Discrimination of familiar human faces in dogs (Canis familiaris)

Faces are an important visual category for many taxa, and the human face is no exception to this. Because faces differ in subtle ways and possess many idiosyncratic features, they provide a rich source of perceptual cues. A fair amount of those cues are learned through social interactions and are used for future identification of individual humans. These effects of individual experience can be studied particularly well in hetero-specific face perception. Domestic dogs represent a perfect model in this respect, due to their proved ability to extract important information from the human face in socio-communicative interactions. There is also suggestive evidence that dogs can identify their owner or other familiar human individuals by using visual information from the face. However, most studies have used only dogs’ looking behavior to examine their visual processing of human faces and it has been demonstrated only that dogs can differentiate between familiar and unknown human faces. Here, we examined the dog's ability to discriminate the faces of two familiar persons by active choice (approach and touch). Furthermore, in successive stages of the experiment we investigated how well dogs discriminate humans in different representations by systematically reducing the informational richness and the quality of the stimuli. We found a huge inter-individual and inter-stage variance in performance, indicating differences across dogs in their learning ability as well as their selection of discriminative cues. On a group level, the performance of dogs significantly decreased when they were presented with pictures of human heads after having learned to discriminate the real heads, and when – after relearning – confronted with the same pictures showing only the inner parts of the heads. However, as two dogs quickly mastered all stages, we conclude that dogs are in principle able to discriminate people on the basis of visual information from their faces and by making active choices.     

The own-species face bias: A review of developmental and comparative data

Face recognition is characterized in part by biases resulting in enhanced perception and memory for individuals within frequently encountered groups and impaired recognition for individuals within uncommonly encountered groups. These biases are found across multiple categories and levels, including species, race, age, and gender (Scherf & Scott, 2012). At the highest level of categorization, human adults and nonhuman primates exhibit improved recognition (Dufour, Pascalis, & Petit, 2006; Pascalis & Bachevalier, 1998) and discrimination abilities (Pascalis, de Haan, & Nelson, 2002) for individuals within their own species, reflecting what has been called an “own-species” or “species-specific” bias in face processing. The own-species bias is a model system for delineating the role of experience and the developmental trajectory of face-processing biases, for further understanding the malleability of face biases in adults, and for examining and comparing face-processing abilities across species. Here, we will review findings from studies investigating the perception of other-species faces during development, into adulthood and across species.     

Prototypes and same-gender bias in perceptions of hiring discrimination

The present study investigated the relative importance of two explanations behind perceptions of gender discrimination in hiring: prototypes and same-gender bias. According to the prototype explanation, people perceive an event as discrimination to the extent that it fits their preconceptions of typical discrimination. In contrast, the same-gender bias explanation asserts that people more readily detect discrimination toward members of their own gender. In four experiments (n = 797), women and men made considerably stronger discrimination attributions, and were moderately more discouraged from seeking work, when the victim was female rather than male. Further, a series of regressions analyses showed beliefs in discrimination of women to be moderately correlated with discrimination attributions of female victims, but little added explanatory value of participant gender, stigma consciousness, or feminist identification. The results offer strong support for the prototype explanation.     

Eye-tracking the own-gender bias in face recognition: Other-gender faces are viewed differently to own-gender faces

Research on the own-gender bias in face recognition has indicated an asymmetrical effect: an effect found only in women. We investigated the own-gender bias, using an eye-tracker to examine whether the own-gender bias is associated with differential processing strategies. We found an own-gender bias in our female participants. Our eye-tracking analysis indicated different scanning behaviours when processing own- and other-gender faces, with longer and more fixations to the eyes when viewing own-gender faces. Our results favour the socio-cognitive model, whilst acknowledging the role of perceptual expertise in the own-gender bias.     

The effect of facial expression and gaze direction on memory for unfamiliar faces

We report data from an experiment that investigated the influence of gaze direction and facial expression on face memory. Participants were shown a set of unfamiliar faces with either happy or angry facial expressions, which were either gazing straight ahead or had their gaze averted to one side. Memory for faces that were initially shown with angry expressions was found to be poorer when these faces had averted as opposed to direct gaze, whereas memory for individuals shown with happy faces was unaffected by gaze direction. We suggest that memory for another individual's face partly depends on an evaluation of the behavioural intention of that individual.     

