Local stability in coordinated rhythmic movements: fluctuations and relaxation times

An experiment was conducted to examine the stability of the anti-phase and in-phase modes of coordination by means of both fluctuations and relaxation times. Participants (n=6) performed a rhythmic bimanual forearm coordination task that required them to oscillate their forearms in-phase and anti-phase while grasping two manipulanda at fixed frequencies ranging from 0.6 to 1.8 Hz. Relaxation times were measured as the time taken to return to a stable mode following the application of a transient mechanical torque. It was found that relaxation times were not different statistically across participants, frequencies, and coordinative modes. However, fluctuations, as indicated by the mean S.D. of relative phase across individual frequency plateaus, were significantly greater in the anti-phase than in the in-phase mode of coordination, p<0.05. Whilst providing new empirical support for the notion that relaxation times should be of the same order of magnitude at frequencies outside transition regions, the findings suggest that the level of stochastic noise in the anti-phase mode is greater than that of the in-phase mode. Implications are made for the future assessment of local pattern stability.     

Attentional load associated with performing and stabilizing a between-persons coordination of rhythmic limb movements

This study addressed the issue of intentional stabilization of between-persons coordination patterns (in-phase/isodirectional and anti-phase/non-isodirectional) and the attentional cost incurred by the nervous system in maintaining and further stabilizing these coordination patterns. Five pairs of participants performed in-phase and anti-phase interpersonal coordination patterns in dual-task conditions (coordination+RT task). Results showed that: (1) isodirectional pattern (in-phase) was more stable than non-isodirectional pattern (anti-phase), (2) both iso- and non-isodirectional pattern were stabilized intentionally, (3) RT was lower for the isodirectional pattern (i.e., the most stable), and (4) attentional manipulation led to a trade-off between pattern stability and RT performance. These results suggest that performing between-persons coordination patterns incurs a central cost that depends on the coupling strength between the limbs. These findings are consistent with the previous studies in intrapersonal coordination.     

Fluctuations and phase symmetry in coordinated rhythmic movements

Pendular, clocking movements typify mammalian terrestrial locomotion. They can be investigated with a procedure in which people swing hand-held pendulums at the wrists, comfortably and rhythmically. Pendular, clocking behavior was examined for in-phase and out-of-phase coordinations. The periodic timing and powering of rhythmic movements in the comfort state follow from different laws (Kugler & Turvey, 1986). One law guides the assembling of the reference frame for "clocking." Another law guides the assembling of the muscular, escapement processes determining the cycle energy. Wing and Kristofferson's (1973) method for parsing periodic-timing variance into independent "clock" and "motor" sources was applied. Mean periodicity was unaffected by phase. "Clock" fluctuations, however, were larger out of phase than in phase. "Motor" fluctuations were indifferent to phase but reflected the departures of individual wrist-pendulum systems from their preferred periods. It appears that an intended phase relation is realized as a constraint on "clock" states. These states are more stable under the in-phase constraint than under the out-of-phase constraint.     

The relationship between reduplicated babble onset and laterality biases in infant rhythmic arm movements

This study examined changes in rhythmic arm shaking and laterality biases in infants observed longitudinally at three points: just prior to, at, and just following reduplicated babble onset. Infants (ranging in age from 4 to 9 months at babble onset) were videotaped at home as they played with two visually identical audible and silent rattles presented at midline for 1.5 min each. Rate of rattle shaking increased sharply from the pre-babble to the babble onset session; but there was no indication that this increase was specific to the right arm. This finding suggests that the link between babble onset and increased rhythmic arm activity may not be the product of language-specific mechanisms, but is rather part of a broader developmental process that is also perceptual and motor.     

Environmental coupling modulates the attractors of rhythmic coordination

A simple instance of coupling behavior to the environment is oscillating the hands in pace with metronome beats. This environmental coupling can be weaker (1 beat per cycle) or stronger (2 beats per cycle). The authors examined whether strength of environmental coupling enhanced the stability of in-phase bimanual coordination. Detuning by manipulanda that produced different left and right eigenfrequencies shifted the relative phase angle from 0 degrees, with the size of the shift larger for higher movement frequencies. Stronger environmental coupling was found to decrease this relative-phase shift, with accompanying increase and reduction, respectively, in recurrence quantification measures related to coordination stability and coordination noise. Stronger environmental coupling also increased oscillation amplitude. Results are considered from the perspective of parametric stabilization.     

Deterministic variability and stability in detuned bimanual rhythmic coordination

We examined the effects of crossing different degrees of cooperation and competition on inphase and antiphase 1:1 frequency locked coordination of left- and right-hand-oscillated pendulums. Degree of cooperation was manipulated through the joint frequency of oscillation specified by a metronome (the higher the frequency, the weaker the cooperation), and degree of competition by length (and, therefore, preferred frequency) differences between the two pendulums (the greater the difference, the stronger the competition). Increasing competition was accompanied by either decreasing cooperation (for six participants) or increasing cooperation (for six different participants). On each trial, a participant attempted to produce a steady-state phase relation phi for a given combination of competition and cooperation. Numerical simulations of the extended Haken-Kelso-Bunz (HKB) equation were used to predict (a) the patterns of shift in phi from either 0 or pi radians due to the different competition-cooperation relations and (b) the patterns of variability in phi. It was expected that the HKB equation would be successful in respect to (a), which it was, but not in respect to (b). The observed failure to confirm (b) was expected from the variability due to the different nonharmonic dynamics of the component oscillators, a source of variability not included in the HKB equation. The experimental results together with simulations and analyses of the phase-plane trajectories of the component oscillators suggest the operation of deterministic in addition to stochastic variability in the phase relation of contralateral limbs.     

