Browsing by Subject "Beamforming"

Objectives: According to previous observations, the magnitude of variation in the working memory-related electroencephalographic (EEG) alpha oscillations between different subjects is remarkable. The purpose of this study was (1) to find out whether the standard deviation in the oscillatory responses between subjects and within the subject is greater in the alpha band than in the lower or higher frequencies, (2) to examine individual oscillatory responses of subjects, and (3) to localize the neural sources of working memory encoding and retrieval related alpha synchronization and desynchronization and study the effects of different source localizations to the observed oscillatory responses during an auditory working memory task. Methods: Eighteen subjects performed a modified version of Sternberg's memory search paradigm in which they were auditorily presented memory sets consisting of five words. The subjects' task was to decide whether the probe word was included in the previously presented memory set. EEG was recorded during the task and the data was analyzed using the event-related synchronization/desynchronization (ERS/ERD), clustering, and beamforming methods. Both averaged and individual alpha band event-related spectral perturbations (ERSP), standard deviations of ERSP responses between and within the subjects, power spectra of the subjects, and source localization results of the ERSP responses were examined. Results and conclusions: ERSP responses averaged across the subjects support previous studies: Alpha-band ERS was perceived during the memory encoding and alpha-band ERD during the memory retrieval. Standard deviations of the ERSP responses both in the group and individual level were greater in the alpha band than in the lower or higher frequencies. The individual ERSP responses varied remarkably which is not observed in studies that examine only averaged ERSP responses. Calculating distances between individual and averaged ERSP responses and clustering the obtained values provided important information on the individual variation of these responses. Memory encoding -related alpha ERS and memory retrieval -related alpha ERD sources were localized in the occipital and parietal brain areas when these responses were similar to those obtained via averaging across the subjects. This finding, which is new in connection with auditorily presented stimuli, corresponds to the results of previously conducted visual working memory studies and reveals important general level knowledge of connections between working memory processes and oscillatory EEG responses.