Response Timing Variability

Leveraging Joint Mechanics Simplifies the Neural Control of Movement

Behaviors we perform each day, such as manipulating an object or walking, require precise control of the interaction forces between our bodies and the environment. These forces are generated by muscle contractions, specified by the nervous system, and by joint mechanics, determined by the intrinsic properties of the musculoskeletal system. Depending on behavioral goals, joint mechanics might simplify or complicate control of movement by the nervous system. Whether humans can exploit joint mechanics to simplify neural control remains unclear. Here we evaluated if leveraging joint mechanics simplifies neural control by comparing performance in three tasks that required subjects to generate specified torques about the ankle during imposed sinusoidal movements; only one task required torques that could be generated by leveraging the intrinsic mechanics of the joint. The complexity of the neural control was assessed by subjects' perceived difficulty and the resultant task performance. We developed a novel approach that used continuous estimates of ankle impedance, a quantitative description of the joint mechanics, and measures of muscle activity to determine the mechanical and neural contributions to the net ankle torque generated in each task. We found that the torque resulting from changes in neural control was reduced when ankle impedance was consistent with the task being performed. Subjects perceived this task to be easier than those that were not consistent with the impedance of the ankle and were able to perform it with the highest level of consistency across repeated trials. These results demonstrate that leveraging the mechanical properties of a joint can simplify task completion and improve performance.

Reaction time and response dynamics

The experiments reported were designed to examine the relationship between reaction time and the response dynamics of a finger-press task. Experiments 1 and 2 manipulated force duration and peak force level in both simple and choice reaction-time paradigms. Experiment 3 constrained both force duration and peak force, leading to independent changes in the rate of force production. The findings from all three experiments suggest that the rate of force production, rather than force duration, is the key response parameter determining reaction time. Reaction time decreased as an exponential function of rate of force production independent of force duration and peak force.

Wingbeat kinematics and motor control of yaw turns in Anna's hummingbirds (Calypte anna)

The biomechanical and neuromuscular mechanisms used by different animals to generate turns in flight are highly variable. Body size and body plan exert some influence, e.g. birds typically roll their body to orient forces generated by the wings whereas insects are capable of turning via left-right wingbeat asymmetries. Turns are also relatively brief and have low repeatability, with almost every wingbeat serving a different function throughout the change in heading. Here we present an analysis of Anna's hummingbirds (Calypte anna) as they fed continuously from an artificial feeder revolving around the outside of the animal. This setup allowed for examination of sustained changes in yaw without requiring any corresponding changes in pitch, roll or body position. Hummingbirds sustained yaw turns by expanding the wing stroke amplitude of the outer wing during the downstroke and by altering the deviation of the wingtip path during both downstroke and upstroke. The latter led to a shift in the inner-outer stroke plane angle during the upstroke and shifts in the elevation of the stroke plane and in the deviation of the wingtip path during both strokes. These features are generally more similar to how insects, as opposed to birds, turn. However, time series analysis also revealed considerable stroke-to-stroke variation. Changes in the stroke amplitude and the wingtip velocity were highly cross-correlated, as were changes in the stroke deviation and the elevation of the stroke plane. As was the case for wingbeat kinematics, electromyogram recordings from pectoral and wing muscles were highly variable, but no correlations were found between these two features of motor control. The high variability of both kinematic and muscle activation features indicates a high level of wingbeat-to-wingbeat adjustments during sustained yaw. The activation timing of the muscles was more repeatable than the activation intensity, which suggests that the former may be constrained by harmonic motion and that the latter may play a large role in kinematic adjustments. Comparing the revolution frequency of the feeder with measurements of free flight yaws reveals that feeder tracking, even at one revolution every 2 s, is well below the maximum yaw capacity of the hummingbirds.

Accurate production of time-varying patterns of the moment of force in multi-finger tasks

We investigated the production of time profiles of the total moment of force produced in isometric conditions by the four fingers of a hand. We hypothesized that these tasks would be associated with multi-finger synergies stabilizing the time profile of the total moment across trials but not necessarily stabilizing the time profile of the total force produced by the fingers. We also expected the multi-finger synergies to prevent an increase in the moment variability with its magnitude. Seated subjects pressed on force sensors with the four fingers of the right hand and produced two time profiles of the total moment of force, starting from a certain pronation effort, leading to a similar supination effort, and back to the initial pronation effort. One of the profiles was a sequence of straight lines (M-Ramp) while the other was a smooth curve (M-Sine). The subjects showed an increase in the total force during each task. This was accompanied by an increase in the force produced by the fingers opposing the required direction of the total moment-antagonist fingers. Variability of the total force and of the total moment showed complex, non-monotonic changes with the magnitude of the force and moment, respectively. In both tasks, the subjects showed patterns of co-variation of commands to fingers that stabilized the required moment profile over trials. The time profile of the total force was stabilized to a lesser degree or not stabilized at all. The share of fingers with larger moment arms (index finger for pronation efforts and little finger for supination efforts) was higher when the fingers acted to produce moments in a required direction but not necessarily when they acted as antagonists. The results demonstrate the existence of multi-finger synergies stabilizing the combined rotational action. They fit a hypothesis that stabilization of rotational actions may be a default strategy conditioned by everyday experience. The data also suggest that the mechanical advantage hypothesis is valid for sets of effectors that act in the required direction but not for sets of effectors that act as antagonists.

