Neural repetition suppression to identity is abolishedby other-race faces
Luca Viziolia, Guillaume A. Rousseleta, and Roberto Caldaraa,b,1
aCentre for Cognitive Neuroimaging (CCNi), Institute of Neuroscience and Psychology, University of Glasgow, Glasgow G12 8QB, United Kingdom; andDepartment of Psychology, University of Fribourg, 1700 Fribourg, Switzerlandbadaptation| face processing | EEG | visual cognitionAlmost 100 y ago, Feingold (1) reported that human beings
living in different geographical locations perceive individuals
belonging to “other-races” (OR) as all looking alike: “Other things
being equal, individuals of a given race are distinguishable fromeach other in proportion to our familiarity, to our contact with therace as whole. Thus, to the uninitiated American all Asiatics lookalike, while to the Asiatics, all White men look alike.” This commonly experienced all look alike “perceptual illusion” for OR faces
is at the root of one of the most robust empirical ﬁndings in facerecognition: the other-race effect (ORE). The ORE refers to themarked behavioral impairment displayed by humans in recognizing OR compared to same-race (SR) unfamiliar faces (i.e., lower
accuracy coupled with higher false identiﬁcations for OR faces).
The scientiﬁc literature has provided clear evidence that the ORE
and the popular belief that OR faces all look alike are notaccounted for by the paucity of anthropometric variations in OR
faces, but by a genuine lack of expertise. Although this theoreticalexplanation has been supported by numerous behavioral (fora review, see ref. 2), computational (e.g., refs. 3–5) and neuroimaging (6–15) studies on the ORE, the neurophysiological correlates of the all look alike perceptual experience, have never been
directly investigated.
This observation is even more surprising considering that the
rapid development of neuroimaging techniques has dramaticallyincreased our knowledge of how the brain achieves visual categorization. Studies using single-cell recordings in primates andwww.pnas.org/cgi/doi/10.1073/pnas.1005751107

functional MRI (fMRI) in humans have shown the existence ofneural populations responding preferentially to faces (e.g., 16–19)
and face identity (20–23). Importantly, genuine race effects require an interaction between the race of the observer and the raceof the faces. Only one fMRI study has used two groups ofobservers, revealing greater responses for SR compared with OR
faces in the face-sensitive cortex during a face memorization task
(9). Crucially, however, the all look alike effect takes place in thesubsequent face recognition stage during which mechanisms related to individual face identiﬁcation are engaged. For example, it
is common experience to misidentify personally familiar individuals belonging to an OR group (i.e., misidentify your Chinese
friend on the street, if you are Westerner), whereas this perceptual
misidentiﬁcation related to the all look alike effect does not occur
during the memorization of familiar faces. In addition, fMRI
prevents from drawing any conclusion on the time-course of neuralsensitivity to race.
Human faces elicit also a particular electrophysiological signature: the N170 event-related potential (ERP) (24, 25; fora review, see ref. 26). The N170 is a bilateral occipito–temporal
negative deﬂection peaking roughly 170 ms after stimulus onset,
larger for faces compared with other visual categories. Activity inthis time window is associated with the early accumulation ofperceptual information leading to visual categorization, which isnecessary for postsensory, decision, and motor stages (27, 28).
Numerous electrophysiological studies have investigated theearly electrophysiological dynamics of the ORE (6, 7, 10–15).
Several studies have failed to show any sensitivity to race on the
N170 (6, 7, 14), or on its frontal counterpart, the vertex positive
potential (11). These results suggest that brain activity in the N170
time window codes only for the detection of a face shape, whereas
race information from faces is extracted in postperceptual stages
occurring roughly between 250 and 300 ms after stimulus onset (6,7, 14). On the other hand, several studies have reported larger
N170 amplitudes for OR faces compared with SR faces (10, 13,15). These ﬁndings, which might be partially due to uncontrolled
physical differences across the stimulus set (29), are also at odds
with many studies that have shown larger N170 to object categories
of expertise (30), a result that would lead to the prediction ofa larger N170 to SR faces. The heterogeneity of these results andthe lack of sensitivity of the N170 may also be explained by the useof diverse task constraints: the detection of catch trials (7, 15),
detection of colored faces (29), explicit race categorization (6), andan old–new face recognition design (14). Furthermore, previous
studies used different types of OR stimuli, including Western

