EEG-blocking before and during voluntary movements: differences between the eyes-closed and the eyes-open condition.

Movement-related EEG periods of 1 s duration were analyzed by fast Fourier transformation in the Bereitschaftspotential (BP) paradigm in 21 healthy, right-handed normals. Spectra of the rest period and the BP period before the onset of the voluntary movement, and of the rest period and the movement period, were compared in order to describe blocking effects before and during movement execution (fast fist closure of the right hand). When comparing the rest period and the movement period with the eyes closed as well as with the eyes open, blocking was pronounced in both conditions within the total frequency range of the alpha-band, especially at positions contralateral to the movement over the motor cortex (C3') and the parietal cortex (P3) as well as at the parietal midline electrode (Pz). Moreover, when comparing the spectra of the rest and the BP periods in the eyes-closed condition, significant differences were again seen, with lower values during the BP period at C3' and P3. We were, however, able to observe over the parietal cortex an increase of alpha power in the 10 Hz range during the BP period with the eyes opened and fixated. This increase during the BP period in the eyes-open condition was not found when the eyes were closed. This paradoxical phenomenon of an increase of power with the eyes opened instead of blocking during the BP period, as observed in the eyes closed, may be due to an additional activation effect on the already activated (blocked) EEG in the eyes-open condition during fixation.