Transcranial Direct Current Stimulation (tDCS) Eliminates The Other‑race Efect (ORE) Indexed By The Face Inversion Efect For Own Versus Other‑race Faces Part 1

We investigate individuals’ reduced ability to recognize faces from other racial backgrounds, a robust phenomenon named the other-race effect (ORE). In this literature, the term “race” is used to refer to visually distinct ethnic groups. In our study, we will refer to two such groups: Western Caucasian (also known as White European) and East Asian e.g., Chinese, Japanese, and Korean. 

First, race is not the only factor that defines people's intelligence and memory. While some have studied areas in which certain races generally perform better, these studies have always been flawed. Demographic data shows that some races may perform slightly better overall than others, but that doesn't mean everyone has the same abilities. We cannot attribute a person's traits and abilities to his or her belonging to a certain race.

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Finally, modern society increasingly emphasizes diversity and inclusion. Our society is increasingly focusing on the unique value of each person and the specific circumstances of that individual. We should consider everyone to have their strengths and weaknesses, regardless of their ethnic background.

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This study applied the tDCS procedure (double-blind, 10 min duration, 1.5 mA intensity, targeting Fp3 location), developed in the perceptual learning literature, specifically used to remove the expertise component of the face inversion effect (FIE), which consists of higher recognition performance for upright than inverted faces. In the tDCS-sham condition (N= 48) we found a robust ORE i.e., significantly larger FIE for own versus other-race faces due to higher performance for upright own-race faces. 

Critically, in the anodal-tDCS condition (N= 48) the FIE for own-race faces was significantly reduced compared to sham due to impaired performance for upright faces thus eliminating the cross-race interaction index of the ORE. Our results support the major role that perceptual expertise, manifesting through perceptual learning, has in determining the ORE indexed by the FIE.

The race of a face is often one of its most salient features. We can rapidly categorize two faces that belong to two different racial groups as, for example, Western Caucasian versus East Asian, using shape and superficial color information1. More importantly, within the racial group with which we are most familiar, we are highly accurate at recognizing previously seen faces. 

However, one of the most robust and well-documented face recognition phenomena is our impaired ability to recognize faces from other races3,4. This has been named the Other-Race effect (ORE) and is one of the best-replicated phenomena in the face recognition literature as established by several reviews and meta-analyses that have considered 30 years of research on the ORE5–7.

In the lab, the ORE is typically demonstrated through a standard recognition paradigm, in which subjects are asked to recognize faces previously seen during a study phase, intermixed with novel faces. The ORE is then indexed by a cross-over interaction between the race of subjects and the race of faces in recognition accuracy8,9. A long-standing debate regarding the nature of the ORE is whether it can be used as a measure of racial bias, or instead is a face recognition phenomenon caused by reduced perceptual expertise that we have for other-race faces. However, some authors have also proposed a hybrid approach where both perceptual expertise mechanisms and social factors play a key role in determining the ORE. 

In what follows, we give the overall background to this literature, and then introduce our contribution, which is to deploy a neurostimulation technique that significantly reduces (perhaps even eliminates) perceptual expertise. The basic idea of this study is that if the ORE is substantially due to perceptual expertise giving the advantage to own-race faces, then the effect should disappear under our neurostimulation manipulation. This would provide direct evidence of the perceptual expertise mechanisms involved in the ORE and advance our understanding of this phenomenon.

Background

In early studies, social scientists had interpreted this phenomenon as an indication of how observers, particularly those with more prejudiced racial attitudes, would not be motivated to differentiate members of other races, which would then lead to a weaker memory for other-race faces10. Throughout the years, various versions of this motivational account linked with social categorization have been proposed for the ORE11,12. 

But, perhaps, the most developed explanation based on social and motivational factors is the one suggested by several authors in recent years, which uses the ORE as a measure of the tendency that individuals have to think categorically about outgroup racial members, leading them to process facial features differently from own-race faces13,14. Whereas ingroup faces (e.g., own-race) facilitate the perceivers to search for facial features that could distinguish one ingroup member from another, outgroup faces (e.g., other-race) are categorized based on category-prototypical features (e.g., race, sex, age) that are common across all the outgroup faces, thus making discrimination more difficult. 