Author contributions: R.C. and L.V. designed research; L.V. performed research; L.V., G.A.R.,
and R.C. contributed new reagents/analytic tools; L.V. analyzed data; L.V. and R.C. developed the stRS approach; and R.C. wrote the paper.
The authors declare no conﬂict of interest.
*This Direct Submission article had a prearranged editor.
Freely available online through the PNAS open access option.
1

To whom correspondence should be addressed. E-mail: roberto.caldara@unifr.ch.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1005751107/-/DCSupplemental.

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Human beings are remarkably skilled at recognizing faces, with themarked exception of other-race faces: the so-called “other-race effect.” As reported nearly a century ago [Feingold CA (1914) Journal of
Criminal Law and Police Science 5:39–51], this face-recognition impairment is accompanied by the popular belief that other-race faces
all look alike. However, the neural mechanisms underlying this highlevel “perceptual illusion” are still unknown. To address this question, we recorded high-resolution electrophysiological scalp signals
from East Asian (EA) and Western Caucasian (WC) observers as they
viewed two EA or WC faces. The ﬁrst adaptor face was followed bya target face of either the same or different identity. We quantiﬁed
repetition suppression (RS), a reduction in neural activity in stimulussensitive regions following stimulus repetition. Conventional electrophysiological analyses on target faces failed to reveal any RS effect. However, to fully account for the paired nature of RS events, we
subtracted the signal elicited by target to adaptor faces for eachsingle trial and performed unbiased spatiotemporal data-driven
analyses. This unique approach revealed stronger RS to same-racefaces of same identity in both groups of observers on the facesensitive N170 component. Such neurophysiological modulation in
RS suggests efﬁcient identity coding for same-race faces. Strikingly,
OR faces elicited identical RS regardless of identity, all looking aliketo the neural population underlying the N170. Our data show thatsensitivity to race begins early at the perceptual level, providing,
after nearly 100 y of investigations, a neurophysiological correlateof the “all look alike” perceptual experience.

NEUROSCIENCE

Edited* by Riitta Hari, Aalto University School of Science and Technology, Espoo, Finland, and approved September 20, 2010 (received for review April
27, 2010)

Caucasian (WC) (6, 7, 10–13, 15, 29), East Asian (EA) (6, 7, 10, 13,29), African American (11, 12, 14, 15, 29), and Hispanic faces (14).
More importantly, all these ERP studies relied exclusively on datafrom the WC population, which is a methodological problem, because any effect could be confounded by physical differences in theface stimuli, preventing any ﬁrm conclusion on the ORE. A fullcrossover interaction between races of observers and the race of
faces is necessary to assess genuine behavioral and neurophysiological ORE. To the best of our knowledge, only one electrophysiological study used two groups of observers and reported sensitivity
to race for inverted faces in two groups of observers (29). Critically,none of these neuroimaging studies has yet used a paradigm optimally tapping into mechanisms devoted to face recognition, whichleaves the neurophysiological bases of the all look alike effect unexplored.
Adaptation is a well-established paradigm to reveal the nature of
information coding at the perceptual and neurophysiological levels(for a review, see ref. 31). Neural activity in stimulus-sensitive
regions is typically reduced when a stimulus is repeated, a phenomenon known as “repetition suppression” (RS) (31–34). RS was
initially observed in monkey single-cell recordings (e.g., refs. 33–
36). More recently it has been reported in human electrophysiological (e.g., refs. 37–41) and fMRI blood-oxygen level-dependentstudies (e.g., refs. 23, 34, 42) using a variety of cognitive tasks. Although the precise neural computations of RS remain unclear, RS
elicited by two stimuli presented in rapid succession indicates theengagement of the same (or at least a largely overlapping) neural
population in the processing of both stimuli (31). Therefore, theamount of RS is related to the capacity of a neural population todiscriminate stimuli and could be compared with a novelty detection mechanism (31), decreasing neural responses’ redundancy
and increasing coding efﬁciency (31, 36, 43–45). Interestingly, theface-sensitive N170 component shows preferential RS to faces but
not to other visual categories (46), as well as to face identity (47–49).
Thus, RS represents a powerful tool to elucidate the time-course
and the nature of the neural representations leading to the all lookalike effect. In this context, it seems logical to predict stronger RS
for SR compared with OR faces in the N170 time window.
To address this question, we recorded high-temporal resolution
scalp ERP signals in EA and WC observers as they viewed
sequences of two faces: an “adaptor” and a “target” face (Fig. S1).
In each sequence, the two faces were either EA or WC, and of thesame or different identities. To minimize the use of trivial image
matching strategies, we changed the facial expression displayedby the adaptor and the target faces. As expected, Western andEastern observers showed an ORE, as assessed in a separate facerecognition task. Previous electrophysiological adaptation studies
compared the ERPs to target faces, ignoring the response toadaptor faces (46–49). In our experiment, this conventional approach failed to reveal any signiﬁcant difference across conditions.
However, because RS is a signal reduction to the second stimulusof a pair, we developed a unique single-trial analysis method. We
subtracted the signal elicited by the presentation of the target faceto that elicited by the adaptor face independently for each pair,resulting in a single-trial RS (stRS) electrophysiological response(Fig. S2). We also used unbiased spatiotemporal data-driven analyses at all electrodes and time-points. In line with previous ﬁndings (47–49), stRS responses in the N170 time window showedlarger RS for SR faces of the same identity compared with any of
the other conditions in both groups of observers. This result suggests a more effective coding of identity for SR faces than OR
faces. Strikingly, however, in both groups of observers OR faces
elicited similar RS responses regardless of a change in facial identity,
suggesting that the neural populations underlying the early facesensitive N170 responses cannot discriminate OR face exemplars.
20082 | www.pnas.org/cgi/doi/10.1073/pnas.1005751107

