ELSEVIER Human Movement Science 15 ( 1996) 25-38

Simultaneity of two effecters in synchronization with a periodic external signal

Magali Billon * , Chantal Bard, Michelle Flew-y, Jean Blouin ‘, Normand Teasdale

Laboratoire de Performance Motrice Humaine, Universite’ Laual, PEPS, Que’bec, Canada, GlK 7P4

Abstract

Several studies have shown that the control of simultaneous movements differ according to the execution context. For instance, when subjects raise simultaneously their index finger and heel as fast as possible after an auditory signal, the simultaneity is controlled by sending synchronously the motor commands to both effecters. On the other hand, when subjects self-pace their movements, the simultaneity is controlled by processing the delay between afferent signals from both movements at the central level (Paillard, 1948). It has been hypothesized that a mode of control similar to the self-paced condition is also used when subjects produce simultaneous and repetitive movements in synchronization with a metronome (Fraisse, 1980). We examined this hypothesis by asking subjects to move simultaneously the index finger and heel in synchronization with metronome sounds. Results showed that the events chronology (i.e., heel movement first, finger movement second and metronome sound third) was a function of the relative distance of the effecters and auditory organ from the central comparator. We deduced that the synchronization and simultaneity was evaluated by computing the time elapsed between the arrival of the sensory feedback of the movement and auditory signal. The second goal of the study was to assess whether, in such a task, each effector is synchronized separately to the metronome sound or together as an unit. A strong positive correlation was found between finger and heel synchroniza- tion errors. This supports the hypothesis that finger and heel movements are synchronized as an

unit to the metronome rather than independently. In conclusion, simultaneity between effecters

* Correspondence address: Laboratoire de Neurosciences Cognitive% CNRS, 31 Chemin Joseph Aiguier,

13402 Marseille Cedex 20, France.

’ J. Blouin is now at Mouvement et Perception, UMR CNRS, FacultC des Sciences du sport, Universite de

la Mtditerrante, 13009 Marseille, France.

0167-9457/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved

SSDI 0167-9457(95)00037-2

26 M. Billon et al. / Humun Movement Science 15 (19961 25-38

and synchronization between effectorx and an external signal, although they share similar processes based on afferent information, are likely to be controlled separately.

PsyclNFO classijication: 2330

Kr~ords: Movement sequence; Timing; Tapping; Temporal coordination; Sensorimotor process

1. Introduction

The existence of a dual mode of motor control according to whether the movements are self-paced or stimulus-driven has been well documented (Bard et al., 1991; Bard et al., 1992; Bizzi et al., 1972; Goldberg, 1985; Paillard, 1948; Romo et al., 1992). Indeed, Paillard’s early experiment (Paillard, 1948) showed that simultaneous index finger extension and heel raising are timed according to movement initiation contexts: heel raising is initiated prior to finger lifting when movements are performed in a self-paced condition, whereas in a stimulus-driven condition (i.e., when subjects react to an auditory signal), finger onset always precedes the heel response. According to Paillard (19481, in the stimulus-driven (reactive) condition, central motor commands are sent simultaneously to both effecters and the precedence of the finger on the heel reflects only the difference in conduction time of the efferent pathways. On the other hand, in the self-paced (predictive) condition, heel movement occurred before that of the finger allow- ing afferent signals derived from both limbs to arrive simultaneously at a central comparator.

Paillard’s results were recently replicated (Bard et al., 1991; Bard et al., 1992). Bard et al. (1992) showed that, in the reactive condition, the delay between finger and heel movement onsets corresponds to the difference between finger and heel reaction times, hence supporting the hypothesis of simultaneous triggering of these effecters in the stimulus-driven condition.

In the self-paced context, the dominant role of afferent signals was also supported in an experiment with a deafferented patient (Bard et al., 1992). In contrast to normal subjects, the deafferented patient initiated finger movement prior to the heel response in the self-paced condition. The absence of sensory feedback in the patient prevented the use of finger and heel afferent signals for evaluating movement simultaneity. Therefore, the deafferented subject con- trolled simultaneity by the synchronized activation of both effecters, as in the reactive condition.

