Does Viewing Mirror-Reflected Body Image Affect Static and Dynamic Standing Balance?

In the present study, we examined the immediate effect of allowing healthy participants to view their mirror-reflected body image on static and dynamic balance. We placed a mirror to allow participants to frontally view their own body image while maintaining a quiet stance or while engaged in a dynamic postural standing task. On measures of body sway during quiet stance, there were no effects of this visual feedback, supporting the view that human beings have no central mechanism for viewing the mirror-reflected body image to control body sway during quiet stance. However, the body deviated forward during quiet stance while viewing the mirror-reflected body image, indicating that viewing the mirror-reflected body image contributed to the anterior-posterior positioning of the body, as mediated by an ankle control strategy. For the dynamic standing task, viewing the body image induced unstable peaks of rhythmic lateral shifting of the body weight over the feet. This indicates that viewing the body image caused unstable motor commands for rhythmic lateral weight shifting. When participants made a transition from a bipedal to a unipedal stance in response to a cue, viewing the body image shortened the onset latency of the body sway. Accordingly, viewing the body image seemed to accelerate the motor execution involved in lateral weight shifting, possibly due to predictive activation of the motor system before movement onset. Considered collectively, we found static and dynamic stance balance to be influenced by viewing one's mirror-reflected body image. Viewing the mirror-reflected body image may be a means of changing static and dynamic balance in patients with impaired postural control.

Effect of Laterally Moving Tactile Stimuli to Sole on Anticipatory Postural Adjustment of Gait Initiation in Healthy Males.

This present study examined the effect of the laterally moving tactile stimuli (LMTS) to the sole on the anticipatory postural adjustment (APA) of the gait initiation. Thirteen healthy males participated in this study. A sound cue was provided at the beginning of each trial. The participants took three steps forward from a quiet stance at their preferred time after the start cue. The LMTS were delivered to the sole after the start cue. The loci of the tactile stimuli moved from the left- to the right-most side of the sole and then moved from the right- to the left-most side of that in a stimuli cycle. The duration of one stimuli cycle was 960 ms, and this cycle was repeated 16 times in a trial. The APA did not onset at the specific direction or phase of the LMTS, indicating that they did not use any specific phase of the stimuli as a trigger for initiating the gait. The LMTS decreased the amplitude and increased the duration of the APA. Simultaneously, the LMTS increased the time between the APA onset and toe-off of the initial support leg, indicating that they moved slowly when initiating gait during the LMTS. Those findings are explained by the view that the suppression of the APA induced via the LMTS to the sole is caused by the slowing down of the gait initiation due to masking the tactile sensation of the sole.

Advanced cueing of auditory stimulus to the head induces body sway in the direction opposite to the stimulus site during quiet stance in male participants.

Under certain conditions, a tactile stimulus to the head induces the movement of the head away from the stimulus, and this is thought to be caused by a defense mechanism. In this study, we tested our hypothesis that predicting the stimulus site of the head in a quiet stance activates the defense mechanism, causing a body to sway to keep the head away from the stimulus. Fourteen healthy male participants aged 31.2 ± 6.8 years participated in this study. A visual cue predicting the forthcoming stimulus site (forehead, left side of the head, right side of the head, or back of the head) was given. Four seconds after this cue, an auditory or electrical tactile stimulus was given at the site predicted by the cue. The cue predicting the tactile stimulus site of the head did not induce a body sway. The cue predicting the auditory stimulus to the back of the head induced a forward body sway, and the cue predicting the stimulus to the forehead induced a backward body sway. The cue predicting the auditory stimulus to the left side of the head induced a rightward body sway, and the cue predicting the stimulus to the right side of the head induced a leftward body sway. These findings support our hypothesis that predicting the auditory stimulus site of the head induces a body sway in a quiet stance to keep the head away from the stimulus. The right gastrocnemius muscle contributes to the control of the body sway in the anterior-posterior axis related to this defense mechanism.

