Abstract
During reactive balance, the central nervous system briefly speeds up neural processing beyond what is typical for voluntary actions. We propose this increased processing speed is accomplished by a phasic increase in norepinephrine (NE) release throughout the brain and spinal cord, whereby NE acts as a neuromodulator to make neurons more responsive to simple inputs. Phasic NE release can be indirectly measured in humans from pupil dilation (PD). We measured PD with a wearable pupillometry device, and lower limb activation with electromyography (EMG) while participants engaged in a reactive balance task. Participants (N=19; mean age = 20.3; 7 women) wore a safety harness attached by a cable to an electromagnet, allowing them to plant their feet and lean away from a wall with the cable as support. In half of 80 trials, a loud beep coincided with the electromagnet being shut off, removing support such that participants had to move a foot forward to reestablish balance. In the other half of trials, there was no electromagnet release, and the loud beep cued participants to step forward as fast as they could without a balance perturbation. PDs were larger on perturbation trials than non-perturbation trials, t(18)=3.0, p<.01. This effect approached significance for the EMG data, t(18)=2.0, p=.06. Interestingly, on perturbation trials, large-PD trials exhibited greater EMG activity than small-PD trials, t(18)=3.2, p<.01. This effect approached significance for no-perturbation trials, t(18)=1.9, p=.07 These results provide preliminary evidence linking NE to reactive balance control.