Alpha oscillations as an indicator of dynamic memory operations - anticipation of omitted stimuli.

Elaborating on a paradigm of Başar in experiments with periodically presented auditory or visual stimuli, 10 subjects performed tasks in which omitted stimuli had to be anticipated and their (virtual) onset time had either to be marked (a) mentally ('mental marking' MM) or (b) by pressing a button ('button pressing' BP). EEG was obtained from 11 sites in frontal, central, temporal, parietal and occipital areas. The experiments were carried out in triplets of MM, BP, MM measurements. Mean temporal errors of motor responses served as the basis for a functional interpretation of EEG patterns. Correlation analysis of sweeps for each individual revealed brief intervals of phase ordering of EEG patterns in the alpha range lasting approximately three periods. For frontal and vertex derivations a close congruence of the location of the phase-ordered patterns to mean errors of motor responding was shown. These results corroborate the claim that 'emitted' alpha oscillations represent intentional processing. The close agreement between temporal locations of phase-ordered EEG segments and those of the motor responses strongly suggests that EEG phases represent memory retrieval of period duration as a common functional component of MM and BP. A lack of topographical correspondence with results from a passive 'missing stimulus' paradigm underlines the specific intentional origin of the observed phase ordering. Theoretical implications of the results are discussed.