Aging Effect on Effective Connectivity within the Prefrontal-Motor Network and Postural-Related Muscle Responses During Dual-Task Paradigms

Abstract

Falls are a significant global public issue for older adults, leading to severe health and social impacts. To understand the underlying mechanism of increased fall risk with aging, loss of postural automaticity has been suggested as increased prefrontal cortex and motor cortices activation, as well as worse balance performance in response to the secondary cognitive task. However, the interaction within these cortical regions and its effects on postural-related muscle responses are poorly understood. This dissertation investigated age-related changes in effective connectivity within the prefrontal-motor network, postural related lower limb muscle response, and biomechanical characteristics in response to the secondary cognitive task. The first experiment of this dissertation measured prefrontal cortex activation and center of pressure data in both age groups during single-task and dual-task standing. The results showed a greater center of pressure displacement, velocity, and oscillation at 0-0.5 Hz in older adults compared to young adults during dual-task standing. For prefrontal cortex activation, only young adults presented greater magnitude in during-task compared to single-task standing. Additionally, the center of pressure oscillations below 0.5 Hz were positively associated with prefrontal cortex activation in both groups. The second experiment of this dissertation measured the cortical activation within the prefrontal-motor network and postural-related lower limb muscle activity in young and older adults during both single-task and dual-task standing. The results indicated that only older adults had greater activation in the prefrontal-motor cortex and greater ankle joint muscle co-activation index during dual-task compared to single-task standing. The secondary analysis further reported that older adults showed stronger top-down effective connectivity index from the dorsolateral prefrontal cortex to the supplementary motor area and primary motor cortex compared to young adults in both task conditions. This failure to inhibit top-down connection in older adults may be associated with increased variability of inter-foot coordination and elevated inter-muscular coherence in the beta and gamma bands during dual-task standing. The third experiment of this dissertation also measured the cortical activation within the prefrontal-motor network and postural-related lower limb muscle activity in young and older adults during signal-task and dual-task standing while maintaining a forward-leaning posture. The results showed that older adults presented greater effective connectivity index from the dorsolateral prefrontal cortex to the supplementary motor area/premotor cortex and smaller ankle joint co-activation index compared to young adults during these tasks. Collectively, this dissertation makes two major contributions: 1) analyzing center of pressure oscillations across different frequency bands may serve as a valuable biomechanical biomarker for clinically assessing age-related loss of postural automaticity; and 2) the age-related loss of postural automaticity may be attributed to deficits in inhibitory control within the prefrontal-motor network, leading to impaired reciprocal control of agonist-antagonist muscles.