© 2007 Nature Publishing Group http://www.nature.com/natureneuroscience

CO R R E S P ON D E N C E

Controlling interstimulus perceptual variance does
not abolish N170 face sensitivity
To the editor:
Numerous studies report a negative eventrelated potential at occipito-temporal scalp
sites between 130 and 200 ms (N170) thatis larger when elicited by faces than by otherobject categories1,2. Thierry and colleagues3
argued that this effect reflects an artifactof uncontrolled interstimulus perceptual(physical) variance (ISPV), which whencontrolled eliminates the difference between
faces and nonfaces. Here we demonstrate that
ISPV was actually controlled in many studies,
yet the N170 effect remained conspicuous(Fig. 1 and Supplementary Figures 1–3
online). Evidently, Thierry and colleagues’
claim is wrong and misleading.
In addition to their factual error, theyfailed to note the striking contradiction
between their hypothesis and the existing
literature. Most notable are the larger N170
for inverted than for upright faces2,4, thelarger N170 for upright than for inverted
Mooney faces, which when inverted areperceived as meaningless objects5, the large
modulations of N170 amplitude to faces
induced by increasing visual expertise with
nonface objects6, and the emergence of N170
after conceptual priming of participants’awareness to the physiognomic value ofstimuli, which were identical before and after
priming7. All these modulations of N170 arerobust despite identical stimuli in different
conditions, hence identical ISPV.
These authors’ position also suffersfrom conceptual weaknesses. It is not clear
why categorical selectivity, but not ISPVinduced jitter, would be seen at an earlyP1 component, whereas the N170, which
manifests higher-level visual processes, would
be modulated only by ISPV. Nor do they
offer any theoretical account of the larger
N170 amplitude for low ISPV than for high

ISPV. A larger jitter between trials evoked byhigh-ISPV stimuli should cause smearingof the component for this condition; thus
causing not only a lower, but also a wider,component, which was not found in theirstudy. In fact, the larger N170 to faces is due
to an increase of power at a fixed latency
rather than a larger phase-resetting of
EEG waveforms for faces as compared withobjects8, an observation incompatible withThierry et al.’s3 speculations.
It is not our task to explain the null
results of Thierry et al.’s3 study. However,
inappropriate measurement of the N170 andinconsistencies across their results should beconsidered. The N170 effect is circumscribedaround occipito-temporal sites, falling offvery rapidly with more medial and superior
locations, and is much smaller (occasionallyabsent) at the medial occipital sites O1 and O2
than at more lateral posterior-temporal sites.
Thierry et al.3 averaged data across ten sites
over each hemisphere, six of which werelocated close to midline, which explains
their extremely small N170 to faces. This
peculiarity is even more striking in Figure 6cof Thierry et al.3, where positive amplitudes
were observed for the N170. Further, if early
P1 differences are found, N170 amplitude
should be measured and analyzed with respect
to the preceding peak to take into accountbaseline differences between conditions.
Figure 1 ISPV does not account for N170face sensitivity. (a) Top, pixel-wise correlations(ISPV) for four categories of stimuli2. Interpixel
correlation was highest for houses (P < 0.001),
equal between faces and greebles (P = 0.80) andlowest for cars (P < 0.001). For faces and objects,correlation values were higher than those reportedby Thierry et al.3, indicating a better control ofISPV. Bottom, the N170 amplitudes, mean pixelwise correlation and spatially averaged stimuli forthe four categories. (b) Top, pixel-wise correlation
for faces and scrambled face stimuli used in aprevious study11, showing higher correlation forfaces than for scrambled faces. Bottom, despite
the ISPV difference (see averaged stimuli), N170
was of equal amplitude for the two conditions.

NATURE NEUROSCIENCE VOLUME 10 | NUMBER 7 | JULY 2007801

© 2007 Nature Publishing Group http://www.nature.com/natureneuroscience

CO R R E S P ON D E N C E
Finally, the off-hand dismissal of theintracranial literature is inexcusable. Thesestudies have identified intracranial facespecific potentials whose response properties
are in agreement with the face-related N170
recorded on the human scalp9, as well aswith neuroimaging and single-cell recording
findings. There is no question as to the validityof those intracranial data and to suggest thatthey could be artifactual as a result of thepopulation studied is inappropriate.
To conclude, it is basic psychophysics
that stimulus variance must be controlled10,
and ERPs can indeed be affected by factors
such as ISPV. However, control of ISPV does
not abolish the N170 face effect. Moreover,
several self-contradictions in the authors’ data(replotted in Supplementary Fig. 4 online)
refute their own proposal. Thus, the reported
results are not valid and are not evidence
against the well-established and reliable effectof larger N170s to faces than to objects.