The effect of face inversion on the detection of emotional faces in visual search

Past literature has indicated that face inversion either attenuates emotion detection advantages in visual search, implying that detection of emotional expressions requires holistic face processing, or has no effect, implying that expression detection is feature based. Across six experiments that utilised different task designs, ranging from simple (single poser, single set size) to complex (multiple posers, multiple set sizes), and stimuli drawn from different databases, significant emotion detection advantages were found for both upright and inverted faces. Consistent with past research, the nature of the expression detection advantage, anger superiority (Experiments 1, 2 and 6) or happiness superiority (Experiments 3, 4 and 5), differed across stimulus sets. However both patterns were evident for upright and inverted faces. These results indicate that face inversion does not interfere with visual search for emotional expressions, and suggest that expression detection in visual search may rely on feature-based mechanisms.     

Differential attentional allocation and subsequent recognition for young and older adult faces

Studies examining own-age recognition biases report inconsistent results and often utilize paradigms that present faces individually and in isolation. We investigated young and older adults' attention towards young and older faces during learning and whether differential attention influences recognition. Participants viewed complex scenes while their eye movements were recorded; each scene contained two young and two older faces. Half of the participants formed scene impressions and half prepared for a memory test. Participants then completed an old/new face recognition task. Both age groups looked longer at young than older faces; however, only young adults showed an own-age recognition advantage. Participants in the memory condition looked longer at faces but did not show enhanced recognition relative to the impressions condition. Overall, attention during learning did not influence recognition. Our results provide evidence for a young adult face bias in attentional allocation but suggest that longer looking does not necessarily indicate deeper encoding.     

The effect of educational environment on identity recognition and perceptions of within-person variability

Individuals from small communities show impoverished face recognition relative to those from large communities, suggesting that the number of faces to which one is exposed has a measurable effect on face processing abilities. We sought to extend these findings by examining a second factor that influences the population of faces to which one is exposed during childhood: educational setting. In particular, we examined whether formerly home-schooled participants show reduced performance relative to non-homeschoolers on the Cambridge Face Memory Test (CFMT) and on a sorting task in which participants sort photographs of two unfamiliar identities into piles representing the number of identities they believe are present. On the CFMT, there was no effect of educational setting. However, formerly home-schooled participants showed significant deficits on the sorting task. Such results suggest that reduced exposure to faces early in life as a function of home-schooling may have lasting effects on the face processing system.     

Pictorial versus structural representations of ingroup and outgroup faces

Mental representations of unfamiliar faces may rely, to a greater or lesser extent, on pictorial cues that are closely linked to the specific image studied, and structural cues that allow for recognition across various transformations. Here, we test participants’ recognition of unfamiliar ingroup and outgroup faces, manipulating image transformation (pose) and exposure duration. The results showed that changes in pose reduced accuracy for outgroup faces but not for ingroup faces. Overall, accuracy increased as exposure duration increased. Accurate responses were made more rapidly than inaccurate responses for both ingroup and outgroup faces, showing that response latency is a useful postdictor of accuracy even for cross-ethnic identifications. The results are discussed in the framework of Bruce and Young's (1986) model of face recognition.     

The own-age face recognition bias in children and adults

Children recognize children's faces more accurately than adult faces, and adults recognize adult faces more accurately than children's faces (e.g., Anastasi & Rhodes, 2005 Anastasi, J. S. and Rhodes, M. G. 2005. An own-age bias in face recognition for children and older adults. Psychonomic Bulletin and Review, 12: 1043–1047. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar]). This is the own-age bias. Research has shown that this bias is at least partially based on experience since trainee teachers show less of an own-age bias than do other adults (Harrison & Hole, 2009 Harrison, V. and Hole, G. J. 2009. Evidence for a contact-based explanation of the own-age bias in face recognition. Psychonomic Bulletin & Review, 16: 264–269. [Crossref], [PubMed], [Web of Science ®] , [Google Scholar]). The present research tested the own-age bias in three groups of children (age 4–6, 7–9, 10–12 years) and a group of adults in the recognition of three age groups of faces (age 7–9, 20–22, and 65–90 years). Results showed an own-age bias for 7- to 9-year-old children and adults. Specifically, children could recognize faces more accurately if they were less than two years different from their own age than if they were more than two years older or younger. These results are discussed in terms of short-term experience with faces creating biases, and this rapidly changes with age.     

