A Large N400 but No BOLD Effect – Comparing Source Activations of Semantic Priming in Simultaneous EEG-fMRI

Sebastian Geukes,Christian Dobel,Markus Junghöfer,Pienie Zwitserlood,René J Huster,Andreas Wollbrink

doi:10.1371/journal.pone.0084029

Sebastian Geukes, Christian Dobel + Show 4 more

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https://doi.org/10.1371/journal.pone.0084029

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Abstract

Numerous studies have reported neurophysiological effects of semantic priming in electroencephalography (EEG) and in functional magnetic resonance imaging (fMRI). Because of differing methodological constraints, the comparability of the observed effects remains unclear. To directly compare EEG and fMRI effects and neural sources of semantic priming, we conducted a semantic word-picture priming experiment while measuring EEG and fMRI simultaneously. The visually presented primes were pseudowords, words unrelated to the target, semantically related words and the identical names of the target. Distributed source analysis of the event-related potentials (ERPs) successfully revealed a large effect of semantic prime-target relatedness (the N400 effect), which was driven by activations in a left-temporal source region. However, no significantly differing activations between priming conditions were found in the fMRI data. Our results support the notion that, for joint interpretations of existing EEG and fMRI studies of semantic priming, we need to fully appreciate the respective methodological limitations. Second, they show that simultaneous EEG-fMRI, including ERP source localization, is a feasible and promising methodological advancement for the investigation of higher-cognitive processes. Third, they substantiate the finding that, compared to fMRI, ERPs are often more sensitive to subtle cognitive effects.

Highlights

The term semantic priming describes the facilitated processing of a meaningful stimulus when it is accompanied by another stimulus related in meaning [1]
The discovery of the N400 effect [7], an event-related potential (ERP) component in the EEG closely related to semantic priming, has greatly contributed to the understanding of the effect’s determinants and its functional significance
Differential EEG source activations for the four semantic priming conditions can be seen in the interval about 400 to 600 ms after picture onset, corresponding with the pattern visible in the sensor data (Fig. 2)

Summary

Introduction

The term semantic priming describes the facilitated processing of a meaningful stimulus when it is accompanied by another stimulus related in meaning [1]. Semantic priming is a cognitive phenomenon that is well established behaviorally [2,3], with electroencephalography (EEG) [4] and, to a lesser degree, with functional magnetic resonance imaging (fMRI) [5,6]. The discovery of the N400 effect [7], an event-related potential (ERP) component in the EEG closely related to semantic priming, has greatly contributed to the understanding of the effect’s determinants and its functional significance. Using a wide array of methodological approaches, increased effort has been spent to identify neural sources of the behavioral semantic priming effect and of the N400 ERP response [5,6]. The number and location of identified brain regions potentially related to semantic priming varies between studies, materials, tasks and methods, but some converging evidence exists. Studies with lesion data (e.g., [8]), intracranial EEG recordings (e.g., [9]), localization based on magnetoencephalography (MEG) [10,11,12] and fMRI [13,14,15,16] all point to a broad region encompassing the left superior and middle temporal gyri (STG/ MTG) as a major generator of the neural priming response

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