Magnetic brain oscillations and auditory memory

Electromagnetic oscillatory activity in the gamma-band range (GBA, about 30–100Hz) has been proposed as a correlate of cortical network synchronization involved in cognitive processing. Increases in electroencephalographic GBA have been demonstrated during a variety of paradigms including gestalt perception, attention, learning and memory. In magnetoencephalogram topographically focal GBA enhancements have been found in fast frequencies (about 50–90Hz) during both bottom-up and top-down driven processing of auditory information. Studies on passive auditory mismatch processing have shown that the detection of changes in sound positions was accompanied by increased GBA over posterior temporo-parietal areas. In contrast, deviations in auditory patterns (e.g., syllables or animal vocalizations) induced oscillatory activity over left anterior temporal and inferior frontal cortex. The topography of these findings supported the notion of dual processing streams for pattern versus spatial information („what“ and „where“ streams, respectively) in the auditory system. During separate delayed matching-to-sample tasks for sound positions and patterns, enhanced GBA was found during the delay phases over posterior parietal and inferior frontal cortex, respectively. In addition, both prefrontal GBA and gamma coherence between putative auditory stream areas and prefrontal cortex were increased during stimulus maintenance in short-term memory. In contrast, an active echoic memory task did not involve prefrontal activations or connectivity increases. This suggests that only the retention of auditory information in short-term memory requires coupling between higher-order sensory areas and putative prefrontal executive networks. In summary, magnetoencephalographic GBA serves as a tool for the investigation of cortical auditory processing at a good spatial and high temporal resolution. Supported by Deutsche Forschungsgemeinschaft (SFB 550/C1).