Lecturers' faces fatigue their students: Face identity aftereffects for dynamic and static faces

Face adaptation has been used as a tool to probe our representations for facial identity. It has also been claimed to play a functional role in face processing, perhaps calibrating the visual system towards encountered faces. However, for this to be so, face aftereffects must be observable following adaptation to ecologically valid moving stimuli, not just after prolonged viewing of static images. We adapted our participants to videos, static image sequences or single images of the faces of lecturers who were personally familiar to them. All three stimulus types produced significant, and equivalent, face identity aftereffects, demonstrating that aftereffects are not confined to static images but can occur after exposure to more naturalistic stimuli. It is also further evidence against explanations of face adaptation effects solely in terms of low-level visual processing.     

Female condition influences preferences for sexual dimorphism in faces of male humans (Homo sapiens)

In some species, female condition correlates positively with preferences for male secondary sexual traits. Women's preferences for sexually dimorphic characteristics in male faces (facial masculinity) have recently been reported to covary with self-reported attractiveness. As women's attractiveness has been proposed to signal reproductive condition, the findings in human (Homo sapiens) and other species may reflect similar processes. The current study investigated whether the covariation between condition and preferences for masculinity would generalize to 2 further measures of female attractiveness: other-rated facial attractiveness and waist-to-hip ratio (WHR). Women with high (unattractive) WHR and/or relatively low other-rated facial attractiveness preferred more "feminine" male faces when choosing faces for a long-term relationship than when choosing for a short-term relationship, possibly reflecting diverse tactics in female mate choice.     

Adolescents’ preferences for sexual dimorphism are influenced by relative exposure to male and female faces

Exposure to a particular population of faces can increase ratings of the normality and attractiveness of similar-looking faces. Such exposure can also refine the perceived boundaries of that face population, such that other faces are more readily perceived as dissimilar. We predicted that relatively less exposure to opposite-sex faces, as experienced by children at single-sex compared with mixed-sex schools, would decrease ratings of the attractiveness of sexual dimorphism in opposite-sex faces (that is, boys at single-sex schools would show a decreased preference for feminised faces, and girls at single-sex schools would show a decreased preference for masculinised faces). Consistent with this prediction, girls at single-sex compared with mixed-sex schools demonstrated significantly stronger preferences for facial femininity in both male and female faces. Boys at single-sex compared with mixed-sex schools demonstrated marginally stronger preferences for facial masculinity in male faces, but did not differ in their ratings of female faces. These effects were attenuated among some single-sex school pupils by the presence of adolescent opposite-sex siblings. These data add to the evidence that long-term exposure to a particular face population can influence judgements of other faces, and contribute to our understanding of the factors leading to individual differences in face preferences.     

Removing the own-race bias in face recognition by attentional shift using fixation crosses to diagnostic features: An eye-tracking study

Hills and Lewis (2011) have demonstrated that the own-race bias in face recognition can be reduced or even removed by guiding participants' attention and potentially eye movements to the most diagnostic visual features. Using the same old/new recognition paradigm as Hills and Lewis, we recorded Black and White participants' eye movements whilst viewing Black and White faces following fixation crosses that preceded the bridge of the nose (between the eyes) or the tip of the nose. White faces were more accurately recognized when following high fixation crosses (that preceded the bridge of the nose) than when following low fixation crosses. The converse was true for Black faces. These effects were independent of participant race. The fixation crosses attracted the first fixation but had less effect on other eye-tracking measures. Furthermore, the location of the first fixation was predictive of recognition accuracy. These results are consistent with an attentional allocation model of the own-race bias in face recognition and highlight the importance of the first fixation for face perception (cf. Hsiao & Cottrell, 2008).     

