Weber (Slope) Analyses of Timing Variability in Tapping and Drawing Tasks

Timing variability in continuous drawing tasks has not been found to be correlated with timing variability in repetitive finger tapping in recent studies (S. D. Robertson et al., 1999; H. N. Zelaznik, R. M. C. Spencer, & R. B. Ivry, 2002). Furthermore, the central component of timing variability, as measured by the slope of the timing variance versus the square of the timed interval, differed for tapping and drawing tasks. On the basis of those results, the authors posited that timing in tapping is explicit and as such uses a central representation of the interval to be timed, whereas timing in drawing tasks is implicit, that is, the temporal component is an emergent property of the trajectory produced. The authors examined that hypothesis in the present study by determining the linear relationship between timing variance and squared duration for tapping, circle-drawing, and line-drawing tasks. Participants (N = 501 performed 1 of 5 tasks: finger tapping, line drawing in the x dimension, line drawing in the y dimension, continuous circle drawing timed in the x dimension, or continuous circle drawing timed in the y dimension. The slopes differed significantly between finger tapping, line drawing, and circle drawing, suggesting separable sources of timing variability. The slopes of the 2 circle-drawing tasks did not differ from one another, nor did the slopes of the 2 line-drawing tasks differ significantly, suggesting a shared timing process within those tasks. Those results are evidence of a high degree of specificity in timing processes.     

Force and Directional Force Modulation Effects on Accuracy and Variability in Low-Level Pinch Force Tracking

The authors investigated how varying the required low-level forces and the direction of force change affect accuracy and variability of force production in a cyclic isometric pinch force tracking task. Eighteen healthy right-handed adult volunteers performed the tracking task over 3 different force ranges. Root mean square error and coefficient of variation were higher at lower force levels and during minimum reversals compared with maximum reversals. Overall, the thumb showed greater root mean square error and coefficient of variation scores than did the index finger during maximum reversals, but not during minimum reversals. The observed impaired performance during minimum reversals might originate from history-dependent mechanisms of force production and highly coupled 2-digit performance.     

Blocking in Rapid Finger Tapping: The Role of Variability in Proximodistal Coordination

In rapid finger tapping, occasional intertap intervals of about twice the normal length or even longer, called blockings, can be observed. Skilled rapid tapping requires that flexor and extensor activity be timed so that they coincide with certain phases of the finger movement. In the present study, the hypothesis examined was that blockings are associated with a deviation from the proper timing relations between the more proximal signals (electromyographic [EMG] bursts) and the more distal signal (position-time curve of the finger). Participants (N = 8) performed up-and-down tapping. Blockings were compared with the preceding normal tapping cycles; a temporal forward shift of the flexor burst in the time interval between two kinematic landmarks—the lifting of the finger and the reversal of the movement—was found consistently in the blockings The phase shift of the flexor burst relative to the kinematic landmarks did not develop gradually in the course of the tapping cycles that preceded the blocking but was an abrupt deviation, which suggests that blockings occur with an increased likelihood as the extremes of the normal variability of the phase relation are approached.     

Motor Performance of Stutterers and Nonstutterers on Timing and Force Control Tasks

Recently it has been suggested that speech and manual timing tasks share a common central process (Franz, Zelaznik, & Smith, 1992). Because stuttering is thought to be related to deficits in motoric processes such as timing, stutterers (n = 15) were compared with a set of age-, education-, and sex-matched nonstutterers on timing and isometric force-production tasks. In the timing tasks, subjects flexed and extended the right index finger at the metacarpophalangeal joint at cycle durations of 600, 500, 400, 300, and 200 ms. In the force-production tasks, subjects generated isometric forces to match target force levels displayed on a cathode-ray tube (CRT) screen. There were five levels of force, ranging from .11 to 7.85 newtons. Overall, there were no differences in timing and force-production performance between stutterers and nonstutterers. These results are similar to those obtained recently by Hulstijn, Summers, van Lieshout, and Peters (1992). We suggest that stuttering is not characterized by a general deficit in rhythmic timing. Instead, the motor deficit associated with stuttering should be viewed as speech specific.     