Essentially, the argument is that there are different facial features used within own and other-race faces that are guided by social categorization based on group membership, in this case, race, but the same analysis could apply to sex and age15.

Cognitive scientists have instead proposed an explanation of the ORE based on the lack of visual experience we have with other-race individuals which would result in reduced perceptual expertise with other-race faces. Researchers provided evidence that the size of the ORE would vary with the amount of interracial contact experience that individuals have in their everyday lives16,17. Interestingly, a recent study also proposed that there would be a specific developmental window (approximately until 12 years of age) where the acquisition of other-race faces is facilitated and would have effects in reducing the ORE18. Furthermore, authors have explored the mechanisms at the basis of the role that perceptual expertise plays in determining the ORE. 

More specifically, it has been proposed that the perceivers would have more expertise in scrutinizing the configurable information (i.e., spatial relationships among the main facial features) for own-race faces versus other-race faces with the latter being processed more naturally (i.e., isolated features)5,15. It is this visual perceptual expertise for configuring the information that we rely on when recognizing faces as demonstrated by the face inversion effect (FIE)19,20. 

Currently, 300+ papers have shown that when we are presented with upside-down faces our recognition performance is significantly reduced compared to when we see the same faces presented in their usual upright orientation. This is the FIE and it is another robust phenomenon in the face recognition literature. The most widespread explanation for the FIE is that when presented upright, we process faces based on their confgural information, however, when inverted confgural information is disrupted resulting in a reliance on individual features and a reduced recognition performance21–24.

Importantly, Rhodes et al. adopted the FIE to investigate the perceptual expertise explanation of the ORE25. The authors hypothesized that if own-race faces are processed more configurable than other-race faces, a reduced FIE should be recorded for other-race faces due to there being less expertise at scrutinizing configure information to be lost on inversion. Western Caucasian and East Asian students were recruited and engaged in a recognition task involving Western Caucasian and East Asian upright and inverted faces. As predicted the results revealed for both subject samples, a larger FIE for own-race versus other-race faces supporting the perceptual expertise linked to the confgural processing explanation of the ORE (for a replication see). 

Vizioli et al.27 using the old/ new recognition task typically adopted to investigate the FIE, showed that for Western Caucasian subjects, the FIE was larger in response to Western Caucasian faces versus that found for East Asian faces mainly because of impaired recognition performance for upright East Asian Faces. Critically, this result was reversed for East Asian subjects, thus a larger FIE was found for East Asian faces versus that found for Western Caucasian faces and this was mainly because of impaired recognition performance for upright Western Caucasian faces. 

Within the same study, Vizioli et al.27 looked at the ORE on the N170 ERP component which is often used in the literature as an index of face recognition performance, and it is found to be the largest at occipital-temporal areas28. Typically, inverted faces elicit a delayed and larger N170 peak compared to that in response to upright faces. The results revealed that for both Western Caucasian and East Asian subjects the FIE on the N170 amplitudes was larger for the same versus other-race faces revealing a similar pattern to the behavioral effects. No signifcant diferences were found on the N170 latencies27.

Vizioli et al.’s behavioral results are particularly interesting in the context of the ORE literature27. More specifically, the reduced expertise for upright other-race faces, which leads to impaired recognition performance, is what leads to a smaller FIE compared to that found for own-race faces. On inversion, because we are not familiar with seeing either own or other-race faces presented inverted, no difference in performance should be expected. However, these results could be also explained by the social accounts of the ORE. One may say that either the lack of motivation to approach individuals from other races, or the tendency to categorize outgroup members based on prototypical elements (e.g., race) could mainly affect upright faces thus leading to a reduced FIE for other-race faces. 

This illustrates a key limitation in this literature thus far, there has been a persistent failure to develop an experimental paradigm that could definitively demonstrate whether the ORE is due to either social factors or cognitive mechanisms by excluding one or the other. Interestingly, recent studies have also attempted to reconcile these two extreme accounts (social motivation and perceptual expertise) proposing that increasing motivation may be helpful only if a sufficient minimum degree of perceptual experience is present, or that motivation contributes over-and-above experience only in cultural settings where the groups differ in socioeconomic status30. 

