Cancelling discrete and stopping ongoing rhythmic movements: Do they involve the same process of motor inhibition?

Motor inhibition is considered to be an important process of executive control and to be implicated in numerous activities in order to cancel prepared actions and, supposedly, to suppress ongoing ones. Usually, it is evaluated using a "stop-signal task" in which participants have to inhibit prepared discrete movements. However, it is unknown whether other movement types involve the same inhibition process. We therefore investigated whether the inhibition process for discrete movements is involved in stopping ongoing rhythmic movements as well. Twenty healthy adults performed two counterbalanced tasks. The first task was used to estimate the stop-signal reaction time (SSRTd) needed to inhibit prepared discrete key-pressing movements. In the second task, participants drew graphic patterns on a tablet and had to stop the movement when a stop-signal occurred. We calculated the rhythmic stop signal-reaction time as the time needed to initiate stopping such ongoing rhythmic movement (SSRTr) and the same latency relative to the period of the rhythmic movement (relSSRTr). We measured these delays under different movement frequencies and motor coordination conditions and further investigated whether they varied as a function of several parameters of the rhythmic movements (speed, mean and variance of the relative phase, and movement phase at several time events). We found no correlation between inhibition measures in the two tasks. In contrast, generalized linear models showed a moderate yet significant influence of the motion parameters on the inhibition of ongoing rhythmic movements. We therefore conclude that the motor inhibition processes involved in cancelling prepared discrete movements and stopping ongoing rhythmic movements are dissimilar.

Keeping your eye on the target: eye-hand coordination in a repetitive Fitts' task.

In a cyclical Fitts' task, hand movements transition from continuous to discrete movements when the Index of Difficulty (ID) increases. Moreover, at high ID (small target), the eyes saccade to and subsequently fixate the targets at every movement, while at low ID (large target) intermittent monitoring is used. By hypothesis, the (periodic) gaze shifts are abandoned for movement times shorter than about 0.350 s due to systemic constraints (i.e., a refractory period and intrinsic latency). If so, the transition in eye and hand movements is independent. To investigate these issues, the present study examined the effects of changing ID via the targets' width or distance as well as hysteresis in eye-hand coordination. To this aim, 14 participants performed a cyclical Fitts' task while their hand and eye movements were recorded simultaneously. The results show that the transition in eye-hand synchronization (at 2.87 bit; 0.25 s) and in hand dynamics (at 4.85 bit; 0.81 s) neither co-occurred nor correlated. Some small width vs. distance dissociations and hysteresis effects were found, but they disappeared when eye-hand synchronization was viewed as a function of movement time rather than ID. This confirms that a minimal between-saccade time is the limiting factor in eye-hand synchronization. Additionally, the timing between the start of the hand movement and the saccade appeared to be relatively constant (at 0.15 s) and independent of movement time, implying a constant delay that should be implemented in a dynamical model of eye-hand coordination.

Effect of table top slope and height on body posture and muscular activity pattern.

The objective of this study was to assess the effect of table top slope and height on body posture and muscular activity pattern. Twelve asymptomatic participants performed a 5-min reading task while sitting, in six experimental conditions manipulating the table top slope (20° backward slope, no slope) and its height (low, medium, up). EMGs recordings were taken on 9 superficial muscles located at the trunk and shoulder level, and the angular positions of the head, trunk and pelvis were assessed using an inertial orientation system. Results revealed that the sloping table top was associated with a higher activity of deltoideus pars clavicularis (P<0.05) and a smaller flexion angle of the head (P<0.05). A tentative conclusion is that a sloping table top induces a more erect posture of the head and the neck, but entails an overload of the shoulder, which might be harmful on the long run.

Covariation of attentional cost and stability provides further evidence for two routes to learning new coordination patterns.

This study investigated how learning a new bimanual coordination pattern affects the attentional resources allotted by the CNS to maintain it throughout the acquisition process. The repertoire of the existing stable coordination patterns was individually evaluated before and after practice in order to detect expected changes with learning. Bistable participants, who initially exhibited stable and accurate coordination patterns at 0 degrees and 180 degrees of relative phase, practiced a 90 degrees pattern, whereas multistable participants, who already mastered the 90 degrees pattern, practiced 135 degrees pattern instead. In a typical dual-task paradigm, all participants had to simultaneously perform a reaction time task that assessed the associated attentional cost. Beyond an overall increase in accuracy, the results revealed a significant decrease in the attentional cost for bistable participants, accompanying the stabilization of the 90 degrees pattern with learning, but not for multistable participants, as the 135 degrees pattern barely stabilized. Pattern stability and attentional cost co-evolve during learning and the process follows two different routes depending on the interplay between the task and the learner's coordination abilities before practice.

