Temporal dynamics of stimulus-specific gamma-band activity components during auditory short-term memory.

Recently we have demonstrated that during auditory short-term memory maintenance, gamma-band activity (GBA) components can be identified which are specific to the retained stimulus. These activations peaked in the middle of the delay phase between sample and test stimuli, and their magnitude during the final part of this period correlated with performance. However, using a constant delay duration did not allow to answer the question whether stimulus-specific GBA components represented responses to sample sounds or anticipatory activations preceding test stimuli. Here we addressed this unresolved issue by investigating the temporal dynamics of stimulus-specific GBA during two delay durations. Magnetoencephalogram was recorded in 18 adults during an auditory spatial short-term memory task involving lateralized sample stimuli presented with two different interaural time delays. Subjects had to decide whether test stimuli presented after retention phases of 800 or 1200 ms had the same lateralization as sample sounds. Statistical probability mapping served to identify oscillatory activations differentiating between the two sample sounds. We found stimulus-specific GBA components over posterior cortex peaking about 400 ms prior to the onset of test stimuli regardless of delay duration. Their magnitude correlated with task performance. In summary, stimulus-specific GBA components with a predictive power for short-term memory performance were observed in anticipation of test stimuli. They may reflect the preparatory activation of memory representations or the shifting of attention to the specific expected location of the test stimulus.