Ageing and brain plasticity. Training-induced alterations in spectral power of the brain and motor coordination

Declines in sensorimotor and cognitive function are well-known consequences of ageing. Improving our understanding of what ageing entails at cellular up to systemic levels is hence timely and urgent. And, finding strategies to decelerate or even reverse the pervasive effects of aging can have a major societal impact. The aim of this thesis was to investigate changes in spectral dynamics of the brain associated with bimanual task difficulty, motor learning and ageing. We provided a concise literature review of the electro- and magneto-encephalographic (EEG/MEG) studies on bimanual coordination. This review revealed converging evidence for (1) stronger suppression of spectral power during motor execution of bimanual compared to unimanual movements, (2) the involvement of supplementary motor area (SMA), premotor cortex, sensorimotor cortices (M1/S1), anterior cingulate cortex and cerebellum during anti-phase compared to in-phase movements, and for (3) increases of inter-hemispheric connectivity in the beta band for bimanual compared to unimanual movements. The review also revealed gaps in the literature, which motivated two experimental studies.

One of these gaps was related to the study of continuous non-isofrequency rotational movements. Therefore, we investigated how the brain copes with task difficulty in non-isofrequency bimanual movements. Beta (15-30 Hz) power in the non-dominant (right) hemisphere was modulated with task difficulty, presumably reflecting stronger control of the right M1/S1 over the non-dominant (left) hand. By using realistic head models, we were able to estimate neural activity in the cortex, and assessed functional connectivity in terms of beta-band synchronisation, in a bilateral network including M1/S1, dorsal premotor cortex and the precuneus. Beta synchronisation increased with increasing task demand. Further confirmation of the increases in beta synchronisation as a coping mechanism of the brain to deal with task difficulty was found in the positive correlation between the connectivity of the network and the performance error.

Another gap identified in the literature review concerned the neural correlates of motor learning without online feedback across extended practice. Importantly, age-related alterations in learning strategies had not been studied in the context of EEG/MEG spectral dynamics across different frequency bands and motor task variations. We therefore investigated learning strategies supported by spectral dynamics in the theta (4-8 Hz), alpha (8-12 Hz) and beta bands, and how these differed between young and older adults. Importantly, we found different spectral dynamics over different brain regions underlying the retrieval of motor memories. Additionally, training-induced changes in the last compared to the first session were observed over the prefrontal cortex (PFC) in the theta band, and over M1/S1 in the alpha and beta bands. Finally, age-related effects were mainly found in the beta band covering parietal, central and medial PFC. All these findings were accompanied by a reduced learning gain in the older compared to the young adults.

In this thesis, we provided an overview of the EEG/MEG literature on bimanual coordination which may serve as a starting point to any scientist new to the subject. We highlighted the role of the beta power and long-range synchronisation in the execution of different precision movements beyond the well-known role in the initiation of movement and sustainment of muscle contractions. We provided a comprehensive view of learning-related spectral changes by investigating theta, alpha and beta bands which usually are studied individually. Finally, at a time when life expectancy is constantly increasing thanks to medical improvements, ageing-related functional changes in the brain ought to be understood. We hope that the present work contributed to this understanding by detailing learning- and age-related modulations in frequency-specific dynamics in the cortex.