Pre-encoding gamma-band activity during auditory working memory

Previous magnetoencephalography (MEG) studies have revealed gamma-band activity at sensors over parietal and fronto-temporal cortex during the delay phase of auditory spatial and non-spatial match-to-sample tasks, respectively. While this activity was interpreted as reflecting the memory maintenance of sound features, we noted that task-related activation differences might have been present already prior to the onset of the sample stimulus. The present study focused on the interval between a visual cue indicating which sound feature was to be memorized (lateralization or pitch) and sample sound presentation to test for task-related activation differences preceding stimulus encoding. MEG spectral activity was analyzed with cluster randomization tests (N = 15). Whereas there were no differences in frequencies below 40 Hz, gamma-band spectral amplitude (about 50-65 and 90-100 Hz) was higher for the lateralization than the pitch task. This activity was localized at right posterior and central sensors and present for several hundred ms after task cue offset. Activity at 50-65 Hz was also increased throughout the delay phase for the lateralization compared with the pitch task. Apparently cortical networks related to auditory spatial processing were activated after participants had been informed about the task.