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K.G.-S., and N.P.C., B.L.H., contributions: Author in activity greater and adaptation from (16). release dissimilar regions a face-selective more adapta- while to presentation, neural lead their greater faces of elicit course the faces over similarity similar tion perceptual more (15). the faces stimuli: by different graded across further of is blocks adaptation to pre- Neural is relative face times, same the multiple where sented blocks example, to a response For (FFA) tuned, diminished area a face (14). are show fusiform the adaptation they in populations neural which neural face-sensitive activity to as reduced stimuli known to exhibit phenomenon of exposure populations exposures repeated Neural repeated after stimuli. to brain same habituate of feature to the core tendency a highlights its work processing: of body large A question. variability. systems intragroup neural such face-sensitive to how respond extant examined yet However, not identities. has research outgroup toward greater per- for sensitivity in reduced and ceptual result identity, identities would ingroup individual toward mechanism sensitivity perceptual a than tuned Such rather broadly members. more outgroup membership, dif- but between category members distinguish ingroup to to for tuned identities narrowly ferent be may perception ...adNPC otiue qal oti work. this to equally contributed N.P.C. and B.L.H. owo orsodnemyb drse.Eal [email protected],ncamp@ [email protected], Email: addressed. be may [email protected] or stanford.edu, correspondence whom To ailigopvru ugopfcs hs eut suggest perception. earliest of the results of stages some These at of emerges faces. deindividuation perception outgroup outgroup that and versus memory ingroup in racial differences and behavioral faces outgroup mirror and early ingroup of racial properties for cortex tuning face-selective different the that conclude in We emerge faces. differences (Black) outgroup and racial (White) brain among ingroup similarity face-selective physical in how variation to assess respond regions to functional paradigm a used adaptation inter- we Here, MRI these understood. of poorly is nature discrim- biases perceptual group and of early stereotyping the component to However, core precursor ination. a a inter- and considered as bias outgroups been intergroup social long of has members changeable view to tendency The Significance iulnuocec rvdstost drs hsunanswered this address to tools provides neuroscience Visual https://osf.io/zsdyf/ b aai Grill-Spector Kalanit , . y . y raieCmosAttribution-NonCommercial- Commons Creative b,e y www.pnas.org/lookup/suppl/doi:10. , NSLts Articles Latest PNAS | f6 of 1

PSYCHOLOGICAL AND COGNITIVE SCIENCES Fig. 1. The volume of face-selectivity in high-level visual cortex is modulated by face race. (A) Thresholded parameter maps show voxelwise t values for the contrast of White faces (Top) or Black faces (Bottom) versus all other stimuli. Data are presented on inflated cortical surfaces of 3 participants (dark regions are sulci; lighter regions are gyri). The region shown is the ventral surface of the temporal lobe, known as the VTC. The blue region outlined in dotted-white is the MFS, whose posterior and anterior tips are anatomical anchors predicting the location of face-selective cortex on the lateral fusiform gyrus. (B) Line plots illustrating the volume of above-threshold (t values > 3) activation in each subject’s VTC defined with the contrasts of White or Black faces.

The present research harnesses this approach to examine the faces, despite being matched for low-level properties. In all but 1 perceptual underpinnings of perceived racial homogeneity. The subject, the volume of face-selectivity in VTC was significantly anatomical consistency of face-sensitive regions across humans lower for Black faces [t(19) = 6.5, P < 5 × 10−6; Fig. 2B]; (17) makes this area of visual cortex an ideal region to test how in 6 subjects, face-selectivity to Black faces was not observed perceived outgroup homogeneity may reflect differential neu- at all. ral tuning properties. If neural adaptation underlies perceived outgroup homogeneity, then neural activity in the FFA should Face Morph Task. Participants next completed a second experi- be less sensitive to physical variation among other- versus own- ment to measure their perceptual sensitivity to physical variation race faces. Specifically, to the extent that neuronal populations in Black versus White faces with neural adaptation. In each 3-s are broadly tuned to outgroup faces, the same population of block, participants viewed a sequence of Black or White faces neurons will respond across distinct outgroup targets, eliciting sampled from a separate stimulus set (22) and presented at 2 Hz, a greater habituation response across a wider range of physical while responding to periodic oddball stimuli. We used photo variability. morphing software (23) to generate sets of Black and White To test this hypothesis, we used a functional MRI (fMRI) faces varying in physical dissimilarity from 0% (i.e., the same adaptation design (14, 18) to measure adaptation to own- and face presented 6 times) to 100% (6 separate identities), with other-race faces at varying levels of physical similarity. Self- intermediate levels at 30, 50, and 70% (Fig. 2A). This design let identified White participants first completed a face-localization us fit response curves measuring neural adaptation across levels task to independently define face-selective regions; in a second of groupwise variability. Given the causal link between face- adaptation experiment, they responded to an oddball stimulus selective cortex in the right hemisphere (24, 25), we hypothesized nested in blocks of own-race (White) and blocks of other-race (Black) faces. Using photo morphing, we parametrically manip- ulated the physical similarity of faces within each block, ranging from repetitions of the same face, to completely unique identi- ties. By measuring the intragroup similarity of faces within each block against the corresponding adaptation response, we could derive the neural sensitivity to physical variation among faces and thus test the extent to which this sensitivity differed for variation in own- and other-race faces.

