Timing goals in bimanual coordination.

Phase coupling between movement trajectories has been proposed as the basic mechanism of hand coordination in the production of bimanual rhythmic movements with a 1:2 frequency ratio. Here a central temporal coupling view is proposed as an alternative. Extending previous models of two-handed synchronic and alternate-hand tapping, we hypothesized that 1:2 tapping is performed under the control of a single internal timekeeper set at the frequency required for the fast hand. The fast hand is assumed to use every signal and the slow hand every other signal of the timekeeper, to produce actions coordinated in time. The model's predictions for the variance-covariance pattern of tap timing within and across hands were tested in an experiment that required tapping with both hands with 1:1 or 1:2 frequency ratio. The finger contact on the response plate was to be short or long, according to instruction. Prolonged finger contact entailed profound modifications in the movement trajectories but failed to modify the variance-covariance pattern of the tap timing. This pattern proved to conform to predictions under both the short and the long contact conditions, thus supporting the central temporal coupling hypothesis.

The coupled oscillator model of between-hand coordination in alternate-hand tapping: a reappraisal.

Single and alternating hand tapping were compared to test the hypothesis that coordination during rhythmic movements is mediated by the control of specific time intervals. In Experiment 1, an auditory metronome was used to indicate a set of timing patterns in which a 1-s interval was divided into 2 subintervals. Performance, measured in terms of the deviation from the target patterns and variability, was similar under conditions in which the finger taps were made with 1 hand or alternated between the 2 hands. In Experiment 2, the modality of the metronome (auditory or visual) was found to influence the manner in which the produced intervals deviated from the target patterns. These results challenge the notion that bimanual coordination emerges from coupling constraints intrinsic to the 2-hand system. They are in accord with a framework that emphasizes the control of specific time intervals to form a series of well-defined motor events.

Timing precision in continuation and synchronization tapping.

Wing and Kristofferson (1973) have shown that temporal precision in self-paced tapping is limited by variability in a central timekeeper and by variability arising in the peripheral motor system. Here we test an extension of the Wing-Kristofferson model to synchronization with periodic external events that was proposed by Vorberg and Wing (1994). In addition to the timekeeper and motor components, a linear phase correction mechanism is assumed which is triggered by the last or the last two synchronization errors. The model is tested in an experiment that contrasts synchronized and self-paced trapping, with response periods ranging from 200-640 ms. The variances of timekeeper and motor delays and the error correction parameters were estimated from the auto-covariance functions of the inter-response intervals in continuation and the asynchronies in synchronization. Plausible estimates for all parameters were obtained when equal motor variance was assumed for synchronization and continuation. Timekeeper variance increased with metronome period, but more steeply during continuation than during synchronization, suggesting that internal timekeeping processes are stabilized by periodic external signals. First-order error correction became more important as the metronome period increased, whereas the contribution of second-order error correction decreased. It is concluded that the extended two-level model accounts well for both synchronization and continuation performance.

Simulating a neural cross-talk model for between-hand interference during bimanual circle drawing.

Studies on drawing circles with both hands in the horizontal plane have shown that this task is easy to perform across a wide range of movement frequencies under the symmetrical mode of coordination, whereas under the asymmetrical mode (both limbs moving clockwise or counterclockwise) increases in movement frequency have a disruptive effect on trajectory control and hand coordination. To account for these interference effects, we propose a simplified computer model for bimanual circle drawing based on the assumptions that (1) circular trajectories are generated from two orthogonal oscillations coupled with a phase delay, (2) the trajectories are organized on two levels, "intention" and "motor execution", and (3) the motor systems controlling each hand are prone to neural cross-talk. The neural cross-talk consists in dispatching some fraction of any force command sent to one limb as a mirror image to the other limb. Assuming predominating coupling influences from the dominant to the nondominant limb, the simulations successfully reproduced the main characteristics of performance during asymmetrical bimanual circle drawing with increasing movement frequencies, including disruption of the circular form drawn with the nondominant hand, increasing dephasing of the hand movements, increasing variability of the phase difference, and occasional reversals of the movement direction in the nondominant limb. The implications of these results for current theories of bimanual coordination are discussed.

