Semmler, JohnOpie, GeorgeLiao, Wei-Yeh Merrick2024-09-122024-09-122024https://hdl.handle.net/2440/142298Learning and modifying complex motor behaviour is a lifelong skill that requires efficient movement planning and execution, but this ability declines with advancing age. One possible reason for this decline is age-related changes in neuroplasticity, which refers to the ability of the brain to modify its structure, function, and connections with new experiences. An important area within the brain that regulates motor plasticity and mediates movement planning is the dorsal premotor cortex (PMd), which passes movement plans to the primary motor cortex (M1) for voluntary movement control. While previous studies using transcranial magnetic stimulation (TMS) have demonstrated reduced plasticity within M1 and weaker PMd-M1 effective connectivity in older adults, the mechanisms that underpin this decline remain elusive. Therefore, the overarching aim of this thesis was to investigate age-related changes in PMd-M1 communication, and whether strengthening this communication can enhance motor learning in older adults. To achieve this, I first characterised the influence of PMd on different M1 intracortical circuits known as indirect (I) wave circuits (early & late I-waves), which are important components of plasticity and learning. I applied the plasticity-inducing paradigm intermittent theta burst stimulation (iTBS) to PMd and assessed the effects on I-wave excitability (study 1, Chapter 2). In a novel finding, I demonstrated that PMd iTBS potentiated the excitability of both early and late I-waves, but the effects on early I-waves were weaker in older adults. As I-wave circuits are important for M1 plasticity, I then investigated the influence of PMd on M1 plasticity in young and older adults (study 2, Chapter 3). I applied PMd iTBS as a priming intervention in young and older adults and assessed how this modulated the neuroplastic response to subsequent M1 iTBS. Interestingly, PMd iTBS lowered the potentiation of early I-waves to M1 iTBS in both young and older adults, suggesting that the influence of PMd on M1 plasticity can be preserved with ageing and may involve early I-wave circuits. Finally, I assessed whether PMd iTBS can enhance performance in older adults during a visuomotor adaptation task (study 3, Chapter 4). The results demonstrated that PMd iTBS attenuated visuomotor adaptation in young, but had no effect in older adults, providing new evidence for an altered functional role of PMd in regulating visuomotor behaviour with advancing age. Taken together, these studies provide new evidence on how ageing modifies different connections within PMd-M1 communication, with important implications for plasticity and behaviour in older adults.enTranscranial magnetic stimulationageingprimary motor cortexneuroplasticitypremotor cortexThesis