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We investigated the effect of movement rate on the time-series properties of human synchronization errors by applying power spectrum analysis and detrended fluctuation analysis (DFA). Participants were required to execute a finger-tapping task with their right index finger in synchrony with periodic sounds under seven conditions of movement rates ranging from 1 to 4 Hz and separated by 0.5 Hz. At slow movement rates (1 and 1.5 Hz), the power spectrum of the synchronization errors flattened in the low-frequency portion, resulting in short-range correlation. At fast movement rates (from 2 to 4 Hz), on the other hand, the time series of the timing errors exhibited a 1/fbeta-type long-range correlation. These results indicate that the movement rate has an effect on the temporal coordination pattern in the human timing control mechanism. We inferred that this change in the coordination pattern reflects the transition between different states in human synchronization movements with an external signal.

We investigated the difference between brain activities in speeded and precisely timed responses to identical visual stimulus using fMRI. Stimulus used was a row of seven light-emitting diodes (LEDs) lightened up one after another with constant speed within a trial but with various speeds between trials. Subjects were asked to execute finger-thumb tapping with the right hand in response to the onset of the first LED light in the reaction time (RT) task and in anticipation of the onset of the last (i.e., seventh) LED light in the timing task. In control condition, they were asked to passively view the stimulus without motor response. Results showed that various movement-related areas including contralateral cingulate motor cortex were commonly activated for both tasks relative to the control condition, suggesting these structures are involved in general perception and response execution rather than specific function for speeded or precisely timed responses. In the RT task, the presupplementary motor area extending to the cingulate sulcus was activated more strongly than in the timing task probably to focus attention to the onset of the first LED light unpredictably presented after random foreperiods. The lateral occipital area extending to the temporo-parieto-occipital junction was activated more strongly in the timing task than in the RT task; the same area was deactivated in the RT task relative to the control condition. Auditory-related areas were also deactivated in the both tasks. This inter- and intramodal task-specific modification including deactivation underscores significance of the context for perception and action and can have an important role in dexterous or skilled performance.

Many investigators have demonstrated that in swing motions such as ball-throwing, the motion of the proximal joint (shoulder) produced assistive interaction torque for the distal joint (elbow). In line with these studies, the shoulder and elbow motions would be expected to produce the assistive interaction torque for the wrist joint as well. However, we recently showed that the interaction torque at the wrist was always counteractive to the wrist muscle torque during ball-throwing. The purpose of this study is to clarify, by means of computer simulation, whether the counteractive relationship at the wrist during ball-throwing is caused by the neural contribution or the musculoskeletal mechanical properties of the human arm. First, we simulated the throwing motions of the normal forearm-hand model by systematically changing the proximal-to-distal delay of muscle activities and could line up two candidates for the determinant of the counteractive relationship: the rest angle (neutral angle) of the wrist and the length and mass of the hand. Second, we simulated the throwing motions of the virtual forearm-hand models, showing that only nonrealistic elongation of these two parameters produced the assistive relationship between the interaction torque and muscle torque. These results suggested that the mechanical properties of the human wrist are the main determinant of the counteractive relationship, which is advantageous for keeping the state of the wrist joint stable in multi-joint upper-limb movements and would lead to avoidance of excessive wrist extension or flexion and simplification of extrinsic finger control.

Carbon radicals from allyl O-(trimethylsilyl)-alpha-bromo-alpha,alpha-difluoroacetal can cyclize onto the olefinic part regiospecifically to give gamma-lactols in good yield. The lactols are then converted to the corresponding alpha,alpha-difluoro-gamma-lactones. Systematic synthesis of multifluorinated-alpha,alpha-difluoro-gamma-lactones has thus been accomplished through intramolecular radical cyclization as a key reaction. Semiempirical MO calculation study suggested a unique nature of alpha,alpha-difluoroacetate in that complete delocalization of the electrons in the SOMO orbital of alpha,alpha-difluoroacetyl radical occurred; this caused unsuccessful cyclization. To apply the present radical reaction, the first synthesis of both enantiomers of difluoroeldanolide, analogues of the sex pheromone of the male African sugarcane borer, has been demonstrated. Electrophysiological tests revealed that the difluorinated analogues were as active as the natural eldanolide on the olfactory receptors.

Many researchers have investigated the variability of discrete goal-directed movements. However, a possible time-dependent property for them has been ignored. We investigated the time-dependent property of the kinematic variability in human rapid goal-directed movements by using fractal analysis. The variability of the peak acceleration had fractal time correlation characterized by a non-negligible frequency-spectral exponent ( β =0.3). This β value was reduced at the peak velocity ( β =0.2) and nearly reached zero at the peak movement amplitude, indicating that the variability became white noise with no time correlation. A similar result was observed in the progress of the positional trajectory. The functional significance of this phenomenon for the goal-directed task demand is discussed.