Some authors proposed that the ORE might be due to a lack of social motivation in some circumstances, and a lack of perceptual experience in others. But perhaps, the most developed hybrid account is based on the Categorization-Individuation Model32. This is still a socially orientated theory of the ORE which is based on subjects’ selective attention to category-level or identity-level facial features, but it proposes that experience influences the ease with which it is possible to selectively attend to identity-level information. 

Thus, in subjects with strong other-race experience, attending to identity-level information will be easy and successful while in subjects with weak other-race experience, attending to identity-level information, even when motivated to do so, could be difficult or unsuccessful. A few studies have shown that if subjects are motivated to apply the same individuation processes (e.g., informing them of the presence of other-race faces and encouraging them not to avoid them) then if they are sufficiently familiar with the race (e.g., Western Caucasian Americans looking at African American) the ORE can be overcome32,33.

In the current study, we aimed to examine further the nature of the ORE directly by removing the perceptual expertise component for own-race faces and seeing whether that would eliminate the ORE. 

To do so, we used a transcranial Direct Current Stimulation (tDCS) procedure developed in the literature in recent years, to directly affect and disrupt the perceptual learning component of the FIE. The specific tDCS montage targeting the DLPFC area with anodal stimulation derives from previous research on categorization learning tasks–36. Specifically, Ambrus et al.36 provided the first evidence for anodal tDCS delivered over the left DLPFC at the Fp3 area influencing categorization learning for sets of prototype-defined pattern configurations. The DLPFC region was chosen based on previous fMRI studies showing this brain region being highly activated throughout the whole learning phase of the categorization task used. The Fp3 area was selected because of being particularly implicated in participants with high categorization performance36,37. 

In Ambrus et al.36 the authors administered the tDCS stimulation for a brief period of 10 min during the learning phase (i.e., online) and made sure that stimulation ended before the beginning of the testing phase. The results revealed that anodal tDCS influenced categorization in the testing phase directly eliminating the prototype distortion effect (better categorization performance for non-pre-exposed category prototypes compared to category exemplars). This finding was later extended by other authors to a prototype-distortion task using the same prototype-defined categories of checkerboards used in the literature to demonstrate an analogous of the FIE39–41. 

McLaren et al.38 showed that anodal tDCS at Fp3 (cathode/return channel placed on the opposite supraorbital area) delivered for 10 min at an intensity of 1.5 mA, during the learning phase reduced later in the testing phase the prototype distortion effect for checkerboards.

Through, a double-blind and between-subjects design Civile, Verbruggen et al.42 extended the same tDCS procedure to modulate the robust inversion effect with checkerboards previously established in the literature as evidence of perceptual learning39–41. Anodal stimulation delivered during the categorization learning task (at Fp3 area, 10 min duration, 1.5 mA intensity) involving prototype-defined categories of checkerboards influenced the checkerboard inversion effect recorded in the subsequent old/new recognition task. Specifically, the checkerboard inversion effect in the anodal group was significantly reduced compared to the robust checkerboard inversion effect found in the sham group. 

Importantly, the reduction of the inversion effect was mainly due to impaired performance for upright familiar checkerboards in the anodal condition versus sham42. Critically, when the same tDCS procedure was extended to the inversion effect for faces a similar pattern of results was demonstrated. Civile et al.43 used the same tDCS procedure while participants were engaged in an old/new recognition task involving Western Caucasian upright and inverted faces (Experiments 1 and 2). A double-blind and between-subjects design was adopted where the tDCS stimulation was delivered throughout the learning phase (study phase). 

The results from the subsequent recognition task revealed a robust FIE in the sham group and a significantly reduced FIE in the anodal group. This time as well this reduction was also due to an impaired performance for upright faces induced by the anodal tDCS43. This effect of the tDCS procedure on the FIE has been replicated across several publications and constitutes established funding in the literature–49. This work established a causal link between the inversion effect for faces and that for checkerboards, by showing that tDCS can systematically reduce both.

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