Dynamics of coordination in cross-country skiing.

The aim of the present study was to identify modes of coordination in cross-country skiing from a dynamical systems perspective. Participants (N=8) skied on a treadmill using classical techniques with varying steepness (i.e., 0 degrees-7 degrees). Coordination was evaluated in terms of the relative frequency and relative phase between upper arms and thighs. Results revealed that the limb movements were systematically attracted towards low integer frequency ratios (i.e., 1:1 and 2:1) and in-phase (phi approximately 0 degrees ) and anti-phase relationships (phi approximately 180 degrees). The increase in steepness produced shifts between the attractive modes of limb movements and a loss of stability was observed during transitions. These results suggest that principles of coordination between limbs in cross-country skiing are akin to those of non-linear coupled oscillators, as documented for a broad range of motor activities. Yet, differences with such classical findings are discussed reflecting the specific biomechanical constraints of cross-country skiing.

Movement-related sensory feedback mediates the learning of a new bimanual relative phase pattern.

On the basis of findings emphasizing the role of perceptual consequences in movement coordination, the authors tested the hypothesis that the learning of a new bimanual relative phase pattern would involve the matching of the movement-related sensory consequences (rather than the motor outflow commands) to the to-be-learned pattern. Two groups of participants (n = 10 in each) practiced rhythmically moving their forearms with a phase difference of 30 degrees . In 1 group, a difference in the arms' eigenfrequencies was imposed such that synchronous generation of the left and right motor commands resulted in the required relative phase (30 degrees ), yielding incongruence between the motor commands and their sensory consequences. In the other group, the experimenter imposed no eigenfrequency difference so that the sensory consequences were congruent with the motor commands. Throughout the practice period, performance of both groups was assessed repeatedly for the congruent situation (i.e., no eigenfrequency difference). On those criterion tests, both groups performed the required pattern equally well. The authors discuss that result, which corroborated the hypothesis, from a dynamical systems perspective.

Attentional demands reflect learning-induced alterations of bimanual coordination dynamics.

This study aimed to investigate the effects of practice on bimanual coordination dynamics and attentional demands. Participants were asked to perform a dual-task associating a cyclic antiphase bimanual pattern and a discrete reaction time task. A pretest determined each individual critical transition frequency. In the training session, participants practised 120 trials. They were instructed to maintain the antiphase coordination pattern at the critical transition frequency. The training session was interrupted and followed by an intermediate test (after 60 trials) and a post-test (30 min after 120 trials), respectively. A retention test was performed 7 days after the end of the training session. Results showed that: (i) the number of transitions decreased as a consequence of practice; and (ii), subjects were able to maintain the antiphase pattern at a higher frequency than in the pretest. Analysis of the trade-off between relative phase variability and reaction time showed that participants were able to maintain a higher level of stability at the same (intermediate and post-test) or a lower attentional cost (retention test). These findings show that phase transition dynamics and pattern stability can be significantly modified as a result of practice. Changes in the trade-off between pattern stability and cost with learning confirm that the attentional cost incurred by the central nervous system to maintain pattern stability decreased with practice. In line with recent neurobiological studies, the present study provides new insights regarding relationships between brain processes, attentional demands and coordinated behaviour in learning bimanual patterns.

Memory dynamics: distance between the new task and existing behavioural patterns affects learning and interference in bimanual coordination in humans.

In order to study memory and learning dynamics, we examined the effects of the distance between a bimanual pattern to be learned, a relative phase of either 90 degrees, 135 degrees, or 158 degrees, and pre-existing patterns at 0 degrees and 180 degrees. Learning was assessed through a practice task, memory through a synchronization-continuation task, and interference through a prompting task recalling nearby patterns. Results were that (1) interference induced a smaller decrease in accuracy and in stability for the pattern closest to the pre-existing patterns; (2) the withdrawal of the stimulus in the continuation task led to consistent changes in accuracy; and (3) learning rate was faster when the to-be-learned pattern was either far from or close to extant patterns. The findings are discussed in terms of flexibility and robustness of underlying coordination dynamics, and of neural-field dynamic models.

