Monday, October 22, 2007

Why Are Faces Special? Bruno Rossion on the N170 Controversy

Are faces special? This question has sparked endless debate among investigators who study object recognition, perceptual expertise, and developmental psychology. Neuroimaging experiments have demonstrated that specific fMRI activations (in the fusiform face area) and EEG responses (the N170 component) are larger for faces than for non-face objects. A controversial paper published in March (Thierry et al., 2007) claimed that the face selectivity shown in over 100 previous studies of the N170 was an artifact of differential interstimulus perceptual variance (ISPV). Namely, they argued that earlier experiments did not adequately control for variability across stimulus classes, i.e., face stimuli have been much more similar to each other than the non-face stimuli.

Dr. Bruno Rossion, a leading researcher in this area, has been highly critical of this finding, and his letter to the editor at Nature Neuroscience (Bentin et al., 2007) was previously covered by The Neurocritic and The Phineas Gage Fan Club. His latest comments on the matter first appeared here, and I have taken the liberty to repost them as a new entry.

A few months ago, you asked me to keep you updated on this issue. We have now in press (Neuroimage) a full paper deconstructing the claim of Thierry et al. that the N170 would not be larger to faces than other visual stimuli :

Rossion, B. & Jacques, C. (in press). Does physical interstimulus variance account for early electrophysiological face sensitive responses in the human brain? Ten lessons on the N170. NeuroImage. PDF

You can get it here :

The short reply that the editors of Nature Neuroscience allowed us to write was not enough, and gave the opportunity to Thierry and colleagues to throw more confusion on this issue (as also acknowledged by the comment on this website). So we thought it deserved a full commentary, deconstructing their paper, and trying to take the positives from this unfortunate publication (i.e. what can we learn about this for N170 research, suggest some kinds of guidelines, clarify a number of theoretical and methodological points).

In a nutshell, in our paper, we :

- Explain clearly the nature of their claim, why it’s ill defined and not to be confused with the real debate about the NATURE of the larger N170 to faces.
- Explain why their claim was not really plausible for reasons that are related to EEG/ERP analysis : an increase of intertrial variance should have delayed/smeared the N170, which is due to a fixed increase of power time-locked to the stimulus onset.
- Show that Thierry et al. were wrong with respect to previous studies not controlling for the factor they mention. In fact, ironically again, one of the few studies that suffered from such limitations, is their only published study before this one.
- Explain clearly why they failed to find a N170 effect in their study (we replicate this « finding » with the wrong electrodes considered)
- Show that they did not control for the factor that was supposed to be controlled and for which they were blaming other studies.
- Provide an account for their « ISPV » finding, which merely reflects a comparison of high-quality images to low quality image sets.
- Discuss why the N170 face effect is not related to low-level visual factors, whereas the earlier P1 effect (emphasized by Thierry et al.) is likely to be related to such factors.
- Emphasize that the larger N170 to faces is an important phenomenon for researchers to understand the time-course of face processing, and that this effect is in line with a large body of data from other sources.

I hope this will set the record straight. The paper of Thierry et al. was accepted in NN it seems precisely because, done by novices in this area it was very controversial and would make a lot of noise (i.e. citations). [NOTE: the title of the university press release is Neuropsychologists set for shock finding.] I believe it is not only intellectually dishonest, but reveal a dangerous trend in some « high impact factors » journals to publish papers first and foremost because they appear novel, catchy or controversial rather than on their scientific credibility.

Perhaps at some point you will be able to reformulate the question as « why are faces special » ?

All the best

Bruno Rossion


Bentin S, Taylor MJ, Rousselet GA, Itier RJ, Caldara R, Schyns P, Jacques C, Rossion B. (2007). Controlling interstimulus perceptual variance does not abolish N170 face sensitivity. Nature Neurosci. 10:801-802.

Thierry G, Martin CD, Downing P, Pegna AJ. (2007). Controlling for interstimulus perceptual variance abolishes N170 face selectivity. Nature Neurosci. 10:505-511.

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