Experimental conditions in which dyads outperform individuals in a task of force produced by two people.

When participants control periodic isometric force cycling between two target forces, they more accurately control force in a joint action than in an individual action. In some other studies, however, individuals tend to outperform dyads in joint action. The present study thus examined experimental conditions in which dyads outperformed individuals in a task of force produced by two people. This study consisted of two tasks with two target conditions and three force production conditions. The individual task was performed by one participant, and the joint task was performed by two participants. In absolute and relative target conditions, the participants made continuous, discrete, and periodic isometric pressing movements with the index finger. Although no difference was seen in force error between tasks in the continuous condition, the joint task had a smaller error than the individual task in the two other conditions. The joint task had a smaller variable force than the individual task in the periodic conditions, but no difference was seen in force variability between tasks in the two other conditions. Participants mainly controlled force in both tasks in the continuous condition. In the periodic or discrete condition at a prescribed interval, however, participants had to control both force and timing in the individual task, and muscle force must be mainly controlled to compensate for force errors by synchronizing interpersonal force outputs in the joint task. Thus, dyads can reduce the dimensionality of the control problem because they can synchronize their action which provides timing information.

Force asymmetry deteriorates complementary force production during joint action.

The present study aimed to determine the effects of force asymmetry on interpersonal force production. This study consisted of an individual task executed by one participant at a time in a pair, and three joint tasks executed by two participants simultaneously under conditions of 1:1, 1:0.75, and 1:0.5. Two individuals produced discrete forces at the same time so that the sum of forces they produced was the target force in the joint task. Under the 1:1 condition, the target force was the sum of the maximum voluntary contraction (MVC) produced by the index finger of each participant × 0.1 (10% MVC). Under the 1:0.75 condition, the investigators manipulated the force produced by only one of the pair, for example, B, but not A. The feedback was also scaled as a result. The target force was the MVC of participant A + the MVC of participant B × 0.75 × 0.1. Similarly, the target force under the 1:0.5 condition was the MVC of participant A + the MVC of participant B × 0.5 × 0.1. The present study found that forces produced by pairs were negatively correlated and the correlation value was higher under the 1:1 condition than the 1:0.75 and 1:0.5 conditions. The absolute error was smaller under the 1:1 condition than the 1:0.5 condition. Complementary force production was attenuated and the error increased as differences between forces produced by two participants increased. Thus, asymmetry of forces produced by pairs deteriorated complementary force production and interpersonal performance.

Load Perturbation Facilitates Interpersonal Error Compensation in a Dyadic Force Production Task.

The present study tested the hypothesis that a load perturbation facilitates interpersonal compensation for force error. Ten groups performed both control and perturbation conditions. In the control condition, a target discrete peak force was the sum of 10% of the maximum voluntary contraction produced by two participants. In the perturbation condition, two cooperative participants a and b produced the same target force as the control condition, and the force produced by a non-cooperative participant c increased or decreased the total forces produced by participants a and b. The load perturbation facilitated interpersonal compensation for force error but negatively affected performance during joint action. The participants further learned to improve the error compensation over experimental blocks in the perturbation condition.

A Common Force-Sharing Pattern in Joint Action That Consists of Four People.

The authors examined the force-sharing patterns in a joint action performed by a group of two, three, or four people compared with a solo action. In the joint actions, 28 participants produced periodic isometric forces such that the sum of forces they produced cycled between 5% and 10% maximum voluntary contraction with the right hand at 1 Hz. In both the three- and four-person tasks, the correlation between forces produced by two of the three or four participants was negative, and the remaining one or two participants produced intermediate forces. The errors of force and interval and force variabilities were smaller in four- and three-people groups than individuals. Four- and three-people groups thus performed better than individuals.

Bidirectional transfer between joint and individual actions in a task of discrete force production.

The present study examined bidirectional learning transfer between joint and individual actions involving discrete isometric force production with the right index finger. To examine the effects of practice of joint action on performance of the individual action, participants performed a pre-test (individual condition), practice blocks (joint condition), and a post-test (individual condition) (IJI task). To examine the effects of practice of the individual action on performance during the joint action, the participants performed a pre-test (joint condition), practice blocks (individual condition), and a post-test (joint condition) (JIJ task). Whereas one participant made pressing movements with a target peak force of 10% maximum voluntary contraction (MVC) in the individual condition, two participants produced the target force of the sum of 10% MVC produced by each of them in the joint condition. In both the IJI and JIJ tasks, absolute errors and standard deviations of peak force were smaller post-test than pre-test, indicating bidirectional transfer between individual and joint conditions for force accuracy and variability. Although the negative correlation between forces produced by two participants (complementary force production) became stronger with practice blocks in the IJI task, there was no difference between the pre- and post-tests for the negative correlation in the JIJ task. In the JIJ task, the decrease in force accuracy and variability during the individual action did not facilitate complementary force production during the joint action. This indicates that practice performed by two people is essential for complementary force production in joint action.

