This study investigated the effect of the time and direction preparation on the electromyographic (EMG) response of the ankle extensor to the backward translation of the support surface in stance. Fifteen healthy adult males aged 35.9 ± 6.2 years participated in this study. In the constant session, the interval between the warning cue and the onset of the backward support surface translation was constant. In the random time session, the interval was randomly assigned in each trial, but the direction was backward across the trials. In the random direction session, the direction was randomly assigned in each trial, but the interval was constant. The EMG amplitude in the time epochs 100–175 ms after translation onset in the random time session was significantly greater than that in the constant session in the soleus, gastrocnemius, and tibialis anterior muscles. The EMG amplitude in the time epochs 120–185 ms after translation onset in the random direction session was significantly greater than that in the constant session in the gastrocnemius and tibialis anterior muscles. This finding indicates that time and direction preparation reduces the late component of the ankle EMG response to backward translation of the support surface. This finding is explained by the supposed process through which uncertainty of the upcoming event causes disinhibition of response or by how time and direction preparation optimizes the magnitude of the long-latency response mediated by the transcortical pathway.
Masakazu Matsuoka, Hiroshi Kunimura, and Koichi Hiraoka
Koichi Hiraoka, Yuta Chujyo, Sakiko Hatano, Kohei Fukuhara, and Yuki Yamanaka
We investigated the effects of contralateral movement on the soleus H-reflex during bilateral movement of the ankles. The soleus H-reflex was evoked during unilateral or in-phase or antiphase bilateral ankle movement in healthy humans. The soleus H-reflex amplitude during antiphase movement was not significantly different from that during in-phase movement. The H-reflex amplitude during inphase movement was not significantly different from that during unilateral movement of the tested ankle. The H-reflex amplitude in the maximal plantar flexion phase of the tested ankle during antiphase movement was significantly smaller than that in the corresponding phase during unilateral movement. The amount of the decrease was 21%. The effect of contralateral movement is prominent during antiphase movement of the ankles.
Keisuke Tani, Yasutomo Jono, Yoshifumi Nomura, Yuta Chujo, and Koichi Hiraoka
This study investigated the effect of monaural auditory stimulus on hand selection when reaching. Healthy right-handed participants were asked to reach to a visual target and were free to use either the right or left hand. A visual target appeared at one of 11 positions in the visual field between -25 and 25 degrees of the horizontal visual angle. An auditory stimulus was given either in the left or right ear 100 ms after the presentation of the visual target, or no auditory stimulus was given. An auditory stimulus in the right ear increased right hand selection, and that in the left ear slightly increased left hand selection when reaching to a target around the midline of the visual field. The horizontal visual angle, where the probabilities of right hand selection and left hand selection were equal when reaching, shifted leftward when an auditory stimulus was given in the right ear, but the angle did not shift in either direction when an auditory stimulus was given in the left ear. The right-ear-dominant auditory stimulus effect on hand selection indicates hemispheric asymmetry of cortical activity for hand selection.
Hitoshi Oda, Yasushi Sawaguchi, Hiroshi Kunimura, Taku Kawasaki, and Koichi Hiraoka
This study examined whether the current movement follows the previous movement and whether this process is enhanced by somatosensory stimulation or is gated while retrieving and using the memory of the previously practiced target end point. Healthy humans abducted the index finger to a previously practiced target (target movement) or abducted it freely without aiming at the target (nontarget movement). The end point of the nontarget movement had a positive correlation with the previous nontarget movement only when somatosensory stimulation was given during the previous movement, indicating that the current nontarget movement follows the previous nontarget movement with somatosensory stimulation. No conclusive evidence of whether this process is gated by retrieving and using the memory of the previously practiced target was found.
Koichi Hiraoka, Masaya Ishimoto, Mai Kishigami, Ryota Sakaya, Asahi Sumimoto, and Kazuki Yoshikawa
This study investigated the process that contributes to the decay of short-term motor memory regarding force reproduction. Participants performed tonic flexion of the right index finger with the target force feedback (criterion phase) and reproduced this force level without feedback 3, 10, 30, or 60 s after the end of the criterion phase (recall phase). The constant error for force reproduction was significantly greater than zero, indicating that information about the somatosensation and/or motor command in the criterion phase is positively biased. Constant and absolute errors were not influenced by the retention interval, indicating that neither bias nor error represents the decay of short-term motor memory over time. Variable error, defined as SD of bias (force in the recall phase minus that in the criterion phase), increased as the retention interval increased. This indicates that the decay of short-term motor memory is represented by the increase in inconsistency of memory bias among the trials. The correlation coefficient of the force between the criterion and recall phases with 3-s retention interval was greater than that with longer intervals. This is explained by the view that the contribution of the information of the practiced force to the force reproduction process is great within 3 s after the end of the practice, but the additional contribution of the noise information becomes greater after this time, causing lesser relative contribution of the information of the practiced force to the force reproduction process.