Short-Term Reproduction of Active Movement with Visual Feedback and Passive Movement with a Therapist's Hands.

Reproducing instructed movements is crucial for practice in motor learning. In this study, we compared the short-term reproduction of active pelvis movements with visual feedback and passive movement with the therapist's hands in an upright stance. Sixteen healthy males (M age = 34.1; SD = 10.2 years) participated in this study. In one condition, healthy males maintained an upright stance while a physical therapist moved the participant's pelvis (passive movement instruction), and in a second condition, the participant actively moved their pelvis with visual feedback of the target and the online trajectory of the center of pressure (active movement instruction). Reproduction errors (displacement of the center of pressure in the medial-lateral axis) 10 s after the passive movement instruction were significantly greater than after the active movement instruction (p < 0.001), but this difference disappeared 30 s after the instruction (p = 0.118). Error of movement reproduction in the anterior-posterior axis after the passive movement instruction was significantly greater than after the active movement instruction, no matter how long the retention interval was between the instruction and reproduction phases (p = 0.025). Taken together, active pelvis movements with visual feedback, rather than passive movement with the therapist's hand, is better to be used for instructing pelvis movements.

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.

Descending motor command to prime mover of dependent finger induces tactile gating in target and distant non-target finger.

This study examined whether tactile gating induced by the descending motor command to one finger spreads out to the other fingers to which the command is not delivered and whether this gating is dependent on the target finger to which the command is delivered. The change in perceptual threshold to the digital nerve stimulation of one finger induced by tonic contraction of the first dorsal interosseous or abductor digiti minimi muscle was examined. The perceptual threshold to the digital nerve stimulation of the thumb or little finger was increased by tonic contraction of the abductor digiti minimi muscle. This finding indicates that the descending motor command to the prime mover of the little finger abduction induces tactile gating not only in the finger to which the command is delivered but also in the other finger to which the command is not delivered. Tonic contraction of the first dorsal interosseous muscle did not change the perceptual threshold to the digital nerve stimulation in any finger. This finding means that tactile gating occurs particularly when the descending motor command is delivered to the dependent finger. Spreading out of tactile gating of one finger, to which the descending motor command is not delivered, is likely mediated by surround inhibition.

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.

Afferent volley from the digital nerve induces short-latency facilitation of perceptual sensitivity and primary sensory cortex excitability.

The present study examined whether the perceptual sensitivity and excitability of the primary sensory cortex are modulated by the afferent volley from the digital nerve of a conditioned finger within a short period of time. The perceptual threshold of an electrical stimulus to the index finger (test stimulus) was decreased by a conditioning stimulus to the index finger 4 or 6 ms before the test stimulus, or by a stimulus to the middle or ring finger 2 ms before that. This is explained by the view that the afferent volleys from the digital nerves of the fingers converge in the somatosensory areas, causing spatial summation of the afferent inputs through a small number of synaptic relays, leading to the facilitation of perceptual sensitivity. The N20 component of the somatosensory-evoked potential was facilitated by a conditioning stimulus to the middle finger 4 ms before a test stimulus or to the thumb 2 ms before the test stimulus. This is explained by the view that the afferent volley from the digital nerve of the finger adjacent to the tested finger induces lateral facilitation of the representation of the tested finger in the primary sensory cortex through a small number of synaptic relays.

Suppression of perceptual sensitivity to digital nerve stimulation induced by afferent volley from digital nerve of contralateral homologous finger.

The purpose of the present study is to investigate whether perceptual sensitivity to digital nerve stimulation is modulated by the afferent volley from the digital nerve of a contralateral finger. Fifteen healthy humans participated in this study. A test stimulus was given to the right-hand index finger, and a conditioning stimulus was given to one of the five fingers on the left hand 20, 30, or 40 ms before the test stimulus. The perceptual threshold of the finger stimulation was measured. The perceptual threshold of the test stimulus was significantly increased by a conditioning stimulus to the left-hand index finger given 40 ms before the test stimulus. In contrast, the threshold was not significantly changed by a conditioning stimulus to any finger other than the index finger. Perceptual sensitivity to digital nerve stimulation is suppressed by the afferent volley from the digital nerve of the contralateral homologous finger. This means that the afferent volley from the digital nerve suppresses the homologous finger representation in the ipsilateral somatosensory areas. These findings can be explained by the view that the afferent volley from the digital nerve of the index finger projects to the index finger representation in the contralateral primary sensory cortex and that the interhemispheric transcallosal inhibitory drive is provided from the secondary sensory cortex to the homologous finger representation in the contralateral secondary sensory cortex.

