Theta oscillations index human hippocampal activation during a working memory task

Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is me diated by a distributed network including frontal and posterior cortices, h ippocampus, and cerebellum. In rodents, the presentation of stimuli in a WM task is followed by a reset of the phase of hippocampal theta. In this pap er we report the observation of a similar phenomenon in normal human subjec ts. Neuromagnetic responses were recorded during presentation of a set of d igits and a subsequent probe of the retained items. All stimuli were presen ted with a fixed temporal pattern. We observed phase reset of approximate t o 7 Hz theta in left hippocampus approximate to 120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible approximate to 80 ms prior to these anticipated stimuli. The duration of stimulus-locked thet a increased with memory load, with a limiting value of approximate to 600 m s for 5-7 retained items. We suggest that, as in rats, stimulus-locked thet a may index involvement of human hippocampal networks in the cognitive proc essing of sensory input. The anticipatory phase reset of theta indicates in volvement of hippocampus in right hemisphere and cerebellar timing networks . Hippocampal structures are essential for orientation to perturbations in the sensory scene, a function that requires use of a context established by a constellation of stimuli, We suggest that the initiation and maintenance of stimulus-locked hippocampal theta observed here may facilitate processi ng of potentially salient and/or novel input with respect to a context esta blished by the contents of WM.