Phase relationships between cortical and muscle oscillations in cortical myoclonus: electrocorticographic assessment in a single case.

AIM: To compare voluntary- and sensory-induced myoclonic jerks using spectral analysis in a subject with cortical myoclonus. METHODS: The coherence, phase and cumulant density estimates were calculated between right electrocorticographic (ECoG) signals and distal left leg muscles in a patient with subdural electrodes inserted over the right sensorimotor cortex. RESULTS: Significant coherence between sensorimotor cortex and muscle was found up to 60 Hz during voluntary induced myoclonic jerks. Additional higher frequency coherence ( approximately 140 and 190 Hz) was found during sensory-induced myoclonic jerks. The cortical signals phase led muscle signals at frequencies >15 Hz by delays consistent with transmission along corticospinal pathways. Below 15 Hz the cortex phase lagged the muscle signals. Polarity reversal of the cumulant density estimate and the ECoG site demonstrating the highest coherence helped to localize the site of the abnormal oscillatory activity to the leg area of the motor cortex. CONCLUSIONS: Oscillations of different frequencies can co-exist at a given location and can both phase lead and lag contralateral muscle. This has implications for cortex-muscle latency measures calculated by back-averaging techniques.