Spontaneous and intentional pattern switching in a multisegmental bimanual coordination task.

Two experiments required right-handed subjects to trace circular trajectories while complying with either a symmetric or asymmetric pattern. In symmetric patterns, circles were traced in a mirror image either inward or outward. In asymmetric patterns, circles were traced in the same direction either clockwise or counterclockwise. Subjects were instructed to trace with spatial accuracy while maintaining a strict temporal relationship to a metronome that scaled movement rates from 1.25 to 3 Hz. The symmetric patterns were more stable than asymmetric patterns; the circularity of trajectories was greater for the dominant side; and there were spontaneous reversals in the direction of circling in the nondominant limb when performing asymmetric patterns. The second experiment examined the same subjects under the instruction of intentionally changing the pattern by reversing the left or right limb circling direction when cued to do so. The degree of interlimb interference was highly asymmetric and contingent on the direction of pattern change. Intentional direction reversals were more expedient and with less disruption to the contralateral limb when asymmetric to symmetric pattern changes were effected through a reversal in the direction of nondominant side. The results are interpreted with reference to evidence that the supplementary motor area mediates descending input to the upper limbs during disparate bimanual actions, but not during symmetric actions.

Bimanual circle drawing during secondary task loading.

Seven right-handed participants performed bimanual circling movements in either a symmetrical or an asymmetrical coordination mode. Movements were paced with an auditory metronome at predetermined frequencies corresponding to transition frequency, where asymmetrical patterns became unstable, or at two-thirds transition frequency where both symmetrical and asymmetrical patterns were stable. The pacing tones were presented in either a high (1000 Hz) or low (500 Hz) pitch, and the percentage of high-pitched tones during a 20 s trial varied between 0% and 70%. Participants were instructed to count the number of high-pitched pacing tones that occurred during a trial of bimanual circling. Overall, the symmetrical pattern was more stable than the asymmetrical pattern at both frequencies. Errors on the tone-counting task were significantly higher during asymmetrical circling than symmetrical circling but only at the transition movement frequency. The results suggest that cognitive processes play a role in maintaining coordination patterns within regions of instability.

Motor timing under microgravity.

Five participants were tested on their ability to produce accurate and regular inter-response intervals in the 350 to 530 ms time range. Three of them were members of the French-Russian CASSIOPEE 96 spaceflight mission, and the other two were control subjects tested on the ground. During spaceflight, the target inter-response intervals were increasingly undershot and the timing became more variable (less regular). The increase in the timing variability was mostly attributable to the internal timekeeping processes rather than those involved in motor execution. The results are discussed with reference to the physiological mechanisms possibly underlying the timing of fast serial movements.

Temporal control and motor control: two functional modules which may be influenced differently under microgravity.

Three subjects performed sequences of periodic movements by synchronizing their movements (button pressing with the thumb) to a series of visual stimuli (induction phase), and by continuing to produce the movements with the same rhythm after the metronome had been switched off (continuation phase). The required inter-response intervals (IRIs) were 450, 550 or 650 ms. Two subjects were members of the EUROMIR 94 spaceflight mission. The inter-response intervals of the continuation phase were analyzed in terms of mean and variability. The mean inter-response intervals did not differ systematically during spaceflight from the pre- and post-flight values. The variability of the inter-response intervals significantly increased during the flight with both experimental subjects. The total variance of the inter-response intervals was partitioned into variance due to the internal timekeeper and variance due to the motor implementation processes, following the method proposed by Wing, A.M., Kristofferson, A.B., 1973. Response delays in the timing of discrete motor responses. Perception and Psychophysics 14, 5-12. The variance attributed to the timekeeper showed a significant increase with both subjects, whereas the variance attributed to the motor processes showed inconsistent trends during the spaceflight. It is concluded that during spaceflight, the functioning of the internal timing module may undergo some changes, as the result of which the regularity of the motor timing is slightly impaired.

The dynamics of bimanual circle drawing.