Results
We carried out ANOVAs independently at all electrodes and alltime-points (see Methods for details). This approach led to a large
number of F, t, and P values. For clarity, we report here only theminimum and maximum F or t values and their associated P
values. Exact P values are reported for signiﬁcant effects, unlessthey were smaller than 0.001.
Behavioral Results. WC observers were better at recognizing SR(d’ = 2.2; SD = 0.44) than OR faces (d’ = 1.7; SD = 0.4).
Likewise, EA observers were better at recognizing SR (d’ = 2.1;
SD = 0.41) than OR faces (d’ = 1.6; SD = 0.49). A repeatedmeasures ANOVA performed on d’ scores, with race of theobservers (WC and EA) as a between-subject factor, and race ofthe face (WC or EA) as within-subject factor, conﬁrmed thesigniﬁcance of this observation [F(2,32) = 7.19; P < 0.001].
ERP Results. Descriptive statistics. Consistent with previous research(26), for both groups of observers the N170 peaked bilaterally
over parietal–occipital electrodes, being largest at the righthemisphere electrode PO8h (Table S1). After artifact rejection,
the mean number of trials per condition was 72 (SD = 9.2).
RS results. Following the conventional RS analysis, we performed
a two-way ANOVA of the amplitude of the ERPs to target faces,
independently at all electrodes and time-points [race of the observer (two: WC, EA) × condition (four: WC same identity, WC
different identity, EA same identity, EA different identity)]. ThisANOVA revealed no signiﬁcant effect across the whole epoch[P > 0.05, corrected for multiple comparisons (MC) by a multivariate cluster analysis, as described in Methods] (Fig. 1). A twoway ANOVA of the amplitude of the ERPs to adaptor faces also
led to null results [race of the observer (two: WC, EA) × race ofthe face (two: WC, EA; P > 0.05, MC corrected)] (Fig. 1).
We carried out a two-way ANOVA of the peak latency of the
P1, N170, and P2 components elicited by the target face. The raceof the observer was the between-subject factor and the four
conditions (as described above) were the within-subject factor.
We performed the ANOVA at the electrodes where the components were the largest: PO8h and PO7h for the N170, and O2 andO1 for P1 and P2. No signiﬁcant latency differences were observed for P1 [F(2,11) = 1.12; P > 0.05], N170 [F(2,11) = 2.11; P >0.05], or P2 [F(2,11) = 1.06; P > 0.05].
We also carried out a two-way ANOVA on the peak latency of
P1, N170, and P2 components elicited by the adaptor face at theelectrodes described above. The factors of the ANOVA were raceof the observers and conditions. Once again, no signiﬁcant latency
differences were observed for P1 [F(2,11) = 2.31; P > 0.05], N170
[F(2,11) = 0.51; P > 0.05], or P2 [F(2,11) = 1.2; P > 0.05].

