Theta oscillations index human hippocampal activation during a working memory task

Abstract

Working memory (WM) is the ability to retain and associate information over brief time intervals. Functional imaging studies demonstrate that WM is mediated by a distributed network including frontal and posterior cortices, hippocampus, 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 paper we report the observation of a similar phenomenon in normal human subjects. Neuromagnetic responses were recorded during presentation of a set of digits and a subsequent probe of the retained items. All stimuli were presented with a fixed temporal pattern. We observed phase reset of Ϸ7 Hz theta in left hippocampus Ϸ120 ms after probe stimuli, whereas reset of theta in right hippocampus was visible Ϸ80 ms prior to these anticipated stimuli. The duration of stimulus-locked theta increased with memory load, with a limiting value of Ϸ600 ms for 5-7 retained items. We suggest that, as in rats, stimulus-locked theta may index involvement of human hippocampal networks in the cognitive processing of sensory input. The anticipatory phase reset of theta indicates involvement 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 processing of potentially salient and͞or novel input with respect to a context established by the contents of WM.

Potentiation of Gamma Oscillatory Activity through Repetitive Transcranial Magnetic Stimulation of the Dorsolateral Prefrontal Cortex

“…Although changes in oscillatory activity in the g-frequency band are most consistently observed, increases in the y-, a-, and b-frequency bands have also been associated with cognitive tasks (Gevins et al, 1997;Klimesch et al, 2001;Klimesch et al, 1997;Klimesch, 1999;Sarnthein et al, 1998;TallonBaudry et al, 1999;von Stein et al, 1999;Tesche and Karhu, 2000;Raghavachari et al, 2001;Jensen et al, 2002;Schack et al, 2002;Howard et al, 2003;Rizzuto et al, 2003;Gruber and Muller, 2005;Kaiser and Lutzenberger, 2005). In line with these studies, we found an increase in d-, y-, a-, and b-activity with WM load.…”

Increased mesiotemporal delta activity characterizes virtual navigation in humans

“…These report reconstructed sources of theta within hippocampal (or parahippocampal) structures during task performance (Tesche & Karhu, 2000;Cornwell et al, 2008). Given that source reconstruction models suffer from important limitations in detecting sources in deep brain structures with a close geometry (Alarcon et al 1994;Wiederin et al 1999) these results are unconvincing.…”

“…This is an important issue since in the human literature there is abundant data on theta that was found during virtual navigation (Kahana et al, 1999;de Araújo et al, 2002;Bischof & Boulanger, 2003;Ekstrom et al, 2005) working memory tasks (Tesche & Karhu, 2000;Raghavachari et al, 2001;Schauseng et al, 2004) and REM sleep (Cantero et al, 2003 ). These studies report theta appearing in short and phasic bursts instead of continuous activity typically seen in animals.…”

Delta and theta oscillations as risk markers in adolescent offspring of alcoholics

“…These neuroelectric oscillations when analyzed in the context of stimulus-related brain function can be termed event related oscillations or EROs. Two competing theories have tried to explain the genesis of averaged event-related potential (ERP) features and dominate the current literature: (a) Evoked model -that ERPs originate from an event-related activation of neural assemblies distinct from background dynamics and (b) Phase-reset model -that ERPs are produced by phase resetting of ongoing oscillatory activity (Basar, 1980;Brandt, 1997;Makeig et al, 2002;Schack and Klimesch, 2002;TallonBaudry et al, 1999;Tesche and Karhu, 2000). The debate is ongoing; however, it is likely that both aspects -event-related activation of neural assemblies and phase resetting of ongoing activity, are involved in the generation of late ERP components.…”