On the coordination of two-handed movements

In a set of three experiments, we show that after an auditory "go" signal, subjects simultaneously initiate and terminate two-handed movements to targets of widely disparate difficulty. This is the case when the movements required are (a) lateral and away from the midline of the body (Experiment 1), (b) toward the midline of the body (Experiment 2), and (c) in the forward direction away from the body midline (Experiment 3). Kinematic data obtained from high-speed cinematography (200 frames/sec) point to a tight coordinative coupling between the two hands. Although the hands move at entirely different speeds to different points in space, times to peak velocity and acceleration are almost perfectly synchronous. We believe that the brain produces simultaneity of action as the optimal solution for the two-handed task by organizing functional groupings of muscles (coordinative structures) that are constrained to act as a single unit.     

Cognitive activity shifts the attractors of bimanual rhythmic coordination

The attractors of bimanual rhythmic coordination are given as the solutions of a motion equation in relative phase. How are those attractors affected by cognitive activity? In 3 experiments, participants (N = 10 in Experiments 1 and 2; N = 5 in Experiment 3) were required to produce in-phase or antiphase coordination while they either did or did not perform an information-reduction task. The average absolute deviation from in-phase (0 degrees ) and antiphase (180 degrees ) satisfying a particular parameterization of the motion equation was amplified by cognitive activity. That amplification of absolute phase shift was the same for both in-phase and antiphase coordination. Furthermore, the amplification (in degrees) increased linearly with the magnitude of cognitive activity (in bits). Cognitive activity had limited influence on the variability of relative phase and did not affect its average signed deviation. Collectively, the results suggest that cognitive activity produces a shift in the attractors of bimanual coordination dynamics that is directionally nonspecific and is independent of movement speed, detuning, and the in-phase-antiphase distinction.     

Effects of velocity and limb loading on the coordination between limb movements during walking

The authors investigated the effects of velocity (increasing from 0.5 to 5.0 km/hr in steps of 0.5 km/hr) and limb loading on the coordination between arm and leg movements during treadmill walking in 7 participants. Both the consistency of the individual limb movements and the stability of their coordination increased with increasing velocity; the frequency coordination between arm and leg movements was 2:1 at the lower velocities and 1:1 at the higher velocities. The mass manipulation affected the individual limb movements but not their coordination, indicating that a stable walking pattern was preserved. The results differed qualitatively from those obtained in studies on bimanual interlimb coordination, implying that the dynamical principles identified therein are not readily applicable to locomotion.     

Eye movements and the integration of visual memory and visual perception

Because visual perception has temporal extent, temporally discontinuous input must be linked in memory. Recent research has suggested that this may be accomplished by integrating the active contents of visual short-term memory (VSTM) with subsequently perceived information. In the present experiments, we explored the relationship between VSTM consolidation and maintenance and eye movements, in order to discover how attention selects the information that is to be integrated. Specifically, we addressed whether stimuli needed to be overtly attended in order to be included in the memory representation or whether covert attention was sufficient. Results demonstrated that in static displays in which the to-be-integrated information was presented in the same spatial location, VSTM consolidation proceeded independently of the eyes, since subjects made few eye movements. In dynamic displays, however, in which the to-be-integrated information was presented in different spatial locations, eye movements were directly related to task performance. We conclude that these differences are related to different encoding strategies. In the static display case, VSTM was maintained in the same spatial location as that in which it was generated. This could apparently be accomplished with covert deployments of attention. In the dynamic case, however, VSTM was generated in a location that did not overlap with one of the to-be-integrated percepts. In order to "move" the memory trace, overt shifts of attention were required.     

Extracting global and local dynamics from the stochastics of rhythmic forearm movements

The authors examined the dynamics governing rhythmic forearm movements that 9 participants performed under a variety of task constraints by using a generic, unbiased analysis technique for extracting the drift coefficients of Fokker-Planck equations from stochastic data. From those coefficients, they reconstructed and analyzed vector fields and phase portraits to identify characteristic, task-dependent kinematic and dynamical features. They first directly estimated the parameters of weakly nonlinear self-sustaining oscillators from the extracted drift coefficients. The estimated parameters that the authors had selected instinctively and then particularized by using averaging methods largely confirmed previously derived limit-cycle models. Next, they ventured beyond limit-cycle models to examine global and local dynamical features that those models cannot adequately address, particularly task-dependent changes in flow strength and curvature and distinct dynamical features associated with flexion and extension. The authors argue that those features should be focal points of researchers' future modeling efforts to formulate a more adequate and encompassing account of the dynamics of rhythmic movement.     

