The subsequent memory paradigm, according to which cerebral activity for later remembered (LR) and later forgotten (LF) items is contrasted, can be used to characterize the processes necessary for successful memory encoding. Previous simultaneous electroencephalography/functional magnetic resonance imaging (EEG/fMRI) memory studies suggest an inverse relationship between frontal theta band power and the blood oxygenation level dependent (BOLD) signal in the default mode network (DMN). The principal aim of this EEG/fMRI study was to test the hypothesis that this putative theta-DMN relationship is less evident in LF compared with LR trials. Fourteen healthy participants performed an episodic memory task in which pictorial stimuli were presented during encoding, and categorized (as LR or LF) by subsequent memory performance. For each encoding trial, the mean of the Hilbert envelope of the theta signal from 400 to 800 ms after stimulus presentation was calculated. To integrate the EEG and fMRI data, general linear models (GLMs) were used to assess the extent to which these single-trial theta values (as modulators of the main effect of stimulus) predicted DMN BOLD signal change, using: (i) whole-head univariate GLMs and (ii) GLMs in which the outcome variable was the time-course of a DMN component derived from spatial independent component analysis of the fMRI data. Theta was significantly greater for LR than LF stimuli. Furthermore, the inverse relationship between theta and BOLD in the DMN was consistently stronger for LR than LF pictures. These findings imply that theta oscillations are key to attenuating processes which may otherwise impair memory encoding.

This study examines EEG low frequency characteristics which have been linked to specific cognitive functions such as stimulus encoding and attention during an auditory oddball task in schizophrenia patients and healthy controls. EEG data was recorded from 17 young schizophrenia patients in a stable phase of their illness and 17 healthy controls performing an auditory oddball task. Evoked and induced delta and theta activity, N100, P300 amplitude were computed. Between 200-500 ms after a stimulus was presented, patients displayed significantly reduced P300, less evoked and induced delta and theta activity than controls. We conclude that the well known finding of P300 reduction in schizophrenia can be linked to reductions in delta and theta activity, which are a manifestation of impaired stimulus evaluation, memory retrieval, and a lack of sustained attention.

No abstract