Hippocampal gamma oscillations increase with working memory load

Functional imaging has implicated a diverse network of brain regions supporting working memory—the capacity to maintain multiple item representations over brief intervals. However, considerable controversy surrounds the role of the hippocampus in working memory. To simultaneously examine both hippocampal and neocorti-cal contributions to working memory, we recorded electroencephalographic activity from 1,454 widespread cortical and subcortical sites as 20 neurosurgical patients performed working memory tasks involving either letters or faces. These recordings revealed significant increases in 28–128 Hz gamma oscillatory power and significant decreases in 4–9 Hz theta oscillatory power with increasing memory load for both letters and faces. Sites exhibiting gamma increases with memory load appeared primarily in the hippocampus and medial temporal lobe, whereas those exhibiting decreases in theta activity appeared primarily in visual and temporal cortex. These findings implicate theta and gamma oscillatory activity in the maintenance of both letters and faces in working memory, and they provide the first direct evidence for modulation of hippocampal gamma oscillations in a working memory task