The difference between discrete actions initiated in self-paced and stimulus- driven conditions lies in the predictability of the moment to act. Similar

M. Billon et al. / Human Movement Science 15 (1996) 25-38 27

predictability as in a self-paced condition is found when movements are

periodically repeated. Fraisse et al. (1958) also attributed the usual precession of the finger tap on the metronome (20-50 ms) during synchronization tasks to differences in conduction time. The synchronization accuracy would be evalu- ated by computing the time elapsed between the arrival of the sensory feedback of the movement and auditory signal. Later, Fraisse (1980) brought support to this hypothesis by showing that when subjects produce simultaneous taps with the finger and the foot in synchronization to a metronome, both motor responses precede the metronome sound, and the magnitude of the synchronization errors

(’ i.e., delay between the tap and the metronome sound) is a function of the distance of the effector with the central comparator: the heel tap preceding the finger tap. The delay between finger and foot taps in Fraisse’s experiment (synchronization condition) was about 20 ms whereas in Bard et al’s experi- ment (Bard et al., 1992, in the self-paced condition), the error of simultaneity was on average 8 ms. The larger delays found in Fraisse’s study could be due to the supplementary temporal constraint brought by the synchronization with a metronome. Moreover, movements of the effecters differed in both studies, raising (Bard et al., 199 1; Bard et al., 1992) and tapping (Fraisse, 1980) movements being used.

The first goal of the present experiments was to further study the temporal control of simultaneous finger and heel raising or tapping in task where synchronization with a metronome was required. The second aim of this study was to bring further insight into the nature of the relation between simultaneity and synchronization control processes by comparing different temporal parame- ters, resulting from a double (finger and foot) synchronization condition with those obtained in conditions having only simultaneity or synchronization con- straints. In finger and heel raising or tapping movement conditions, foot movements should precede finger movements. However, different processes could result in a foot precedence over the finger in the synchronization condition (see Fig. 1). On the one hand, simultaneity (between effecters) and synchroniza- tion (between effecters and the auditory signal) could be inferred together by the delay between finger, heel and auditory afferent signals. In such a scenario, the delay between heel and finger no more constitutes the controlled variable as it does in the self-paced condition. Here, the signal from each effector would be referred to the metronome and the controlled variables should be both the heel and finger synchronization errors (separate processing of effector delays). On the other hand, simultaneity and synchronization could also be controlled separately. Here, the delay between heel and finger would remain the controlled variable for the simultaneity as in the self-paced condition (combined processing

28 M. Billon et al. /Human Movement Science 15 (1996125-38

A. Separate processing of

effector delays

Metronome 54 6. Combined processing of

effector delays

Fig. 1. Schematized representation of separate and combined processing of effector delays. See the text for

details.

of effector delays), and the output of the finger-heel unit would be compared to the metronome for evaluating the synchronization. From the literature on bimanual movements, we know that when different movements, discrete as well as continuous, are performed simultaneously with both hands, their spatio-tem- poral characteristics often interact. For instance, consider the task where subjects draw repeatedly a line with one hand and a circle with the other hand simultaneously. The circle will become more line-like and the line will become more circle-like. Moreover, movement direction of both hands will be reversed synchronously (Franz et al., 199 1). Similarly, when pointing simultaneously with both hands, movements onsets and offsets tend to be synchronous even if the hands cover different distances (Kelso et al., 1979; Marteniuk et al., 1984; Sherwood, 1991). Sherwood (1989) also found the same assimilation duration phenomenon when movements were performed with upper and lower limbs. Hence, it appears that the timing dependency between simultaneous movements is very tight. From these observations, we may expect that simultaneity control

M. Billon et d/Human Movement Science 15 (1996125-38 29

is made first and independently of the synchronization control as in the combined processing of effector delays (see Fig. I).

2. Experiment 1

2. I. Methods

2.1. I. Subjects

Eight subjects (three men and five women), aged 22 to 3.5 years (mean age: 271, participated in the experiment on a voluntary basis.

2.1.2. Tasks Subjects simultaneously raised their preferred index finger and the ipsilateral

heel in four experimental conditions: (a) reactive, where subjects performed both movements as rapidly as possible in response to an auditory signal, (b) self-paced, where subjects initiated both movements whenever they were ready, (c) double rhythmic, where subjects synchronized both movements with a metronome set at a frequency of 1.33 Hz (i.e., within a frequency range allowing accurate synchronization; Fraisse, 19741, and (d) single rhythmic, where subjects syn- chronized single effector raising (i.e., finger or heel, separately) with the metronome.

