Scott E. Boebinger
BME PhD Defense Presentation

Date: 2025-01-17
Time: 10am
Location / Meeting Link: HSRB-II N600 / https://emory.zoom.us/j/91068578484

Committee Members:
Lena H. Ting; J. Lucas McKay; Michael R. Borich; Svjetlana Miocinovic; Lewis A. Wheaton


Title: Cortical Contributions to Balance Control Across Aging and Parkinson's Disease

Abstract:
Control of balance is a fundamental aspect of human motor control as it underlies many other activities and balance impairments can profoundly impact an individual’s quality of life. Balance is not controlled by a single neural substrate. Instead, it relies on the integration of multiple substrates throughout the nervous system, which are subject to different delays associated with neural transmission and processing. As balance performance declines, such as in aging and impairments like Parkinson’s disease (PD), the control of balance shifts from being primarily mediated by brainstem sensorimotor circuits to involve higher order circuits such as the basal ganglia and cortex. However, we lack a mechanistic understanding of how this shift to involve higher order centers during balance control alters motor output, which limits our ability to develop effective rehabilitation strategies. A more comprehensive, mechanistic approach to dissociate higher order contributions to balance control may improve our assessments of balance health, which could subsequently lead to earlier rehabilitation interventions prior to the occurrence of a fall. In a series of studies, I tested the hypothesis that parallel sensorimotor feedback loops engaging brainstem and higher-order circuitry contribute to balance control with higher-order contributions increasing with challenge level, aging, and PD. First, I developed a neuromechanical model in a healthy young adult dataset that is able to decompose perturbation-evoked agonist muscle activity into hierarchical components based on latency. This work showed that similar transformations from center of mass motion to motor output occur at multiple levels of the nervous system at latencies consistent with brainstem and cortical mediation and that the contribution of cortically-mediated muscle activity increases balance challenge. I then applied this agonist neuromechanical model to data collected from older adults (OAs) with and without PD and showed that both groups also have muscle activity that can be explained by the sensorimotor transformation of center of mass kinematics at latencies consistent with cortical mediation. In addition to the agonist neuromechanical model, I also applied a similar neuromechanical model to reconstruct perturbation-evoked antagonist muscle activity and showed that these hierarchical components are related to clinical measures of an individual’s balance ability in older adults, but not individuals with PD, where more activity from higher order centers being related to lower measures of balance ability. This basic science work presented in this dissertation aims to improve assessments of balance health. Engagement of cortical resources during balance control is larger in aging and increases further with impairment. Balance function could potentially be maintained without noticeable detriment to clinical assessments of balance health if engagement of higher order centers sufficiently compensates for inadequate subcortical sensorimotor circuits. Following this logic, it is feasible that the availability of both cortical and subcortical resources has already reached critically low levels by the time of an individual’s first fall. Since it is unclear when this shift in the hierarchical control of balance occurs, individuals may not seek rehabilitation interventions until their balance ability has substantially declined. Therefore, developing a model that can assess contributions of different neural substrates during balance control could lead to earlier identification of cortical engagement prior to clinically noticeable detriments to balance health. This would subsequently improve our ability to assess balance health, which is a key step to develop better assessments of balance health and guide mechanistically-based rehabilitation interventions.