Abstract
Recent studies demonstrated that the corticospinal pathway is one of the key nodes for the feedback control of human standing and that the excitability is flexibly changed according to the current state of posture. However, it has been unclear whether this pathway is also involved in a predictive control of human standing. Here, we investigated whether the corticospinal excitability of the soleus (SOL) and tibialis anterior (TA) muscles during standing would be modulated anticipatorily when perturbation was impending. We measured the motor-evoked potential (MEP) induced by transcranial magnetic stimulation over the motor cortex at six stimulus intensities. Three experimental conditions were set depending on predictabilities about perturbation occurrence and onset: No perturbation, No Cue, and Cue conditions. In the Cue condition, an acoustic signal was given as timing information of perturbation. The slope of the stimulus–response relation curve revealed that the TA-MEP was enhanced when postural perturbation was expected compared to when the perturbation was not expected (No Perturbation vs. No Cue, 0.023 ± 0.004 vs. 0.042 ± 0.007; No Perturbation vs. Cue, 0.023 ± 0.004 vs. 0.050 ± 0.009; Bonferroni correction, p = 0.01, respectively). In addition, two-way analysis of variance (intensity × condition) revealed the main effect of condition (F(1,13) = 6.31, p = 0.03) but not intensity and interaction when the MEP amplitude of the Cue and No Cue conditions was normalized by that in No Perturbation, suggesting the enhancement more apparent when timing information was given. The SOL-MEP was not modulated even when perturbation was expected, but it slightly reduced due to the timing information. The results of an additional experiment confirmed that the acoustic cue by itself did not affect the TA- and SOL-MEPs. Our findings suggest that a prediction of a future state of standing balance modulates the corticospinal excitability in the TA, and that the additional timing information facilitates this modulation. The corticospinal pathway thus appears to be involved in mechanisms of the predictive control as well as feedback control of standing posture.
Highlights
MATERIALS AND METHODSIt has been established that the corticospinal pathway is involved in the regulation of human bipedal standing
In the No Cue and Cue conditions, the tibialis anterior (TA)-motor-evoked potential (MEP) were clearly enhanced compared to those in the No Perturbation condition, whereas no significant difference was found in the SOL-MEPs
The subsequent post hoc test revealed that the slopes of the TA-MEP in the No Cue and Cue conditions were both significantly larger than that in the No Perturbation condition (No Perturbation 0.023 ± 0.004 vs. No Cue 0.042 ± 0.007, p = 0.01; No Perturbation 0.023 ± 0.004 vs. Cue 0.050 ± 0.009, p = 0.01), whereas no significant difference was observed in the slope of the TA-MEP between the No Cue and Cue conditions (No Cue vs. Cue, p = 0.14)
Summary
MATERIALS AND METHODSIt has been established that the corticospinal pathway is involved in the regulation of human bipedal standing. The excitability of the corticospinal pathways in the tibialis anterior (TA) and soleus (SOL) muscles was enhanced while the subjects stood on an unstable rocking plate (Solopova et al, 2003), whereas it was decreased during supported standing (Tokuno et al, 2009). These findings suggest that the contribution of the corticospinal pathway increases when standing posture is exposed to unstable conditions. Since the long-latency EMG response is mediated by the transcortical pathway (Petersen et al, 1998; Taube et al, 2006), the effects of prediction on the corticospinal pathway may be inferred in part based on the long-latency EMG response
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