Mu rhythm, visual processing and motor control

The Mu-rhythm has been proposed as both an inhibitor ("idling hypothesis") and as a promoter ("processing hypothesis") of information processing in the primary sensory-motor cortex (SM-C). We tested these possibilities by analyzing the phase-locked and non-phase-locked Mu response during the execution of a visual-motor task.

EEG was recorded in 13 subjects during the visual presentation of an arrow which indicated the direction of the finger motion to be executed after the presentation of a second stimulus. The EEG activity in the α-range (Mu-α) and β-range was evaluated by a method which segregated the phase-locked and the non-phase-locked response. The event-related Mu-response observed during this task was compared with that computed when the subjects saw the same arrow-stimuli but did not perform any task (passive test).

Visual stimuli induced a phase-locked α-oscillation which began ≈50ms after the stimulus onset and persisted for about 150-200ms. This response was much higher when stimuli were used for motion planning than when they were passively observed, and was more marked in the α-range than in the β-range. The phase-locked response was followed by a persistent decrease of the non-phase-locked Mu-activity similar to that previously reported with the event-related desynchronization/synchronization method.

The Mu-wave is not a single phenomenon. It was segregated here into two components, one with an early and short-lasting phase locked-response to visual stimuli, which increased during the task execution, and the other without phase-locked responses which persistently decreased during the task execution.

Present data suggest that Mu-activity performs a double action, increasing the information processing of one task (according to the "processing hypothesis") and decreasing the computation of other potentially interfering tasks (according to the "idling hypothesis"), with task selection being achieved by choosing their phase-association to the Mu-wave.