Responses of stance leg muscles induced by support surface translation during gait.

This study determined the presence of the muscle responses to the support surface translation in the stance leg during gait and examined the effect of the direction and time point of the translation and that of the cognitive process on the responses. The rectus femoris (RF), biceps femoris (BF), soleus (SOL), and tibialis anterior (TA) muscles in the stance leg were tested. There was no significant effect of cognitive process on the electromyographic (EMG) activity induced by the translation of the support surface. In all muscles except the SOL, the EMG amplitude increased 0-300 â€‹ms after the support surface translation at the initial stance (IS) or middle stance (MS) of the tested leg. This means that the EMG activity in the leg muscles other than the SOL occurs after the support surface translation at the IS or MS no matter the direction of the translation. The EMG amplitude was not changed after the translation at the late stance, indicating that the translation does not influence the EMG amplitude at the double limb support phase with the tested leg behind the other. In the SOL, the EMG amplitude increased after the backward translation at the IS and after the forward translation at the MS, but decreased after the forward translation at the IS, indicating that the support surface translation-induced change in the EMG amplitude of the SOL is dependent on its direction. The change in the EMG amplitude of the TA and RF induced by the forward translation was greatest when the translation was given at the IS. In the SOL, the decrease in the EMG amplitude after the forward translation and the increase in the amplitude after the backward translation were greatest at the IS. Taken together, the change in the EMG amplitude induced by the support surface translation is greatest when the translation is given at the IS. The increase in the EMG amplitude in the TA and RF after the forward translation was greater than that after the backward translation at the IS, indicating that the EMG activity of the frontal leg muscles after the forward translation is greater when the translation is given at the IS.

Tactile Perception of Right Middle Fingertip Suppresses Excitability of Motor Cortex Supplying Right First Dorsal Interosseous Muscle.

The present study examined whether tactile perception of the fingertip modulates excitability of the motor cortex supplying the intrinsic hand muscle and whether this modulation is specific to the fingertip stimulated and the muscle and hand tested. Tactile stimulation was given to one of the five fingertips in the left or right hand, and transcranial magnetic stimulation eliciting motor evoked potential in the first dorsal interosseous muscle (FDI) or abductor digiti minimi was given 200 ms after the onset of tactile stimulation. The corticospinal excitability of the FDI at rest was suppressed by the tactile stimulation of the right middle fingertip, but such suppression was absent for the other fingers stimulated and for the other muscle or hand tested. The persistence and amplitude of the F-wave was not significantly influenced by tactile stimulation of the fingertip in the right hand. These findings indicate that tactile perception of the right middle fingertip suppresses excitability of the motor cortex supplying the right FDI at rest. The suppression of corticospinal excitability was absent during tonic contraction of the right FDI, indicating that the motor execution process interrupts the tactile perception-induced suppression of motor cortical excitability supplying the right FDI. These findings are in line with a view that the tactile perception of the right middle finger induces surround inhibition of the motor cortex supplying the prime mover of the finger neighboring the stimulated finger.

Influence of the Inter-Trial Interval, Movement Observation, and Hand Dominance on the Previous Trial Effect.

Previous studies have shown that current movement is influenced by the previous movement, which is known as the previous trial effect. In this study, we investigated the influence of the inter-trial interval, movement observation, and hand dominance on the previous trial effect of the non-target discrete movement. Right-handed healthy humans abducted the index finger in response to a start cue, and this task was repeated with constant inter-trial intervals. The absolute difference in the reaction time (RT) between the previous and current trials increased as the inter-trial interval increased. The absolute difference in RT reflects the reproducibility of the time taken for the motor execution between two consecutive trials. Thus, the finding supported the view that there is a carryover of movement information from one trial to the next, and that the underlying reproducibility of the RT between the two consecutive trials decays over time. This carryover of movement information is presumably conveyed by implicit short-term memory, which also decays within a short period of time. The correlation coefficient of the RT between the previous and current trials decreased with an increase in the inter-trial interval, indicating that the common responsiveness of two consecutive trials weakens over time. The absolute difference was smaller when the response was performed while observing finger movement, indicating that a carryover of the visual information to the next trial enhances the reproducibility of the motor execution process between consecutive trials. Hand dominance did not influence the absolute difference or correlation coefficient, indicating that the central process mediating previous trial effect of hand movement is not greatly lateralized.