Motor control hierarchy in joint action that involves bimanual force production.

The concept of hierarchical motor control has been viewed as a means of progressively decreasing the number of variables manipulated by each higher control level. We tested the hypothesis that turning an individual bimanual force-production task into a joint (two-participant) force-production task would lead to positive correlation between forces produced by the two hands of the individual participant (symmetric strategy) to enable negative correlation between forces produced by two participants (complementary strategy). The present study consisted of individual and joint tasks that involved both unimanual and bimanual conditions. In the joint task, 10 pairs of participants produced periodic isometric forces, such that the sum of forces that they produced matched a target force cycling between 5% and 10% of maximum voluntary contraction at 1 Hz. In the individual task, individuals attempted to match the same target force. In the joint bimanual condition, the two hands of each participant adopted a symmetric strategy of force, whereas the two participants adopted a complementary strategy of force, highlighting that the bimanual action behaved as a low level of a hierarchy, whereas the joint action behaved as an upper level. The complementary force production was greater interpersonally than intrapersonally. However, whereas the coherence was highest at 1 Hz in all conditions, the frequency synchrony was stronger intrapersonally than interpersonally. Moreover, whereas the bimanual action exhibited a smaller error and variability of force than the unimanual action, the joint action exhibited a less-variable interval and force than the individual action.

Visual and Proprioceptive Adaptation of Arm Position in a Virtual Environment.

The authors examined the resolution of a discrepancy between visual and proprioceptive estimates of arm position in 10 participants. The participants fixed their right shoulder at 0°, 30°, or 60° of transverse adduction while they viewed a video on a head-mounted display that showed their right arm extended in front of the trunk for 30 min. The perceived arm position more closely approached the seen arm position on the display as the difference between the actual and visually displayed arm positions increased. In the extreme case of a 90° discrepancy, the seen arm position on the display was very gradually perceived as approaching the actual arm position. The magnitude of changes in sensory estimates was larger for proprioception (20%) than for vision (< 10%).

Perceptual changes of interaction between hand and object in an experimental phantom hand.

The authors examined whether the wrist and elbow were perceived as flexed when a stick was fixed to the hand while the joints were extended during anesthesia. Ten healthy participants lay on their back on a bed with their eyes closed, and a stick was fixed to their right hand. Surprisingly, while the perceived position of the wrist and elbow moved toward flexion from 10 to 40 min after the ischemic block, the stick fixed to the hand was also perceived as having moved toward flexion from 10 to 20 min after the block. Such coupling the change in the perceived stick position with the change in body image suggests a new type of hand-object illusion.

A leader-follower relationship in joint action on a discrete force production task.

The present study examined the development of a leader-follower relationship in joint action performed by participants with different skill levels. Two participants were instructed to produce discrete isometric forces such that the sum of the forces was the target force. The task did not prescribe the onset time or share of force each participant contributed to the target force. Although novices with low force variability did not produce an earlier force than those with high force variability in the novice-novice group, experienced participants produced an earlier force than novices in the novice-experienced group. While participants with low force variability always produced a stronger force than those with high force variability in both the groups, there was no significant difference in force distributions between participants with low and high force variabilities. Although a novice-experienced pair produced force more complementarily than a novice-novice pair in the first practice block, the difference between pairs vanished after the first practice block, suggesting that leader-follower relations were not always beneficial to task performance. In addition, practice of the joint action did not transfer to individual action.

Effects of speech on both complementary and synchronous strategies in joint action.

If two people row a boat, they often call to each other to synchronize their strokes. It is anticipated that such a call promotes periodic joint action. The present study thus examined the effects of speech on both complementary and synchronous strategies in joint action using the same task as we used previously (Masumoto and Inui in J Neurophysiol 109:1307-1314, 2013a). Ten pairs of participants produced periodic isometric forces such that the sum of the forces they produced was the target force cycling between 5 and 10 % of maximum voluntary contraction with an interval of 1,000 ms with the right hand. There were three speech conditions crossed with the presence or absence of visual information. Whereas two participants synchronized an utterance/ba/with the peak and valley forces in the 'Both' condition, one synchronized it with both forces in the 'One-side' condition, and nobody uttered it in the 'None' condition. When the total force was visible, the One-side and Both conditions exhibited lower correlations than the None condition, although the correlation between forces produced by two participants was negative in all conditions. When the total force was invisible, although the coherence between force and time series produced by two participants was low under the None condition, it was high at 1 and 3 Hz under the One-side and Both conditions. Thus, although periodically uttering a syllable worsened complementary force production when the target was visible, it promoted synchronization of their performance to each other's timing when the target was invisible.

Effects of movement duration on error compensation in periodic bimanual isometric force production.

