Time-Frequency Analysis of Motor-Evoked Potential in Patients with Stroke vs Healthy Subjects: a Transcranial Magnetic Stimulation Study

Conventional analysis of motor-evoked potential (MEP) is performed in time domain using amplitude and latency, which encapsulates information relevant to the cortical excitability of the brain. The study investigated the importance of time-frequency analysis by comparing MEPs in time-frequency domains (TFD) of healthy versus stroke survivors. Six healthy subjects and ten patients with stroke were enrolled. Single-pulse transcranial magnetic stimulation (TMS) at resting motor threshold (RMT) was given at extensor digitorum communis muscle cortical representation to obtain MEP. MEPs were obtained at resting motor threshold (100% RMT subjects and patients), supra-threshold range (100–170% RMT), and different voluntary contractions (100% RMT) to subjects. Fast Fourier transform and continuous wavelet transform (CWT) were used for analysis. Frequency spectrum showed 98% and 66% of signal power in 0–100 Hz for subjects and patients, respectively. Top 10, top 25, and top 50 percentile power of CWT were calculated for each MEP. Frequency spectrum of top 10 and top 25 percentile power of subjects were different (p < 0.05) and dispersed to 0–500 Hz for patients; both groups having a 40-Hz peak. Total power of MEP was found to be low (p < 0.05) in patients as compared to subjects and top 10, top 25, and top 50 percentile power showed decrease. Clinical scores—MAS and FM—were observed to be correlated to frequency and time-frequency features (p < 0.05). Frequency spectrum belonging top 10 percentile power of different level voluntary contractions showed statistical significance (p < 0.05). However, no significant differences were observed for MEPs at different supra-threshold intensities. Results suggest time-frequency analysis might provide objective ways to quantify TMS measures for stroke patients.