How the Thatcher illusion reveals evolutionary differences in the face processing of primates

Face recognition in humans is a complex cognitive skill that requires sensitivity to unique configurations of eyes, mouth, and other facial features. The Thatcher illusion has been used to demonstrate the importance of orientation when processing configural information within faces. Transforming an upright face so that the eyes and mouth are inverted renders the face grotesque; however, when this “Thatcherized” face is inverted, the effect disappears. Due to the use of primate models in social cognition research, it is important to determine the extent to which specialized cognitive functions like face processing occur across species. To date, the Thatcher illusion has been explored in only a few species with mixed results. Here, we used computerized tasks to examine whether nonhuman primates perceive the Thatcher illusion. Chimpanzees and rhesus monkeys were required to discriminate between Thatcherized and unaltered faces presented upright and inverted. Our results confirm that chimpanzees perceived the Thatcher illusion, but rhesus monkeys did not, suggesting species differences in the importance of configural information in face processing. Three further experiments were conducted to understand why our results differed from previously published accounts of the Thatcher illusion in rhesus monkeys.     

The Thatcher illusion and face processing in infancy

Adults readily detect changes in face patterns brought about by the inversion of eyes and mouth when the faces are viewed upright but not when they are viewed upside down. Research suggests that this illusion (the Thatcher illusion) is caused by the interfering effects of face inversion on the processing of second-order relational information (fine spatial information such as the distance between the eyes). In the current study, 6-month-olds discriminated 'thatcherized' faces when they were viewed upright but not when they were viewed upside down. These results are consistent with the notion that 6-month-olds are sensitive to second-order relational information while processing faces.     

The effect of inversion on the encoding of normal and “thatcherized” faces

In the “Thatcher illusion” a face, in which the eyes and mouth are inverted relative to the rest of the face, looks grotesque when shown upright but not when inverted. In four experiments we investigated the contribution of local and global processing to this illusion in normal observers. We examined inversion effects (i.e., better performance for upright than for inverted faces) in a task requiring discrimination of whether faces were or were not “thatcherized”. Observers made same/different judgements to isolated face parts (Experiments 1-2) and to whole faces (Experiments 3-4). Face pairs had the same or different identity, allowing for different process- ing strategies using feature-based or configural information, respectively. In Experiment 1, feature-based matching of same-person face parts yielded only a small inversion effect for normal face parts. However, when feature-based matching was prevented by using the face parts of different people on all trials (Experiment 2) an inversion effect occurred for normal but not for thatcherized parts. In Experiments 3 and 4, inversion effects occurred with normal but not with thatcherized whole faces, on both same- and different-person matching tasks. This suggests that a common configural strategy was used with whole (normal) faces. Face context facilitated attention to misoriented parts in same-person but not in different-person matching. The results indicate that (1) face inversion disrupts local configural processing, but not the processing of image features, and (2) thatcherization disrupts local configural processing in upright faces.     

Effects of face configuration change on shape perception: a new illusion

We report two experiments indicating that varying the configuration of face features changes perception of an oval aperture windowing the face: as the eyes and mouth of a frontal-view face photograph are moved vertically toward face boundaries, the oval appears increasingly clongated, taller, and narrower; when eyes and mouth are moved toward the nose, the oval appears increasingly rounder, shorter, and wider. This shape illusion is maximised when faces appear upright within the oval and major face features (eyes, nose, and mouth) appear in their correct relative locations. These results establish that processing of a face configuration can affect perception of a geometric shape that shares visual space with a face. Whether the illusion is face-specific or a special case of a more general geometric illusion is discussed.     

Holistic face processing can be independent of gaze behaviour: Evidence from the composite face illusion

People tend to perceive identical top halves (i.e. above the nose) of two face stimuli as being different when they are aligned with distinct bottom halves. This composite face illusion is generally considered as the most compelling evidence that facial features are integrated into a holistic representation. Here, we recorded eye‐movements during the composite face illusion in a delayed matching task of top halves of faces. Behavioural results showed a strong composite face effect, participants making more mistakes and taking longer time to match two identical top halves of faces when they were aligned (vs. misaligned) with different bottom halves. Importantly, fixation sites and eye‐movements were virtually identical when the top and bottom parts were aligned (composite illusion) or misaligned (no illusion), indicating that holistic face processing can be independent of gaze behaviour. These findings reinforce the view that holistic representations of individual faces can be extracted early on from information at a relatively coarse scale, independently of overt attention.     

