Article

Simultaneous EEG-fMRI during a Working Memory Task: Modulations in Low and High Frequency Bands

Ecole Polytechnique Fédérale de Lausanne, Switzerland
PLoS ONE (Impact Factor: 3.23). 04/2010; 5(4):e10298. DOI: 10.1371/journal.pone.0010298
Source: PubMed

ABSTRACT

EEG studies of working memory (WM) have demonstrated load dependent frequency band modulations. FMRI studies have localized load modulated activity to the dorsolateral prefrontal cortex (DLPFC), medial prefrontal cortex (MPFC), and posterior parietal cortex (PPC). Recently, an EEG-fMRI study found that low frequency band (theta and alpha) activity negatively correlated with the BOLD signal during the retention phase of a WM task. However, the coupling of higher (beta and gamma) frequencies with the BOLD signal during WM is unknown.

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    • "The main features found in this study are consistent with literature reports of working memory. In [9], it is discussed that changes due to working memory load are often observed in the theta power of midline electrodes, and alpha power in occipital electrodes. "
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    ABSTRACT: We investigate the neural correlates of visual working memory using electroencephalography (EEG). Our objective is to develop a cognitive Brain-Computer Interface (BCI) able to monitor visual working memory load in real-time. A system with these properties would eventually have different applications, such as training, rehabilitation, or safety while operating dangerous machinery. The BCI performances were evaluated using cross-validation. With an appropriately chosen classification threshold, it is possible to detect high working memory load with a sensitivity of 68% and a specificity of 72%. However, it is well known that some subjects are BCI illiterate, meaning that up to 30% of the users have too high signal variability to use EEG-based BCI systems. If we analyse each subject individually, it is possible to detect high working memory load with a sensitivity of 78% and a specificity of 81% (accuracy = 81%) for a typical good subject. Changes due to visual working memory load were observed in frontal, parietal, and occipital regions.
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    • "However no significant difference in the θ-band was observed [22]. Moreover, in an EEG/fMRI study, θ, β and γ bands demonstrated positive load effects, while lower alpha showed negative effects [23]. "
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    • "Neverthe less , one hypothesis can be derived from the literature : Based on studies showing increased pre - stimulus DMN activity leading to deteriorated task performance ( Esposito et al . , 2006 ; Li et al . , 2007 ) , and the findings on load - dependent increases in theta power during WM tasks ( Onton et al . , 2005 ; Meltzer et al . , 2007 ; Michels et al . , 2008 , 2010 , 2012 ; Huang et al . , 2013 ) , we expected load - dependent decreases in pre - stimulus DMN activity to be predictive of load - dependent EEG theta power increases during the task ."
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    ABSTRACT: Working memory (WM) processes depend on our momentary mental state and therefore exhibit considerable fluctuations. Here, we investigate the interplay of task-preparatory and task-related brain activity as represented by pre-stimulus BOLD-fluctuations and spectral EEG from the retention periods of a visual WM task. Visual WM is used to maintain sensory information in the brain enabling the performance of cognitive operations and is associated with mental health. We tested 22 subjects simultaneously with EEG and fMRI while performing a visuo-verbal Sternberg task with two different loads, allowing for the temporal separation of preparation, encoding, retention and retrieval periods. Four temporally coherent networks (TCNs)-the default mode network (DMN), the dorsal attention, the right and the left WM network-were extracted from the continuous BOLD data by means of a group ICA. Subsequently, the modulatory effect of these networks' pre-stimulus activation upon retention-related EEG activity in the theta, alpha, and beta frequencies was analyzed. The obtained results are informative in the context of state-dependent information processing. We were able to replicate two well-known load-dependent effects: the frontal-midline theta increase during the task and the decrease of pre-stimulus DMN activity. As our main finding, these two measures seem to depend on each other as the significant negative correlations at frontal-midline channels suggested. Thus, suppressed pre-stimulus DMN levels facilitated later task related frontal midline theta increases. In general, based on previous findings that neuronal coupling in different frequency bands may underlie distinct functions in WM retention, our results suggest that processes reflected by spectral oscillations during retention seem not only to be "online" synchronized with activity in different attention-related networks but are also modulated by activity in these networks during preparation intervals.
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