (i.e., the expected sensory consequences). Granit (1972) has expressed a similar view. Certainly, the obstruction to movement should lead to an elevated excitability in the firing of both tendon organs and muscle spindles beyond the level that would normally occur (Goodwin et al., 1972; Vallbo, 1971). If this information is useful to the subject, failure to execute the movement should be consistently perceived. In contrast, if the obstruction to planned displacement is not perceived, and if the subject proceeds to reproduce the movement originally planned (but not executed), a role for feed-forward information in preselected movement may be hypothesized. In this case, with no peripheral input to inform him otherwise, the subject presumably perceives the movement has taken place on the basis of the anticipated sensory consequences alone. Such a finding may be conceived of as the direct limb-movement analogue of the visual experiments reported long ago by

531

Helmholtz (1962) and von Hoist (1954) in which an attempt to move a paralyzed eye gave rise to apparent movement of the visual field. Thus, a shift in the visual field is perceived on the basis of the expected sensory consequences contingent on voluntary eye movement. Experiment 1 then, seeks first to examine the role of joint inputs in the reproduction of voluntary, preselected movements, a question not yet investigated (Monster, Herman, & Altland, 1973). Of added interest is the relative contribution of muscle-afferent and internal, feed-forward information. Although it would be unwise to fail to recognize that all such inputs are complexly involved in motor control, the issue is one of determining the relative contribution of each source of information to the reproduction performance of preselected movements. Experiment 1 Method Subjects. The subjects were 12 right-handed graduate and undergraduate students at the University of Wisconsin—Madison who volunteered for the experiment and were not paid for their services. Prior to participation subjects were informed that the wrist-cuff procedure would involve some unusual sensations similar to those encountered when the limb falls asleep. In an attempt to insure a satisfactory level of motivation, only those subjects who felt they could handle these sensations participated in the experiment. Apparatus, A curvilinear positioning task involving flexion around the metacarpophalangeal joint of the index finger was used. The wrist-cuff technique clearly determines the task utilized, since it is necessary to examine movement about a joint that lies distal to the cuff, if the technique is to be effective. The basic apparatus lay on a 73 X 36-cm base of 1.75-cm-thick plywood, which in turn rested on a tabletop 75 cm in height. Inserted in the base were two vertical steel rods, 27 cm in length and 1.5 cm in diameter, whose function was to support, via standard retort clarnp equipment, the major portion of the apparatus. Extending from the leftmost rod (facing the subject) was a 17.5-cm horizontal steel bar 1.25 cm in diameter, at the end of which was a 3.5-cm diameter hollow circular disc. The latter was bent so as to fit into the palm of the subject's hand and served as a reference point for placing the subject's finger in the positioning apparatus. A retort clip maintained horizontally by a clamp placed on the right-hand rod (facing the subject) held a Spectral Electronics potentiometer (resis-

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J. A. SCOTT KELSO

tance = 60 Kfi). A 10-cm-long aluminum splint, sufficiently flexible to accommodate different finger widths, was screwed to the base of the potentiometer. A brass pointer 1.25 cm in length was welded to the end of the splint and allowed the experimenter to record criterion and reproduction movements on a IS-cm-radius protractor (\°=2 mm), supported at a height of 7.5 cm by two wooden blocks. For experimental trials the subject was seated facing the apparatus, right arm slightly extended, hand supported in a relaxed position in the vertical plane, palm resting against the reference disc. The splint maintained the subject's index finger in a straight position such that the near frictionless movements were made entirely around the metacarpophalangeal joint, which was centered at the midpoint of the potentiometer base. An adjustable stop served as the starting position referent and was situated such that when the subject's finger was in a straight-ahead extended position the reading on the protractor was 90°. The output of the potentiometer was amplified via a conventional operational amplifier and recorded on a Brush Instruments four-channel recorder. An adult (15 cm wide) blood pressure cuff applied to the upper arm of the subject at a pressure of 180 mm Hg. (Laszlo, 1966) was used to render the limb anoxic distal to the cuff. A child's cuff (8 cm wide) was then applied to the wrist and inflated to a pressure of 240 mm Hg., keeping the hand anoxic but allowing the forearm muscles to recover. This procedure had been found in previous studies and pilot experiments to reduce the total time of ischemia in comparison with a situation in which the wrist cuff was utilized throughout (Goodwin et al., 1972). A Gralab timer and conventional dark glasses to exclude visual inputs completed the basic apparatus. Procedure. Subjects participated in two sessions, one primarily involving familiarization with the wrist-cuff technique and the other entailing a series of finger-positioning trials under normal and cuff conditions. The purpose of the familiarization session was to acquaint the subject with the peculiar sensations associated with the wrist-cuff technique and to reduce any concerns regarding recovery from it. The adult-size cuff was applied to the upper arm, as high above the elbow as possible, observing the necessary precautions referred to by Laszlo and Bairstow (1971). The subject was then placed in the apparatus and after 15 min. sensory testing commenced at 1-minute intervals. Subjects were instructed to label the fingers one through five (from thumb to little finger) and to specify the sensation tested (e.g., TOUCH FOUR, MOVEMENT ONE). Tactile sensation was tested using a cotton wool covered stick applied to the skin. Movementposition sense was similarly tested by moving the digits in specific directions at variable rates. The order of stimulus presentation was random for both modalities to further insure discriminating responses on the part of the subject.