Neuroimaging Results Localizer Task. Participants (n = 20) first completed 3 runs of a standard localizer experiment (19). Subjects viewed 4-s blocks of Black or White male faces (20) or of non-face stimuli (bod- ies, limbs, cars, guitars, houses, corridors, words, and numbers) at a rate of 2 Hz (SI Appendix, Fig. S1). To localize face- selective cortex, t-value parameter maps were produced in each subject contrasting faces versus all nonface stimuli. Data were unsmoothed. Face-selective cortex was defined on the cortical surface in each participant as clusters of voxels on the posterior (pFus-faces) and middle (mFus-faces) fusiform gyrus, just lateral to the mid-fusiform sulcus (MFS): a set of regions of interest (ROIs) commonly referred to as the FFA. To avoid bias in subse- quent data analyses, face-selective ROIs were defined using both White and Black faces. Fig. 2. (A) Experimental design of the adaptation experiment. In each In each subject, we also analyzed the volume of face-selectivity block, 6 faces were presented at 1 of 5 levels of groupwise dissimilarity. An example of the morphing scheme is presented on the Left, and 2 example within bilateral ventral temporal cortex (VTC) using contrast morph lines are shown on the Right. (B) Plots mapping the percentage of maps for White and Black faces, as the VTC plays a causal role signal change in right hemisphere face-selective cortex in each participant in face perception (21). As Fig. 1A illustrates, the volume of for White (Left) and Black (Right) faces, normalized to the zero-morph level activation to Black faces was a fraction of the volume to White (full adaptation), along with the summary curve in black.

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PSYCHOLOGICAL AND COGNITIVE SCIENCES experience and would be impossible to access via traditional categorical or stereotyped manner (40, 41); changes in this moti- measures alone. vational context could accompany differences in adaptation to We conclude that racial disparities in perceptual individua- outgroup members. tion emerge in the tuning of face-selective cortex and mirror More broadly, fMRI adaptation can shed light on higher-level behavioral differences in memory and perception. These results social cognitive processes and their interaction with lower-level suggest that biases for other-race faces emerge at some of the perception. Indeed, extant work has applied this technique to earliest stages of sensory processing and add further nuance test whether higher-order representations of social targets draw and specificity to the mechanisms guiding intergroup percep- on overlapping neuronal populations as the self (42) or mem- tion. For example, past work documents differential activation bers of one’s own group (43, 44). The extent to which these in FFA and other face-sensitive regions for encoding own- ver- social categories affect neural adaptation in early visual per- sus other-race (22, 28, 29) and group (26, 30) targets. Such ception, however, had not been tested. We suggest that the research has generally relied on a coarse neural measure of failure to deeply process racial outgroup members as individuals individuation: overall activity in FFA collapsed across all own- could drive neural adaptation in response to dissimilar outgroup versus other-group targets (cf. ref. 31). While overall FFA acti- targets. This perspective is consistent with research demonstrat- vation suggests individual versus group-level processing (32), the ing more shallow processing of individuating information about adaptation paradigm used here allowed us to measure a more outgroup members and reduced prefrontal neural activity in fine-grained mechanism of outgroup perception: neural sensitiv- response to such information (45, 46). Future research can for- ity to graded variations in dissimilarity across Black and White mally test the connections between neural adaptation in low-level faces. A further limitation of past work is that, since participants perception observed here and in higher-order representations of were explicitly instructed to memorize (22) or categorize (26) social groups. Recent work raises the possibility that these pro- targets, group differences in FFA activity could have stemmed cesses may be connected: stereotypic associations across race, from cognitive or motivational regulation evoked by experimen- gender, and emotion may influence the representation of social tal demand, rather than perceptual sensitivity. In contrast, our category exemplars in VTC (47). design allowed us to assess perceptual sensitivity by removing The extent to which people individuate or conflate members of such demand effects and instead having participants complete other social groups has a range of adverse consequences. Inaccu- an oddball task orthogonal to the encoding of social targets. racies in distinguishing outgroup members can lead to mix-ups in The increased sensitivity to variation among