EMG Activity in Selected Target Muscles During Imagery Rising on Tiptoes in Healthy Adults and Poststrokes Hemiparetic Patients

The authors sought to gain further knowledge about activation of target muscles during imagery engagement in a motor task. Six hemiparetic patients and 9 healthy participants performed 3 real rises on tiptoes and then, after pausing, 3 imagery rises on tiptoes. Metronome beats guided the rate of rises and descents. Electromyographic (EMG) activity from the medial gastrocnemius and the rectus femoris muscles were monitored bilaterally throughout the performance of both tasks. In 3 healthy participants and 3 individuals with hemiparesis, EMG activity was related to the imagery task in at least 1 of the target muscles. Conversely, in the other participants, motor imagery practice was not accompanied by task-related EMG activity in the monitored muscles. In all cases, the increment in activation level during motor imagery practice was very low in comparison with that of real performance. The findings were not unequivocal; therefore, EMG activity may sometimes, but not always, be recorded during motor imagery practice both in healthy individuals and in poststroke hemiparetic participants. Further research is needed to align motor imagery practice with the objectives of motor rehabilitation.

An examination of rapid positioning movements with spatiotemporal constraints

Unidirectional positioning movements with spatiotemporal constraints were examined as a test of impulse-timing theory (Schmidt, 1976; 1980; Wallace, 1981). Movements were examined at the kinematic, kinetic, and neuromuscular levels in three experiments. In the first experiment, displacement was held constant while five different movement times were examined. Both amplitudes and durations of the EMG and the kinetic variables were related to movement time. The results generally support the impulse-timing model. In the second experiment, movements were performed to a target at each of four distances in a constant movement time. EMG and force amplitudes and, unexpectedly, accelerative-force duration were modulated to achieve changes in displacement when movement time was constant. In the third experiment, movement time and displacement were simultaneously varied resulting in four conditions with equal average velocities. The results of this experiment were not as clear and exhibited individual differences. EMG duration did not always vary with changes in movement time. The results of all three experiments could not be adequately accounted for by the impulse-timing model.

Distributed Control in Rapid Sequential Aiming Responses

The preparation and on-line control of short, rapid sequential aiming responses were studied in 3 experiments. Participants (N = 12 in Experiments 1 and 2, and 20 in Experiment 3) produced 3-segment responses (a) within self-initiation, simple reaction time (RT), and choice RT paradigms (Experiment 1); (b) without visual feedback under self-initiation conditions (Experiment 2); and (c) with and without visual feedback under simple RT conditions (Experiment 3). In all conditions in which participants initiated movement in response to an external imperative signal, the 2nd response segment was performed consistently slower than preceding and succeeding response segments. That pattern of segmental movement times was found whether or not visual feedback was available but was not evident when participants self-initiated their responses with or without visual feedback. The findings rule out the possibility that subjects' use of visual feedback is responsible for the slowing of the 2nd response segment under RT conditions and suggest that the programming of rapid sequential aiming responses can be distributed in pre- and postinitiation intervals.

Response Timing Accuracy as a Function of Movement Velocity and Distance

In two experiments, patterns of response error during a timing accuracy task were investigated. In Experiment 1, these patterns were examined across a full range of movement velocities, which provided a test of the hypothesis that as movement velocity increases, constant error (CE) shifts from a negative to a positive response bias, with the zero CE point occurring at approximately 50% of maximum movement velocity (Hancock & Newell, 1985). Additionally, by examining variable error (VE), timing error variability patterns over a full range of movement velocities were established. Subjects (N = 6) performed a series of forearm flexion movements requiring 19 different movement velocities. Results corroborated previous observations that variability of timing error primarily decreased as movement velocity increased from 6 to 42% of maximum velocity. Additionally, CE data across the velocity spectrum did not support the proposed timing error function. In Experiment 2, the effect(s) of responding at 3 movement distances with 6 movement velocities on response timing error were investigated. VE was significantly lower for the 3 high-velocity movements than for the 3 low-velocity movements. Additionally, when MT was mathematically factored out, VE was less at the long movement distance than at the short distance. As in Experiment 1, CE was unaffected by distance or velocity effects and the predicted CE timing error function was not evident.

