The temporal, spatial, and frequency dimensions of neural oscillations associated with verbal working memory

Cognitive operations engage neural generators oscillating at different frequencies distributed in time and space. Accordingly, oscillatory activity detected by magnetoencephalography (MEG)/electroencephalography (EEG) should be analyzed along frequency, time, and spatial dimensions. MEG data were obtained from 19 healthy individuals while performing a modified Sternberg paradigm. The stimuli were letters, which constituted words or pronounceable nonwords. We applied tridimensional analysis of oscillations and also computed event-related fields (ERFs) in areas where significant changes in oscillatory activity were observed. Verbal working memory for visual verbal stimuli was associated with oscillatory interplay between the bilateral occipital lobes and the left frontoparietotemporal areas. Spatially stable occipital desynchrony was noted during information encoding, while a left hemisphere desynchronization, increasing in amplitude and spatial extent over time, was observed during information encoding and maintenance. No ERF changes were detected during information maintenance. Oscillatory activity associated with verbal working memory is consistent with the above hypothesis. These findings underscore the importance of multidimensional evaluation of oscillations. The findings also indicate that combining electrophysiological methods increase the chance of signal detection.