Perturbation-evoked cortical responses are associated with balance ability in healthy young adults and in older adults with Parkinson's disease

Abstract

Balance and cognitive impairments negatively impact quality of life in old age and in Parkinson's disease (PD) and are associated through unknown mechanisms. Measuring brain activity during reactive balance recovery may yield insight into the relationship between balance and cognitive function, facilitating the development of better treatment strategies. Electroencephalography (EEG) recordings show that sudden perturbations to standing balance reliably evoke a cortical N1 response localized to the supplementary motor area. The cortical N1 response is known to be influenced by cognitive processes because it is smaller when perturbations are predictable and enhanced when people are afraid or paying more attention to balance. Because the cortical N1 response is evoked by balance perturbations and influenced by cognition, it has the potential to reflect a site of interaction between balance and cognitive function. Despite knowledge of these cognitive influences on the cortical N1 response, there are no existing theories of how the cortical N1 response might influence subsequent balance recovery behavior. Through a series of studies, I present a novel hypothesis that perturbation-evoked cortical responses reflect cortical contributions to balance recovery, which are greater in people with lower balance ability. First, I show in healthy young adults (HYA) that a very small proportion of the cortical N1 response amplitude can be explained by the magnitude of sensory inputs, and that cortical N1 responses differ to a much larger extent between individuals. Then, I demonstrate that cortical N1 responses are larger for HYA who have lower balance ability, and that the cortical N1 response is larger when people take compensatory steps, suggesting that the cortical N1 response may reflect the need for cortically-mediated compensatory motor outputs. Then, I show that perturbation-evoked cortical responses in older adults with and without PD contain two component peaks, with smaller amplitudes of the second peak reflecting balance impairment in PD. Finally, I show that perturbation-evoked cortical responses are impacted by PD in a different manner than a related cortical response called the error-related negativity, which occurs when mistakes are made in computer-based cognitive tasks. The basic science work presented in this thesis may inform future studies into the relationship between balance and cognitive impairments, thereby facilitating the development of better rehabilitation strategies for balance impairments in Parkinson's disease.