Recent studies using bimanual force production have examined how factors influence redundancy in the nervous system. The present study examined effects of different movement durations on bimanual force control strategies. Ten healthy male participants produced periodic isometric forces such that the sum of two finger forces was a target cycling between 5 and 10 % of maximum voluntary contraction during five movement durations (500, 750, 1,000, 1,250, and 1,500 ms). Correlations between the two finger forces changed from positive to negative with an increase in duration. The polynomial regression analysis indicates that while the correlations between two finger forces were most negative at the target duration of 1,250 ms, they became more positive as the durations deviated from 1,250 ms. Similarly, while force variability was smallest at the target duration of 1,250 ms, it increased as the durations deviated from 1,250 ms. These findings suggested that while the duration of 1,250 ms might be a natural frequency of both fingers, bimanual force strategies changed from force error compensation to force coupling as the durations deviated from 1,250 ms. In addition, while the variance in the sum of two finger forces (the task-relevant variance) decreased with movement duration, the difference between both the finger forces (the task-irrelevant variance) did not change with the duration. Thus, a decrease in the task-relevant variance with movement duration resulted in the negative correlation between the two finger forces and the small force variability.

Effects of visual information on perceived posture of an experimental phantom foot.

Our previous studies showed that a fully extended finger, wrist, and elbow became a flexed phantom hand and arm with ischemic anesthesia, and vice versa (Inui et al. in J Physiol 589:5775-5784, 2011, Exp Brain Res 221:369-375, 2012a, Exp Brain Res 218:487-494, 2012b). It was anticipated that if the ankle and knee were fixed in full extension or flexion before and during ischemic anesthesia, the perceived positions would move in the opposite direction. The present study examined what happened when participants looked at their fixed foot and leg at the end of the anesthesia. Using the left ankle and knee, ten healthy participants demonstrated the perceived postures of the right joints during an ischemic block of the right thigh (40 min) and after they looked at the right joints at the end of the block. When the right ankle and knee were fully extended before and during the block, the final joints were perceived as flexed by all participants, and vice versa. Although there was no significant difference between joints for the magnitude of the perceived changes in flexion, the magnitude in the knee was larger than that in the ankle in extension. At the end of the experiment, when participants were allowed to see their foot, its perceived position reverted to that indicated by them earlier, during the first 25 min of cuff inflation. This new finding suggests that the position of limbs is coded by visual input more dominantly than by proprioceptive input in the brain.

Two heads are better than one: both complementary and synchronous strategies facilitate joint action.

If two people lift and carry an object, they not only produce complementary forces on the object but also walk in synchrony. Previous studies have not examined how two types of coordination strategy are adopted simultaneously. The present study thus tested the hypothesis that complementary and synchronous strategies simultaneously facilitate the action coordination performed by two people. Ten pairs of participants produced periodic isometric forces such that the sum of forces they produced was the target force cycling between 5% and 10% of maximum voluntary contraction with an interval of 1,000 ms (joint action), while individuals alone produced the same target forces with the right hand (individual action). The correlation between forces produced by two participants was highly negative when the total force was visible, indicating that the two participants produced complementary forces. When the image of the total or partner force was presented, the coherence between force-time series produced by two participants was highest at 1 Hz. The relative phase angles were also distributed at the 0-20° phase region. These innovative findings indicate that two participants simultaneously adopted both complementary and temporal synchronous strategies exclusively when the total force was visible. With the vision of total force, surprisingly, while the joint action exhibited a less variable force than the individual action, the joint action exhibited a smaller absolute error of forces than the individual action. These new findings indicated that the joint action controlled force more accurately than the individual action.

Adolescents with Down syndrome exhibit greater force and delay in onset of tapping movements.

This study examined the control of force and timing during finger tapping sequences of adolescents with Down syndrome. Participants performed both unimanual and bimanual tapping tasks with one self-paced test trial after three audible-synchronized practice trials with concurrent feedback of force output. All tasks consisted of a target force of 2N and a target intertap interval of 500 msec. Adolescents with Down syndrome exhibited a greater magnitude of positive constant error and variable error for peak force than typical adolescents. They also exhibited a greater magnitude of negative constant error and variable error for intertap interval than typical adolescents. Although normally developing comparison adolescents exhibited a linear relationship between peak force and press duration or time-to-peak force, the relationship was not familiar to adolescents with Down's syndrome. This may suggest differences in the manner of motor unit recruitment between the group with Down's syndrome and comparison adolescents.

Loss of large-diameter nerve sensory input changes perceived posture.