When the subject was no longer capable of reporting touch or localizing position and movement in the digit being tested on two consecutive occasions, the experimenter defined this point as the respective cutoff point for that digit. When all digits succumbed to this criterion, the final end point was assumed and its time of occurrence recorded. At this point the child's cuff was applied at the wrist, with the styloid process of the ulna serving as a referent for the distal edge of the cuff. The upper cuff was then deflated in the manner recommended by Laszlo & Bairstow (1971). There then followed a short period of time (5-10 min.) in which the subject experienced pins and needles in the forearm and hand. This was accompanied by a partial and temporary recovery of sensation in the hand, a finding also reported by Goodwin et al. (1972) and thought to be due to the greater protection provided by the bony processes of the wrist against pressure-induced ischemia. Once the subject reported the cessation of pins and needles, sensory testing was resumed at 1-minute intervals in the manner alluded to for the upper arm cuff. Precisely the same criteria for cutoff was assumed. In contrast to the upper-cuff situation, in which virtually no movement was possible at assignment of sensory elimination, subjects with the wrist cuff were capable of producing a full range of flexion (up to 75°) with vision absent. Following a brief period of such movement (no knowledge of results was given), the wrist cuff was deflated and the blindfold removed. The subject remained seated for a mandatory recovery period of 10 min. and then left the laboratory. The second session took place at least 4 days later and involved two experimental phases. Phase 1 entailed a short period of practice plus a series of finger-positioning trials under normal conditions, with the cuffs in place but deflated. Subjects were given the commands READY and SELECT, the latter determining the distance and end-point of the chosen movement. Selection of the criterion movement was voluntary, that is, defined by the subject, with the restriction that the subject conform to the specification provided by the experimenter. Thus, SELECT SHORT allowed the subject to move to any spot within the short sector (0°-24.5°), SELECT MEDIUM allowed the subject to move to any spot within the medium sector (25°-49.5°), and SELECT LONG allowed the subject to move to any spot within the long sector (50°-74.5°). At least two passive presentations were given within each area, to familiarize the subject with the specification desired. Subjects were further instructed to disperse their selections within each sector and not to repeat the same selection if possible. On the command MOVE the subject moved the finger at a slow, smooth speed. When the desired movement was executed, the subject said "there,"' and this position was recorded as defining the criterion movement. After remaining at this point for 2 sec, the experimenter issued a RELAX command and returned the subject passively to the starting position, which remained

MOVEMENT REPRODUCTION constant throughout. On the command RECALL, given 3 sec later, the subject reproduced the initial response at a slow, smooth speed, following which the experimenter recorded the error to the nearest .5° and returned the subject to the starting position for the duration of the IS-sec intertrial interval. On completion of 12 normal positioning trials, the blindfold was removed and the subject given a brief 2-min rest. Phase 2 commenced and involved the cuff procedure with which the subject was already familiar. Following the demise of tactile and movement-position sensitivity, the subject performed preselected movements under cuff conditions. The experimental commands and procedures were identical to those given in Phase 1, with one exception. Interspersed with the standard trials was a set of 12 randomly placed restrained trials in which exactly the same sequence of commands was given, except the subject was held at the criterion starting position and was not allowed to define the movement specified by the experimenter. On the command RECALL, however, the subject was free to reproduce the planned (but not executed) criterion movement. The subject's response to this condition was designed to reveal whether the subject perceived the obstructed movement on the basis of proprioceptive information. Apart from the theoretical issues involved, it also served as an internal control on the efficacy of the sensory-testing procedures. The manner of terminating the session was identical to that employed in the familiarization condition. Design. Subjects performed 12 preselected trials under both normal and cuff conditions. In addition, 12 trials were performed under restrained cuff conditions. Since these did not involve a criterion movement, they were analyzed separately for descriptive purposes. Reproduction errors in each sector were collapsed for inspection of absolute error —the absolute amount by which the subject was in error, regardless of sign—and constant or signed error. In addition a measure of response consistency was obtained by calculating the standard deviation around the mean constant error within a sector. This index provided an estimate of variable error. The error data were thus analyzed, using a within-subjects design with subjects as the first factor, movement conditions (normal and cuff) as the second factor, and response sector (short, medium, and long) as the third factor, in a 12 X 2 X 3 analysis of variance (ANOVA). In addition to reproduction errors, a measure of mean movement velocity was obtained for each trial by dividing the criterion movement and reproduction movement distance by the time taken for each movement, as measured by the potentiometer output. A special program was written for this purpose on a Hewlett-Packard calculator-digitizer unit. Movement velocity for each movement type (criterion movement and reproduction movement) was collapsed across four trials in each sector and analyzed using a 12(Subjects) X2(Movement Conditions X 3 (Response Sectors) X2(Movement Type) ANOVA.

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Table 1 Means (in degrees) and Standard Deviations of Movements Selected Under Normal, Cuff, and Restrained (Cuff) Conditions . Selected movement Short (0°-24.5°) M SD Medium (25°-49.5°) M SD Long (50°-74.5°) M SD

Movement condition Normal

Cuff

Restrained

14.92 5.984

18.42 7.273

16.97 5.998

36.41 5.477

37.14 7.269

36.96 7.921

56.30 3.890

56.28 6.365

51.80 8.606

Results Before presenting the reproduction error data, it would seem important to establish that first, the wrist-cuff technique was effective in terms of the purpose of the experiment, and second, subjects dispersed their movement selections within the specified response sectors in both normal and cuff conditions. With regard to the former, a key issue pertained to the subjects' responses under restrained cuff conditions. The invariant feature of this data was that subjects "recalled" into the sector specified by the experimenter, in spite of the fact that no criterion movement had ever been made. Indeed, not a single subject reported that they had failed to move when restrained in this manner, even though the perceived movements (as reflected in the reproduction responses) ranged over 70° (see Table 1). This finding is difficult to reconcile with the notion of movement and position information provided by sensory receptor organs being available to subjects performing under cuff conditions. Had such information been present, one would have expected an immediate response on the part of subjects to the effect that they were unable to move. The means and standard deviations of the responses actually chosen by subjects under normal, cuff, and restrained (cuff) conditions are shown in Table 1. It can be seen that subjects responded into the area speci-

