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 spectr al analysis in a subject with cortical myoclonus. Methods: The coherence, phase and cumulant density estimates were calculate d between right electrocorticographic (ECoG) signals and distal left leg mu scles in a patient with subdural electrodes inserted over the right sensori motor cortex. Results: Significant coherence between sensorimotor cortex and muscle was f ound up to 60 Hz during voluntary induced myoclonic jerks. Additional highe r frequency coherence (similar to 140 and 190 Hz) was found during sensory- induced myoclonic jerks. The cortical signals phase led muscle signals at f requencies >15 Hz by delays consistent with transmission along corticospina l pathways, Below 15 Hz the cortex phase lagged the muscle signals. Polarit y reversal of the cumulant density estimate and the ECoG site demonstrating the highest coherence helped to localize the site of the abnormal oscillat ory 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 imp lications for cortex-muscle latency measures calculated by back-averaging t echniques. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.