Our previous studies (Inui et al. in J Physiol 589:5775-5784, 2011, Exp Brain Res 218:487-494, 2012) showed that a fully flexed or extended finger, wrist, and elbow became perceived as an extended or flexed 'phantom' hand and arm as ischemic anesthesia progressed. Here, we examined what happened if the wrist was fixed in full extension while the elbow was in full flexion before and during the anesthesia, and vice versa. Ten healthy participants demonstrated the perceived postures of their right wrist and elbow during an ischemic block of the right upper arm with the left hand and arm. If the actual wrist was fully extended while the actual elbow was fully flexed, then the perceived position of the wrist moved toward flexion and that of the elbow moved toward extension. Conversely, if the actual wrist was fully flexed while the actual elbow was fully extended, then the wrist was perceived to extend and the elbow was perceived to flex. Following the loss of the afferent signal coming from the main muscles acting at the two joints, the two perceived postures moved toward the opposite direction independently. The changes in the perceived postures are a shift in the body schema depending on the balance of the proprioceptive inputs that determine limb posture.

Effects of force levels on error compensation in periodic bimanual isometric force control.

The authors examined whether force level interacts with the presence or absence of vision in bimanual force control. Participants produced periodic isometric forces such that the sum of the 2 finger forces was the target force under 4 force levels cycling between lower levels (5-40%) of maximum voluntary contraction with an interval of 1000 ms. Without vision, the correlation between the 2 finger forces was strongly positive over all force levels. However, with vision the correlation changed from negative to positive with force level. The result with vision indicated that the strategy of the bimanual force control changed from force error compensation to force coupling and the available redundancy thus decreased with an increase in force.

Systematic changes in the perceived posture of the wrist and elbow during formation of a phantom hand and arm.

Our previous study showed that a fully flexed or extended hand became perceived as an extended or flexed 'phantom' hand as ischemic anesthesia progressed (Inui et al. in J Physiol 589:5775-5784, 2011). Here, we examined what happened if the hand was held in the midposition before and during the anesthesia. Twenty healthy participants reported the perceived postures of their right wrist and elbow during an ischemic block of the right upper arm using the left hand and arm. If the actual arm and hand were fully extended, then the perceived position of the elbow and wrist moved toward flexion. Conversely, if they were fully flexed, then the perceived position of the joints moved toward extension. However, when the hand was held in the midposition before and during the anesthesia, the position of the wrist was perceived to be in the same position. Hence, the fully flexed or extended position of a limb was essential for systematic changes in the perceived posture of the limb during the anesthesia. Because the start of these changes occurred as somatosensory inputs were declining, the changes depended on the fading inputs from strongly stretched muscle and skin during the anesthesia.

Effects of practice on magnitude and structure of force variability during periodic unimanual isometric force production.

The present study examined practice effects on the magnitude and structure of force variability using multiscale entropy analysis. Ten male participants (M age = 22.5 yr., SD = 1.7) produced unimanual isometric forces cycling between low levels of maximum voluntary contraction. The participants practiced the motor task over ten blocks: one block consisted of one self-paced recall trial immediately after nine audibly-synchronized practice trials with feedback of force output. Although the coefficient of variation of the force-time series in the recall trial had a greater magnitude than that in the practice, the recall trial had lower sample entropy values of force output (higher time structure of force variability). Multiscale entropy analysis did not find a practice effect on the time structure of force variability on a short time scale (10 msec.) but did on a long time scale (20-60 msec.). Different tasks have different time scales of force output, and the task demands of the present experiment emphasized the long time scales.

Effects of aging on control of timing and force of finger tapping.

The present study examined whether the elderly produced a hastened or delayed tap with a negative or positive constant intertap interval error more frequently in self-paced tapping than in the stimulus-synchronized tapping for the 2 N target force at 2 or 4 Hz frequency. The analysis showed that, at both frequencies, the percentage of the delayed tap was larger in the self-paced tapping than in the stimulus-synchronized tapping, whereas the hastened tap showed the opposite result. At the 4 Hz frequency, all age groups had more variable intertap intervals during the self-paced tapping than during the stimulus-synchronized tapping, and the variability of the intertap intervals increased with age. Thus, although the increase in the frequency of delayed taps and variable intertap intervals in the self-paced tapping perhaps resulted from a dysfunction of movement timing in the basal ganglia with age, the decline in timing accuracy was somewhat improved by an auditory cue. The force variability of tapping at 4 Hz further increased with age, indicating an effect of aging on the control of force.

Practice effects on decreasing and increasing force-control during periodic isometric movements of the index finger.

The present study examined whether improvement in control while decreasing force to achieve a lower force target would be facilitated by comparison of performance while increasing force to achieve a higher force target. Participants practiced control of isometric force and timing during a unimanual force production task cycling between 5 and 10% of maximum voluntary contraction with a target interval of 500 msec. Although errors and variability of both peak and valley forces and interval decreased during early practice, the valley force was still more inaccurate and variable than the peak force in the final practice. Variabilities of both forces did not decrease when the valley force was synchronized with an audible metronome pulse but did decrease when the peak force was synchronized with it.