Antiphase synchrony increases perceived entitativity and uniqueness: A joint hand-clapping task.

In- and antiphase are the dominant patterns identified in the study of synchrony in relative phases. Many previous studies have focused on in-phase synchrony and compared it to asynchrony, but antiphase synchrony has yet not been the subject of much research attention. The limited findings on antiphase synchrony suggest that its role or nature is unclear or unstable in human interaction. To account for this factor, this study examined the possibility that antiphase synchrony simultaneously induced perceived entitativity and uniqueness. The results of an experiment employing a joint hand-clapping task supported this prediction. Further, the elevated feeling of uniqueness in those who experienced antiphase synchrony may have increased the self-other overlap for those who felt oneness with their partner, but it decreased overlap for those who did not. The theoretical implications for synchrony literature are discussed.

Postural instability via a loss of intermittent control in elderly and patients with Parkinson's disease: A model-based and data-driven approach.

Postural instability is one of the major symptoms of Parkinson's disease. Here, we assimilated a model of intermittent delay feedback control during quiet standing into postural sway data from healthy young and elderly individuals as well as patients with Parkinson's disease to elucidate the possible mechanisms of instability. Specifically, we estimated the joint probability distribution of a set of parameters in the model using the Bayesian parameter inference such that the model with the inferred parameters can best-fit sway data for each individual. It was expected that the parameter values for three populations would distribute differently in the parameter space depending on their balance capability. Because the intermittent control model is parameterized by a parameter associated with the degree of intermittency in the control, it can represent not only the intermittent model but also the traditional continuous control model with no intermittency. We showed that the inferred parameter values for the three groups of individuals are classified into two major groups in the parameter space: one represents the intermittent control mostly for healthy people and patients with mild postural symptoms and the other the continuous control mostly for some elderly and patients with severe postural symptoms. The results of this study may be interpreted by postulating that increased postural instability in most Parkinson's patients and some elderly persons might be characterized as a dynamical disease.

Postural flexibility during quiet standing in healthy elderly and patients with Parkinson's disease.

Postural instability is one of the predominant symptoms of Parkinson's disease (PD). Despite its significant impact on the deterioration in quality of life in PD patients, mechanistic causes of the instability have not been clarified. Joint inflexibility at ankle and hip joints might be such a major cause, leading to small variability in the center of pressure (CoP) during quiet stance. However, this conjecture is still controversial. Thus, quantitative characterization of CoP patterns during quiet stance in PD patients remains a matter of research. Here we performed a linear discriminant analysis for CoP data in PD patients and age-matched healthy elderly during quiet stance, and showed that CoP variations in PD patients and those in healthy elderly could be well distinguished with an accuracy of about 90%, to which appropriately selected sway indices characterizing aspects of power spectrum for the CoP variations contributed. Specifically, major factors responsible for the discrimination were all associated with increase in the power at a high-frequency band (near and over 1 Hz) along with reduction at the low-frequency regime (lower than about 0.7 Hz). Then, the power-ratio, defined as the relative spectral power in a band around 1 Hz, was examined, since the power in this band reflects postural sway with anti-phase coordinated motions of the ankle and hip joints. We showed that the power-ratio values were significantly smaller in the PD patients than those in the healthy subjects. This difference as well as the results of the linear discriminant analysis suggest joint inflexibility in PD patients, particularly at hip joint, which diminished anti-phase coordination between trunk and lower extremity, leading to postural instability in PD patients.

A model of postural control in quiet standing: robust compensation of delay-induced instability using intermittent activation of feedback control.

The main purpose of this study is to compare two different feedback controllers for the stabilization of quiet standing in humans, taking into account that the intrinsic ankle stiffness is insufficient and that there is a large delay inducing instability in the feedback loop: 1) a standard linear, continuous-time PD controller and 2) an intermittent PD controller characterized by a switching function defined in the phase plane, with or without a dead zone around the nominal equilibrium state. The stability analysis of the first controller is carried out by using the standard tools of linear control systems, whereas the analysis of the intermittent controllers is based on the use of Poincaré maps defined in the phase plane. When the PD-control is off, the dynamics of the system is characterized by a saddle-like equilibrium, with a stable and an unstable manifold. The switching function of the intermittent controller is implemented in such a way that PD-control is 'off' when the state vector is near the stable manifold of the saddle and is 'on' otherwise. A theoretical analysis and a related simulation study show that the intermittent control model is much more robust than the standard model because the size of the region in the parameter space of the feedback control gains (P vs. D) that characterizes stable behavior is much larger in the latter case than in the former one. Moreover, the intermittent controller can use feedback parameters that are much smaller than the standard model. Typical sway patterns generated by the intermittent controller are the result of an alternation between slow motion along the stable manifold of the saddle, when the PD-control is off, and spiral motion away from the upright equilibrium determined by the activation of the PD-control with low feedback gains. Remarkably, overall dynamic stability can be achieved by combining in a smart way two unstable regimes: a saddle and an unstable spiral. The intermittent controller exploits the stabilizing effect of one part of the saddle, letting the system evolve by alone when it slides on or near the stable manifold; when the state vector enters the strongly unstable part of the saddle it switches on a mild feedback which is not supposed to impose a strict stable regime but rather to mitigate the impending fall. The presence of a dead zone in the intermittent controller does not alter the stability properties but improves the similarity with biological sway patterns. The two types of controllers are also compared in the frequency domain by considering the power spectral density (PSD) of the sway sequences generated by the models with additive noise. Different from the standard continuous model, whose PSD function is similar to an over-damped second order system without a resonance, the intermittent control model is capable to exhibit the two power law scaling regimes that are typical of physiological sway movements in humans.