J. A. SCOTT KELSO

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Table 2 Means (in degrees') and Standard Errors for Preselected Movement Conditions and Response Sectors Movement condition

Normal

Response sectors Main effects M SE Short (0°-24.5°) M SE Medium (25°-49.5°) M SE Long (50°-74.5°) M SE

AE

CE

2.98 .51 2.79 .60

.30 .79 1.92

.76

3.02 .44 3.13 .73

.40 .87 -1.42

.73

Cuff

VE'

AE

CE

VE'

3.03

3.37

.37

.67 .66

3.75

.41 2.63

3.24

1.20

3.50

.40

.27

.68

.46

.44 .48

3.99

.37 .33

3.77

2.93

3.30

.43

.43

3.53

3.48

.41

.40

.53

.66 .47

Note. AE = absolute error, CE = constant error, VE' = within-sector consistency.

fied by the experimenter for both types of movement. Furthermore, the variability of responses chosen under cuff conditions was noticeably larger than the variability of normal response selection. This would not have been expected had subjects been zeroing in on particular locations in the cuff condition. In contrast, one might predict that the variability of reproduction movements and hence reproduction errors would be greater in cuff conditions than in normal conditions. That this was not the case is emphatically apparent in Table 2, which contains the means and standard errors (in degrees) of the two movement conditions (normal and cuff) presented as a function of response sector. Reproduction errors. Analyses of absolute error, constant error, and within-sector consistency revealed no significant differences between movement conditions, F(l, 11) = .76, .55, and 2.34, respectively, p > .05. The individual data were in accordance with the variance analyses of movement conditions. With a single exception, the overall differences in absolute accuracy between normal and cuff movements were small, that is, less than 2°. One subject, however, had noticeably larger reproduction error when performing under cuff conditions (M = 4.85°) than under normal conditions (M = 0.83°).

For constant error, the individual subject data revealed no systematic patterns in reproduction error between normal and cuff movements. When performing normal movements, 5 subjects tended to undershoot the criterion target, and two revealed such a tendency in the cuff condition. Only one subject tended to undershoot in both conditions. In general the size of the constant errors was small except in the case of 2 subjects, both of whom demonstrated large positive constant errors in normal (M — 5.00° and 4.08°) and cuff (M = 4.61° and 2.42°) reproduction. For within-sector consistency, thfere was a definite, though nonsignificant, trend toward increased variability in performance under cuff conditions. Eight of 12 subjects revealed such a trend, although, as with absolute error and constant error, the size of the difference was small. Again, only 1 subject (referred to earlier in the absolute error analysis) revealed a sizeable increase in variability of reproduction under cuff conditions (M - 5.34°) compared with normal (M = 1.65°). The main effect of response sectors was significant for constant error, F(2, 22) = 6.34, p < .01, but not for absolute error, F(2, 22) = .31, p > .05, or within-sector consistency, F(2, 22) = .87, p > .05. Post hoc analysis of constant error, using Tukey's

MOVEMENT REPRODUCTION

test, revealed that short movements (M = 1.56°) were significantly different from long movements (M = —.53°) but not from medium movements (M = .42°), these last two not being different from each other. The interaction between movement conditions and response sectors was not significant for absolute error, constant error, or within-sector consistency, F(2, 22) = .02, 2.94, and .44, respectively, p > .05. Reproduction of normal and cuff movements was not, therefore, differentially affected by the extent variable. Mean velocity. For mean movement velocity, normal movements were made at a slightly faster rate than cuff movements, F(l, 11) = 25.43, p < .01. The interaction of movement type and movement condition was not significant, F(l, 11) = .22, p > .05. Criterion- and reproduction-movement means under normal conditions were 10.80/ sec and 12.7°/sec, compared with 7.9°/sec and 8.6°/sec under cuff conditions. The nonsignificant difference between criterion- and reproduction-movement velocities held across response sectors, F(2, 22) = .32, p > .05. Sensory testing. In the case of the upper cuff, tactile sensations were eliminated slightly earlier (M = 23.9 min.) than sensations of movement and position (M = 27.3 min.), an apparently reliable finding (Glencross & Oldfield, 1975; Kelso et al., 1974; Laszlo & Bairstow, 1971). For the wrist cuff, tactile loss (M = 42.3 min.) also occurred prior to loss of position and movement sense (M = 52.8 min.), although the difference in the timing of these events was somewhat larger than the difference that occurred for the upper cuff. Further, the between-subject variability in both tactile and kinesthetic elimination was much greater with the wrist cuff. Discussion The major finding in the present experiment was the nonsignificant difference between preselected movements under normal and cuff conditions for all of the dependent variables. This result suggests that joint afferent information is not crucial in preselected, voluntary movement and is in con-

