Generalizability of perturbation-evoked cortical potentials: Independence from sensory, motor and overall postural state.

Following disturbances to postural stability, balance recovery reactions are evoked by numerous sensory inputs and characterized by motor reactions involving different patterns of activity, depending on postural task conditions. It remains unknown whether well-documented cortical responses to instability share common spatio-temporal characteristics, despite variations in the sensory, motor, and postural components of the reactions. The objective was to explore the spatio-temporal profile of cortical potentials evoked by instability requiring either upper- or lower-limb compensatory responses. The hypothesis that upper- and lower-limb balance-correcting reactions are associated with evoked cortical potentials (N1, P2) featuring similar spatio-temporal characteristics was tested by inducing postural perturbations in seated (SIT) or standing (STAND) positions. For both conditions, N1 amplitude was greatest at FCz, with no significant differences in the timing of N1 peak (SIT: 142.4+/-7.95ms; STAND: 148.4+/-4.10ms) or N1 amplitude (SIT: 37.16+/-6.99microV; STAND: 39.08+/-4.51microV). The amplitude of the P2 potential (measured at CPz) was significantly larger in the STAND condition (37.87+/-6.14microV) than in the SIT (23.66+/-6.21microV) condition. Significant differences in P2 peak time between tasks were absent (SIT: 319.9+/-11.45ms; STAND: 322.7+/-7.61ms). Though differences in the amplitude of components of evoked potentials may reflect the extent of cortical involvement in different aspects of postural control, similarities in the spatio-temporal components of cortical potentials between tasks reflects generalizable cortical processing of unexpected stimuli independent of the sensory, motor, or postural aspects of the response.

Phasic electrodermal responses associated with whole-body instability: presence and influence of expectation.

While much is understood about somatic contributions to postural control, there is less consideration for the potential involvement of the autonomic nervous system (ANS) as integral for maintenance of stability. The purpose of this study was to examine autonomic responses, as measured through electrodermal recordings, evoked in response to whole-body perturbations to standing balance. We hypothesized that phasic electrodermal responses (EDRs) would be consistently observed in response to evoked perturbations and that response amplitude would depend on the capacity to predict perturbation timing. Temporally unpredictable and self-initiated (predictable) backward perturbations evoked in healthy participants (n=15) elicited compensatory feet-in-place reactions with tibialis anterior activation 125.1+/-60.2 ms following perturbation onset. EDRs were consistently observed starting 1883.6+/-329.1 ms after perturbation and reaching their peak at 4016.6+/-896.9 ms. Amplitude was significantly larger in the unpredictable task (1.1+/-0.84 micromho) compared to the predictable task (0.45+/-0.55 micromho, P<0.001). Amplitude was largest in the first block of five trials (P<0.0001), then remained constant for subsequent trials in each condition. Post-hoc analysis indicated that trials with an unplanned compensatory step (3.5%) were 137.0+/-176.6% larger than feet-in-place reactions (P=0.02). Elevated EDRs during initial trials and unanticipated reactions suggest that these measures could be used to assess the perceived 'novelty' of applied perturbations, having implications for interpreting characteristics of the evoked somatic reactions. The persistence of perturbation-evoked EDRs even after thirty trials may also highlight an important role for phasic ANS responses in compensatory postural control.