Fig. 1. Mean ERPs elicited by the adaptor (continuous line) and the target
faces (dotted line) measured at PO8h for the four conditions (Asian faces ofsame identities, red line; Asian faces of different identities, green line; Caucasian faces of same identities, blue line; Caucasian faces of different identities,
yellow line) for WC and EA observers. No signiﬁcant differences were observed
across all conditions for the ERPs elicited by both adaptor and target faces.

Vizioli et al.

Single-trial RS results. Only trials accepted for both adaptor andtarget faces were included in the computation of the stRS ERP(see results at electrode PO8h in Fig. 2A). The mean number oftrials accepted per condition thus dropped from 72 to 54 (minimum = 46; maximum = 68; standard = 6.4).
The ANOVA (race of the observer × condition) carried out on
the stRS amplitude independently at all electrodes and timepoints revealed a signiﬁcant interaction [maximum F(2,11) =11.06; minimum F(2,11) = 2.75, P = 0.049, MC corrected], which
was maximal at the latency of the stRS peak amplitude at PO8h
(Fig. 2). This interaction was signiﬁcant over a cluster of centerand right occipito–parietal electrodes (Fig. 2D). The time-courseof the F values of the race of the observer × condition interaction
revealed a burst of signiﬁcant F values in the N170 time window,
synchronous with the largest RS responses (Fig. 2B). Post hoc
paired t tests revealed that regardless of the race of the observers,
SR same-identity trials elicited signiﬁcantly larger stRS responses
compared with the other conditions [at PO8h − minimum: WC:t(11) = −2.73; P = 0.02; EA: t(11) = −2.29; P = 0.04; maximum:
WC: t(11) = −3.92; P = 0.002; EA: t(11) = −4.22; P = 0.001] overa cluster of right occipito–parietal electrodes only (i.e., P8, P8h,
P10h, PO6, PO8, PO8h, PO10h, PO10) (Fig. 2D). RS wasequivalent across the remaining conditions (Fig. 2A). Moreover,stRS responses were time-locked to stimulus onset and reliably
present throughout the duration of the experiment (Fig. S3). No
other signiﬁcant amplitude differences were observed.
We also carried out a two-way ANOVA (race of the observer ×
condition) on the peak latency of the stRS response at theelectrodes showing signiﬁcant race of the observer × conditioninteraction on the stRS amplitude (as described above). No
signiﬁcant latency differences were observed.
Finally, to rule out any potential signiﬁcant contribution of thedifferent facial expressions portrayed in the adaptor faces in modulating the stRS responses, we carried out two-way ANOVAs (twogroups of observers, WC and EA, × ﬁve facial expressions of emotion: angry, sad, happy, surprised, and disgust) at all electrodes andA mv 2

WC observers

EA observers

time-points on stRS signals. Our data revealed that facial expressions
did not modulate stRS signals (P > 0.05, MC corrected) (Fig. S4).
Discussion
This cross-cultural study investigated the early neural dynamics ofthe all look alike perceptual phenomenon, the root of the marked
recognition impairment observed in humans for OR faces. We
used an adaptation paradigm and recorded electrophysiologicalsignals. In line with previous behavioral studies on the ORE (e.g.,
refs. 15, 30, 50–54), our WC and EA observers were more accurate at recognizing SR than OR faces. Importantly, in both groupsof observers, our component-free spatiotemporal EEG analyses
revealed larger repetition suppression (stRS) following adaptation to SR faces of the same facial identity compared with SR
faces of a different identity. Consistently with previous electrophysiological studies, adaptation to face identity occurred in the
N170 time window (47, 49).
In addition, we found a unique result: RS responses did notdiscriminate between “same” and “different” identities for OR
faces in both groups of observers. This crossover interaction can beconsidered as an early neurophysiological signature of the perceptual all look alike effect, occurring in a time window typicallyassociated with early categorical face processing (e.g., refs. 27, 47–
49, 55). Importantly, the full crossover interaction between therace of the observers and the race of the faces demonstrates thatthe differences in stRS are genuinely related to race and culture of
the observers, and not a consequence of differences in the visualproperties of faces from different races (see also ref. 29). It is also
worth noting that RS and the all look alike perceptual phenomenon
were not modulated by the facial expression of the adaptor faces.
Identity-dependent RS occurred at right occipito–temporalelectrodes only, in line with lesion studies suggesting that the rightanterior temporal cortex is critical for face identiﬁcation (e.g., 56–
58). These early scalp electrophysiological effects were similar inobservers from two cultures. This ﬁnding suggests that the consecutive presentation of any pair of faces elicits a comparable
stRS (represented in Fig. 2E by the nonﬁlled bars). Neural pop-