Sexual dimorphism and attractiveness in Asian and White faces

Marriages between White men and Asian women are over twice as frequent as those between White women and Asian men. Recent research has proposed that this imbalance may be explained by the finding that, on average, White men are perceived as more attractive than Asian men, and Asian women are perceived as more attractive than White women, possibly because Asian faces are perceived as more feminine than White faces. Here, we explore whether Asian faces are perceived as more feminine than White faces. Thirty-five Malaysian Chinese (20 male) and 30 Australian White (12 male) participants manipulated 100 face photographs (50 Asian; 50 White; half male) on a masculinity/femininity axis to optimize attractive appearance. As predicted, White women’s faces were increased more in femininity than Asian women’s faces, and White men’s faces were feminized more than Asian men’s faces to optimize attractiveness. These findings suggest that White faces are perceived as more masculine than Asian faces.     

Reducing the own-race bias in face recognition by attentional shift using fixation crosses preceding the lower half of a face

Hills and Lewis (2006) reduced White participants’ own-race bias (ORB) in face recognition by training them to attend to features critical for Black faces (lower portion of the face). Here, the ORB was investigated following a brief fixation cross either in the upper portion of the face (critical for White faces) or the lower portion of the face. Results showed that when the cross preceded the lower portion of the face, Black faces were recognized more accurately than White faces and vice versa when it preceded the upper portion of the face. A second experiment demonstrated that this effect disappears if the participants are forced to delay their responses by 4 s. These results suggest that an immediate attentional mechanism can attenuate the ORB when immediate attention is paid to diagnostic features but this can be overridden with increased time spent viewing faces.     

Adolescents’ preferences for sexual dimorphism are influenced by relative exposure to male and female faces

Exposure to a particular population of faces can increase ratings of the normality and attractiveness of similar-looking faces. Such exposure can also refine the perceived boundaries of that face population, such that other faces are more readily perceived as dissimilar. We predicted that relatively less exposure to opposite-sex faces, as experienced by children at single-sex compared with mixed-sex schools, would decrease ratings of the attractiveness of sexual dimorphism in opposite-sex faces (that is, boys at single-sex schools would show a decreased preference for feminised faces, and girls at single-sex schools would show a decreased preference for masculinised faces). Consistent with this prediction, girls at single-sex compared with mixed-sex schools demonstrated significantly stronger preferences for facial femininity in both male and female faces. Boys at single-sex compared with mixed-sex schools demonstrated marginally stronger preferences for facial masculinity in male faces, but did not differ in their ratings of female faces. These effects were attenuated among some single-sex school pupils by the presence of adolescent opposite-sex siblings. These data add to the evidence that long-term exposure to a particular face population can influence judgements of other faces, and contribute to our understanding of the factors leading to individual differences in face preferences.     

Examining the hemispheric distribution of semantic information using lateralised priming of familiar faces

The way in which the semantic information associated with people is organised in the brain is still unclear. Most evidence suggests either bilateral or left hemisphere lateralisation. In this paper we use a lateralised semantic priming paradigm to further examine this neuropsychological organisation. A clear semantic priming effect was found with greater priming occurring when semantically related prime faces were presented to the left visual field than when presented to the right visual field. Possible explanations for this finding are discussed in terms of the bilateral distribution of different classes of semantic information, a possible role of associative processes within semantic priming and interhemispheric transfer.     

Detection of emotional faces: low perceptual threshold and wide attentional span

In two experiments, prime face stimuli with an emotional or a neutral expression were presented individually for 25 to 125 ms, either in foveal or parafoveal vision; following a mask, a probe face or a word label appeared for recognition. Accurate detection and sensitivity (A') were higher for angry, happy, and sad faces than for nonemotional (neutral) or novel (scheming) faces at short exposure times (25-75 ms), in both the foveal and the parafoveal field, and with both the probe face and the probe word. These results indicate that there is a low perceptual threshold for unambiguous emotional faces, which are especially likely to be detected both within and outside the focus of attention; and that this facilitated detection involves processing of the affective meaning of faces, not only discrimination of formal visual features.