Abstract
Transcranial magnetic stimulation (TMS) activates neurons in the cortex non-invasively. TMS has proven a valuable therapeutic tool for the treatment of neurological and psychiatric disorders. One example is the investigation and treatment of motor disorders such as stroke. However, responses to TMS are subject to inter- and intra-individual variability, and these trends become accentuated in stroke, which warrants a real-time approach that accounts for ongoing fluctuations in neural activity. Based on previous findings in healthy volunteers, we hypothesize that the phase of neural oscillations at the motor cortex modulates responses to TMS in stroke survivors. Therefore, the objective of this study was to evaluate the contribution of real-time oscillation phase to TMS responses in persons with unilateral, chronic stroke. To investigate the relationship between oscillation phase and cortical excitability, we systematically investigated responses to TMS at different oscillation phases of the Mu (8-13 Hz) rhythm in both hemispheres of the motor cortex. We used a novel real-time TMS-electroencephalogram (EEG) system, which allows for instantaneous triggering of TMS based on EEG signals. Cortical excitability at different phases was tested by measuring the motor-evoked potential (MEP) by using single pulse TMS at the hotspot of the first dorsal interosseus (FDI) muscle. Preliminary results (n = 4) suggested significant effect of phase on MEP amplitude (F = 11.31, p < 0.001). MEPs tended to be larger at the trough compared to the peak phase of the mu oscillation. However, significant variability between individuals and between paretic and non-paretic hand are observed. These results suggest that optimization based on ongoing brain state crucially determines TMS responses. Thus, controlling for neural oscillations phase in real-time could hold the key to effective TMS-based therapies for neurological impairments.