Cortical Activity Associated With Changes in Sensory Contributions During Standing Balance Control

Maintenance of safe upright static stance regardless of sensory input is imperative for completing activities of daily living required for high quality of life. The current body of work aims to the potential involvement of the cortex in standing balance by examining the frequency of cortical activity associated with manipulation of task challenge and sensory contributions. The current study is focused manipulating sensory input (haptic touch, and vision) to explore if there was evidence of changes at the level fo cortex that may implicate cortical contributions in sensory processing during the control of stationary standing. Since altering sensory inputs will alter challenge it was necessary to determine the impact of changes in cortical activity related to changes in balance challenge independent of sensory status. It was hypothesized that increasing task challenge, by altering base of support, would result in an increase in theta frequency power ratio and a decrease in alpha frequency power ratio, specifically at frontocentral regions of the cortex. It was predicted that by including haptic touch there would be an increase in theta frequency power ratio and a decrease in alpha frequency power ratio, with accompanying increase in power in beta, delta, and gamma frequency bands around the Pz electrode. Finally, it was hypothesized that increasing visual availability would result in an increase in theta frequency power ratio and a decrease in alpha frequency power ratio, with accompanying increase in power in beta, delta, and gamma frequency bands around the Oz electrode. The study tested 12 healthy young subjects who performed 7 different tasks: 1) Tandem stance with eyes closed, 2) Narrow stance with eyes closed, 3) Standard stance with eyes closed, 4) Tandem stance with restricted visual field, 5) Tandem stance with eyes open but vision occluded, 6) Tandem stance eyes open, 7) Tandem stance with haptic touch. Cortical activity was measured using a 32 channel electroencephalography (EEG) system and balance control was measured from ground reaction forces used to calculate centre of pressure (COP). Frequency analysis was conducted and plotted topographically for qualitative evaluation. Mean power, within each band (delta, theta, alpha, beta, gamma) was calculated for specific electrode sites (Fz, Pz, Oz) and compared across task conditions. Mean ML (mediolateral) and AP (anteroposterior) COP and velocity was compared across task conditions. Overall, the manipulation of task challenge did result in significant changes in COP as a measure of task challenge, but there were only modest changes in cortical excitation that were largely characterized by an increase power of theta and alpha frequency. The main effect seen in frontocentral increase in power was observed for manipulation of task challenge (altered based of support) and changes in sensory inputs. With respect to changes in sensory information, the availability of haptic information was used to determine evidence of unilateral parietal cortical involvement that would have been consistent with the spatial specificity of the sensory input. Results supported the hypothesis of an increase in theta power and decrease in alpha power over the Pz electrode. With respect to visual inputs there was an expectation of varying activity in occipital power of theta and alpha frequency; when vision is available it was expected that theta power would increase and alpha power would decrease in the posterior regions of the cortex. Overall the current study confirms the expected impact of BOS and sensory input changes in on COP sway. However, the changes in cortical activity were more modest. Common across tasks of varying challenge (BOS or sensory) was increase in theta power in frontocentral region which may be associated with the N1 responses that are associated with reactive control. For sensory conditions there was some modest difference frequency of activity for haptic (Pz) and visual (Oz) though these were not statistically significant. The current study reinforces the involvement of the cortex in the control of reactive balance control but does not confirm a potential role for sensory processing. Future studies investigating delta, beta, and gamma frequency bands and sensory manipulation during standard stance should be undertaken to improve statistical power and reduction of potential confounding influence of differences associated with task challenge