Author(s): MALONE, Q., TUTT, H., REEKIE, P.K., DAKIN, C.J., MCNEIL, C.J., DALTON, B.H., Institution: UNIVERSITY OF BRITISH COLUMBIA: OKANAGAN, Country: CANADA, Abstract-ID: 1438

INTRODUCTION:
Vestibular control of balance is temporarily downregulated upon initiation of a goal-directed movement (e.g., reaching, gait, and posture transition), which fits with the optimal feedback control theory. This theory states that a sensation that does not contribute to, or causes interference with, a planned movement is downregulated (i.e., not used to plan the movement). With the previously examined tasks, it was not possible to isolate the effects of cognitive planning and transitions from the motor response with the previously examined tasks. As such, this experiment was designed to determine if a predominantly cognitive task also causes this vestibular downregulation. We hypothesised that initiation of a cognitive task response would cause a downregulation of vestibular control of balance.
METHODS:
While standing on a force plate measuring ground reaction forces (GRFs), participants (N=18) received binaural, bipolar stochastic (0-25Hz, root mean squared≈1mA) electrical vestibular stimulation (EVS). Meanwhile, participants performed two tasks: 1) a simple reaction time (SRT) task requiring a button-press in response to a visual cue, and 2) a Stroop task requiring a verbal response to the textual colour of a presented word. These tasks were chosen because both involve a minimal motor response, yet require varying levels of cognitive complexity; a factor which influences many motor outcomes. Mean coherence between the EVS signal and the mediolateral GRF signal from 0-10Hz was calculated in 5ms steps over 5000ms centered around the time of movement onset. Two-tailed, paired-sample t-tests were used to compare mean coherence 1000-600ms before movement onset and from 200ms before to 200ms after movement onset.
RESULTS:
Contrary to our hypothesis, there was a significant increase in coherence upon initiation of the SRT task response (t17=2.95, p=0.01) and no differences for the Stroop task (t17 = 1.92, p=0.07).
CONCLUSION:
These results imply that previously reported drops in vestibular control of balance are likely due to the transition to a gross movement, rather than a result of cognitive processes occurring during a transition. Potential explanations for the vestibular downregulation during a transition to a gross movement include: 1) a technical artifact arising from the data analysis technique used, or 2) a result of changes in head movement variability (which is positively associated with EVS-GRF coherence). That is, initiation of a gross movement causes anticipatory postural adjustments that stabilize the body and reduce head movement variability, which could downregulate the vestibular control of balance (lower EVS-GRF coherence). In contrast, during the transition to fine movements, there is no voluntary command to drive the head in a particular direction, so the balance controller remains reliant on vestibular cues to maintain posture.