Motor Imagery and Corticospinal Excitability: A Brain-state-dependent TMS approach based on the μ-Rhythm

This thesis investigated how ongoing tonic and phasic fluctuations of sensorimotor brain activity influence corticospinal excitability. Specifically, the study examined motor imagery-related activity and sensorimotor μ-rhythm as well as their relationship with corticospinal excitability. Electroencephalography (EEG) was used to continuously record brain activity and estimate the instantaneous phase of sensorimotor μ rhythm. At specific phases of this rhythm, corresponding to its positive and negative peaks, transcranial magnetic stimulation (TMS) was delivered over the primary cortex. The resulting corticospinal response was measured through motor-evoked potentials (MEPs). This brain-state-dependent TMS approach was applied during two different conditions: motor imagery and a control condition. This design allowed us to investigate whether the TMS-evoked corticospinal response was influenced by the neurophysiological state of the sensorimotor cortex at the time of stimulation, including both the tonic fluctuations associated with motor imagery and phasic fluctuations indexed by the instantaneous phase of the μ-rhythm. The thesis is organized into four chapters. Chapter 1 introduces the main concepts related to cortical and corticospinal excitability and presents the main techniques used to study them, with particular focus on TMS and the state-dependency framework. This framework proposes that the effects of brain stimulation may depend on the state of brain activity at the moment stimulation is delivered. Chapter 2 focuses on two factors that may influence corticospinal excitability: motor imagery, the mental simulation of a movement without actually performing it, and sensorimotor μ-oscillations, rhythmic patterns of brain activity associated with motor processing. Chapter 3 describes the experimental study, including its methodology, results, and discussion. Thirty participants completed Motor Imagery and Motor Control conditions while brain activity was continuously recorded using EEG. TMS pulses were delivered in relation to different phases of the ongoing sensorimotor μ-rhythm, and corticospinal excitability was assessed through MEP amplitude. The results showed greater MEP amplitudes during the Motor Imagery condition than during the Motor Control condition, indicating a tonic difference in corticospinal excitability between the two conditions. In addition, during the Motor Control condition, corticospinal excitability varied according to the phase of the μ rhythm; this phase-dependent effect was not observed during the Motor Imagery condition. These findings suggest that phasic fluctuations in ongoing sensorimotor activity can index distinct instantaneous brain states and that their relationship with corticospinal excitability depends on the tonic state associated with motor imagery. Finally, Chapter 4 summarizes the main findings of the thesis and discusses their meaning within the state-dependency framework, together with the main limitations of the study and possible directions for future research.