Distribution of programming in a rapid aimed sequential movement

Studies indicate that rapid sequential movements are preprogrammed and that preprogramming increases with complexity, but more complex sequences that require on-line programming have been seldom been studied. The purpose of this investigation was to determine whether on-line programming occurs in a 7-target sequence in which there is a unique target constraint and if so, to determine how different task constraints affect the distribution of additional programming. Subjects contacted seven targets with a hand-held stylus as quickly as possible while maintaining a 90% hit rate. Initiation-band execution-timing patterns and movement kinematics were measured to determine when the additional programming took place. Results indicated that additional programming occurred before initiation and during movement to the first target when the constraint required more spatial accuracy (small target). A different type of unique target (a triple hit of one target) caused the additional programming to occur on-line one or two segments before its execution. Different positions of the unique target also affected timing patterns. Results were discussed in terms of: (1) capacity of processing; (2) control of movement variance; and (3) mean velocity as a programmed parameter in sequential aiming movements.

Evidence Limiting the Subtended-Angle Hypothesis of Response-Programming Delays

In 1991 Sidaway proposed an alternative explanation to account for the traditional effect of response complexity on programming time. He suggested that rather than number of movement parts per se, it is the directional accuracy demand of a response, as quantified by the “subtended angle,” that constrains the movement initiation and produces programming delays. In Sidaway's experiments targets were positioned so that the first target was always closest to the starting position, with subsequent targets positioned further away from the start. This study tested the subtended-angle hypothesis by placing the first target furthest from the starting position, with subsequent targets struck by reversing direction and coming back toward the start. 30 subjects performed two aiming responses, a discrete 1-TAP condition and a serial 3-REVERSE condition. Because the subtended angle was identical in both conditions, there should be no differences in programming time. Our analyses, however, showed a significant effect for number of targets. Mean reaction time was longer for 3-REVERSE than for 1-TAP. These results argue against a strict constraint interpretation based on subtended angle and suggest that, under certain conditions, number of movement parts can still affect delays in response programming.

Evidence limiting the subtended-angle hypothesis of response-programming delays

In 1991 Sidaway proposed an alternative explanation to account for the traditional effect of response complexity on programming time. He suggested that rather than number of movement parts per se, it is the directional accuracy demand of a response, as quantified by the "subtended angle," that constrains the movement initiation and produces programming delays. In Sidaway's experiments targets were positioned so that the first target was always closest to the starting position, with subsequent targets positioned further away from the start. This study tested the subtended-angle hypothesis by placing the first target furthest from the starting position, with subsequent targets struck by reversing direction and coming back toward the start. 30 subjects performed two aiming responses, a discrete 1-TAP condition and a serial 3-REVERSE condition. Because the subtended angle was identical in both conditions, there should be no difference in programming time. Our analyses, however, showed a significant effect for number of targets. Mean reaction time was longer for 3-REVERSE than for 1-TAP. These results argue against a strict constraint interpretation based on subtended angle and suggest that, under certain conditions, number of movement parts can still affect delays in response programming.

Physiological differences between stutterers and nonstutterers in perceptually fluent speech: EMG amplitude and duration

Electromyograph (EMG) signals of the m. orbicularis oris inferior evoked by lip-rounding gestures were analyzed to see whether stutterers in their perceptually fluent speech had higher levels of EMG and longer EMG durations. The relationship between levels of EMG and durations of elevated muscle activity was investigated, and a search for the best discriminating EMG measure was made. In contrast to some previous studies on the EMG signals of stutterers, a relatively large group of stutterers (n = 15) and control speakers (n = 20), matched for age and gender, was examined. Both groups took part in a reaction time experiment using verbal items of different length (syllables, words, and sentences) in two time-stress conditions. Measures were taken for lip muscle activity during lip-rounding gestures for the Dutch /o/ sound. Only perceptually fluent trials were analyzed. The results showed that stutterers had significantly higher EMG levels at the moment of speech onset and during speech production than nonstutterers. A much larger difference between the two groups, however, was found for the EMG peak latency measure, which proved to be a very powerful distinctive feature in differentiating stutterers from nonstutterers. The results were discussed with respect to previous findings and recent theories about (speech) motor control strategies.