A bimanual circle drawing task was employed to elucidate the dynamics of intralimb and interlimb coordination. Right-handed subjects were required to produce circles with both hands in either a symmetrical (mirror) mode (i.e. one hand moving clockwise, the other counter-clockwise) or in an asymmetrical mode (i.e. both hands moving clockwise or counter-clockwise). The frequency of movement was scaled by an auditory metronome from 1.50 Hz to 3.25 Hz in 8 (8-sec) steps. In the asymmetrical mode, distortions of the movement trajectories, transient departures from the target pattern of coordination, and phase wandering were evidence as movement frequency was increased. These features suggested loss of stability. Deviations from circular trajectories were most prominent for movements of the left hand. Transient departures from the required mode of coordination were also largely precipitated by the left hand. The results are discussed with reference to manual asymmetries and mechanisms of interlimb and intersegmental coordination.

The Timing Effects of Accent Production in Periodic Finger-Tapping Sequences.

When subjects are required to produce short sequences of equally paced finger taps and to accentuate one of the taps, the interval preceding the forceful tap is shortened and the one that immediately follows the accent is lengthened. Assuming that the tapping movements are triggered by an internal clock, one explanation attributes the rnistiming of the taps to central factors: The momentary rate of the clock is accelerated or decelerated as a function of motor preparation to, respectively, increase or decrease the movement force. This hypothesis predicts that the interresponse intervals measured between either tap movement onsets or movement terminations (taps) will show the same timing pattern. A second explanation for the observed interval effects is that the tapping movements are triggered by a regular internal clock but the timing of the successive taps is altered because the forceful movement is completed in less time than the other tap movements are. This "peripheral" hypothesis predicts regular timing of movement onsets but distorted timing of movement terminations. In the present study, the trajectories of the movements performed by subjects were recorded and the interresponse intervals were measured at the beginning and the end of the tapping movements. The results of Experiment 1 showed that neither model can fully explain the interval effects: The fast forceful movements were initiated with an additional delay that took into account the small execution time of these movements. Experiment 2 reproduced this finding and showed that the timing of the onset and contact intervals did not evolve with the repetition of trial blocks. Therefore, the assumption of an internal clock that would trigger the successive movements must be rejected. The results are discussed in the framework of a modified two-stage model in which the internal clock, instead of triggering the tapping movements, provides target time points at which the movements have to produce their meaningful effects, that is, contacts with the response key. The timing distortions are likely to reflect both peripheral and central components.

The role of sensory information in the production of periodic finger-tapping sequences.

A subject lacking proprioceptive and tactile sensibility below the neck and a group of control subjects performed sequences of periodic finger taps involving a pattern of accentuation. The required intertap interval was 700 ms. In some situations, the taps were synchronized with the clicks of a metronome. Feedback conditions were manipulated by either allowing or not allowing the subjects to hear the taps and see their finger movements. We recorded the trajectory of the subjects' finger displacement in the vertical plane, and the force and moment of contact of the finger with the response key. The control subjects achieved precise timing of the finger taps by trading off downstroke onset for movement duration, e.g., they initiated shorter-duration tapping movements with a delay. This strategy did not vary depending on task demands (e.g., synchronization) or feedback conditions. The deafferented patient produced intertap intervals on average close to the required value. However, his tap timing was characterized by increased variability and severe distortion (lengthening) after the accentuated tap, regardless of feedback conditions. He did not manifest the compensatory strategy whereby, in control subjects, movement onset was adjusted to movement duration. Thus, such a strategy in controls seems to depend on intact proprioceptive and/or tactile information from the moving limb. Upon withdrawal of visual and acoustic feedback, the deafferented subject increased the force of the taps and the amplitude of tapping movements; his mean synchronization error with the metronome also increased. However, he did not lose correct phasing between the taps and the clicks of the metronome. These findings suggest that, under normal circumstances, sequential movements are timed by an internal timekeeper which paces sensory consequences relating to the occurrence of behaviorally important events (e.g., finger taps), and not the onset of the movements eliciting those events. In the synchronization task, the timekeeper may be phase locked to the periodic acoustic stimuli by direct entrainment. Feedback information may be needed, however, for keeping any synchronization error as small as possible.

Execution-time updating of motor program for rapid serial output.

