Perturbation-evoked cortical activity reflects both the context and consequence of postural instability

The cerebral cortex may play a role in the control of compensatory balance reactions by optimizing these responses to suit the task conditions and/or to stimulus (i.e. perturbation) characteristics. These possible contributions appear to be reflected by pre-perturbation and post-perturbation cortical activity. While studies have explored the characteristics and possible meaning of these different events (pre- vs. post-) there is little insight into the possible association between them. The purpose of this study was to explore whether pre- and post-perturbation cortical events are associated or whether they reflect different control processes linked to the control of balance. Twelve participants were presented temporally-predictable postural perturbations under four test conditions. The Block/Random tasks were designed to assess modifiability in CNS gain prior to instability, while the Unconstrained/Constrained tasks assessed responsiveness to the magnitude of instability. Perturbations were evoked by releasing a cable which held the participant in a forward lean position. The magnitude of pre-perturbation cortical activity scaled to perturbation amplitude when the magnitude of the perturbation was predictable [F(3,11)=2.906, P<0.05]. The amplitude of pre-perturbation cortical activity was large when the size of the forthcoming perturbation was unknown (13.8 + or - 7.9, 11.4 + or - 9.9, 16.9 + or - 9.3, and 16.1 + or - 10.6 muV for the Block Unconstrained and Constrained and Random Unconstrained and Constrained, respectively). In addition, N1 amplitude scaled to perturbation amplitude regardless of whether the size of the forthcoming perturbation was known (30.1 + or - 17.7, 11.4 + or - 7.1, 30.9 + or - 18.4, 12.4 + or - 6.1 muV). This is the first work to examine modifiability in the pre-perturbation cortical activity related to postural set alterations. The cerebral cortex differentially processes independent components prior to and following postural instability to generate compensatory responses linked to the conditions under which instability is experienced.