Coordination dynamics of learning and transfer across different effector systems.

If different effector systems share a common task-specific coordination dynamics, transfer and generalization of sensorimotor learning are predicted. Subjects learned a visually specified phase relationship with either the arms or the legs. Coordination tendencies in both effector systems were evaluated before and after practice to detect attractive states of the coordination dynamics. Results indicated that learning a novel relative phase with a single effector system spontaneously transferred to the other, untrained effector system. Transfer was revealed not only as improvements in performance but also as modifications of each system's initial (prelearning) coordinative landscape. What is learned, appears to be a high-level but neurally instantiated dynamic representation of skilled behavior that proves to be largely effector independent, at least across anatomically symmetric limbs.

Shared dynamics of attentional cost and pattern stability.

This paper examines the informational activity devoted by the CNS to couple oscillating limbs in order to sustain and stabilize bimanual coordination patterns. Through a double-task paradigm associating a bimanual coordination task and a reaction time (RT) task, we investigated the relation between the stability of preferred bimanual coordination patterns and the central cost expended by the CNS for their stabilization. Ten participants performed in-phase and anti-phase coordination patterns in a dual task condition (coordination + RT) at several frequencies (0.5, 0.75, 1.0, 1.5, and 2.0 Hz), thereby decreasing the stability of the bimanual patterns. Results showed a U-shaped evolution of pattern stability and attentional cost, as a function of oscillation frequency, exhibiting a minimum value at the same frequency. These findings indicate that central cost and pattern stability covary and may share common, high order dynamics. Moreover, the attentional focus given to the bimanual coordination and the RT task was also manipulated by requiring either shared attention or priority to the coordination task. Such a manipulation led to a tradeoff between pattern stability and RT performance: The more stable the pattern, the more costly it is to stabilize. This suggests that stabilizing a coordination pattern incurs a central cost that depends on its intrinsic stability. Conceptual consequences of these results for understanding the relationship between attention and coordination are drawn, and the mechanisms putatively at work in dual tasks are discussed.

A dynamical framework to understand performance trade-offs and interference in dual tasks.

This study demonstrated that the dynamic pattern approach may reconcile resource and outcome conflict theories to explain performance in dual tasks. Participants performed a bimanual coordination task and a reaction time task with different conditions of attentional priority. Results showed a trade-off between pattern variability and reaction time when priority was given to the coordination task. Such a trade-off was indicative of resource allocation. An analysis of perturbation in the bimanual coordination revealed interference, a reputed sign of outcome conflict. Moreover, interference diminished substantially when priority was given to the bimanual task. The coexistence of performance trade-off and outcome conflict suggests that these two phenomena are not mutually exclusive. Rather, both may follow from modifying the coupling between the limbs through attention.

Effects of attention on phase transitions between bimanual coordination patterns: a behavioral and cost analysis in humans.

The present study aimed to obtain a behavioral analysis of the effects of attentional focus on the dynamics of phase transitions in bimanual coordination and to evaluate the central cost expended by the central nervous system to maintain and stabilize such coordination patterns before and after the transition. Eight subjects were asked to execute an anti-phase coordination pattern (180 degrees of relative phase), while gradually increasing the frequency of oscillation. The central cost was assessed using a dual-task paradigm associating the bimanual coordination task with a reaction time task. Results showed that: (1) the transition process was significantly altered by focusing attention on the bimanual coordination task; and (2) the cost involved in sustaining the bimanual patterns was determined by their coordination dynamics.

Coordination dynamics of learning and transfer: collective and component levels.

The dynamics of learning a new coordinated behavior was examined by requiring participants to perform a visually specified phase relationship between the hands. Results showed that learning may involve qualitative or quantitative alterations in the layout of the coordination dynamics depending on whether such dynamics are bistable or multistable before exposure to the learning task. In both cases, the process stabilized the to-be-learned behavior and its symmetry partner, even though the latter had not actually been practiced. Kinematic analyses of hand motion showed that previously existing coordination tendencies were exploited during learning in order to match visual requirements. These findings and the concepts presented here provide a framework for understanding how learning occurs in the context of previous experience and allow individual differences in learning to be tackled explicitly.