Carryover of scanning behaviour affects upright face recognition differently to inverted face recognition

Face perception is characterized by a distinct scanpath. While eye movements are considered functional, there has not been direct evidence that disrupting this scanpath affects face recognition performance. The present experiment investigated the influence of an irrelevant letter-search task (with letter strings arranged horizontally, vertically, or randomly) on the subsequent scanning strategies in processing upright and inverted famous faces. Participants’ response time to identify the face and the direction of their eye movements were recorded. The orientation of the letter search influenced saccadic direction when viewing the face images, such that a direct carryover-effect was observed. Following a vertically oriented letter-search task, the recognition of famous faces was slower and less accurate for upright faces, and faster for inverted faces. These results extend the carryover findings of Thompson and Crundall into a novel domain. Crucially they also indicate that upright and inverted faces are better processed by different eye movements, highlighting the importance of scanpaths in face recognition.     

Pulling faces: an investigation of the face-distortion aftereffect

After adaptation to a face distorted to look unnaturally thin or fat, a normal face appears distorted in the opposite direction (Webster and MacLin 1999 Psychonomic Bulletin & Review 6 647-653). When the adapting face is oriented 45 degrees from vertically upright and the test face 45 degrees in the opposite direction, the axis of perceived distortion changes with the orientation of the face. The magnitude of this aftereffect shows a reduction of approximately 40% from that found when both adapting and test faces are tilted identically. This finding suggests that to a large degree the aftereffect is mediated not by low-level retinotopic (image-based) visual mechanisms but at a higher level of object-based processing. Aftereffects of a similar magnitude are obtained when adapting and test images are both either upright or inverted, or for an upright adapter and an inverted test; but aftereffects are smaller when the adapter is inverted and the test upright. This pattern of results suggests that the face-distortion aftereffect is mediated by object-processing mechanisms including, but not restricted to, configurational face-processing mechanisms.     

Face processing at birth: a Thatcher illusion study

The present study was aimed at exploring newborns’ ability to recognize configural changes within real face images by testing newborns’ sensitivity to the Thatcher illusion. Using the habituation procedure, newborns’ ability to discriminate between an unaltered face image and the same face with the eyes and the mouth 180° rotated (i.e. thatcherized) was investigated. Newborns were able to discriminate an unaltered from the thatcherized version of the same face when stimuli were presented in the canonical upright orientation (Experiment 1), but failed to discriminate the same stimuli when they were presented upside‐down (Experiment 2). The results indicate that sensitivity to fine spatial information (defined as second‐order relational information) in processing upright faces is already present at birth.     

Face processing in adolescents with autistic disorder: the inversion and composite effects

Two experiments with upright and inverted face and object images were carried out to investigate whether face processing in autism is more feature-based than in individuals with typical development. Participants were 17 high-ability adolescents with autistic disorder (16-24 years), 24 typically developing children (9-10 years) and 16 adults (18-33 years). In Experiment 1, a normal inversion effect was found for the adolescents with autism in a standard face recognition paradigm with reduced memory demands, except for a subgroup with low social intelligence who were not better in recognizing upright relative to inverted photographs of faces. In Experiment 2, the group with autism did not show the composite effect like the adult group did: they recognized face halves as well in aligned composite faces as in non-aligned composite faces. The results on the inversion task suggest that most adolescents with autism form a normal configuration-based face representation, but the absence of the composite effect indicates that they are less prone to use the contextual information of the face in a visual-search task.     

In the Dock: Chimeric Image Composites Reduce Identification Accuracy

The aim of presenting chimeric images (formed from opposing halves of a pair of same or different faces) in court settings is to optimise the accuracy of identification decisions based on CCTV evidence. The experiments reported here examined the utility of this technique. Experiment 1 examined the accuracy of face matching with vertically split, aligned chimeric images, misaligned hemi‐faces and full‐face images. Experiment 2 replicated the first experiment but replaced the misaligned images with opposing hemi‐faces separated by a gap. The final experiment used horizontally split faces. All three experiments showed that matching was less accurate with aligned chimeric images than with full‐face images. Furthermore, the pattern of responses obtained with chimeric images differed significantly from full‐face matching and misaligned/separated hemi‐face matching. Chimeric images produced a bias towards same responses even when the face halves were different. The results suggest caution in the use of chimeric images in court. Copyright © 2011 John Wiley & Sons, Ltd.     

The effect of face orientation on holistic processing

Holistic processing, a hallmark of face processing, can be measured by the composite face effect: adults have difficulty recognising the top half of a face when it is aligned with a new bottom half, unless holistic processing is disrupted by misaligning the two halves. Like the recognition of facial identity, holistic processing is impaired when faces are inverted. To obtain a more refined measure of the influence of orientation on holistic face processing, we administered the composite face task to adults at each of seven orientations. In both experiment 1 (in which orientations were randomly intermixed) and experiment 2 (in which orientations were blocked), the composite face effect decreased linearly with rotation. We conclude that holistic processing is tuned to upright faces, diminishes as faces deviate from upright, and becomes insignificant once faces reach a sideways orientation. We propose a hierarchical model of face perception in which linear decreases in holistic processing underlie qualitative shifts in other aspects of face perception.     