2.1.3. Materials Two electric contacts detected the onset of finger and heel movement.

Prespaced (2.5 cm) Ag-AgCl surface electrodes were fixed to the central portion of the extensor digitorum longus and the lateralis gastrocnemius. The elec- tromyographic (EMG) signal was preamplified at the source (35x1, filtered with a time constant of 2.5 ms, and band-passed between 20 Hz-4 KHz. The signals (finger and heel contacts, EMG and metronome) were collected at 500 Hz (12 bit A/D conversion). A metronome connected to a loudspeaker situated in front of the subject delivered clicks of 3 ms duration every 700 ms. The EMG onset was determined when the signal exceeded 5% of the peak signal. All onsets were visually verified and trials with an ill-defined EMG onset were rejected (2.85% in the self-paced and the reactive situations, and 8.57% in the double rhythmic condition).

2.1.4. Procedure Subjects performed 35 trials in each experimental condition over two testing

sessions. In a first session, the reactive, self-paced, and double rhythmic

30 M. Billon et al. /Human Movement Science I5 C 1996125-38

conditions were performed. The order of presentation of the self-paced and reactive conditions was counterbalanced across subjects. The double rhythmic condition was always presented last. Subjects had a few practice trials (about five) before data recording. In the reactive condition, three randomly presented preparatory periods (0.75, 1.2 and 1.7 s) preceded the auditory signal to reduce anticipatory responses. Trials with finger reaction times shorter than 150 ms or with heel reaction times shorter than 175 ms were considered as anticipatory trials and were automatically rejected (Bard et al., 1992). At the start of each self-paced trial, subjects received a verbal signal from the experimenter indicat- ing that they could initiate the movements whenever ready (data recording lasted 5 s). Subjects were instructed not to respond immediately following the signal, to pay attention to their movements and to avoid automatism and periodic responses (most of the responses were initiated 2-3 s following the verbal signal). In both single and double conditions, the rhythmic performance was recorded during 30 s. Finally, in the second session, the single rhythmic condition was performed with finger and heel responses counterbalanced across subjects.

2.1.5. Measures Two measures were used to evaluate the delay between finger and heel

movements: (a) response time difference, i.e. the delay between the break of heel and finger electrical contacts, and (b) premotor time difference, i.e. the delay between the EMG onsets of the extensor digitorum longus and the lateralis gastrocnemius. Response and premotor time differences were nega- tively signed when the heel preceded the finger. In the rhythmic conditions, the synchronization errors were defined as the delay elapsed between the sound of the metronome and the break of the electrical contact of the finger and/or the heel. Negative values were given when the raising of the effector preceded the metronome sound.

2.2. Results

2.2. I. Response and premotor time differences Fig. 2 shows the response and premotor time differences for the reactive,

self-paced and rhythmic conditions. Both response and premotor time differ- ences followed similar variations. These two measures were similar and nega- tively signed in self-paced and double rhythmic conditions, but different than in reactive condition where the response and premotor time differences were positively signed. A 2 X 3 [Measure (Response and Premotor) X Condition

M. Billon et al. /Human Movement Science I5 (I 996) 25-38 31

.15

-25 -L I

-35 1 A-

Reactive Self-paced Double rhythmic

Fig. 2. Delay between finger and heel movement onsets and delay between finger and heel EMG onsets for the

reactive, self-paced and double rhythmic conditions. Negative values indicate that heel movements occurred

prior to movements of the finger. Error bars represent standard deviations between subjects.

(Reactive, Self-paced and Rhythmic)] analysis of variance (ANOVA) revealed only a main effect of condition (F(2,14) = 22.08, p < 0.001). Post hoc analyses (Tukey LSC test, p < 0.05) confirmed that the response and premotor time differences were significantly different in reactive condition compared to self- paced and rhythmic conditions.