Variability and Noise in Continuous Force Production

In the present 3 experiments, the authors examined the hypothesis, derived from information theory, that increases in the variability of motor responses result from increases in perceptual-motor noise. Three different groups of participants (Ns = 10, 9, and 10, respectively, in Experiments 1, 2, and 3) produced continuous isometric force under either low, intermediate, or high target force levels. When considered together, the results showed that force variability (SD) increased exponentially as a function of force level. However, an index of information transmission (M/SD), as well as measures of noise in both the time (approximate entropy) and the frequency (power spectrum) domains, changed according to an inverted-U-shaped function over the range of force levels. The findings provide further evidence that increased information transmission is related to increases, and not to decreases, in the noisiness of the structure of force output.     

Timing and the Control of Rhythmic Upper-Limb Movements

Accurate timing of limb displacement is crucial for effective motor control. The authors examined the effects of movement velocity, duration, direction, added mass, and auditory cueing on timing, spatial, and trajectory variability of single- and multijoint rhythmic movements. During single-joint movements, increased velocity decreased timing and spatial variability, whereas increased movement duration increased timing variability but decreased spatial variability. For multijoint movements, regardless of condition, increasing velocity decreased joint timing, spatial, and trajectory variability, but all hand variabilities were unaffected by velocity, duration, load, or direction. Timing, spatial, and trajectory variability was greater at the shoulder compared with the elbow and minimal at the hand, supporting the notion that reaching movements are planned in hand space as opposed to joint space.     

Effects of Practice Variability on Unimanual Arm Rotation

High variability practice has been found to lead to a higher rate of motor learning than low variability practice in sports tasks. The authors compared the effects of low and high levels of practice variability on a simple unimanual arm rotation task. Participants performed rhythmic unimanual internal-external arm rotation as smoothly as possible before and after 2 weeks of low (LV) or high (HV) variability practice and after a 2-week retention interval. Compared to the pretest, the HV group significantly decreased hand, radioulnar, and shoulder rotation jerk on the retention test and shoulder jerk on the posttest. After training the LV group had lower radioulnar and shoulder jerk on the posttest but not the retention test. The results supported the hypothesis that high variability practice would lead to greater learning and reminiscence than low variability practice and the theoretical prediction of a bifurcation in the motor learning dynamics.     

Sequential and Biomechanical Factors Constrain Timing and Motion in Tapping

The authors examined how timing accuracy in tapping sequences is influenced by sequential effects of preceding finger movements and biomechanical interdependencies among fingers. Skilled pianists tapped sequences at 3 rates; in each sequence, a finger whose motion was more or less independent of other fingers' motion was preceded by a finger to which it was more or less coupled. Less independent fingers and those preceded by a more coupled finger showed large timing errors and change in motion because of the preceding finger's motion. Motion change correlated with shorter intertap intervals and increased with rate. Thus, timing of sequence elements is not independent of the motion trajectories that individuals use to produce them. Neither motion nor its relation to timing is invariant across rates.     

Evidence of Common Timing Processes in the Control of Manual,Orofacial, and Speech Movements

Recent investigations of timing in motor control have been interpreted as support for the concept of brain modularity. According to this concept, the brain is organized into functional modules that contain mechanisms responsible for general processes. Keele and colleagues (Keele & Hawkins, 1982; Keele & Ivry, 1987; Keele, Ivry, & Pokorny, 1987; Keele, Pokorny, Corcos, & Ivry, 1985) demonstrated that the within-subject variability in cycle duration of repetitive movements is correlated across finger, forearm, and foot movements, providing evidence in support of a general timing module. The present study examines the notion of timing modularity of speech and nonspeech movements of the oral motor system as well as the manual motor system. Subjects produced repetitive movements with the finger, forearm, and jaw. In addition, a fourth task involved the repetition of a syllable. All tasks were to be produced with a 400-ms cycle duration; target duration was established with a pacing tone, which then was removed. For each task, the within-subject variability of the cycle duration was computed for the unpaced movements over 20 trials. Significant correlations were found between each pair of effectors and tasks. The present results provide evidence that common timing processes are involved not only in movements of the limbs, but also in speech and nonspeech movements of oral structures.     

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 mistiming 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 interre-sponse 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.     

Temporal Precision in Tapping and Circle Drawing Movements at Preferred Rates is Not Correlated: Further Evidence Against Timing as a General-Purpose Ability

Recently, researchers have discovered that individuals who are consistent timers in a tapping task are not necessarily consistent timers when they perform a continuous drawing task. In other words, nonsignificant correlations were found among tapping and drawing movements for timing precision (S. D. Robertson et al., 1999). In the present experiment, the authors investigated whether or not consistency in timing for tapping and drawing was correlated when participants (N = 24) were allowed to move at their preferred rate of movement. There were no significant correlations between tapping and drawing in terms of timing precision. That result lends further support to the notion that timing behavior is specific to the nature of the task, and thus further weakens the idea that timing is a generalized ability that can be imposed on a variety of different types of tasks.     