535

flict with those studies that have implicated joint receptors in movement coding (Browne, Lee, & Ring, 1954; Marteniuk & Roy, 1972; Provins, 1958). It should be noted that the latter studies were concerned with passive movement of anesthetized joints (Browne et al., 1954; Provins, 1958) or the reproduction of active, constrained movements (Marteniuk & Roy, 1972) in which prior information regarding the terminal locus of the movement was absent. Some support for joint receptors' involvement in passive movements can be gleaned from the present experiment. Although no measure of movement rate was taken during sensory testing with the wrist cuff in place, passive displacement of the fingers up to an estimated 90°/sec failed to evoke a response on the part of the subject indicating movement had occurred. Similarly, that joint receptors may be involved in the coding of constrained movements can be supported by data on constrained reproduction under cuff conditions (see Table 3). Four subjects who did not participate in Experiment 1 were entirely incapable of detecting an experimenter-defined stop under cuff conditions. The procedures in this experiment were similar to those used in the present experiment and required subjects to perform preselected and constrained movements with and without the cuff. In each case the movements chosen in the preselection condition were presented to the same subject in the constrained condition. For the latter, the subject moved to a stop and, after 2 sec, was passively returned by the experimenter to the starting position. In every case the subjects reported an inability to perceive the stop during the criterion movement and merely kept moving until instructed to relax while the experimenter returned them to the starting position. This resulted in gross overshooting at recall. It is possible, however, that the high degree of reproduction accuracy under cuff conditions was due to movement cues emanating from other sources apart from the joint of interest—for example, the wrist, forearm, or remaining fingers. While this criticism is difficult to refute, it should be emphasized that every effort was taken to

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J. A. SCOTT KELSO

Table 3 Mean Reproduction Errors of Preselected and Constrained Movements (in degrees) Under Normal and Cuff Conditions Preselected Normal Response sector no.

AE

1 (0°-24.5°) 2 (2S°-49.S°) 3 (50°-74.5°)

2.38 3.35 3.22

CE

Constrained Cuff

Normal

VE'

AE

CE

VE'

AE

1.63 2.61 -.48 4.83 -.56 4.83

2.90 3.67 3.77

-.90 3.68 .33 4.84 1.61 4.05

3.81 5.30 4.33

CE

Cuff VE'

.56 5.13 -2.44 5.99 .56 5.37

AE

CE

14.74 13.47 13.05 12.14 12.22 11.11

VE'

14.20 10.28 8.21

Note. AE = absolute error, CE = constant error, VE' = within-sector consistency.

insure that movement was restricted to the index finger. Furthermore, the sensory cutoff criterion applied to all digits, in which case it seems unlikely that useful information regarding the movement was available from them. While tactile sensitivity was not reduced above the cuff, it seems unlikely this information was sufficiently salient to mediate such accurate performance. Although several subjects reported occasional pressure sensations in the forearm, they did not feel this subsidiary information had any bearing on their ability to reproduce movement. If joint receptors are minimally involved in preselected movement reproduction, other sources of information must play a primary role. The potential peripheral contribution arises from the recent implication of certain muscle receptor organs in the conscious perception of position and movement (Goodwin et al., 1972; Granit, 1972), in spite of the still classical physiological position that they are not involved (e.g., Skoglund, 1973). Goodwin et al. (1972), for example, using the same technique as the present experiment, reported that, whereas some subjects failed to detect passive movement of the finger joint of 90°, others were capable of doing so, especially if the muscles acting on the joint were lightly tensed. This finding, which was interpreted as support for a conscious muscle receptor contribution, is in contrast to one reported by Merton (1964) in which "the subject [became] quite insensitive to passive movements of the joint of whatever range or rapidity" (p. 394). In the present experiment the passive manipulation of the digits under wrist-cuff conditions was not

perceived by the subject when cutoff was assigned, thus supporting the Merton report. Furthermore, it seems readily apparent that subjects in the Goodwin et al. (1972) study were also largely insensitive to passive movement, rendering their study's conclusion questionable. It has been argued, however, that information from muscle receptors can only be interpreted in relation to the commands to the muscles programmed in alpha-gamma linkage (Granit, 1972). Only when a mismatch occurs between the demand of the system (as reflected in the motor commands to extra- and intrafusal muscle fibers) and its accomplishment (the 1 A discharge arising from muscle spindle receptors) does muscle afferent information access central mechanisms. The restrained condition of the present experiment was designed to induce a mismatch between the planned movement and its accomplishment. Since an unexpected obstruction to movement should lead to an increase in firing of both spindles and tendon organs to above normal levels, perception of the obstruction would be expected to reveal itself. As the present data clearly indicate, this was not so, neither for the restrained condition in the experiment proper, nor in the Table 3 data on constrained movements. Viewed in light of the present findings, the contribution of muscle receptor information must be deemed questionable in explaining the performance of preselected movements under cuff conditions. However, one possible drawback to this conclusion exists that cannot be disregarded in the present study. This pertains to the finding that

MOVEMENT REPRODUCTION anoxia of the thumb using the wrist-cuff technique greatly diminishes the stretch reflex of its long flexor muscle, even though the latter is assumed to be unaffected since it lies high in the forearm. It may be, therefore, that under the conditions of the present experiment, muscle-afferent information cannot access central mechanisms due, perhaps, to the elimination of a normal central facilitation by cutaneous and joint inputs (Merton, 1974). Tentative support for this position may be obtained from consideration of 5 subjects in whom the metacarpophalangeal joints of the right hand were surgically removed and replaced, a procedure involving the removal of the joint capsule. The data for these subjects who performed preselected and constrained movements in a reproduction paradigm present sharp contrast to the constrained-cuff performance shown in Table 3. In terms of absolute accuracy, the mean reproduction error under constrained conditions was 3.13°, compared to a mean of 13.33° for subjects in the constrained-cuff condition. Whether tactile information, per se, is sufficient for mediating the reproduction performance shown by these subjects is open to question. A possible alternative interpretation is that intact cutaneous afferents provide the necessary facilitation to fusimotor neurons or to synapses acted upon by muscle afferents (Merton, 1974). While interpretation of the present data must be tempered in light of the foregoing possibility, it nevertheless seems justifiable to conclude that under wrist-cuff conditions muscle-afferent information plays a negligible role. Rather, the major mediating influence in the performance of preselected movements under cuff conditions appears to derive from central sources. The finding that all subjects perceived criterion movements up to 75° flexion were executed under restrained conditions supports this conclusion. It does not seem unreasonable to suggest that when the subject makes a voluntary, preselected movement, he generates the expected sensory consequences on the basis of the intended movement and prepares to interpret incoming information accordingly. When peripheral movement-position infor-

