Human judgment of the temporal order of two sensory signals is liable to change depending on our prior experiences. Previous studies have reported that signals presented at short intervals but in the same order as the most frequently repeated signal are perceived as occurring simultaneously. Here we report opposite perceptual changes that conform to a Bayesian integration theory in judging the order of two stimuli delivered one to each hand.

. A sensorimotor control task often requires an accurate estimation of the timing of the arrival of an external target (e.g., when hitting a pitched ball). Conventional studies of human timing processes have ignored the stochastic features of target timing: e.g., the speed of the pitched ball is not generally constant, but is variable. Interestingly, based on Bayesian theory, it has been recently shown that the human sensorimotor system achieves the optimal estimation by integrating sensory information with prior knowledge of the probabilistic structure of the target variation. In this study, we tested whether Bayesian integration is also implemented while performing a coincidencetiming type of sensorimotor task by manipulating the trial-by-trial variability (i.e., the prior distribution) of the target timing. As a result, within several hundred trials of learning, subjects were able to generate systematic timing behavior according to the width of the prior distribution, as predicted by the optimal Bayesian model. Considering the previous studies showing that the human sensorimotor system uses Bayesian integration in spacing and forcegrading tasks, our result indicates that Bayesian integration is fundamental to all aspects of human sensorimotor control. Moreover, it was noteworthy that the subjects could adjust their behavior both when the prior distribution was switched from wide to narrow and vice versa, although the adjustment was slower in the former case. Based on a comparison with observations in a previous study, we discuss the flexibility and adaptability of Bayesian sensorimotor learning. I N T R O D U C T I O NOur sensorimotor system often requires that motor responses are timed precisely in accordance with the behavior of a certain external target. For example, to hit a pitched ball while playing baseball or cricket, the batter has to control the timing of the swing based on the speed of the ball. Such timing behavior, which is referred to as coincidence timing, has been studied extensively in sports science (e.g., Ripoll and Latiri 1997; Williams et al. 2002). Because the external environment that we usually encounter is variable, not constant, we need to monitor visual and other sensory signals to estimate the current behavior of the external target accurately. Naturally, these sensory signals are exposed to internal and external noise (Körding and Wolpert 2004; van Beers et al. 2002), so they cannot always provide sufficient information for precise estimates. To compensate for the sensory uncertainty, prior knowledge or experience of the target behavior is helpful information for the estimation. By observing the external target behavior for a long time, we can determine its predictable probabilistic structure. Considering various phenomena in our world, such as human behavior and physical events, the trial-by-trial variability has a certain probabilistic structure, such as a Gaussian distribution (e.g., Chen et al. 1997). That is, the speeds of all pitched balls do not appear with the same probab...

We have investigated the etiology of lower-back muscle fatigue using simultaneous recordings of electromyography (EMG), mechanomyography (MMG), and near-infrared spectroscopy (NIRS) in an attempt to shed some light on the electrophysiological, mechanical, and metabolic characteristics, respectively. Eight male subjects performed isometric back extensions at an angle of 15 degrees with reference to the horizontal plane, for a period of 60 s. Surface EMG, MMG and NIRS signals were recorded simultaneously from the center of the erector spinae at the level of L3. NIRS was measured to determine the level of muscle blood volume (BV) and oxygenation (Oxy-Hb). The root mean square amplitude value (RMS) of the EMG signal was significantly increased at the initial phase of contraction and then fell significantly, while mean power frequency (MPF) of the EMG signal decreased significantly and progressively as a function of time. There were also significant initial increases in RMS-MMG that were followed by progressive decreases at the end of fatiguing contractions. MPF-MMG remained unchanged. Muscle BV and Oxy-Hb decreased dramatically at the onset of the contraction and then remained almost constant throughout the rest of the contraction. These results, obtained by simultaneous recordings of EMG, MMG, and NIRS, demonstrate that the restriction of blood flow due to high intramuscular mechanical pressure is one of the most important factors in muscle fatigue in the lower-back muscles. In addition, the simultaneous recording system described here can be used to obtain more reliable information regarding the mechanism(s) of lower-back muscle fatigue.

n = 4-27) are characterized by IR spectroscopy of OH stretches in the gas phase. Spectral changes with cluster size demonstrate that the chain structures at small sizes (n < 10) develop into two-dimensional net structures (10 < n < 21), and then into nanometer-scaled cages (n > 21). -(MIYAZAKI, M.; FUJII*, A.; EBATA, T.; MIKAMI, N.; Sci.

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