Effects of memory load on cortical oscillatory activity during auditory pattern working memory.

The present magnetoencephalography study investigated memory load-dependent changes in cortical oscillatory activity during a modified auditory version of the Sternberg paradigm. Twelve subjects were presented with test stimulus sets of 1-3 syllables spoken in a natural female voice. After an 800-ms delay period, a probe syllable was presented and subjects had to judge whether the probe had been included in the preceding test set. Compared to a non-memory-control task, memory trials elicited an increase of beta activity over right temporal regions and an increase of alpha activity over right middle prefrontal cortex at the end of the delay phase. Monotonic increases in spectral amplitude as a function of memory load were revealed for the beta band over right temporal sensors and the alpha band over right frontal sensors during the delay period. The results demonstrate the relevance of both beta and alpha oscillations for the memorization of multiple stimuli. The former may be associated with the representation of task-relevant stimulus features, while the latter may reflect the top-down control of these representations.