537

mation is not available, as seems quite likely in the present experiment, the subject has to rely on the internal signal alone as the principal reproduction cue. Thus the subject perceives movement on the basis of what he expected to happen and not on the basis of what actually did happen. It may be that under conditions in which the subject has prior knowledge of the terminal locus of the movement, he relies primarily on central information, rather than the actual sensory consequences of the movement, since the latter may be somewhat redundant if the movement has been accurately preplanned. This finding under restrained conditions seems directly analogous to experiments in visual perception in which the visual field is perceived as shifting in spite of the fact that eye movement, though commanded, did not take place owing to paralysis (Helmholtz, 1962; von Hoist, 1954). Thus the subject perceives not the scene in front of him that has not altered, but the scene expected on the basis of his intended glance. Experiment 2 Although the results of Experiment 1 suggest an important role for central mechanisms in preselected movement reproduction, a principal question concerns the nature of the information carried in such signals. Jones (1974) has suggested that central commands define a movement extent rather than a spatial location. Such a conclusion was reached on the basis of an experiment that demonstrated subjects could reproduce selected amplitudes equally well from constant and variable starting positions. This finding would not have been expected had subjects been remembering the end location of the standard criterion movement in preference to the efference from the movement (Jones, 1974). Although more recent data has shown subjects' reproduction of location to be much more accurate than their reproduction of distance under normal, preselected conditions (Stelmach, Kelso, & Wallace, 1975), the efficacy of the mechanism proposed by Jones has yet to be tested under conditions in which proprioceptive location information is unavailable.

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J. A. SCOTT KELSO

An alteration of reproduction-movement starting position presents one possible method for clarifying the source of information primarily used. Since the Jones (1974) model posits a stored efferent representation for voluntary, preselected movements, an alteration of reproduction-movement starting position should have no effect on accuracy as long as the movement extent remains the same. Under this condition any information coded in the efference (such as duration and velocity) would remain unaffected. However, were starting position to be altered and the subject asked to reproduce spatial location, stored extent information would not be reliable for reproduction purposes, since the movement parameters required for accurate reproduction would of necessity be altered. The present experiment, then, sought to examine the reproduction of preselected distance and location under wrist-cuff conditions. If location reproduction were superior to distance reproduction, evidence would be provided that the central monitoring of efference notion—at least as proposed by Jones (1974)—is not a viable interpretation of preselected performance under deprived feedback conditions. Rather, an interpretation focusing on central command signals conveying spatial location to the peripheral musculature would seem a more likely alternative. Method Subjects. The eight right-handed subjects— seven of whom had participated in Experiment 1— volunteered for the experiment and were not paid for their services. The remaining subject participated in an earlier pilot experiment and was thus familiar with the cuff procedure. Apparatus. The apparatus was identical to that used in Experiment 1. Procedure. The basic procedure was similar to the experimental session in Experiment 1 and entailed the performance of distance and location trials under normal and cuff conditions. Half of the subjects performed these trials under normal conditions first, and the remainder performed initially under cuff conditions. In the former group the first phase of the experiment entailed a block of 12 distance and 12 location trials under normal conditions. Prior to a MOVE command, subjects were instructed to SELECT a given extent (distance) or endpoint (location) from a starting position of

20° flexion (equivalent to 70° on protractor scale, with 90° denning full extension). Selection of the criterion movement was voluntary, with the restriction that the subject conform to the specification provided by the experimenter. That is, SELECT 1 allowed the subject to choose any distance or location in the first sector (.5°-15°), and SELECT 2 allowed the subject to choose any distance or location in the second sector (15.5°-30°). As in Experiment 1, subjects were further instructed to disperse their selections within each sector. On reaching the chosen distance or location, the subject waited 2 sec and, following a RELAX instruction, was returned passively by the experimenter to a different starting position, which was either 5° or 15° backward from the criterion starting position. For distance trials, the subject was instructed to reproduce the selected extent of the criterion movement. For location, the instruction was to reproduce the criterion end point. A period of practice was given prior to each movement condition to insure that the subject understood the instructions. On completion of the normal positioning trials, the subject underwent the cuff procedure, following which a series of distance and location trials were performed under cuff conditions. The procedures were identical to those previously described. For those subjects who performed under cuff conditions first, the only change in procedure involved the order of experimental events. That is, subjects underwent the cuff procedure and then performed distance and location trials in the manner previously described. It should be emphasized that, following the demise of movement-position sense, conscious information about starting position changes was unavailable to subjects. On completing the trials, the cuff was deflated and the subject allowed a period of recovery of approximately 15 min., following which he or she performed under normal conditions. The range of recovery times was 15-25 min. Design. The eight subjects performed 12 trial blocks of distance and location trials under both normal and cuff conditions. Distance and location blocks were balanced in an ABBA order. Reproduction movements were categorized into two sectors, according to the specification the experimenter determined by a single random order. For each sector, 3 trials were performed from a reproduction movement starting position 5° backward from the criterion starting position (—5°), and 3 trials were performed from 15° backward (—15°). Reproduction errors from each starting position were collapsed for inspection of absolute error, constant error, and within-sector consistency. The error data were thus analyzed using an 8(Subjects) X 2 (Movement Condition [normal and cuff]) X 2(Response Modes [distance and location]) X 2(Response Sectors) X 2(Starting Position) analysis of variance for each dependent variable. As in Experiment 1 an analysis was also made of movement velocity using a similar factorial design.