Priming global and local processing of composite faces: revisiting the processing-bias effect on face perception

We used the composite-face illusion and Navon stimuli to determine the consequences of priming local or global processing on subsequent face recognition. The composite-face illusion reflects the difficulty of ignoring the task-irrelevant half-face while attending the task-relevant half if the half-faces in the composite are aligned. On each trial, participants first matched two Navon stimuli, attending to either the global or the local level, and then matched the upper halves of two composite faces presented sequentially. Global processing of Navon stimuli increased the sensitivity to incongruence between the upper and the lower halves of the composite face, relative to a baseline in which the composite faces were not primed. Local processing of Navon stimuli did not influence the sensitivity to incongruence. Although incongruence induced a bias toward different responses, this bias was not modulated by priming. We conclude that global processing of Navon stimuli augments holistic processing of the face.     

The role of holistic processing in judgments of facial attractiveness

Previous work has demonstrated that facial identity recognition, expression recognition, gender categorisation, and race categorisation rely on a holistic representation. Here we examine whether a holistic representation is also used for judgments of facial attractiveness. Like past studies, we used the composite paradigm to assess holistic processing (Young et al 1987, Perception 16 747-759). Experiment 1 showed that top halves of upright faces are judged to be more attractive when aligned with an attractive bottom half than when aligned with an unattractive bottom half. To assess whether this effect resulted from holistic processing or more general effects, we examined the impact of the attractive and unattractive bottom halves when upright halves were misaligned and when aligned and misaligned halves were presented upside-down. The bottom halves had no effect in either condition. These results demonstrate that the perceptual processes underlying upright facial-attractiveness judgments represent the face holistically. Our findings with attractiveness judgments and previous demonstrations involving other aspects of face processing suggest that a common holistic representation is used for most types of face processing.     

Face processing in humans and new world monkeys: the influence of experiential and ecological factors

This study tests whether the face-processing system of humans and a nonhuman primate species share characteristics that would allow for early and quick processing of socially salient stimuli: a sensitivity toward conspecific faces, a sensitivity toward highly practiced face stimuli, and an ability to generalize changes in the face that do not suggest a new identity, such as a face differently oriented. The look rates by adult tamarins and humans toward conspecific and other primate faces were examined to determine if these characteristics are shared. A visual paired comparison (VPC) task presented subjects with either a human face, chimpanzee face, tamarin face, or an object as a sample, and then a pair containing the previous stimulus and a novel stimulus was presented. The stimuli were either presented all in an upright orientation, or all in an inverted orientation. The novel stimulus in the pair was either an orientation change of the same face/object or a new example of the same type of face/object, and the stimuli were shown either in an upright orientation or in an inverted orientation. Preference to novelty scores revealed that humans attended most to novel individual human faces, and this effect decreased significantly if the stimuli were inverted. Tamarins showed preferential looking toward novel orientations of previously seen tamarin faces in the upright orientation, but not in an inverted orientation. Similarly, their preference to look longer at novel tamarin and human faces within the pair was reduced significantly with inverted stimuli. The results confirmed prior findings in humans that novel human faces generate more attention in the upright than in the inverted orientation. The monkeys also attended more to faces of conspecifics, but showed an inversion effect to orientation change in tamarin faces and to identity changes in tamarin and human faces. The results indicate configural processing restricted to particular kinds of primate faces by a New World monkey species, with configural processing influenced by life experience (human faces and tamarin faces) and specialized to process orientation changes specific to conspecific faces.     

Configurational information in face perception

A new facial composites technique is demonstrated, in which photographs of the top and bottom halves of different familiar faces fuse to form unfamiliar faces when aligned with each other. The perception of a novel configuration in such composite stimuli is sufficiently convincing to interfere with identification of the constituent parts (experiment 1), but this effect disappears when stimuli are inverted (experiment 2). Difficulty in identifying the parts of upright composites is found even for stimuli made from parts of unfamiliar faces that have only ever been encountered as face fragments (experiment 3). An equivalent effect is found for composites made from internal and external facial features of well-known people (experiment 4). These findings demonstrate the importance of configurational information in face perception, and that configurations are only properly perceived in upright faces.     

Holistic processing of faces in preschool children and adults

Contrary to the encoding-switch hypothesis, recent research demonstrates that 6-year-olds do not rely solely on parts-based encoding to recognize upright faces. This research shows better recognition of face parts presented in the whole face than in isolation, indicating use of holistic encoding. The present study examined whether children younger than 6 years also recognize faces holistically. Four-year-olds, 5-year-olds, and adults were administered a part-whole face recognition task. Children below the age of 6 remembered parts from upright faces better when tested in the whole-face context than in isolation. This whole-face advantage did not occur when faces were inverted. Although children showed a smaller inversion decrement than adults and generally performed more poorly than adults, the different age groups showed similar patterns of performance, indicating that young preschoolers, like older children and adults, are able to recognize faces holistically.     

