Elsevier

NeuroImage

Volume 35, Issue 4, 1 May 2007, Pages 1495-1501
NeuroImage

Technical Note
MEG/EEG sources of the 170-ms response to faces are co-localized in the fusiform gyrus

https://doi.org/10.1016/j.neuroimage.2007.01.034Get rights and content

Abstract

The 170-ms electrophysiological processing stage (N170 in EEG, M170 in MEG) is considered an important computational step in face processing. Hence its neuronal sources have been modelled in several studies. The current study aimed to specify the relation of the dipolar sources underlying N170 and M170. Whole head EEG and MEG were measured simultaneously during the presentation of unfamiliar faces. An Independent Component Analysis (ICA) was applied to the data prior to localization. N170 and M170 were then modelled with a pair of dipoles in a four-shell ellipse (EEG)/homogeneous sphere (MEG) arranged symmetrically across midline. The dipole locations were projected onto the individual structural MR brain images. Dipoles were localized in fusiform gyri in ten out of eleven individuals for EEG and in seven out of eleven for MEG. N170 and M170 were co-localized in the fusiform gyrus in six individuals. The ICA shifted some of the single-subject dipoles up from cerebellum to fusiform gyrus mainly due to the removal of cardiac activity. The group mean dipole locations were also found in posterior fusiform gyri, and did not differ significantly between EEG and MEG. The result was replicated in a repeated measurement 3 months later.

Introduction

Perceiving and recognizing faces is considered one of the most complex and demanding visual processes. Electroencephalographic (EEG) studies have reported a positive face-specific event-related potential (ERP) at the vertex around 150 ms after face presentation onset (the vertex positive potential, VPP) (Botzel and Grusser, 1989, Jeffreys, 1989, Jeffreys and Tukmachi, 1992). Further, a negative deflection at right and left posterior electrodes can be recorded around 170 ms after face presentations (N170, e.g. Bentin et al., 1996). The N170 has been related to the configural processing of face features and their integration into a holistic face percept (Bentin and Deouell, 2000, Schweinberger et al., 2002, Carbon et al., 2005). N170 and VPP depend on the same neural generator, a main factor contributing to apparent functional differences between both components being the choice of the reference electrode (Joyce and Rossion, 2005).

Using magnetoencephalography (MEG), face-specific event-related magnetic fields (ERFs) have been recorded at posterior sensors around the same latency as the N170 (e.g. Lu et al., 1991, Sato et al., 1999, Watanabe et al., 1999, Halgren et al., 2000, Liu et al., 2002, Hoshiyama et al., 2003, Lewis et al., 2003). These ERFs have been related to configural face encoding and to the identification of faces (Halgren et al., 2000, Liu et al., 2002).

Several studies have modelled the cortical sources underlying N170 and M170 as equivalent current dipoles (ECDs). N170 sources have been localized to the fusiform gyri (FG) (Rossion et al., 2003), or to the FG and additional structures such as the lingual gyri (Mnatsakanian and Tarkka, 2004). Other EEG studies found entirely different generators such as the lateral occipitotemporal cortex (Schweinberger et al., 2002) or the superior temporal sulci (Itier and Taylor, 2004). In contrast, the M170 source has been consistently localized in or close to the FG (Linkenkaer-Hansen et al., 1998, Sato et al., 1999, Watanabe et al., 1999, Lewis et al., 2003, Halgren et al., 2000, Hoshiyama et al., 2003). The reason for the larger heterogeneity of results in localization studies of the N170 than the M170 is unclear. Critical factors may be task and stimulus as well as recording technique.

MEG has become more widely used over the last years in the study of face processing. The current study aimed at a specification of the relation between face-related N170 and M170 sources by localizing them on simultaneously recorded EEG and MEG data. The data quality was improved by removing noise with the help of Independent Component Analysis (ICA). To account for individual differences in functional neuroanatomy, dipoles were localized on single-subject data. The robustness of the localizations was examined by repeating the experiment after 3 months with the same participants.

Section snippets

Material and methods

The recent study is part of a broader research project on electrophysiological correlates of face priming processes. Its design will be described here as long as it is relevant for the reported results.

Results

MEG and EEG were recorded from 11 subjects in Session 1 and from 10 subjects in Session 2. The error rate for the age decisions was relatively high (mean value = 21%, S.D. = 12%), indicating that the task was difficult. Indeed, this was reported by the majority of participants after the experiment. There was no change of the error rate across the face repetitions [ANOVA main effect for repetitions with short lag: F(3,30) = 0.71, p = .45; long lag: F(3,30) = 1.27, p = .30].

Discussion

The aim of the present study was to compare on a single subject and on a group level the neuronal sources of face processing in the 170-ms latency range as measured by EEG and MEG in a simultaneous recording. Furthermore, the impact of artefact reduction by Independent Component Analysis on the localization results was examined. In addition, the re-test-reliability of the localizations was tested in a repeated measurement design.

The ECDs corresponding to N170 and M170 were localized in the

Conclusion

In the current study sources of N and M170 were co-localized in the fusiform gyrus. This result was not only obtained for mean source locations, but more importantly in a considerable number of single subjects. Moreover, the result was replicated within the same group. Possible explanations for the high quality of the co-localization are a high number of trials, sufficient artefact reduction by the use of ICA and a predominant activation of the fusiform gyrus by the task used. The necessity of

Acknowledgments

We thank Walter Endl, Vienna, for the face photographs. We also thank C. Carbon, T. Bengner, B. Pelzer, and both anonymous reviewers for their detailed comments on earlier versions of the manuscript. This work is supported by DFG grant number GRK 432/2 and BMBF grant number 01GO0208 BNIC.

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