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539

Table 4 Means (in degrees) and Standard Errors for Movement Conditions, Response Mode, Sectors, and Starting Positions Movement condition Normal Distance Variable

Cuff Location

Location

Distance

CE

VE'

VE'

AE

CE

VE'

AE

VE'

AE

CE

3. 35 1.48 3.50 .51 .88 .69

2.98 .53

-1.06 .90

2.72 .40

5.79 3.67 4.53 .85 1.41 1.00

3.27 .57

-.24 3.61 .81 .77

2.86 .56

-.94 2.27 1.00 .35

5.26 4.21 2.94 .70 1.38 .78

3.56 .68

-.31 3.99 .96 .98

.94 4.16 .96 .64

3.12 .50

-1.19 3.16 .80 .46

6.13 3.12 6.13 .99 1.44 1.22

2.98 .45

-.17

,81 3.46 .81 .67

2.83 .45

-.62 2.49 .88 .26

4.35 .83

1.52 4.35 1.27 1.10

2.89 .60

-.13 3.31 .72 .80

3.48 2.15 3.54 .69 .94 .70

3.13 .62

7.23 5.81 4.71 .86 1.54 .91

3.65 .53

-.35 3.90 .90 .74

AE

CE

Overall performance M SE

Sector 1 (.5°-15.0c) M SE

3.02 2.02 3.84 .67 .79 .73

Sector 2 (15.5°-30.0°) M SE

3.69 .35

.66

3.22 .56

— 5° Starting position

M SE

3.23 .33

— 15° Starting position

M SE

-1.50 .92

2.94 .54

Note. AE = absolute error, CE = constant error, VE' = within-sector consistency. Results Reproduction errors. The means and standard errors (in degrees) for movement conditions, response mode, response sectors, and starting positions are presented in Table 4. The main effect of movement conditions was significant for absolute error, F(l, 7) = 23.97, p < .01, but not for constant error, F(l, 7) =4.45, p > .05, or within-sector consistency, F(l, 7) = 5.12, p > .05. Movements under cuff conditions revealed significantly greater absolute errors (M = 4.53°) than did normal movements (M = 3.17°). The main effect of response mode was significant for absolute error, F(l, 7) = 13.75, p < .01, and constant error, F(l, 7) = 12.73, p < .01, but not for within-sector consistency (F < 1). For absolute error, distance reproduction (M — 4.57°) was inferior to reproduction of location (M — 2.12°~). Distance reproduction also revealed an overshooting tendency (M = 2.57°) in relation to location (M = — .65°), in terms of signed error.

The sector main effect was sigilificanl t for within-sector consistency only, F(l, 7) = 11.05, p < .05. Reproduction errors within the second sector were more variable (M = 4.17°) than those within the first sector (M = 3.01°). Errors in reproduction from the — 5° starting position were significantly smaller than those from the —15° starting position for absolute error (M = 3.32° vs. M = 4.32°) and constant error (M =.40° vs. M=1.53°), F(l, 7) =8.80 and 8.37, respectively, p < .05. The foregoing main effects can be most meaningfully interpreted in terms of interactions between relevant independent variables. Of particular interest was the Movement Conditions X Response Mode interaction, which was significant for absolute error only, F(l, 7) =6.94, p < .05. A simple effects analysis of this interaction revealed there were no significant differences between normal and cuff conditions for location reproduction, whereas distance reproduction deteriorated significantly under cuff conditions. Further, although no differences were de-

J. A. SCOTT KELSO

540

Table 5 Individual Subjects' Mean Absolute Error (in degrees) for Distance and Location Reproduction Under Normal and Cuff Conditions Normal

Cuff

Distance Location

Distance Location

Subject no. 1 2 3 4

5 6 7 8

4.08 3.00 4.58 3.50 2.58 2.42 4.00 2.67

1.83 2.92 2.08 2.58 2.92 4.83 3.58 3.08

5.33 5.17 7.08 4.67 4.75 5.83 9.08 4.42

4.50 2.42 3.17 2.83 4.33 2.50 3.83 2.58

tected between distance and location reproduction under normal conditions, location reproduction was superior under cuff conditions. These effects are strongly present in the individual subject data shown in Table 5. Normal distance reproduction was superior to cuff distance reproduction in all eight subjects. On the other hand, location reproduction was superior to distance reproduction in five of eight subjects under normal conditions, but all subjects exhibited superior location performance under cuff conditions. A final interesting aspect of the error data is revealed by an analysis of the starting position effect on reproduction accuracy (see Table 4). Under normal conditions, location reproduction shows a mean increase in negative error of .87° from the — 5° to — 15° starting position, whereas distance reproduction shows an increase in a positive direction of 1.34° between the two starting positions. With cuff movements however, location reproduction shifts only .23° in a negative direction, while distance reproduction reveals a positive shift of 4.29°. Thus although location recall is relatively unaffected by changes in starting position, distance recall reveals a positive response bias, which is especially apparent under cuff conditions. It should be noted that this bias is in the direction expected, were subjects basing reproduction on location information. As would be expected from the large effect present in the group data, all eight subjects portrayed a positive response set between the two starting positions for distance reproduction under cuff conditions.