own- versus the classroom or faulty eyewitness testimony. Given the role that other-race faces in face-selective cortex observed here likely categorical processing plays in intergroup biases more broadly, represents the sharpening of neural tuning to own-race exem- the biased perceptual mechanisms we document here could cas- plars. That is, individuals are more sensitive to physical variation cade into a range of harmful outcomes, from the application among own-race faces and, conversely, have broader tuning to of group stereotypes to the spillover of fear or other responses other-race faces, habituating to them as repeated instances of across individuals. the same social category rather than distinct individuals. This account is consistent with different theoretical explanations of Materials and Methods the other-race effect (9, 10, 33). First, these biases could stem, Protocols were approved by the Stanford Institutional Review Board for in part, from perceptual expertise with own-race faces, result- Human Subjects Research, and informed consent was obtained from all ing from a lifetime of experience interacting with racial ingroup participants; 24 participants (mean age, 19.7 y; 14 female) were recruited members. Recent research consistent with this account finds through a paid participant pool. We determined our sample size in advance of data collection, based on the robustness of visual stimulus-driven effects that structural and functional properties of face-selective cor- and from past work on neural adaptation to faces (18, 19, 48). All partic- tex are tuned by one’s visual experience across development (18, ipants self-identified as White in a prescreening eligibility survey and had 34). Alongside these experiential accounts, top-down processes normal or corrected-to-normal vision. Participants completed a scanning could also contribute to the biases we observed. For exam- sequence consisting of a T1-weighted anatomical scan, 3 runs of a face- ple, recent work finds that patterns of FFA activity can accu- localizer task (5 min, 24 s per run), and 4 runs of the fMRI adaptation rately predict the race of face identities but only among more experiment (4 min per run). Following the scanning protocol, participants prejudiced individuals (35). Racial prejudice, then, may lead to completed behavioral tasks outside the scanner. Of the 24 subjects, 3 were divergent representations of own- versus other-race faces. Our unable to complete the full set of scans and 1 subject’s data could not be current findings suggest one way these representations differ: properly reconstructed after collection. 3 of the remaining 20 subjects had atypical data from the face morph experiment (resulting from either fatigue populations of neurons in FFA are more narrowly tuned toward in the scanner or venous artifacts inducing signal variability), such that typ- the individual identity of racial ingroup members and more ical fMRI adaptation to face similarity could not be observed. Of these 17 broadly tuned toward social category information for outgroup participants, face-selectivity could be localized on the fusiform gyrus for all members. participants in the left hemisphere and all but 1 for the right. The fMRI adaptation approach applied here opens avenues for examining the flexibility of neural tuning across perceivers Localizer Task. Face-selective ROIs were identified for each participant in a and their social context. Individuating experiences with racial functional localizer task adapted from prior work (19). In each run, par- outgroup members, for example, attenuate other-race biases in ticipants viewed seventy-eight 4-s blocks of gray-scaled images randomly positioned in the visual field, presented against a scrambled noise texture perception and memory (4, 36), and lack of exposure to racial at a rate of 2 Hz. In each block, participants viewed stimuli from 1 of 7 cat- outgroups can exacerbate them (37). It is possible, then, that egories: Black male faces, White male faces, character strings (numbers and cross-group contact over the course of development could lead words), bodies (limbs and headless figures), objects (cars and guitars), scenes to enhanced neural sensitivity to outgroup members. Future (buildings and corridors), and baseline blocks of background texture. Facial research can also test how the social and motivational con- stimuli for the localizer were selected from the Bainbridge Face Database text in which others are perceived influences neural tuning. For (20) and Mind, Culture, and Society laboratory database (49) to capture instance, perceivers are more likely to individuate other-race tar- Black and White faces at various angles (direct, oblique) and expressions gets with whom they share a superordinate group identity (26, (neutral, smiling) to capture voxels that respond selectively to faces. Angles 38, 39). This would suggest that identical targets would elicit and expressions of face stimuli were balanced across race. broader neural tuning if they were categorized as members of Face Morph Experiment. Stimuli for the adaptation experiment were drawn a rival team or political party than if they were designated as from a separate set of Black and White male faces used in previous imag- members of one’s ingroup. In a similar vein, intergroup competi- ing research (22). We created 6 White and 6 Black sets of faces. In each tion often leads individuals to view outgroup members in a more set, 1 “source” face was morphed with 6 remaining “target” faces in a

