Direct measurements of the brain mechanisms of motor control in ageing

This research investigates the modulatory effects of cortico-cortical paired associative stimulation (ccPAS) on interhemispheric inhibition (IHI) and cortical excitability within the human motor cortex (M1), focusing on variations due to stimulation direction and participant age. The study comprises two distinct age groups: young adults and older adults, aiming to understand the age-dependent characteristics of neuroplasticity induced by ccPAS. The young adult group was divided into three subgroups to compare left-to-right M1-M1 ccPAS, right-to-left M1-M1 ccPAS, and a control group receiving sham stimulation. The older adult group received only left-to-right M1-M1 ccPAS and a sham condition to mitigate fatigue concerns. The research adopts a postpositivist epistemological foundation, utilising empirical, observable evidence and quantitative methods to gather data. Through rigorous experimental design, including randomisation and blinding procedures, the study ensures the validity and reliability of its findings. A systematic review of the existing ccPAS literature was conducted to inform the study design and methodology. This review revealed that ccPAS has consistently demonstrated the capacity to modulate corticospinal excitability and interhemispheric inhibition in targeted brain regions. Findings from prior studies underscore the importance of stimulation directionality, the temporal specificity of effects, and the potential clinical applications of ccPAS in neurorehabilitation. The review also identified research gaps, including the limited understanding of age-related differences and temporal dynamics, which guided the formulation of the study’s hypotheses and experimental protocols. The findings from both experiments revealed that M1-M1 ccPAS, particularly in the right-to-left direction, significantly modulates corticospinal excitability and interhemispheric inhibition in young adults. In older adults, left-to-right ccPAS affects cortical excitability in the motor cortex but does not modulate interhemispheric inhibition. Moreover, there was no significant age-dependent difference in the modulation of cortical excitability by left-to-right ccPAS, indicating that the effects of this intervention are consistent across different age groups. These effects develop over time, indicating temporal dynamics in cortical modulation. Overall, the study underscores the complexity of cortico-cortical communication and the factors influencing neuroplasticity. It highlights the potential of ccPAS as a non-invasive intervention to modulate cortical excitability and interhemispheric inhibition while also pointing out the limitations and variations in effectiveness. These insights are crucial for developing more effective, age-specific neurorehabilitation strategies and advancing our understanding of the neural mechanisms underlying plasticity and cortical communication.