2.2.2. Synchronization errors In the double rhythmic condition, the analyses of synchronization errors

showed that the finger and the heel preceded the tones of the metronome by 133 ms (SD between subjects = 85 ms) and 142 ms (SD between subjects = 88 ms), respectively. In the single rhythmic condition, these effecters preceded the metronome by 71 ms for the finger (SD between subjects = 21 ms) and 110 ms for the heel (SD between subjects = 65 ms). A 2 X 2 [Effector (finger and heel) X Condition (single and double)] analysis of variance revealed no signifi- cant main effect of Condition and no significant interaction ( p > 0.05). How- ever, the effect of Effector was marginally significant at p = 0.06 ( F(1,7) = 5.50). Simple effect statistic for both single and double rhythmic conditions were not significant (F(1,7) = 3.03 and 2.91, p > 0.05, respectively). The

32 M. Billon et al./Human Moaement Science 15 (1996) 25-38

failure to reach significant difference may result from the high variability

between subjects.

2.2.3. Separate or combined processing of efector delays? The synchronization errors of the foot were plotted against the finger

synchronization errors to investigate whether both errors were strongly corre- lated as is predicted by the hypothesis of combined processing of effector delays. Regression analyses were applied to data obtained by each subject. Individual R* and slopes of the regression line were averaged over all subjects. The probabilities for all individual regression line were smaller than 0.005. R*

value was 0.76 (ranging from 0.55 to 0.94) indicating that most of the variation in the foot synchronization errors can be accounted for by the finger synchro- nization errors. Of particular significance is the fact that the slope of the regression line approached unity (i.e., 0.87, ranging from 0.74 to 0.97). This finding shows that variations in the foot synchronization errors were associated with similar changes in the finger synchronization errors.

According to the hypothesis of separate processing of effector delays, which predicts that each effector is independently controlled in relation to the metronome, the variability in the finger and heel synchronization errors should vary independently in both the single and double rhythmic conditions. Con- versely, in the combined processing of effector delays hypothesis, because the synchronization is believed to be evaluated by comparing the delay between the finger-heel unit output and the metronome, the variability of both the finger and heel synchronization errors should be similar in the double rhythmic condition. An Effector (finger and heel) X Condition (single and double) analysis of variance applied to the intra-subject variability (SD) of the synchronization errors, revealed significant main effects ( F( 1,71 = 19.38 and 7.85, ps < 0.05 for Effector and Condition, respectively) and a significant Effector X Condition interaction (F(1,7) = 39.69, p < 0.05). A decomposition of the interaction showed that the variability of synchronization errors was different for the finger and the heel in the single rhythmic condition (F( 1,7) = 28.47, p < 0.01; finger = 14 ms, SD between subjects = 5 ms; heel = 51 ms, SD between subjects = 18 ms) but was very similar in the double rhythmic condition (F( 1,7) = 0.45, p > 0.05; finger = 39 ms, SD between subjects = 8 ms; heel = 38 ms, SD between subjects = 10 ms).

Moreover, according to the separate processing of effector delays hypothesis, because the finger-heel delay is not the controlled variable, temporal adjust- ments between each effector and the metronome should tend to increase the variability of this delay for the double rhythmic condition in comparison to the

M. Billon et al. /Human Mor;ement Science 15 (I 996) 25-38 33

self-paced condition. In contrast, according to the combined processing of effector delays hypothesis, the variability should be similar to the self-paced condition. Two one way ANOVAs showed no reliable effect of Condition for the variability of both response and premotor time differences (F(1,7) = 0.13 and 2.08, ps > 0.05, for response and premotor time differences, respectively). The variability of the response time difference was 18 ms (SD between subjects = 4 ms) and 16 ms (SD between subjects = 6 ms) for the self-paced and double rhythmic conditions, respectively. The variability of the premotor time difference was 19 ms (SD between subjects = 5 ms) and 25 ms (SD between subjects = 6 ms) for the self-paced and double rhythmic conditions, respectively.

