Functional dissociation of lower and upper frequency mu rhythms in relation to voluntary limb movement

Objective: The goal of this study is to investigate the reactivity of central rhythms in the alpha band during self-paced voluntary finger and foot movement and to give an answer to the question, whether different types of mu rhythms exist. Methods: The effect of self-paced, voluntary finger and foot movement was studied in a group of 12 right-handed healthy volunteers. The EEG was recorded from a grid of 34 electrodes placed over sensorimotor areas with inter-electrode distances of approximately 2.5 cm. The event-related desynchronization (ERD) was quantified in the 8–10 and 10–12 Hz bands. Results: Both frequency components are blocked prior to and during movement and therefore, they have to be considered as mu rhythms. The lower frequency component results in a widespread movement-type non-specific ERD pattern, whereas the upper frequency component shows a more focused and movement-type specific pattern, clearly different with finger and foot movement. Conclusions: The distinct reactivity patterns provide evidence for the existence of two types of mu rhythms, a somatotopically non-specific lower frequency mu rhythm and a somatotopically specific mu rhythm characteristically found in the upper alpha frequency band.