Behavioral/Systems/Cognitive Hippocampal Gamma Oscillations Increase with Memory Load

Although the hippocampus plays a crucial role in encoding and retrieval of contextually mediated episodic memories, considerable controversy surrounds the role of the hippocampus in short-term or working memory. To examine both hippocampal and neocortical contributions to workingmemory function, we recorded electrocorticographic activity fromwidespread cortical and subcortical sites as 20 neurosurgical patients performed working memory tasks. These recordings revealed significant increases in 48–90 Hz gamma oscil-latory powerwithmemory load for two classes of stimuli: letters and faces. Sites exhibiting gamma increaseswithmemory load appeared primarily in the hippocampus andmedial temporal lobe. These findings implicate gammaoscillatory activity in themaintenance of both letters and faces inworkingmemory and provide the first direct evidence formodulation of hippocampal gamma oscillations as humans perform a working memory task