EEG correlates of verbal and nonverbal working memory

Background: Distinct cognitive processes support verbal and nonverbal working memory, with verbal memory depending specifically on the subvocal rehearsal of items. Methods: We recorded scalp EEG while subjects performed a Sternberg task. In each trial, subjects judged whether a probe item was one of the three items in a study list. Lists were composed of stimuli from one of five pools whose items either were verbally rehearsable (letters, words, pictures of common objects) or resistant to verbal rehearsal (sinusoidal grating patterns, single dot locations). Results: We found oscillatory correlates unique to verbal stimuli in the theta (4-8 Hz), alpha (9-12 Hz), beta (14-28 Hz), and gamma (30-50 Hz) frequency bands. Verbal stimuli generally elicited greater power than did nonverbal stimuli. Enhanced verbal power was found bilaterally in the theta band, over frontal and occipital areas in the alpha and beta bands, and centrally in the gamma band. When we looked specifically for cases where oscillatory power in the time interval between item presentations was greater than oscillatory power during item presentation, we found enhanced beta activity in the frontal and occipital regions. Conclusion: These results implicate stimulus-induced oscillatory activity in verbal working memory and beta activity in the process of subvocal rehearsal.