Movement velocity. The main effects of both movement conditions (normal vs. cuff) and response mode (distance vs. location) were not significant, F(l, 7) = 1.64 and .36, respectively />s > .05, nor was the interaction (F < 1.00). The respective mean velocities of criterion and reproduction movements for distance were 5.29° and 5.78°/sec under normal conditions and 6.10° and 7.90°/sec for cuff movements. For location, the respective velocities were 5.19° and 6.37°/sec (normal) and 6.32° and 7.80°/sec (cuff). Analysis of these means indicated that criterion- and reproduction-movement velocities for both distance and location were significantly slower under normal conditions (P < .05). Sensory testing. The mean cutoff times for the tactile modality for upper- and lowercuff applications were 23.5 min. (SD = 2.06) and 49.0 min. (SD = 3.77), respectively. Loss of movement-position sense always followed the demise of tactile input, with means of 27.0 min. (SD = 3.25) and 75.5 min. (SD — 14.24) for upper and lower cuffs, respectively. These data are reasonably congruent with previous studies (Goodwin et al, 1972; Merton, 1964). For the lower cuff however, assignment of tactile and movement-position sense loss was noticeably prolonged in comparison with Experiment 1. All seven of the subjects who participated in both experiments showed an upward increase in cutoff time that was significant for the tactile, t ( 6 ) — 3.23, p < .02, and kinesthetic modalities, t(6) = 4.95, p < .01 (two tailed in both cases). Discussion The dominant finding in Experiment 2 was that preselected location reproduction was unaffected by the cuff manipulation, whereas reproduction of distance deteriorated dramatically from normal. The superiority of location is all the more significant in light of the failure of subjects to report any perception of changed starting position under cuff conditions. Thus, as in Experiment 1, it seems unlikely that useful proprioceptive information was available from joint, muscle, or cutaneous sources to mediate cuff performance.

MOVEMENT REPRODUCTION Precisely the opposite result would have been predicted had subjects been relying on central command signals coded in terms of extent or duration (e.g., Jones, 1974). With this control mode, knowledge of starting position is immaterial as long as the same efferent commands can be elicited for a given extent. Such an explicatory mechanism is suggested by Taub, Goldberg, and Taub (1975) for the observation that deafferented monkeys seemed to adopt the same limb position prior to moving to an unseen target; a strategy that would maintain movement amplitude at a constant level. However, this data can equally well be explained by central command signals specifying desired position to the peripheral musculature (Houk, 1972). That the termination of movement may be centrally determined is corroborated by recent neurophysiological and behavioral data. Bizzi, Polit, and Morasso (1976) applied constant and inertial load disturbances to visually triggered eye-head movements in normal and deafferented monkeys. Although the imposed perturbations led to characteristic overshoots and undershoots, the final head position was correctly attained even in the complete absence of sensory cues from vestibular, periosteum, and visual sources. This finding suggests that the program for terminal location was maintained throughout load application and was not readjusted by altering proprioceptive stimulation. Furthermore, that achievement of terminal location under the predictive conditions of the present study is independent of starting position is corroborated by Bizzi et al.'s (1976) work on deafferented monkeys and Fel'dman's (1966) study on normal humans, in which sudden changes in initial conditions (e.g., passive extension of the joint) did not affect the final equilibrium state of the joint. Hence, as in the present experiment, a command signal to the peripheral apparatus can be viewed as establishing a particular joint position in absolute coordinates independent of starting position. When a change in these coordinates is required for movement reproduction—as in the distance condition of the present experiment—accuracy diminishes.

541

General Discussion The present experiments were designed to investigate the nature of the processes underlying the accuracy and control of voluntary, preselected movements. Experiment 1 asked what happens to reproduction of preselected movements when incoming proprioceptive information is reduced: Can postulated central processes mediate performance under these circumstances? With customary caution the answer was in the affirmative; the implication being that when prior knowledge of the movement is available, movement reproduction is less dependent on peripheral information. Experiment 2 sought to evaluate the nature of the information carried by postulated central signals. The results indicated that terminal location rather than amplitude or duration was centrally determined. Indeed, since the reproduction errors in Experiment 1 with both distance and location available for preselection were nearly identical to those for location alone in Experiment 2, it seems likely that similar processes were operating in both experiments. The crucial question that appears to invite some speculation and further analysis pertains to the underlying mechanisms by which the limb reaches a desired location independent of perceived starting position and in the absence of sensory cues. One possibility, designated the target hypothesis in speech control (MacNeilage, 1970), is the muscles attain desired locations by "presetting" muscle spindle receptors on the basis of fusimotor (gamma) command signals. The main muscles can then be driven via the stretch reflex arc (monosynaptic connections of 1 A sensory afferents with alpha motoneurons) to the specified target location, regardless of their length preceding the gamma efferent discharge. This basic closedloop servomechanism can function effectively despite alterations of starting position and is not dependent upon return information to higher cortical centers for its operation. There are, however, a number of problems with this account (see MacNeilage, 1972). Probably most relevant to the present discussion is the clear evidence that muscle spindle discharge occurs simultaneously or (more often) follows the activation of main

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J. A. SCOTT KELSO

muscle (e.g., Vallbo, 1971). This datum rules out the gamma-muscle spindle feedback loop in the initiation of movements, although possibly not in their termination (Smith, Roberts, & Atkins, 1972). More recent data, however, indicate that the achievement and maintenance of terminal limb position is directly programmed through the alpha system, independent of closed-loop servomechanisms (Bizzi et al., 1976; Fel'dman, 1974a, 1974b). The production of final location in the present experiment may then be viewed as determined by factors such as the length-tension characteristics of agonist and antagonist finger muscles, the passive elastic forces in muscle tissue that tend to counteract the net tension generated in these muscles, and, probably most crucial, the firing rate and recruitment pattern of alphamotoneurons. Bizzi et al. (1976) have shown that the alpha system is primarily responsible for specifying terminal location by selecting the appropriate length-tension relationship for each muscle (Asatryan & Fel'dman, 1965). The final resting length of the muscles is therefore determined when the tensions on agonists and antagonists are equal and opposite. An apparent advantage of a "tension programming" proposal over closed-loop hypotheses is that the initial position of the finger does not have to be taken into account, since terminal location depends only upon the tension difference in relevant agonist and antagonist muscles. The foregoing discussion does not rule out the role of peripheral feedback mechanisms such as Golgi tendon organs and muscle spindles in the process of locating a limb, particularly when load compensation is required. The point that must be emphasized in the present experiments is there is no need for such feedback to access central awareness. Location accuracy can be attained whether cues for motion are present or not. This conclusion applies only to preselected movements however. Under constrained, exploratory conditions sensory information about the location of the limb is crucial for accurate reproduction. Although comparisons may be tenuous at this point, these findings appear congruent with recent neurophysiological work (Bizzi, 1974; Brooks, 1975) revealing a similar underlying principle.

