Abstract
In simultaneous electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) studies, average reference (AR), and digitally linked mastoid (LM) are popular re-referencing techniques in event-related potential (ERP) analyses. However, they may introduce their own physiological signals and alter the EEG/ERP outcome. A reference electrode standardization technique (REST) that calculated a reference point at infinity was proposed to solve this problem. To confirm the advantage of REST in ERP analyses of synchronous EEG-fMRI studies, we compared the reference effect of AR, LM, and REST on task-related ERP results of a working memory task during an fMRI scan. As we hypothesized, we found that the adopted reference did not change the topography map of ERP components (N1 and P300 in the present study), but it did alter the task-related effect on ERP components. LM decreased or eliminated the visual working memory (VWM) load effect on P300, and the AR distorted the distribution of VWM location-related effect at left posterior electrodes as shown in the statistical parametric scalp mapping (SPSM) of N1. ERP cortical source estimates, which are independent of the EEG reference choice, were used as the golden standard to infer the relative utility of different references on the ERP task-related effect. By comparison, REST reference provided a more integrated and reasonable result. These results were further confirmed by the results of fMRI activations and a corresponding EEG-only study. Thus, we recommend the REST, especially with a realistic head model, as the optimal reference method for ERP data analysis in simultaneous EEG-fMRI studies.
Highlights
In electroencephalogram (EEG) and event-related potential (ERP) research, the reference issue is an important problem
The results revealed bilateral activations in postcentral gyrus (PC), superior temporal gyrus (STG), middle temporal gyrus (MT), and superior occipital gyrus (SOG), as well as left hemisphere activations in superior parietal lobule (SPL), and right hemisphere activations in supramarginal gyrus (SMG), and insula (INS; Table 1)
The results showed that the location repeated enhancement was reflected at bilateral posterior N1 and the visual working memory (VWM) load suppression was reflected at central P300, which is consistent with the ERP results in the present EEG-functional magnetic resonance imaging (fMRI) study when using average reference (AR) and two reference electrode standardization technique (REST) references
Summary
In electroencephalogram (EEG) and event-related potential (ERP) research, the reference issue is an important problem. Using simulated data, previous studies showed that AR and LM references lead to significant distortion of scalp power distribution and scalp network structure (Yao et al, 2005; Qin et al, 2010). This occurs because using scalp recordings as reference, like AR and LM, would bring their own physiological dynamic signals into the EEG signal and affect the spatial and temporal aspects of the EEG signal (Yao, 2001; Thatcher, 2012). To minimize the effect of physical reference on EEG signals, Yao proposed a reference electrode standardization technique (REST) which calculated a reference point at infinity (Yao, 2001)
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