2.3. Discussion

Even if results on response and premotor time differences were considerably different in the reactive condition in comparison to the rhythmic and self-paced conditions, we cannot conclude from these data that simultaneity in the rhythmic condition is regulated by afferent signals issued from the movements since no reliable difference was found between heel and finger synchronization errors. The anticipation of the foot on the finger is a general tendency in both the rhythmic and self-paced conditions. However, in the present experiment, the finger movement occurred before that of the foot, but the magnitude of the finger anticipation remains well under that found in reactive conditions. In the self-paced and rhythmic conditions, 3 subjects out of 8 exhibited a slight positive mean finger-foot delay (i.e., finger movements occurred before those of the foot) whereas in Fraisse’s experiment (Fraisse, 1980) 3 out of 10 subjects also obtained a positive mean error. Intra-subject variability is also important in these studies. Fraisse (1980) attributed the large variability to the lack of accuracy in the simultaneity evaluation at the central level. Paillard (1948) reported that the perception of the simultaneity between finger and heel raising is preserved with delays as large as 60 ms. Moreover, the movement feedback is composed of different sensory modalities (tactile, kinesthetic and auditory). Variations in these inputs, such as when the force of the impact is increased, change the content of the afferent message (Johansson and Westling, 1991). Also, subjects might direct their attention to different sources of information during a series of movements. Indeed, a recent study by Aschersleben and Prinz (1995) showed that the error of synchronization increased markedly (about 30 ms) when subjects could not hear the auditory feedback of their tapping movements. However, despite the substantial variations in finger-foot delays, the

34 M. Billon et al./ Human Mocement Science 15 (1996) 25-38

finger-foot asynchrony in self-paced and rhythmic conditions was different than in the reactive condition where onset of the finger movements occurred largely before that of the foot.

The failure to reach statistically significant differences between the synchro- nization errors of the finger and the heel is likely due to the high variability between subjects. This large variability is also associated with large synchro- nization errors compared to those reported in the literature for experiments using metronome intervals between 700 and 800 ms. In the single rhythmic condition, the synchronization errors for the finger usually ranged from 30 to 50 ms and from 50 to 70 ms for the foot (Fraisse, 1984; Hary and Moore, 1987; Peters, 1989). Similar values were reported by Fraisse (1980) in double rhythmic conditions: the synchronization errors were 50 ms and 70 ms for the finger and the foot, respectively. We also found the same range of synchronization errors in both double and single rhythmic conditions. Therefore, the lengthening of the synchronization error observed in our experiment is presumably not due to the additional complexity of the double commands involving two different body segments. It may rather result from our experimental task, which differs from synchronization tasks used in the past. In the present experiment, subjects raised their finger and heel in synchrony with the metronome whereas, in previous studies, tapping movements were used (e.g., Fraisse, 1980, Fraisse, 1984; Hary and Moore, 1987; Peters, 1989). To test whether these differences had an effect, we conducted a second experiment in which subjects produced raising or tapping finger and heel movements in the double and single rhythmic condi- tions.

3. Experiment 2

3.1. Method

Four new subjects (mean age: 29 years) produced sequences of either raising or tapping movements with their preferred index finger and the ipsilateral heel in both double (i.e., finger and heel movements simultaneously in synchronization with a metronome) and single (finger or heel movements in synchronization with a metronome) rhythmic conditions. The metronome cadence and the recording duration of the rhythmic performance were similar to those of the first experiment. Electrical contact detected both the onset of raising and tapping movements. Synchronization errors of the finger and the heel were measured between the rupture time (raising movements) or the electrical contact occur-

M. Billon et al./Human Mocement Science 15 (1996) 25-38 35

-160-

-1 40- 3 E - -120- 8 5 -lOO-

8 ‘J R

-ao- ._

j

-6O-

2 -4O-

-2o-

O-

Index and heel synchronization errors

Double Single Double Single

Raising Tapping

Conditions

Fig. 3. Synchronization errors of the finger and heel in double and single rhythmic conditions with tapping or

raising movements.

rence (tapping movements) and the metronome sound. Subjects performed 35 trials in each experimental conditions. Double rhythmic conditions were always tested first. Two subjects began with the raising movements and two with the tapping movements. The order of effecters used in the single rhythmic condition was also counterbalanced across subjects.

3.2. Results and discussion

3.2.1. Synchronization errors

In single and double rhythmic conditions, the synchronization error was, on average, 46 ms longer when subjects produced periodic raising rather than periodic tapping (see Fig. 3). In this latter situation, results were similar to those observed in previous experiments (e.g., Fraisse, 1980). Nevertheless, an ANOVA with three factors, Effector (finger and heel) X Movement (raising and tapping) X Condition (single and double) only revealed an Effector effect (F(1,3) = 288,

p < 0.001). An important outcome of this experiment is that the difference between finger and heel synchronization errors were larger than in the previous experiment. The heel synchronization error was significantly longer than the finger synchronization error (see Fig. 3). These results support the hypothesis according to which in double rhythmic condition simultaneity between two

36 M. Billon et ul. / Humm Mocement Scirnce 15 (IYWl 25-38

movements is evaluated on the timing of their afferent signals at a central level. Therefore, finger and heel movements were delayed accordingly.