Contribution of vision and tactile sensation on body sway during quiet stance.

[Purpose] This study examines the contribution of vision and tactile sensation on body sway during quiet stance. [Participants and Methods] Sixteen healthy participants maintained quiet stance. The mean distance between the neutral center of pressure (COP) and that at the peak deviated position, indicating how quickly humans initiate the swaying of the body back to the neutral position, was calculated (COPpeak). [Results] The displacement of the COP in both the anterior-posterior and medial-lateral axes was greater when vision was occluded. The anterior or posterior COPpeak was also greater when vision was occluded. The leftward COPpeak was greater when the tactile sensation of the sole was masked. Visual occlusion decreased the tactile perception threshold of the sole. There was no significant interaction between the effect of vision and that of tactile sensation on body sway during quiet stance. [Conclusion] Vision plays a role in returning the body to the neutral position, particularly in the anterior-posterior axis. Tactile sensation contributes particularly to recovery from the leftward body sway during quiet stance. Tactile sensitivity is enhanced by visual occlusion through inter-modal reweighting. However, inter-modal reweighting between vision and tactile sensation is not specifically for postural control during quiet stance.

Sympathetic Response to Postural Perturbation in Stance.

The purpose of the present study was to elucidate whether the sympathetic response to perturbation in stance represents multiple mental responses, whether perturbation-induced fear of fall is one of the mental responses, and whether the sympathetic response is task specific. While healthy humans maintained stance, the support surface of the feet translated in the forward or backward direction. The phasic electrodermal response (EDR), representing the sympathetic response, appeared 1-1.5 s after the support surface translation. Mostly, perturbation-induced EDRs comprised one peak, but some EDRs were comprised of two peaks. The onset latency of the two-peak EDR was much shorter than that of the one-peak EDR. The second peak latency of the two-peak EDR was similar to the peak latency of the one-peak EDR, indicating that the first peak of the two-peak EDR was an additional component preceding the one-peak EDR. This finding supports a view that perturbation-induced EDR in stance sometimes represents multiple mental responses. The amplitude of the EDR had a positive and significant correlation with fear, indicating that perturbation-induced EDR in stance partially represents perturbation-induced fear of fall. The EDR amplitude was dependent on the translation amplitude and direction, indicating that perturbation-induced EDR in stance is a task specific response. The EDR appeared earlier when the participants prepared to answer a question or when the perturbation was self-triggered, indicating that adding cognitive load induces earlier perturbation-induced mental responses.

Current Movement Follows Previous Nontarget Movement With Somatosensory Stimulation.

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.

Supplementary motor area contributes to carrying previous movement information over to current movement.

The purpose of the present study was to determine the cortical areas contributing to the influence of the previous movement on the current movement. Right-handed healthy human participants abducted and then adducted the left index finger in response to a start cue. Twenty consecutive trials with 10 s intertrial intervals were performed in each trial block. An odd-numbered trial was considered to be the previous trial, and a trial immediately after the previous trial (even-numbered trial) was the current trial. In each trial block, transcranial magnetic stimulation (TMS) was given over one of the seven TMS sites with the start cue in the previous trial. The TMS site was over the supplementary motor area (SMA), right dorsolateral prefrontal cortex, right dorsal premotor cortex, right or left posterior parietal cortex or right primary sensory cortex. Sham TMS, producing magnetic stimulation with the coil tilting 90 degrees off the scalp, was delivered over the Cz. In the current trial, TMS was not delivered. The correlation coefficient of the reaction time between the previous and current trials was positive and significant in the sham TMS trial block. This indicates that the current movement is partially dependent on the previous movement. The correlation coefficient of the reaction time between the previous and current movements in the SMA trial block was significantly different from that in the sham TMS trial block, indicating that the SMA contributes to the influence of the previous movement on the current movement. The SMA contributes to carrying the responsiveness level in the previous movement over to the current movement.

Cortical contribution to motor process before and after movement onset.