C 1 p<.05 uncorrected

12

D
12

128
1 p<.05 corrected
ASIAN SameASIAN Different
CAUCASIAN SameCAUCASIAN Different

−7
−100

0

400 −100

0

400ms

128
−100

E

B

0

F

.

. ....
...

n.s.
400ms

F
n.s.

Adaptation to identity
Adaptation to faces

0400 −100

0400ms

Same Different Same Different
WC observers

Same Different Same Different
EA observers

Fig. 2. (A) Single-trial RS responses measured at PO8h, the electrode with the largest N170 for WC and EA observers. The stRS responses are obtained byaveraging the single-trial differences between adaptor and target faces. Note that the more negative is the amplitude, the stronger the adaptation. (B) Timecourse of the F values for the group × condition interaction from the ANOVA on the stRS amplitude at PO8h, the electrode with the largest effect size. Notethe burst of signiﬁcant F values coincided with the stRS peak latency. (C) Signiﬁcant F values of the group × condition interaction on the stRS amplitude
obtained from the ANOVA carried out independently at all electrodes and time-points, uncorrected (Left) and corrected (Right) for multiple comparisons.
Electrodes are stacked up along the y axis and time is shown along the x axis. The electrode order is not based on their spatial topographical positions (whichinstead is correctly shown in D). (D) Topography of the F values at the latency of the maximum F, showing the electrodes displaying a signiﬁcant group ×
condition interaction on the stRS amplitude. The white dots are the right occipito–temporal electrodes that displayed signiﬁcantly larger RS responses for SR
same identity faces compared with any other condition. The largest white dot indicates the electrode showing the largest effect. (E) Model. A general level ofadaptation to faces is observed for all conditions (empty bars). The stRS speciﬁc to identity is only observed for same-race faces (ﬁlled bars), which is related togreater coding efﬁciency for this visual category.

