Many electrophysiology studies have examined neural oscillatory activity during
the encoding, maintenance, and/or retrieval phases of various working memory tasks.
Together, these studies have helped illuminate the underlying neural dynamics, although
much remains to be discovered and some findings have not replicated in subsequent work. In
this study, we examined the oscillatory dynamics that serve visual working memory
operations using high-density magnetoencephalography (MEG) and advanced time-frequency and
beamforming methodology. Specifically, we recorded healthy adults while they performed a
high-load, Sternberg-type working memory task, and focused on the encoding and maintenance
phases. We found significant 9–16 Hz desynchronizations in the bilateral occipital
cortices, left dorsolateral prefrontal cortex (DLPFC), and left superior temporal areas
throughout the encoding phase. Our analysis of the dynamics showed that the left DLPFC and
superior temporal desynchronization became stronger as a function of time during the
encoding period, and was sustained throughout most of the maintenance phase until sharply
decreasing in the milliseconds preceding retrieval. In contrast, desynchronization in
occipital areas became weaker as a function of time during encoding and eventually evolved
into a strong synchronization during the maintenance period, consistent with previous
studies. These results provide clear evidence of dynamic network-level processes during
the encoding and maintenance phases of working memory, and support the notion of a dynamic
pattern of functionally-discrete subprocesses within each working memory phase. The
presence of such dynamic oscillatory networks may be a potential source of inconsistent
findings in this literature, as neural activity within these networks changes dramatically
with time.