A quantitative characterization of postural sway during human quiet standing using a thin pressure distribution measurement system.

The accuracy of the center of pressure (CoP) measurement during human quiet standing using an ultra-thin film pressure measurement system (Tekscan, I-Scan) was examined. To this end, CoP sway was measured simultaneously by I-Scan and a force platform (FP) to quantify differences in CoP obtained by the two methods. The sway amplitudes of I-Scan were slightly smaller than those of FP. The differences were systematic, allowing us to identify a filter that could bring every CoP trajectory of I-Scan close to that of FP. We concluded that, with the use of the filter, I-Scan could be used for accurate measurement of CoP sway during quiet standing.

Phase-dependent chronotropic response of the heart during running in humans.

Heartbeat modulation by muscle contraction during rhythmic exercise involving a small muscle mass is phase-dependent, reflecting the timing of the muscle contraction within the cardiac cycle, but it remains unclear whether such modulation occurs during whole body exercise. To determine whether phase-dependent chronotropic changes in the heart would occur during running, we investigated the relationship between R-R interval (RRI) and the timing of vastus lateralis muscle contractions within the cardiac cycle. Seven healthy subjects were examined during high intensity running where the target heart rate was 160 beats . min(-1). The running pitch was made to wax and wane periodically in the neighborhood of the target heart rate to scan the effect of footfall timing within the cardiac cycle on heart period. We found that when muscle contraction occurred early in the cardiac cycle, RRI was reduced from the mean RRI (P<0.05). Conversely, when muscle contraction occurred in the latter half of the cardiac cycle, RRI tended to increase (P>0.05). Thus, the curve reflecting this phase-dependent relationship between heart period and timing of muscle contraction showed a positive slope within the first one-quarter to three-quarters of the cardiac cycle. Our results suggest the existence of a mechanism that provides beat-by-beat regulation of RRI even when it is very short (approximately 375 ms), i.e., a cardio-locomotor synchronization develops during running, when the frequencies of the two rhythms approach one another.

Interferential current stimulation for sensory communication between prosthetic hand and man.

Our purpose is to clear whether the interferential current (IFC) method is useful for information transmission of a myoelectric prosthetic hand to the user. IFC stimulation is composed of two AC output waves. In previously, we demonstrated that IFC method using the two waves with different frequencies could vary the intensity of perceived stimulus. In the present study, we evaluated that the relationship between a movement of position of perceived stimulus and IFC method, using two waves with the same median frequency and a different phase. Moreover, we investigated the effects of a distance of recording electrodes from stimulating electrodes on a degree of interference of IFC with EMG signals. The movement of position of perceived stimulus was caused by the phase difference changing. IFC interfered with EMG signals. Especially, the interference of IFC with EMG of the muscle under stimulating electrodes was large. In contrast, when recording electrodes was distant from the stimulating electrodes, the interference of IFC was small. Moreover, the interference was eliminated by the low-pass filter (< 500Hz). These results support the usefulness of a combination of the IFC method with EMG recording system equipped with the low-pass filter as a sensory feedback system for myoelectric prosthetic hand.

Comparison of cardio-locomotor synchronization during running and cycling.

By comparing the characteristics of cardiac-locomotor synchronization (CLS) in running and cycling individuals, we tested whether the characteristics of CLS occurring during rhythmic exercise adhere to the central origin hypothesis, which postulates a direct interaction between cardiovascular centers in the brain and the pattern generator in the spinal cord. Ten healthy subjects performed both exercises at the same intensity (150 beats.min(-1)) and cadence (150 steps.min(-1) during running and 75 rpm during cycling), while electrocardiograms and electromyograms from the right vastus lateralis muscle were monitored continuously. An examination of the occurrence of heart beats with respect to the locomotor phase revealed that, in running subjects, CLS exists for relatively prolonged periods at specific phases, whereas, in cycling subjects, it occurs intermittently and is not phase-specific [maximum duration of CLS: 113.6 (66.5) and 58.0 (29.3) s ( P<0.05), respectively]. Determining the probability of CLS by chance as a function of its duration, we also found that, during running, CLS likely results from entrainment, whereas, during cycling, it results from chance, occurring when the cardiac rhythm approached the locomotor rhythm. Our result indicated that the duration of muscle contraction during cycling [317.0 (18.1) ms] was significantly longer than during running [205.6 (20.2) ms]. These results indicated that the difference in the CLS characteristics between running and cycling might be influenced by differences in peripheral inputs between exercise modes.