Evolution of behavioral attractors with learning: nonequilibrium phase transitions.

Learning a bimanual coordination task (synchronization to a visually specified phasing relation) was studied as a dynamical process over 5 days of practicing a required phasing pattern. Systematic probes of the attractor layout of the 5 Ss' coordination dynamics (expressed through a collective variable, relative phase) were conducted before, during, and after practice. Depending on the relationship between the initial coordination dynamics (so-called intrinsic dynamics) and the pattern to be learned (termed behavioral information, which acts as an attractor of the coordination dynamics toward the required phasing), qualitative changes in the phase diagram occurred with learning, accompanied by quantitative evidence for loss of stability (phase transitions). Such effects persisted beyond 1 week. The nature of change due to learning (e.g., abrupt vs. gradual) is shown to arise from the cooperative or competitive interplay between behavioral information and the intrinsic dynamics.

Learning as change of coordination dynamics: theory and experiment.

Learning of coordination patterns was investigated theoretically from the point of view of a dynamic theory of biological coordination and with reference to recent experiments on the learning of relative timing patterns. The theory is based on theoretical and experimental work showing that coordinated movement is characterized not only by the actually performed pattern of coordination but by an entire dynamics of coordination. Theoretically, such dynamics are captured as equations of motion of relevant collective variables. Experimentally, signatures of these underlying dynamics can be found in the temporal stability of coordination patterns, which can be assessed through various stability measures as well as through processes of pattern change. We argue that not only intrinsic coordination tendencies, but also specific behavioral requirements, be they perceived, memorized, or intended, must be expressed in terms of such dynamics. The concept of behavioral information captures such requirements as part of the coordination dynamics. We expound two hypotheses on the nature of learning in this framework. First, we assume that at each point during the learning process the system is governed by a well-defined coordination dynamics. This equation of motion evolves with learning so as to acquire an attractor solution near the to-be-learned pattern. Second, we hypothesize that this change of the coordination dynamics, captured by the time course of memorized behavioral information, can itself be ascribed to an additional layer of dynamics, the slower learning dynamics. Testable consequences of these views are discussed in the light of recent experimental findings on the learning of a relative phase in rhythmic movement: (a) Learning affects dynamic properties of performed coordination patterns, in particular, their stability; (b) the change of the coordination dynamics due to learning leads to specific changes of behavior also under conditions other than the learned condition, namely, to systematic deviation toward the learned patterns; (c) learning may lead to instabilities in the coordination behavior if initial and learned performance differ sufficiently; and (d) the dynamic properties of the performed coordination patterns are distinct on the two time scales of learning and of performance.

Tracking with and without target in 6- to 15-year-old boys.

It is well documented that, in adults, manual tracking of a visual periodic target is very accurate in a wide variety of experimental conditions. With children between 5 and 9 years of age, however, the response lags significantly behind the stimulus. The first study presented here attempts to described the acquisition of this skill by children between 9 and 15 years of age. Within this age span, adult proficiency in pursuing a rapid target was approached through an improvement in response synchronization. Yet adults were distinguishable from the oldest children by a fundamentally different mode of movement execution: The former maintained a smooth modulation of the proper motor pattern, whereas the latter relied mainly on corrections through visual feedback. The second experiment showed that these different perceptuomotor strategies may be related to the availability of a more accurate and stable motor pattern with age. Young children had difficulties reproducing the stimulus after it was withdrawn, with performance deteriorating as the trial progressed. The initial mismatch and the following drift tended to decrease with age, even though the oldest children's stationary performance was still not as consistent with the target motion as the adult response.

Spontaneous covariations of movement parameters in 5- to 7-year-old boys.

The kinematic parameters of to and fro forearm movements in the horizontal plane were investigated in 5- to 7-year-old boys. In one condition, the period of the movements was induced by an acoustic stimulus, while the amplitude was left free. In a second condition, the amplitude of the swing was suggested by a visual frame, but the movement was self-paced. In both conditions, the unconstrained variable (amplitude and period, respectively) was found to covary spontaneously with the constrained variable. The results suggest that, within the age range considered here, motor control processes do not conceive of amplitude and velocity as two independent variables.