Plane of motion mediates the coalition of constraints in rhythmic bimanual coordination

The authors hypothesized that the modulation of coordinative stability and accuracy caused by the coalition of egocentric (neuromuscular) and allocentric (directional) constraints varies depending on the plane of motion in which coordination patterns are performed. Participants (N = 7) produced rhythmic bimanual movements of the hands in the sagittal plane (i.e., up-and-down oscillations resulting from flexion-extension of their wrists). The timing of activation of muscle groups, direction of movements, visual feedback, and across-trial movement frequency were manipulated. Results showed that both the egocentric and the allocentric constraints modulated pattern stability and accuracy. However, the allocentric constraint played a dominant role over the egocentric. The removal of vision only slightly destabilized movements, regardless of the effects of directional and (neuro)muscular constraints. The results of the present study hint at considering the plane in which coordination is performed as a mediator of the coalition of egocentric and allocentric constraints that modulates coordinative stability of rhythmic bimanual coordination.     

Interaction between eye and body movements to perform visual tasks in upright stance

Synergistic interactions between visual and postural behaviors were observed in a previous study during a precise visual task (search for a specific target in a picture) performed upright as steady as possible. The goal of the present study was to confirm and extend these novel findings in a more ecological condition with no steadiness requirement. Twelve healthy young adults performed two visual tasks, i.e. a precise task and a control task (free-viewing). Center of pressure, lower back, neck, head and eye movements were recorded during each task. The subjective cognitive workload was assessed after each task (NASA-TLX questionnaire). Pearson correlations and cross-correlations between eyes (time-series, characteristics of fixation) and center of pressure/body movements were used to test the synergistic model. As expected, significant negative Pearson correlations between eye and head-neck movement variables were only observed in searching. They indicated that larger precise gaze shifts were correlated with lower head and neck movements. One cross-correlation coefficient (between COP on the AP axis and eyes in the up/down direction) was also significantly higher, i.e. stronger, in searching than in free-viewing. These synergistic interactions likely required greater cognitive demand as indicated by the greater NASA-TLX score in searching. Moreover, the previous Pearson correlations were no longer significant after controlling for the NASA-TLX global score (thanks to partial correlations). This study provides new evidence of the existence of a synergistic process between visual and postural behaviors during visual search tasks.     

Discourse-mediation of the mapping between language and the visual world: Eye movements and mental representation

Two experiments explored the mapping between language and mental representations of visual scenes. In both experiments, participants viewed, for example, a scene depicting a woman, a wine glass and bottle on the floor, an empty table, and various other objects. In Experiment 1, participants concurrently heard either ‘The woman will put the glass on the table’ or ‘The woman is too lazy to put the glass on the table’. Subsequently, with the scene unchanged, participants heard that the woman ‘will pick up the bottle, and pour the wine carefully into the glass.’ Experiment 2 was identical except that the scene was removed before the onset of the spoken language. In both cases, eye movements after ‘pour’ (anticipating the glass) and at ‘glass’ reflected the language-determined position of the glass, as either on the floor, or moved onto the table, even though the concurrent (Experiment 1) or prior (Experiment 2) scene showed the glass in its unmoved position on the floor. Language-mediated eye movements thus reflect the real-time mapping of language onto dynamically updateable event-based representations of concurrently or previously seen objects (and their locations).     

Attentional strategies and the visual control of discrete movements

The relationship between the availability of visual information and attention demands during the production of a discrete motor act was examined. Subjects were required to move a linear slide a distance of 15 cm in both 150 and 600 msec conditions under three light manipulations, viz., light always on; light always terminated as movement began; or, subject was unsure as to whether light would stay on or terminate. A simple key press by the index finger of the opposite hand to a tone was used as a secondary task and a measure of attention demands. The light manipulation influenced attention demands on the rapid 150 msec movement such that more attention was demanded when the subject knew the light would terminate. No such attentional strategy differences were found with the slow 600 msec movement. These findings suggest that task constraints in the form of kinematic criteria, together with the perceived availability of visual information, contribute to determining attentional strategy in movement production.     

Synergistic influences of sensory and central stimuli on non-voluntary rhythmic arm movements

In recent years, neuromodulation of the cervical spinal circuitry has become an area of interest for investigating rhythmogenesis of the human spinal cord and interaction between cervical and lumbosacral circuitries, given the involvement of rhythmic arm muscle activity in many locomotor tasks. We have previously shown that arm muscle vibrostimulation can elicit non-voluntary upper limb oscillations in unloading body conditions. Here we investigated the excitability of the cervical spinal circuitry by applying different peripheral and central stimuli in healthy humans. The rationale for applying combined stimuli is that the efficiency of only one stimulus is generally limited. We found that low-intensity electrical stimulation of the superficial arm median nerve can evoke rhythmic arm movements. Furthermore, the movements were enhanced by additional peripheral stimuli (e.g., arm muscle vibration, head turns or passive rhythmic leg movements). Finally, low-frequency transcranial magnetic stimulation of the motor cortex significantly facilitated rhythmogenesis. The findings are discussed in the general framework of a brain-spinal interface for developing adaptive central pattern generator-modulating therapies.