PURPOSE: This study determined the cortical areas contributing to the process of the reaction time (RT), movement time, onset-peak time, peak velocity and amplitude of the movement. METHODS: Eighteen healthy right-handed humans abducted the left index finger in response to a start cue with transcranial magnetic stimulation (TMS). RESULTS: There was a significant and positive correlation coefficient between the peak velocity and amplitude, indicating that movement velocity increases with the size of the movement to maintain the consistent time taken for the movement. There was no significant correlation between the RT and movement time, and thus, hypothesis that those are under common motor process was not supported. The RT in the trials with TMS over the dorsal premotor cortex, dorsolateral prefrontal cortex, or posterior parietal cortex was significantly shorter than the RT in the trials with sham TMS, indicating that those areas contribute to the motor process in the RT. The onset-peak time in the trials with TMS over the posterior parietal cortex was significantly shorter than that in the trials with sham TMS, indicating that the posterior parietal cortex contributes to the motor process that determines the time taken for the acceleration phase of the movement. CONCLUSION: The findings support a view that the cortical areas both in front of and behind the primary motor cortex contribute to the motor process before the movement onset, but the areas behind the primary motor cortex particularly contributes to the motor process during the acceleration phase of the movement.

Rhythmic movement and rhythmic auditory cues enhance anticipatory postural adjustment of gait initiation.

The purpose of this study was to determine whether the rhythmic movements or cues enhance the anticipatory postural adjustment (APA) of gait initiation. Healthy humans initiated gait in response to an auditory start cue (third cue). A first auditory cue was given 8 s before the start cue, and a second auditory cue was given 3 s before the start cue. The participants performed the rhythmic medio-lateral weight shift (ML-WS session), rhythmic anterior-posterior weight shift (AP-WS session), or rhythmic arm swing (arm swing session) in the time between the first and second cues. In the rhythmic cues session, rhythmic auditory cues with a frequency of 1 Hz were given in this time. In the stationary session, the participants maintained stationary stance in this time. The APA and initial step movement preceded by those rhythmic movements or cues were compared with those in the stationary session. The temporal characteristics of the initial step movement of the gait initiation were not changed by the rhythmic movements or cues. The medio-lateral displacement of the APA in the ML-WS and arm swing sessions was significantly greater than that in the stationary session. The anterior-posterior displacement of the APA in the rhythmic cues and arm swing sessions was significantly greater than that in the stationary session. Taken together, the rhythmic movements and cues enhance the APA of gait initiation. The present finding may be a clue or motive for the future investigation for using rhythmic movements or cues as the preparatory activity to enlarge the small APA of gait initiation in the patients with Parkinson's disease.

Effect of Time and Direction Preparation on Ankle Muscle Response During Backward Translation of a Support Surface in Stance.

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.

Effects of Internal and External Attentional Focus on Postural Response to a Sliding Stance Surface.

The present study examined whether an internal or external attentional focus would affect participants' feet-in-place balance response to postural stance perturbations. A movable platform automatically slid forward or backward while healthy participants stood on it and (a) performed no cognitive activity (control), (b) focused on the pelvis or upper body sway (internal focus), (c) memorized a number displayed immediately before the platform slid (external focus), or (d) kept the equilibrium of an unstable cylinder over the arm (external focus). The forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on their pelvic sway, although such effect was absent when they focused on their upper body sway, indicating that the internal focus was effective for the postural response when attention was paid to the pelvic sway. Regarding an external attention focus, the forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on the equilibrium of an unstable object over the arm, but this effect was absent when they focused on the number, indicating that an external focus was only effective when the unstable object focused upon was relevant to the equilibrium of one's own body. No attentional intervention was effective during backward sliding of the support surface, indicating that central set for responding to postural perturbation depends on the direction of the postural perturbation.

Effort to perceive the position of one visual horizontal line relative to another appearing close causes an earlier postural response to backward perturbation.