Directional Variability of the Isometric Force Vector Produced by the Human Hand in Multijoint Planar Tasks

Numerous studies have examined control of force magnitude, but relatively little research has considered force direction control. The subjects applied isometric forces to a handle and the authors compared within-trial variability when force is produced in different directions. The standard deviation of the force parallel to the prescribed direction of force production increased linearly with the targeted force level, as did the standard deviation of the force perpendicular to the instructed direction. In contrast, the standard deviation of the angle of force production decreased with increased force level. In the 4 (of 8) instructed force directions where the endpoint force was generated due to a joint torque in only 1 joint (either the shoulder or elbow) the principal component axes in force space were well aligned with the prescribed direction of force production. In the other directions, the variance was approximately equal along the 2 force axes. The variance explained by the first principal component was significantly larger in torque space compared to the force space, and mostly corresponded to positive correlation between the joint torques. Such coordinated changes suggest that the torque variability was mainly due to the variability of the common drive to the muscles serving 2 joints, although this statement needs to be supported by direct studies of muscle activation in the future.     

Timing Precision in Circle Drawing Does Not Depend on Spatial Precision of the Timing Target

The authors manipulated the width of a timing target in continuous circle drawing to determine whether a more stringent spatial-timing criterion would produce an increase in participants' (N = 30) temporal variability. They also examined the effect of the computational method of determining cycle duration. There was no effect of spatial precision on temporal variability in circle drawing, and tapping and circle drawing were found to use the same criterion. Those findings lend strong support to the earlier view of R. B. Ivry, R. M. Spencer, H. N. Zelaznik, and J. Diedrichsen (2002), who argued that continuous tasks such as circle drawing are timed differently from discrete-like tasks such as tapping. Therefore, the results of the present study provide support for the event and emergent timing frameworks.     

The Relationship Between Force and Force Variability in Minimal and Near-Maximal Static and Dynamic Contractions

The critical assumption of linearity between force and force variability for rapid movements made by Schmidt, Zelaznik, and Frank (1978) was studied in four experiments in the present study. The first three experiments extended earlier work showing linearity between force and force variability for submaximal force levels in static and dynamic contractions. However, at near-maximal force levels, when force was increased, force variability leveled off and then decreased in both static and dynamic conditions. A fourth experiment using the rapid-timing paradigm showed that increased submaximal loads on the movement produced slight decreases in VE. But when the loads were larger, where force and force variability were no longer linearly related, increased load led to larger decreases in VE. These observations led to the hypothesis that VEt is linearly related to the ratio of force variability to force; data from two experiments are presented that support this idea. The motor-output variability theory seems to hold for a wide range of sub-maximal force values, but needs modification for those conditions where near-maximal forces are required.     

Experience with Event Timing Does not Alter Emergent Timing: Further Evidence for Robustness of Event and Emergent Timing

Although, event and emergent timings are thought of as mutually exclusive, significant correlations between tapping and circle drawing (Baer, Thibodeau, Gralnick, Li, &; Penhune, 2013 Baer, L. H., Thibodeau, J. L. N., Gralnick, T. M., Li, K. Z. H., &; Penhune, V. B. (2013). The role of musical training in emergent and event-based timing. Frontiers in Human Neuroscience, 7(191), 1–10. doi:10.3389/fnhum.2013.00191.[Crossref], [PubMed] , [Google Scholar]; Studenka, Zelaznik, &; Balasubramaniam, 2012 Studenka, B. E., Zelaznik, H. N., &; Balasubramaniam, R. (2012). The distinction between tapping and circle drawing with and without tactile feedback: An examination of the sources of timing variance. The Quarterly Journal of Experimental Psychology, 65(6), 1086–1100. doi:10.1080/17470218.2011.640404.[Taylor &; Francis Online], [Web of Science ®] , [Google Scholar]; Zelaznik &; Rosenbaum, 2010 Zelaznik, H. N., &; Rosenbaum, D. A. (2010). Timing processes are correlated when tasks share a salient event. Journal of Experimental Psychology: Human Perception and Performance, 36(6), 1565–1575. doi:10.1037/a0020380.[Crossref], [PubMed], [Web of Science ®] , [Google Scholar]) suggest that emergent timing may not be as robust as once thought. We aimed to test this hypothesis in both a younger (18–25) and older (55–100) population. Participants performed one block of circle drawing as a baseline, then six blocks of tapping, followed by circle drawing. We examined the use of event timing. Our hypothesis that acute experience with event timing would bias an individual to use event timing during an emergent task was not supported. We, instead, support the robustness of event and emergent timing as independent timing modes.     