Changes in the motor pattern of learned and unlearned responses following cerebellar lesions: a kinematic analysis of the nictitating membrane reflex

Kinematic and dynamic analyses were employed to study the effects of cerebellar lesions on conditioned and unconditioned nictitating membrane responses in the rabbit. It was found that conditioned responses acquired to an auditory stimulus accelerated in two bursts as indicated by two distinct peaks of acceleration. The second peak of acceleration was very weak during the early portions of conditioning but became a prominent feature of the conditioned response over 16 sessions of conditioning. The second peak of acceleration in the conditioned response was more sensitive to cerebellar damage than was the first peak. When lesions of the cerebellum permanently reduced the amplitude of conditioned responses, but did not affect their frequency, the second peak of acceleration was nearly abolished while the first peak was unaffected. When cerebellar lesions profoundly impaired both the amplitude and frequency of conditioned responses, large and permanent impairments occurred in both peaks of acceleration. Lesions of the anterior interpositus nucleus most severely impaired both peaks of acceleration in the conditioned response and significantly reduced the acceleration of unconditioned responses across a wide range of intensities of corneal air puff. The deficit in the acceleration of unconditioned responses became manifest only after membrane extension exceeded 0.12 mm. The impairment in the amplitude of the unconditioned response after cerebellar lesions more closely approximated the impairment in the amplitude of the conditioned response when the force-generating properties of the conditioned and unconditioned stimuli were equated. It was hypothesized, therefore, that one reason why conditioned responses are so easily disrupted by cerebellar lesions is because they are of low force and not simply because they are learned. It was proposed that the two peaks of acceleration that characterize the conditioned response represent the function of two distinct anatomical systems. The first, a short-latency system, initiates the response and is most likely mediated by circuits that traverse the pontomedullary reticular formation. The second, a longer-latency system, amplifies response amplitude and its neural basis remains to be elucidated. The two components of the conditioned response may reflect two sequential bursts of activity in the accessory abducens nucleus, the principal site of the motoneurons for the retractor bulbi muscle, or may reflect the synergistic activity of the accessory abducens nucleus and the motor nuclei of the other extraocular muscles. It was concluded that the vulnerability of the second component of the conditioned response to cerebellar damage reflects an important role for the cerebellum in modulating the degree to which long-latency neural systems contribute to the ongoing performance of learned and unlearned behaviors.

Motor Programming as a Function of Constraints on Movement Initiation

Three experiments are reported that test the hypothesis that under certain conditions programming time is a function of the directional accuracy demand of a response, directional accuracy being quantified by the minimal angle subtended at the point of movement initiation by the circular targets within the response. Subjects in three simple reaction time experiments were required to tap a single target or a series of circular targets as rapidly as possible with a hand-held stylus. Experiments 1 and 3 showed that the subtended angle (SA) of a response can have a more powerful effect on programming time, as indexed by reaction time and premotor time, than the number of movement parts in the response. The results of Experiment 2 revealed that the locus of the directional accuracy effect was SA and not target size or movement distance. In all three experiments, response SA was a better predictor of programming time than was number of movement parts, target size, movement distance, movement time, and average movement velocity. The findings support the notion that constraints placed upon movement initiation by the directional accuracy demand of the task can play an important role in determining the length of the programming process.

Programming time in serial tapping responses as a function of pathway constraint

This study varied the accuracy demand within a linear series of targets to investigate the effect of movement-pathway constraints on response-programming time. Sidaway, Christina, and Shea (1988) have suggested that constraints placed upon movement initiation by the demand for response precision may play an important role in determining the length of the programming process. By varying the subtended angles of a series of three targets, this experiment tested the specific prediction of Sidaway et al. that programming time may be a function of the target, within a line of targets, that subtends the smallest angle at the start position. It is this target that demands the greatest precision in the movement pathway. Subjects participated in a series of conditions in which the size and placement of the target that imposed the maximal constraint was varied. In each condition the subjects were required to strike a series of three targets with a stylus in a simple reaction-time paradigm. Analysis of the reaction-time results revealed a significant effect of size of constraint, but no effect of position of constraint. Analysis of the movement-time data dispelled movement-duration and movement-velocity interpretations of the results and intimated a possible online trajectory-correction process.

Effects of Average Movement Velocity on Reaction Time and Spatiotemporal Accuracy in Single-Aiming and Rapid-Timing Movement Tasks

The effects of instructed movement speed were investigated in two experiments. First, rapid-timing and single-aiming movement tasks were compared. Unlike rapid timing, single aiming implies spatial accuracy. The aim of the first experiment was twofold: (a) to examine whether the requirement of accurate placement termination in single aiming affects the negative relationship between instructed average velocity and reaction time found in rapid timing, and (b) to test the speed-accuracy relationships predicted by the symmetric impulse variability model of these movement tasks. For this purpose, four average velocities (5, 24, 75, and 140 cm/s) were investigated in both types of movement tasks in a two-choice reaction task. The effects of average velocity on reaction time were similar in both single-aiming and rapid-timing tasks, and the predicted linear relationship between instructed average velocity and spatial accuracy was not found. The results suggest that the movement control mode, that is, open loop or closed loop, interferes with effects of instructed average velocity. The movement control mode explanation was confirmed in the second experiment with respect to the effect of paired velocities on reaction time. It is argued that the type of movement control mode must be considered in the interpretation of effects of instructed average velocity on reaction time and spatiotemporal measures.