Effect of visual experience on face processing: a developmental study of inversion and non-native effects

In adults, three phenomena are taken to demonstrate an experience effect on face recognition: an inversion effect, a non-native face effect (so-called 'other-race' effect) and their interaction. It is crucial for our understanding of the developmental perception mechanisms of object processing to discover when these effects are present in childhood. Three- to 5-year-old Caucasian children (N = 64) were asked to recognize upright and inverted Caucasian and Asian faces. Recognition was tested with a forced-choice procedure. Overall performance improved with age. However, there was an interaction between the inversion and non-native effects that did not change with age between ages 3 and 5: (a) the inversion effect with native (Caucasian) faces was larger than with non-native (Asian) faces, and (b) upright native faces were recognized better than upright non-native faces. These results show that face orientation and morphology constrain face processing in 3- to 5-year-olds. The first 3 years of life during which the brain and the environment interact are sufficient to build a face-processing system that constrains recognition.     

Evidence for holistic processing of faces viewed as photographic negatives

Inversion and photographic negation both impair face recognition. Inversion seems to disrupt processing of the spatial relationship between facial features ('relational' processing) which normally occurs with upright faces and which facilitates their recognition. It remains unclear why negation affects recognition. To find out if negation impairs relational processing, we investigated whether negative faces are subject to the 'chimeric-face effect'. Recognition of the top half of a composite face (constructed from top and bottom halves of different faces) is difficult when the face is upright, but not when it is inverted. To perform this task successfully, the bottom half of the face has to be disregarded, but the relational processing which normally occurs with upright faces makes this difficult. Inversion reduces relational processing and thus facilitates performance on this particular task. In our experiments, subjects saw pairs of chimeric faces and had to decide whether or not the top halves were identical. On half the trials the two chimeras had identical tops; on the remaining trials the top halves were different. (The bottom halves were always different.) All permutations of orientation (upright or inverted) and luminance (normal or negative) were used. In experiment 1, each pair of 'identical' top halves were the same in all respects. Experiment 2 used differently oriented views of the same person, to preclude matches being based on incidental features of the images rather than the faces displayed within them. In both experiments, similar chimeric-face effects were obtained with both positive and negative faces, implying that negative faces evoke some form of relational processing. It is argued that there may be more than one kind of relational processing involved in face recognition: the 'chimeric-face effect' may reflect an initial 'holistic' processing which binds facial features into a 'Gestalt', rather than being a demonstration of the configurational processing involved in individual recognition.     

The importance of being upright: Use of environmental and viewer-centered reference frames in shape discriminations of novel three-dimensional objects

We investigated the frame of reference that people use to make shape discriminations when their heads are either upright or tilted. Observers madesame-different judgments of pairs of novel threedimensional objects that were aligned along their length within the frontal-parallel plane and rotated in depth around an axis parallel to their own axes of elongation. The aligned objects were displayed vertically, tilted 45°, or horizontally with respect to the environmental upright, but the distance of each pair from the upright was irrelevant to resolving the angular disparity between the stimuli for thesame-different judgment. Nevertheless, when observers’ heads were upright, the time to encode the stimuli was a linear function of the distance of the stimuli from the environmental upright, whereas when observers’ heads were tilted 45°, encoding times for tilted and vertical stimuli did not differ and were faster than the times to encode horizontal stimuli. We interpreted these data to mean that observers either rotate or reference the top of an object to the environmental upright, and they can use either a gravitational or retinal reference frame to do so when either they or the objects are not upright.     

Configural features in the context of upright and inverted faces

When faces are turned upside down, recognition is known to be severely disrupted. This effect is thought to be due to disruption of configural processing. Recently, Leder and Bruce (2000, Quarterly Journal of Experimental Psychology A 53 513-536) argued that configural information in face processing consists at least partly of locally processed relations between facial elements. In three experiments we investigated whether a local relational feature (the interocular distance) is processed differently in upside-down versus upright faces. In experiment 1 participants decided in which of two sequentially presented photographic faces the interocular distance was larger. The decision was more difficult in upside-down presentation. Three different conditions were used in experiment 2 to investigate whether this deficit depends upon parts of the face beyond the eyes themselves; displays showed the eye region alone, the eyes and nose, or the eyes and nose and mouth. The availability of additional features did not interact with the inversion effect which was observed strongly even when the eyes were shown in isolation. In experiment 3 all eyes were turned upside down in the inverted face condition as in the Thatcher illusion (Thompson, 1980 Perception 9 483-484). In this case no inversion effect was found. These results are in accordance with an explanation of the face-inversion effect in which the disruption of configural facial information plays the critical role in memory for faces, and in which configural information corresponds to spatial information that is processed in a way which is sensitive to local properties of the facial features involved.