Humans make an effort to stabilize the body when a stable eye position is required to improve visibility. Perceiving the position of one visual object relative to another appearing close is relatively difficult compared with perceiving those objects appearing at a distance. Thus, humans must make an effort to stabilize their body to improve visibility when they attempt to perceive the position of one object relative to another appearing close. This process may enhance the response to postural perturbation. The present study tested this hypothesis. A total of 15 healthy participants maintained a position standing over the platform and gazed at a horizontal fixation line in front of them. One of the two warning sound cues was given in each trial; one predicted the forthcoming appearance of a horizontal line 2 mm above or below the horizontal fixation line (indistinct condition), and another predicted the forthcoming appearance of the horizontal line 2 cm above or 2 cm below the horizontal fixation line (distinct condition). After the warning sound cue, the support surface moved in the forward or backward direction. A horizontal line appeared 40 ms after the onset of the support surface motion at the level above or below the horizontal fixation line with the distance from the horizontal fixation line as the warning cue predicted. The participants verbally stated the position of the horizontal line relative to the horizontal fixation line after the offset of the support surface movement. The displacement of the head and sacrum was measured with acceleration sensors. A test of the simple main effect following an analysis of variance revealed that the peak downward displacement of the pelvis in the indistinct condition was significantly earlier than that in the distinct condition. This finding indicated that the effort to stabilize the body for improvement of visibility enhances the response of the pelvis to the forward movement of the support surface. This effect may be used for improvement of the postural response in patients with problems with postural control.

Effect of variability of sequence length of go trials preceding a stop trial on ability of response inhibition in stop-signal task.

This study investigated whether the variability of the sequence length of the go trials preceding a stop trial enhanced or interfered with inhibitory control. The hypotheses tested were either inhibitory control improves when the sequence length of the go trials varies as a consequence of increased preparatory effort or it degrades as a consequence of the switching cost from the go trial to the stop trial. The right-handed participants abducted the left or right index finger in response to a go cue during the go trials. A stop cue was given at 50, 90, or 130 ms after the go cue, with 0.25 probability in the stop trial. In the less variable session, a stop trial was presented after two, three, or four consecutive go trials. In the variable session, a stop trial was presented after one, two, three, four, or five consecutive go trials. The reaction time and stop-signal reaction time were not significantly different between the sessions and between the response sides. Nevertheless, the probability of successful inhibition of the right-hand response in the variable session was higher than that in the less variable session when the stop cue was given 50 ms after a go cue. This finding supports the view that preparatory effort due to less predictability of the chance of a forthcoming response inhibition enhances the ability of the right-hand response inhibition when the stop process begins earlier.

Postural perturbation does not reset stepping rhythm in humans, but brief intermission does.

In the present study, we tested a hypothesis that the rhythm generator in humans keeps the rhythm of periodic motor output during brief inactivation of the pattern generator. This investigation was made through testing whether the step rhythm was reset after an interruptive event. A reset of the step rhythm was defined as an observation that the step re-emerges at random timing after an interruptive event regardless of the step rhythm before the interruption. This observation reflects an intermission of rhythm-keeping activity. Healthy participants stepped on a platform that could translate forward or backward. They continued stepping after the platform translation (non-stop session) or stopped briefly after the translation before resuming step with their own timing (stop session). In the non-stop session, the second step after the platform translation appeared at the integer multiple of the pre-existing step period in most participants, indicating that step rhythm was not reset. This finding indicates that postural perturbation does not interfere the rhythm-keeping activity. In the stop session, the step immediately after the intermission of stepping appeared at random time regardless of the step rhythm before the intermission in most participants. The actual side of the first step after the intermission was consistent with the predicted first step side at a 0.5 probability. Those findings indicate that step rhythm is reset after brief intermission of stepping, and contradict with the hypothesis that the activity of the rhythm generator is maintained, while the pattern generator is temporally inactive during a brief intermission of periodic motor output. This analysis could help to determine whether rhythm-keeping activity is inactivated by an interruptive event during periodic motor activity.