The consistency and coordination of release parameters in ball-throwing movements were investigated. The authors used a newly developed index of coordination for release parameters (ICRP) that quantifies the degree of improvement of performance consistency caused by compensatory relationships among parameters (i.e., not caused by consistency of parameters). Eight participants practiced for 150 trials, with the nondominant hand, a ball-throwing task aimed at a stationary target. The magnitude of the ball-release velocity vector, among release parameters, as well as the performance was found to become consistent with practice. The ICRP score suggested that the release parameters were complementarily coordinated with one another, and that the coordination improved with practice. Those results indicate that compensatory relationships among varying release parameters contribute to reducing the variability of performance in a ball-throwing task whose goal is accuracy.

Subjects made a fast elbow extension movement to designated target in response to a go signal. In 45% of trials a stop signal was presented after the go signal, to which subjects were asked to stop the movement as rapidly as possible. The interstimulus interval (ISI), or time interval between the go and stop signals, was randomly varied between 0 and 200 ms. Electromyographic (EMG) activity was recorded from biceps brachii and triceps brachii. Subjects could sometimes completely inhibit initiation of the movements when the ISI was 0 ms, but could rarely do so when the ISI exceeded 100 ms. For responses that were initiated but stopped on the way, the amplitude of the movement decreased linearly as the time interval (=modification time) from the stop signal to EMG onset increased. The peak velocity increased linearly as the movement amplitude increased. This tendency was similar to those previously reported in step-tracking movements with various amplitudes. In spite of the similarity in the kinematics of the movement, the EMG pattern was different from that of step-tracking movement. While the initial agonist burst (AG1) decreased linearly after the modification time exceeded 100 ms, the antagonist burst (ANT) increased compared with the go trial for the modification time from 0 to 200 ms and decreased after the modification time exceeded 300 ms. This change of activation is analogous to functional modification of middle-latency reflex EMG response to load, or cutaneous perturbation. In conclusion, it is suggested that adaptive mechanisms, which would functionally modify the reflex responses, are also continuously working during voluntary movements in response to sudden changes in environmental information.

The aim of this study was to investigate whether whole-body action-perception coordination is governed by dynamical principles using basic street dance movement. Six skilled street dancers and 8 novice controls performed 2 movement patterns: knee-flexion-on-the-beat (down movement) and knee-extension-on-the-beat (up movement) in the standing position, and they did not intervene in the pattern change. The beat rate increased/decreased between 60 and 220 beats per minute (bpm) in steps of 20 bpm. The relative phase between knee movements (as measured by twin-axis electrogoniometer) and the beat were calculated. In the ascending beat rate condition of the up movement, phase transition from knee-extension-on-the-beat to knee-flexion-on-the-beat occurred at averages of 125 bpm in non-dancers and 164 bpm in dancers. Critical fluctuation and hysteresis were also observed. These results suggest that whole-body action-perception pattern formation is governed by general dynamical principles and that critical frequency could be a parameter of proficiency in street dance.

Two experiments were conducted to investigate the effects of instruction and practice on the length-reproduction task using the Müller-Lyer figure. The task consisted of reproducing visually presented lines by positioning the arm. In Exp. 1, 20 subjects were asked to move a stylus exactly the same length as the line presented in the illusory figure. Instructed subjects were told to ignore the visual illusion, while uninstructed subjects were told nothing. No differences in groups' constant errors were observed; however, subjects in both groups reproduced longer or shorter lines in accordance with the visual illusion. In Exp. 2, we investigated the effect of visual illusion in relation to learned movement. After practicing a length reproduction task 200 times, subjects performed a test session in which illusory figures were among the presented lines. Subjects were instructed to ignore any visual illusions, but this again had no effect on the reproduced length, that is, longer or shorter lines were reproduced in accord with the illusion. The findings show that visual illusion still had an effect after subjects were instructed to ignore it or had practiced the task extensively.

Human movements are produced in variable external/internal environments. Because of this variability, the same motor command can result in quite different movement patterns. Therefore, to produce skilled movements humans must coordinate the variability, not try to exclude it. In addition, because human movements are produced in redundant and complex systems, a combination of variability should be observed in different anatomical/physiological levels. In this paper, we introduce our research about human movement variability that shows remarkable coordination among components, and between organism and environment. We also introduce nonlinear dynamical models that can describe a variety of movements as a self-organization of a dynamical system, because the dynamical systems approach is a major candidate to understand the principle underlying organization of varying systems with huge degrees-of-freedom.