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0−100

NEUROSCIENCE- - - - p<.05- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -ulations coding for face shapes might be involved in these early RS
responses. Whereas the presentation of any face caused somelevel of adaptation, the presentation of faces with “same identity”
was responsible for an additional amount of adaptation only iffaces were from the same race as the observer (represented in Fig.
2E by the plain bars). This race-preferential adaptation might bea result of the recruitment of a supplementary neural population
coding selectively for SR facial identity. Importantly, our results
converge with evidence showing the existence of face-preferential
cortical areas (e.g., refs. 18, 59, 60), as well as distinct populations
speciﬁcally coding for face identity (e.g., refs. 20–23). It thenbecomes natural to ask why identity-dependent RS responses are
abolished by OR faces.
According to an inﬂuential theoretical model of the ORE (61,62), individual faces are represented in an arbitrary multidimensional space shaped by visual and social experience. In this space,location codes for facial identity and distance from the mean face(i.e., a prototypical average face) codes for distinctiveness. In themodel, SR faces have a wide multidimensional distribution,
whereas OR faces have a denser, more clustered distribution. For
example, SR faces are coded along many diagnostic dimensions,such as hair and eye color. In contrast, OR faces tend to lack diagnostic information (i.e., all EA faces have black hair and darkeyes), which leads to more prototypical coding, hence harder-todiscriminate faces (3). This encoding strategy emerges throughexperience and increases coding efﬁciency for SR identity to thedetriment of OR faces. Evidence for such norm-based faceencoding has been found in single-cell recordings in monkeys (59)and fMRI in WC observers confronted with SR faces (42). In
particular, both studies show that neural responses to face identity
increase with distinctiveness (i.e., the distance from the face
prototype). Such evidence is in line with our observations, because
we found larger stRS responses to SR faces, but not to OR faces.
At the neurophysiological level, numerous adaptation studies
have interpreted RS as reﬂecting decreased neural redundancy
and sharpening of sparser stimulus’ representation (for a review,see ref. 31), which would result in more efﬁcient neural information coding (36, 43–45). Within this framework, RS has
been considered as a neural mechanism devoted to novelty detection (31). The lack of stRS to OR faces suggests that they
trigger prototypal responses, preventing novelty detection in faceexemplars. In contrast, stRS to SR faces suggests a more efﬁcientcoding of this visual category.
The explanations offered above are also compatible with two
major theoretical accounts of the ORE in psychology: the socialexperience model (63, 64) and the visual-expertise framework (14,65). The social-experience model holds that the rapid extractionof race information, at the cost of individuating information,
would account for the OR recognition deﬁcit. Following this logic,
OR faces would not tap into the neural population coding for
identity. This prediction is clearly supported by our electrophysiological data. Along the same lines, the visual-expertise framework (14, 65) postulates that SR faces are categorized at thesubordinate level (i.e., John, Jack, etc.), whereas OR faces are
categorized at the basic level (i.e., East Asian, Black American,etc.). This theoretical account is also compatible with our data, inwhich only SR faces tap into individual face recognition.
It is worth noting that conventional electrophysiological RS
measures are based on amplitude differences for target faces only
(47–49) and have so far failed to show any signiﬁcant effect for
both groups of observers in the N170 time window. Using thisapproach, we also failed to replicate previously reported RS differences for SR faces of the same identity (e.g., ref. 49) (Fig. 1).
The use of different images of the same individual between theadaptor and the target face (expressive vs. neutral) might explain
this observation. This control increased the validity of the facerecognition task, but decreased the sensitivity of conventional
electrophysiological RS measures on the target face.
20084 | www.pnas.org/cgi/doi/10.1073/pnas.1005751107

