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.

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.

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.

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.

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).

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.

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.

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.

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.

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).

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

Control of increasing or decreasing force during periodic isometric movement of the finger.

The present study examined the control of force and timing during periodic isometric force production of the right index finger. Ten right-handed male participants performed three tasks cycling between lower levels (5-40%) of maximum voluntary force with a target peak-to-peak or valley-to-valley interval of 500 ms. The analysis showed that the valley force was markedly more variable than the peak force over all tasks.

Interactions of speech and manual movement in a syncopated task.

The present study examined interactions of speech production and finger-tapping movement, using a syncopated motor task with two movements in 10 male right-handed undergraduate students (M age = 21.0 yr.; SD = 1.

Coupling of force variability in bimanual tapping with asymmetrical force.

An experiment was conducted to examine the coupling of force variability in bimanual finger tapping sequences with asymmetrical forces. Right-handed participants were trained to produce bimanual finger tapping sequences consisted of an intertap interval of 500 ms and eight force conditions: two alternating force left high, two alternating force right high, two simultaneous force left high, and two simultaneous force right high conditions. During practice, visual force feedback was provided for both hands performing the bimanual tapping sequences.

Contextual effects on force control and timing in a finger-tapping sequence with an accentuated- or attenuated-force tap.

An experiment was conducted to examine contextual effects of the magnitude of changes in force on force control in a finger-tapping sequence with an accentuated- (accentuated-force condition) or attenuated-force tap (attenuated-force condition). Participants were trained to produce a finger-tapping sequence with an intertap interval of 500 ms and four force patterns. During practice, visual force feedback pertaining to the two target forces in the tapping sequences was provided.

Timing of bimanual alternating finger-tapping sequences in adolescents with mental retardation: a pilot study.

We examined differences between the synchronization and self-pacing tasks using repetitive tapping with alternate hands. 9 adolescents with mental retardation made faster and more variable movements on the self-pacing task than on the synchronization task at target intervals of 400 and 800 msec. This suggests that they were able to coordinate the timing of individual responses with individual stimuli on the synchronization task but on the self-pacing task they controlled the timing by memory or generation or setting of an incorrect timer rate.