Spectral analysis of event-related EEG responses during short-term memory performance

Averaged event-related EEG frequency response profiles were generated from normalized spectral analysis data obtained in 2 studies of short-term memory. A continuous performance task was used requiring a keyboard response to targeted letter sequences presented on a video screen at 2 sec. intervals. Seven 2 Hz frequency bands between 5-15 Hz were evaluated. In the first study (n=14) response profiles from control and target stimuli were compared. This analysis disclosed 1) a short latency increase at 5-7 and 7-9 Hz in posterior cortex that was identical in both conditions and was attributed to frequency manifestations of the extrinsic visual evoked response (VEP), 2) a separate long-latency increase of uncertain origin at 5-7 Hz in anterior cortex only in the task condition, and 3) a generalized mid-latency alpha frequency suppression and recovery pattern (event-related desynchronization, or ERD) in all bands during both conditions. The ERD in bands between 9-13 Hz was significantly increased at left central and bilateral parietal cortex during target recall. The second study (n=26) compared good vs. poor performance in the target recall task Findings disclosed significant differences at 7-9 Hz localized to posterior temporal cortex bilaterally. These differences included reduced magnitude in the VEP component and increased suppression in the ERD component among good performers.