To increase the sensitivity of RS analyses, we implemented
a unique approach. Because RS is a reduction in neural response toa target stimulus following the presentation of an adaptor stimulus,
we subtracted the signal elicited by the presentation of the target
face to that elicited by the adaptor face. This analysis was only
performed for trials not contaminated by artifacts for both events.
The amplitude of the stRS increases the signal-to-noise ratio andreﬂects a genuine amount of RS per event, which is a function ofboth amplitude and latency differences between the signals elicitedby the two successively presented faces. Changes in amplitude,
synchrony, or both of local ﬁeld potentials may explain the modulations across events (31). Therefore, the unique stRS approach
may help to elucidate the nature of the neural information coding
underlying responses in electrophysiology, magnetoencephalography, and single-cell recordings, as all these techniques can ﬁnely
exploit the temporal dynamics of single events.
Conclusion
The present study investigated the neural correlates of the perceptual all look alike effect, which represents the core facet of theORE. We used an adaptation paradigm involving Western andEastern faces and quantiﬁed single-trial spatiotemporal electrophysiological RS responses in Western Caucasian and East Asianobservers. Our data show RS in the time window of the early facesensitive N170 component, with sensitivity to face identity only for
SR faces. OR faces did not show RS modulation, indicating a lackof sensitivity to face identity for this category. These results showthat the discrimination of same- and other-race faces begins early,
at the perceptual level. After nearly 100 y of investigations, ourresults are also unique in providing a neurophysiological correlateof the all look alike perceptual experience.
Methods
Participants. Twenty-four right handed subjects took part in the experiment:
12 East Asians (six female), with an age range of 18–33 and a mean age of 25,
and 12 Western Caucasian (six female), with an age range of 19–31 anda mean age of 23. All EA participants were Chinese who had been in the
country for less than 1 mo and had previously never been in contact witha Western society. All participants provided written informed consent andhad normal or corrected-to-normal vision. The ethical committee of theFaculty of Information and Mathematical Sciences at the University ofGlasgow approved the experiments.
Stimuli. The stimuli consisted of 20 front-view grayscale photographs of WC
and EA faces (5 identities × 2 sexes × 2 races), ≈3.75° × 4.25° of visual angle,
taken from the JACFEE database (66).
To limit the possibility that ERP repetition effects were the result of pixelbased low-level adaptation, instead of high-level adaptation to face identity,each identity was equally presented as a neutral face, or a face displaying ﬁve
possible emotions: happy, anger, sad, disgust, and surprise. All emotions werecounterbalanced across face races and conditions. All faces were cropped toremove external features by the application of the same oval mask; none hadparticular distinctive features and male faces were clean-shaven. The stimuli
were centered in a 5.2° × 5.2° and color normalized with Adobe Photoshop CS4,
by constraining all of the images in the same average template color space.
EEG Study. Procedure. Participants sat in a dimly lit, sound-attenuated electrically shielded booth. Viewing distance was maintained at 80 cm bya chinrest. Each trial consisted of two faces of same or different identities (Fig.
S1) presented sequentially on a Samsung SyncMaster 1100 MB monitor (resolution 2,048 × 1,536 pixels, 23.5° × 30.1°, background of average luminance25.4 cd/m2, refresh rate 80 Hz). A trial started with a black ﬁxation cross ≈0.3°
of visual angle, presented at the center of the screen for 300 ms. The ﬁrst face,
the adaptor, was then presented for 350 ms, followed by an interval of random duration (100–300 ms), and then by the second face, the target, for
300 ms. The offset of the second face was followed by a randomized intertrial interval between 1,300–1,500 ms. Target and adaptor faces’ identities
matched in half of the trials. To minimize low-level adaptation, the adaptor
face portrayed an emotion, whereas the target face was neutral. Face raceand sex were consistent within trials. There were four conditions [two races(EA and WC) × two identities (same and different): SR same, SR different, OR

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Vizioli et al.

Behavioral Study. Subsequent to the EEG experiment, the same participants
performed a behavioral task that directly assessed the ORE. In an encoding
phase, participants had to memorize 20 faces of each race, presented for 3 seach, with 5-s inter-stimulus interval (ISI). Subsequently, subjects performed
a forced-choice old–new recognition task with 20 old and 20 new faces.
Faces were selected from a set of stimuli different from the set used in the
EEG experiment (69). Participants did two blocks of the old/new face recognition task per race. The order of the blocks was pseudorandom and counterbalanced across observers. Faces were blocked by race during both theencoding and the recognition phase. The encoding stage was followed by a 1min pause, after which participants pressed the “s” key on the computer
keyboard for old faces and the “k” key for new faces. Each face remained on
the computer screen for 200 ms (3-s ISI). Participants did not know the ratio ofold to new faces and did not receive feedback on their responses.
ACKNOWLEDGMENTS. R.C. was supported by The Economic and Social
Research Council and Medical Research Council (ESRC/RES-060-25-0010).

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PNAS | November 16, 2010 | vol. 107 | no. 46 | 20085

