Reduction of variability of EEG occipital, parietal, and central alpha rhythms by visual feedback stimulation.

Two experiments, each with 24 normal right-handed adults, examined variability of the response of EEG alpha rhythms during repeated visual stimulations that were contingent on the occurrence of those rhythms. Within-trial variability of alpha durations and no-alpha (alpha blocking) durations were recorded from bipolar derivations along two bilateral posterior-anterior lines. Variability was significantly lower for: (1) the contingent EEG connected to the stimulus compared to the contralateral EEG, which was recorded simultaneously but was not connected to the stimulus; (2) occipitoparietal EEGs compared to parietocentral EEGs; (3) alpha durations compared to no-alpha durations. Differences in variability among the four EEG locations on the left or the right side were significant for contingent EEGs but not for contralateral nonconnected EEGs. The results were interpreted to be a demonstration that feedback EEG method can be applied to research on the functional topography of an EEG response to sensory stimuli in terms of the reduction of variability of the response that can be achieved with feedback.