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(to Awards and (pFus-faces, ACKNOWLEDGMENTS. partic- ROI each per for and estimated faces) were White limit or a mFus-faces). (Black and type exponent stimulus an per Thus, ipant y. the parameter and is through L r sweeping using and for by curve exponent, subject’s error the each minimizing is for least-squares, y nonlinear fit limit, a was the function is This L response. step, voxel L(1-e-y*m), level predicted = morph r the as Black is modeled to were m of data responses where exponential for function The each separately a faces. White model as For and exponential responses faces an GLM. of fit the amplitude We the level. from measured morph estimated we ROI, betas and from subject derived were experiment also We maps. contrast in of used volume face- were the localizing faces analyzed of used White purposes and and For hemispheres Black experiment. both both selectivity, (mFus-faces), face-morph in the gyrus defined in were mid-fusiform analyses ROIs in for FFA-2. region as a to and referred (pFus-faces), FFA-1, col- also gyrus as (19), fusiform posterior to stream in referred ventral region (t also a in FFA: regions categories the as face-selective other known 2 lectively of subject each images in than defined faces to more different in variance responses residual comparing and maps values contrast conditions. generate beta variance to the residual GLM used and the We from course. (betas) time condition voxel’s estimated each we each data, of for the to amplitudes predictors the response fit the to GLM a Using predictors. gen- erate the to with (https://www.fil.ion.ucl.ac.uk/spm) convolved function each was response model matrix hemodynamic to design For (GLM) experimental model change. The linear course. signal generalized time of voxel’s a percentage ran we of data, units subject’s converted into of each were units voxel motion each scanner with for arbitrary courses datasets Time In from included. only motion. were voxels experiments, head 2 morph than between-run less face and and within- brain localizer for both native corrected subject’s were each Data in space. (http://github.com/vistalab) toolbox mrVista Analysis. Data MRI of activity of boundary visualization the for inflated folds. from was sulcal surface within generated This matter. was gray or and surface missing (http://www.itksnap.org/white cortical for ITK-SNAP fixed of using manually voxels Reconstruction and matter inspected white were mislabeled surfaces matter White (http://surfer.nmr.mgh.harvard.edu). tool segmentation FreeSurfer the using whole- each of obtain volume to to anatomical used high-resolution used were the participant. which with was scan), data prescription functional (inplane align images same tempo- T1-weighted coverage The superior anatomical whole-brain 76 the brain allowing data. = to technique functional angle multiplexing parallel flip of a oriented ms, using slices, 78.6 sulcus, spi- oblique = ral echo 48 time 2.4 gradient collected echo of ms, We T2*-sensitive resolution 1,000 a a = using with time coil sequence and pulse obtained scanner ral were same scans the anterior the Functional with to aligned plane. were data commissure Anatomical mm]. commissure–posterior 240 = (FOV) view of field .R .Mlas .Kaiz eonto o ae fonadohrrace. other and own of faces for Recognition Kravitz, J. Malpass, S. R. from 6. Insights learning: face cross-race in Deficits other-race Papesh, H. and M. He, contact Y. Goldinger, Social D. S. Nobre, 5. C. A. Hewstone, M. Silvert, L. Walker, M. P. 4. × ooti ntmclsas ecletdawoeban ntmclvol- anatomical whole-brain, a collected we scans, anatomical obtain To epneapiue o xeietlcniin nteadaptation the in conditions experimental for amplitudes Response significantly responded that voxels as defined were voxels Face-selective matter gray and white into segmented were images anatomical T1 Psychol. (2009). pupillometry. and movements eye brain. human the (2007). in processing face m ieo neso 5 s i nl 12 = angle flip ms, 450 = inversion of time mm, 1 3–3 (1969). 330–334 13, mgn aawr nlzduigteMATLAB-based the using analyzed were data Imaging hsrsac a upre ySafr Dean’s Stanford by supported was research This t > oeseiie yec aeseparately. race each by elicited voxels 3 .Ep sco.Lan e.Cognit. Mem. Learn. Psychol. Exp. 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