Motor program updating was studied by asking 2 subjects to modify the accent pattern of rapid finger-tapping sequences during execution. The tap to be accentuated was changed unpredictably in one-third of the trials; on these trials the signal indicating the new accent position was delivered upon the onset of the first tap. The probability of placing the new accent correctly increased when the tap to be accentuated was shifted towards the end of the sequence. The probability of cancelling the initially prepared accent showed a similar pattern. Successful updating of the accent pattern could be achieved without any mistiming of the successive taps. These findings are taken to indicate the existence of a temporal overlap between the execution of the initial part of the sequence and the programming of the later part.

Distributed planning of movement sequences.

Three experiments are reported that examine whether fast finger-tapping sequences are entirely planned before execution starts (advance planning), or if they can be started while planning is still under way (distributed planning). Subjects performed finger tapping sequences of three to eight taps at a high rate, under both simple and 2-choice reaction time (RT) conditions. The sequences differed in the location of an accentuated element within them. The RT to choose between sequences with different accent locations progressively decreased as an inverse function of the time-distance between the initial tap and the first point at which the alternative sequences differed. The shortening in choice reaction time (CRT) was never accompanied by noticeable changes in the inter-response times or force patterns of the tapping sequences. The RT to initiate sequences with accent location known beforehand (SRT condition) showed, in two of three experiments, a weak decreasing trend as the accentuated tap shifted away from the beginning of the sequence. The SRT results suggest a possible predominance of advance planning when the same sequence is repeated over a series of trials. The CRT results are taken as evidence that planning of the sequence beyond the unpredictable tap could be distributed before and after sequence initiation. Several factors are discussed that may influence the balance between planning in advance of, and planning in parallel with, sequence execution.

Planning and timing of finger-tapping sequences with a stressed element.

Advance planning and execution-time organization of sequences of five finger taps were studied in four experiments. Intertap intervals were required to be equal. In some experimental conditions, one of the taps had to be stronger than the other four. Serial position of the stressed tap, number of alternative stress positions, and tapping rate were manipulated. Time to initiate the sequence after presentation of a reaction stimulus (RT), intertap intervals, and force of the taps were measured. the different effects of stress production and choosing between alternative stress locations on the RT of fast as compared to slow sequences suggest that a plan was selected and activated for the whole sequence only when it had to be executed at a fast rate. Additional organization of the fast sequences during execution was inferred from the intertap intervals, force patterns, and stress location errors, that were all different from those observed in slow sequences. The effects of stress production on timing are discussed in relation to existing timing models.

On controlling force and time in rhythmic movement sequences: the effect of stress location.

Accentuation involves modulation of motor intensity. It differentiates a movement from others within a motor sequence. Does the serial position of the accent characterize the whole sequence as a particular response? How are the control of time and force coordinated in the motor sequence? Subjects produced sequences of four fingertaps on a key. Time of onset and force of each tap were recorded. Tapping rate was imposed by a string of four clicks delivered at 180-msec intervals before each trial. A flashed digit served as go signal. It indicated to the subject which of the four taps had to be tapped stronger (stress +) or weaker (stress -) than all the others. These conditions were run in separate series. Reaction time (RT) of the sequence increased when the number of equally likely locations of the stress increased from 2 to 4. RT was also longer under the stress - than under the stress + condition. Tapping intervals were longer before and after the stressed tap than elsewhere in the series. The first and last intervals tended to be longer than the second one. These effects were the same under both stress conditions. The RT data indicate that the motor sequence is identified as a particular response before it starts. Timing is partly force-independent, but is modulated by central processes that control force.

Dual effect of response preparation on conditioned H reflex.

The late or intercurrent facilitation of the soleus H reflex recovery cycle was studied during the preparatory period of a visual reaction time task. Response preparation produced three kinds of effect on the conditioned reflex elicited 200 msec before the end of the preparatory period: depression of the conditioned reflex which was maximum at the 150 msec conditioning interval; enhancement of the conditioned reflex which was maximum at the 300 msec conditioning interval; and, when the soleus muscle was involved in the execution of the voluntary response, an additional depression of the conditioned reflex at each conditioning interval. These different effects of response preparation are discussed with reference to the neural mechanisms which are thought to mediate the intercurrent facilitation.