Shlomo Bentin1, Margot J Taylor2,
Guillaume A Rousselet3, Roxane J Itier4,
Roberto Caldara3, Philippe G Schyns3,Corentin Jacques5 & Bruno Rossion5
1Department of Psychology, Hebrew University,

Mount Scopus, Jerusalem, 91905, Israel.
2Diagnostic Imaging, Hospital for Sick Children,
555 University Avenue, Toronto, Ontario
M5G 1X8, Canada. 3Centre for Cognitive
Neuroimaging, Department of Psychology,
University of Glasgow, 58 Hillhead Street,Glasgow, G12 8QB, UK. 4Baycrest Centre
for Geriatric Care, The Rotman ResearchInstitute, 3560 Bathurst Street, Toronto,Ontario M6A 2E1, Canada. 5Psychology and
Neurophysiology, Université Catholique deLouvain, 10 Place Cardinal Mercier,
Louvain-la-Neuve, 1348, Belgium.
email: shlomo.bentin@huji.ac.il

Note: Supplementary information is available
on the Nature Neuroscience website.

COMPETING INTERESTS STATEMENT
The authors declare no competing financial interests.

1. Bentin, S., McCarthy, G., Perez, E., Puce, A.
& Allison, T. J. Cogn. Neurosci. 8, 551–565 (1996).
2. Rossion, B. et al. Neuroreport 11, 69–74 (2000).
3. Thierry, G., Martin, C.D., Downing, P. & Pegna, A.J.
Nat. Neurosci. 10, 505–511 (2007).
4. Itier, R.J., Latinus, M. & Taylor, M.J. Neuroimage 29,667–676 (2006).
5. Latinus, M. & Taylor, M.J. J. Cogn. Neurosci. 17,1316–1327 (2005).
6. Rossion, B., Kung, C-C. & Tarr, M.J. Proc. Natl. Acad.
Sci. USA 101, 14521–14526 (2004).
7. Bentin, S., Sagiv, N., Mecklinger, A., Friederici, A.
& von Cramon, D.Y. Psychol. Sci. 13, 190–193(2002).
8. Rousselet, G.A., Husk, J.S., Bennett, P. & Sekuler, A.
Neuroimage, published online 23 March 2007
(doi:10.1016/j.neuroimage.2007.02.052).
9. Allison, T. et al. J. Neurophysiol. 71, 821–825(1994).
10. Schyns, P.G., Jentzsch, I., Johnson, M.,
Schweinberger, S.R. & Gosselin, F. Neuroreport 14,1665–1669 (2003).
11. Zion-Golumbic, E. & Bentin, S. Cereb Cortex,
published online 24 October 2006 (doi:10.1093/cercor/bhl100).

Is the N170 sensitive to the human face or to several
intertwined perceptual and conceptual factors?
Reply:
In our study1, minimizing interstimulus
perceptual variance (ISPV) eliminated N170
category effects, whereas ISPV differences
modulated this component. Thus, previous
studies on the N170 in which ISPV was notsimilarly controlled are hard to interpret. We
do not claim that the N170 is sensitive only toISPV; inversion2,3, symmetry4, interpretability5and expertise6 all affect N170 amplitudes,
possibly in a face-selective fashion. Althoughsome previous studies have used low-ISPV
stimuli2,7, comparisons between conditions
differing both on object category and otherdimensions can be misleading.
Consistent with our results, two studies
comparing frontal views of cars and faces2,8found no category effect in the N170 range.
Notably, in Bentin et al.’s Figure 1a, theN170 amplitude difference between faces
and cars was not significant (the remaining
conditions were asymmetrical and so were
the scrambled faces of Figure 1b).
We re-emphasize that we do not equate ISPV, a
psychophysical concept, with interstimulus pixelwise correlations, a physical measure. Clearly,equivalent pixel changes at different image
locations will have different psychophysiological

802Figure 1 ERPs and N170 topographies recorded to faces and to cars in the low- and high-ISPV conditionsof experiment 1 of Thierry et al.1. (a) ERPs at electrodes P7, P8, PO9 and PO10. In all cases, there was noeffect of category (all Ps > 0.1), a significant effect of ISPV (all Ps < 0.001) and a significant interaction(all Ps < 0.01), which replicates analyses conducted over ten electrodes. Notably, the N170 was notlarger to faces than to cars when ISPV was controlled (low ISPV). (b) N170 scalp topographies in the four
conditions of experiment 1. The topographies were highly similar between conditions and differed onlywith respect to their strength, indexed using the global field power. All electrodes included in the statisticalanalysis of Thierry et al.1, except PO3, showed negative amplitudes in all conditions in the N170 range.

consequences. Thus, variation among visual
stimuli along different dimensions will induce
different (and not necessarily linearly increasing)N170 modulations. Understanding the effect ofISPV per se on N170 amplitude requires a fuller

understanding of perceptual similarity, which
is beyond the scope of our study. The point
remains that even simple efforts to minimize
ISPV have a dramatic effect on apparent N170
category selectivity.

VOLUME 10 | NUMBER 7 | JULY 2007 NATURE NEUROSCIENCE