NEUROSCIENCE

EEG recording and analysis. We acquired EEG data with a 128-channel Biosemi
Active Two EEG system (BioSemi). Four additional electrodes (UltraFlat Active
electrodes; BioSemi) attached below and at the outer canthi of both eyes
measured the vertical and horizontal electro-oculograms. Analog signal was
digitized at 1,024 Hz and band-pass ﬁltered online between 0.1 and 200 Hz.
Electrode offsets were kept between ± 20 μV. Participants were asked tominimize blinking, head movement, and swallowing.
We used EEGLAB (68), Matlab 7.5 (2007b), and BESA 5.2 to perform EEG
analyses. In BESA, EEG data were referenced to an average reference. Noisy
electrodes were rejected on a subject-by-subject basis. The signal was lowpass ﬁltered at 40 Hz with a slope of 6dB. Single trials were corrected forhorizontal and vertical eye movements and blinking artifacts by principle
components analysis, as implemented in BESA. First, we identiﬁed eye
movements on the continuous signal using their speciﬁc topographical distribution. Then we manually selected portions of the signal showing the
topographical conﬁguration of interest. Finally, we averaged the selected
epochs to create a subject-speciﬁc template for each artifact. The ﬁrst
principle components analysis component of each artifact was removed. This
component accounted for 89–99% of the variance (mean = 91%). Artifacts
were rejected based on absolute abnormal values larger than 120 μV. Trials
were averaged across an epoch of −100 ms to +500 ms, and the average 100
ms of prestimulus activity was removed from every time-point, independently at each electrode. Trials including inverted faces were excluded
from the analysis. Channels contaminated by artifacts were interpolatedusing the EEGLAB topoplot function. Consistently with previous studies, weﬁrst analyzed the ERP amplitude to the adaptor and to the target faces
separately. For adaptor faces, we carried out a two-way ANOVA with twogroups of observers (WC and EA) × two races of faces (WC and EA). We
performed another two-way ANOVA for target faces, with two groups ofobservers (WC and EA) × four conditions (WC faces same identities, WC faces
different identities, EA faces same identities, EA faces different identities).
These ANOVAs were performed independently at all electrodes and timepoints, with ERP amplitude as a dependent variable. This analysis makes noa priori assumption about where and when to look for effects in the ERP
signal. Post hoc t tests were then carried out between paired conditions.

The ANOVA on both the target and adaptor faces failed to demonstrate
any RS effects across the whole ERP. However, this type of analysis makes theimplicit assumption that adaptor and target faces are independent andtherefore fails to recognize the paired nature of the experimental design. We
thus developed a stRS response. Based on the deﬁnition of RS, which refers toa stimulus speciﬁc reduction of neural activity, we computed the stRS response by subtracting the activity elicited by the target face from the activity
elicited by the adaptor. We rejected from the analysis the signal elicited byboth adaptor and target face if either one of the two epochs was contaminated by artifacts (for more details about the procedure see Fig. S2). We thencarried out a two-way ANOVA [two races of the observers (EA and WC) ×
four conditions (WC faces same identities; WC faces different identities; EA
faces same identities; EA faces different identities)] independently at allelectrodes and time-points, with stRS as a dependent variable. To correct for
the increase in type I errors because of multiple comparisons, we usedbootstrap (67) and a multivariate clustering technique (68). We centered thedata so that each condition had a mean of zero; then we used the bootstrap
to derive an estimate of the sampling distribution of our statistic in a condition in which the null hypothesis of no difference in means is true. In eachbootstrap, we sampled subjects with replacement and carried out the repeated-measures ANOVAs described above, independently at all electrodes
and time-points. Then the signiﬁcant F values (P < 0.05) were grouped inspatiotemporal clusters (68). For each bootstrap, we computed the sum of F
values in every cluster and saved the maximum cluster sum across clusters.
We repeated this procedure 599 times, leading to 600 F cluster sums for eachmain effect and for each interaction. After sorting the 600 cluster sums, weselected the 95th percentile as our cluster threshold to assess statisticalsigniﬁcance. The signiﬁcant F values from the original ANOVAs were clustered, and the sum of F values inside each cluster was compared with thebootstrap cluster threshold for that test. If an observed cluster sum wasequal or larger than the threshold sum obtained under H0, all of the timepoints and the electrodes inside that cluster were considered signiﬁcant.

PSYCHOLOGICAL AND
COGNITIVE SCIENCES

same, and OR different (where same and different refers to the identities of
the two subsequently presented faces)]. Each identity equally appeared in thesame and different conditions. There were 80 trials per condition, and theorder of the conditions was randomized within each block.
Subjects performed an orthogonal task that required pressing the “s” keyon the keyboard every time one of the two faces within a trial was presented upside-down and the “k” key when both faces were inverted. Thisorthogonal task was designed to avoid potential signal modulations becauseof attentional confounds linked to the race of the stimuli (9). Inverted faces
appeared in ≈12.5% of the trials: 20 with the adaptor or the target face
inverted and 20 with both faces inverted. Each identity appeared twice
as inverted.
The experiment consisted of eight blocks of 45 trials each (360 trials in total
with 80 trials × 4 conditions and 40 trials with inverted faces) and lasted≈20 min.

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