3.2.2. Separate or combined processing of eSfector delays? A regression analysis has been applied to the synchronization error of the

finger and that of the heel in the double rhythmic condition with raising and tapping movements. All individual regression line probabilities were smaller than 0.005. Averaged across subjects, R* values were 0.77 (ranging from 0.68 to 0.93) and 0.74 (ranging from 0.61 to 0.80) and slopes of the regression line were of 0.87 (ranging from 0.83 to 0.96) and 0.88 (ranging from 0.78 to 0.90) for raising and tapping movements, respectively. These results corroborate those of Experiment 1.

4. General discussion

When subjects have to simultaneously move the ipsilateral index and heel in synchronization with a metronome, the heel movement precedes that of the finger and both finger and heel motions anticipate the metronome. This events chronology is respected regardless of whether the motor task consists in raising or tapping movements. However, the effecters anticipation on the metronome tends to be greater with raising than with tapping movements. In the literature, important differences have been found between heel and finger asynchrony produced both in self-paced (Bard et al., 1992) and rhythmic (Fraisse, 1980) conditions, whereas in the present study with a direct comparison between self-paced and rhythmic conditions, heel and finger asynchrony was found similar. This similarity as well as the difference between finger and heel synchronization errors support the hypothesis that heel and finger movement simultaneity, when synchronized with a periodic external signal, is attained by synchronizing afferent signals issued from the limb movements at the central level.

Our results support a combined processing of effector delays. First, to attain simultaneity, the timing of the motor commands to each effector must be adjusted so that the subsequent afferent signals arrive simultaneously at a central comparator, the output of which would then be compared with the metronome tone (see Fig. 1). The feedback signal provided by the finger-heel unit that is used for the synchronization with the metronome remains unknown. It could originate either from the effector systems (finger or heel) or from a composite

M. Billon et al. /Human Movement Science I5 (1996) 25-38 37

reference emerging from the finger-heel afferent signals. Therefore, we may

speak in terms of a dual-loop: (a) the simultaneity loop contributing to the temporal patterning between the effecters of the two limbs involved, as in the simultaneity found in the self-paced condition, and (b) the synchronization loop triggering the finger-heel unit in order to synchronize its sensory output with the auditory afferent signal.

Common to the self-paced and the synchronization contexts is a knowledge of the moment of movement onsets (in opposition to the unpredictable stimulus for the reactive condition). This prior information might be necessary for organizing the temporal parameters of finger and heel movements according to the prede- fined goal of the action (simultaneity and synchronization). It also allows participation of higher-order processes such as the preparation and anticipation required for a predictive mode of control (Paillard, 1990). On the other hand, in the reactive condition, the time constraint stemming from the external and unforeseeable stimulus prevents the complex comparator computation.

Relatively large synchronization errors between the metronome and both effecters (in both the single or the double rhythmic conditions) were found in Experiment 1 compared to that reported in the literature (Fraisse, 1984; Hary and Moore, 1987; Peters, 1989). These discrepancies stem from the use of different tasks. In Experiment 1, subjects synchronized the finger and/or the heel raising with the metronome, whereas, in previous studies and in Experiment 2, finger or heel taps were synchronized with the metronome. The better synchronization found in tapping tasks may result from the possibility to modulate the duration of the movements once triggered (Billon et al., 19961, which is not possible for raising tasks. Moreover, the contact of the effector with a solid surface in tapping tasks triggers cutaneous, proprioceptive and auditory signals that may mark the timing of the tap with better precision than signals arising from raising movements.

Acknowledgements

This study was supported by NSERC and FCAR grants to Chantal Bard, Michelle Fleury and Normand Teasdale. Special thanks to Benoit Genest and Gilles Bouchard for programming and technical expertise. The authors wish to thank Drs. Andras Semjen, Jacques Paillard and Paul van Donkelaar for numerous helpful comments made on a previous version of this paper.

38 M. Billon et al. /Human Movement Science 15 (19%) 25-38

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