Attention to Visual Feedback in Motor Learning

Visual guidance and movement to a stop were used to train subjects to make a simple movement without experiencing error in practice. Movement to a stop led to test performance as accurate as that after training with KR, but visual guidance did not. If a continuous visual cue as well as a stop were present during practice, subjects also performed less accurately, although they did not need to attend to the visual cue. All types of training were better than no training at all. Results are discussed in terms of the role of visual feedback in the development and assessment of programs for movement.     

Timing in Eyeblink Classical Conditioning and Timed-Interval Tapping

Abstract—The cerebellum is implicated in interval timing for diverse tasks including eyeblink classical conditioning (EBCC) and repetitive tapping. We examined performance on both tasks across identical intervals ranging from 325 to 550 ms. In five weekly sessions, 23 participants used a different interval each week, both as the target for tapping and as the delay interval in EBCC. Changes in variability as a function of the tapping or delay interval were assessed using regression analyses. The slope for repetitive tapping was comparable to two measures of temporal acuity in EBCC, onset and peak latency of the conditioned response. Each of 80 additional participants was assessed in one session at one of four tapping and delay intervals. Results were similar to those observed in the repeated measures group. These findings provide further evidence that EBCC and repetitive tapping utilize common mechanisms for representing temporal information.     

Variability and Determinism in Motor Behavior

In investigations into perception-action systems, variability of observable behavior may be considered to (a) interfere with inquiry, (b) be neither detrimental nor particularly useful to inquiry, or (c) play a crucial role in inquiry. The authors underscore recent suggestions that alternative (c) is a preferred strategy for the study of many motor behaviors. In tutorial fashion, the authors review the concepts of variability and determinism with respect to postural and rhythmic movements. Study of the variability of those behaviors has revealed crucial features suggestive of underlying mechanisms and control, such as particular blends of noise and determinism (piecewise determinism). It has also revealed general lessons (for example, more variable does not mean more random and more controllable does not mean more deterministic) that may extend to other classes of perceptual-motor behavior.     

Variation of Isometric Response Force in the Rat

Hungry, unrestrained rats (N = 7) were rewarded for pressing a response beam in excess of 11 different force requirements. Changes in peak force production as a function of peak force requirement were examined by analyses of the first four moments of distributions of peak response forces: constant error, the within-subject standard deviation, skewness, and kurtosis. Results were similar to those previously obtained with human subjects: Constant error was positive at low and negative at high force requirements, the within-subject standard deviation increased as a negatively accelerating function of force requirement, and skewness and kurtosis were positive at low force requirements and decreased to negative values at the highest increments. Additional analyses of response kinetics indicated that rats, like humans, meet increasing force requirements by altering the rate of rise of force. The performance similarities suggest that common processes are engaged by the human and rat motor control systems to solve the problem of generating forces that are appropriate to the prevailing environmental constraints.     

Changes in Muscle Activity in Response to Assistive Force during Isometric Elbow Flexion

Abstract

This study investigated the muscle activity and force variability in response to perturbation of assistive force during isometric elbow flexion. Sixteen healthy right-handed young men (age: 22.0?±?1.1?years; height: 171.9?±?4.8?cm; weight 68.4?±?11.2?kg) were recruited and the muscle activity of biceps brachii and triceps brachii were assessed using surface electromyography. Workload force and assistive force applied on isometric elbow flexion significantly affected the changes in both biceps and triceps muscle activities. A higher assistive force was shown to result in reduced biceps muscle activity compared to the unassisted period. In contrast, the efficiency of the assistive force acting on the biceps decreased as the assistive force increased. In general, the force variability of the biceps muscle remained approximately the same at lower workload force conditions than that at higher workload force conditions. In conclusion, higher assistive force may not yield a higher performance efficiency in human-assistive force interaction.