In summary, the present results suggest the operation of two fundamentally different mechanisms in movement reproduction. Until recently, the bulk of the literature using reproduction paradigms has examined constrained movements, which seem to demand closed-loop control. In contrast, preselected movements seem to be predominantly under central, feed-forward control, in which the availability of a movement plan acts to guide the execution of the movement (Greenwald, 1970). It is of interest to this notion that Allen and Tsukahara (1974) have recently provided evidence that different areas of the cerebellum are involved in these types of movement. For preplanned movement in which an accurate "internal model" of performance is available, the lateral cerebellum is thought to be involved primarily because of its extensive connections with the association cortex and its ability to function without the aid of direct peripheral input. In contrast, exploratory movements of the constrained type are thought to be dependent on continual cerebral intervention and also on the pars intermedia of the cerebellum to constantly update the intended movement. Thus the neurophysiological evidence easily allows —rather provides for—-different motor control mechanisms for preselected and constrained movements. References Adams, J. A. A closed-loop theory of motor learning. Journal of Motor Behavior, 1971, 3, 111-150. Allen, G. L, & Tsukahara, N. Cerebrocerebellar communication systems. Physiological Review, 1974, 54, 957-1006. Asatryan, D. G., & Fel'dman, A. G. Functional tuning of nervous system with control of movement or maintenance of a steady posture: 1. Mechanographic analysis of the work of the joint on execution of a postural task. Biophysics, 1965, 10, 925-935. Bizzi, E. Common problems confronting eye movement physiologists and investigators of somatic motor functions. Brain Research, 1974, 71, 191194. Bizzi, E., Kalil, R. E., & Morasso, P. Two modes of active eye-head coordination in monkeys, Brain Research, 1972, 40, 45-48. Bizzi, E., Polit, A., & Morasso, P. Mechanisms underlying achievement of final head position. Journal of N euro physiology, 1976, 39, 435-444. Brooks, V. B. Roles of cerebellum and basal ganglia in initiation and control of movements. Co-

MOVEMENT REPRODUCTION nadian Journal of Neurological Sciences, 1975, 2, 265-277. Browne, K., Lee, J., & Ring, P. A. The sensation of passive movement at the metatarsophalangeat joint of the great toe in man. Journal of Physiology (London), 1954, 126, 448-459. Fel'dman, A. G. Functional tuning of the nervous system with control of movement or maintenance of a steady posture : III. Mechanographic analysis of the execution by man of the simplest motor tasks. Biophysics, 1966, 11, 766-775. Fel'dman, A. G. Change of muscle length due to shift of the equilibrium point of the muscle-load system. Biofisika, 1974, 19, 534-538. (a) Fel'dman, A. G. Control of muscle length. Biofisika, 1974, 19, 749-753. (b) Festinger, L., & Canon, L. K. Information about spatial location based on knowledge about efference. Psychological Review, 1965, 72, 373-384. Glencross, D. J., & Oldfield, S. R. The use of ischemic nerve block procedures in the investigation of the sensory control of movements. Biological Psychology, 1975, 2, 227-236. Goodwin, G. M., McCloskey, D. L, & Matthews, P. B. C. The contribution of muscle afferents to kinaesthesia shown by vibration induced illusions of movement and by the effects of paralyzing joint afferents. Brain, 1972, 95, 705-748. Granit, R. Constant errors in the execution and appreciation of movement. Brain, 1972, 95, 649660. Greenwald, A. G. Sensory feedback mechanisms in performance control: With special reference to the ideo-motor mechanism. Psychological Review, 1970, 77, 73-99. Helmholtz, H. von. [Helmholtz's treatise on physiological optics (Vol. 3)] (J. P. C. Southall, Ed. and trans.). New York: Dover Press, 1962. Houk, J. C. On the significance of various command signals during voluntary control. Brain Research, 1972, 40, 49-53. Jones, B. Role of central monitoring of efference in short-term memory for movements. Journal of Experimental Psychology, 1974, 102, 37-43. Keele, S. W. Movement control in skilled motor performance. Psychological Bulletin, 1968, 70, 387-403. Keele, S. W., & Ells, J. G. Memory characteristics of kinesthetic information. Journal of Motor Behavior, 1972, 4, 127-134. Kelso, J. A. S., & Stelmach, G. E. Central and peripheral mechanisms in motor control. In G. E. Stelmach (Ed.), Motor control: Issues and trends. New York: Academic Press, 1976. Kelso, J. A. S., Wanamaker, W. M., & Stelmach, G. E. Behavioral and neurological parameters of the nerve compression block. Journal of Motor Behavior, 1974, 6, 179-190. Laabs, G. J. Retention characteristics of different reproduction cues in motor short-term memory. Journal of Experimental Psychology, 1973, 100, 168-177. Lashley, K. S. The problem of serial order in be-

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