Holistic face processing is mature at 4 years of age: evidence from the composite face effect

Although it is acknowledged that adults integrate features into a representation of the whole face, there is still some disagreement about the onset and developmental course of holistic face processing. We tested adults and children from 4 to 6 years of age with the same paradigm measuring holistic face processing through an adaptation of the composite face effect [Young, A. W., Hellawell, D., & Hay, D. C. (1987). Configurational information in face perception. Perception, 16, 747-759]. In Experiment 1, only 6-year-old children and adults tended to perceive the two identical top parts as different, suggesting that holistic face processing emerged at 6 years of age. However, Experiment 2 suggested that these results could be due to a response bias in children that was cancelled out by always presenting two faces in the same format on each trial. In this condition, all age groups present strong composite face effects, suggesting that holistic face processing is mature as early as after 4 years of experience with faces.     

The composite face illusion: A whole window into our understanding of holistic face perception

Two identical top halves of a face are perceived as being different when their bottom halves belong to different faces, showing that the parts of a face cannot be perceived independently from the whole face. When this visual illusion is inserted in a matching task, observers make more mistakes and/or are slower at matching identical top face halves aligned with different bottom halves than when the bottom halves are spatially offset: The composite face effect. This composite face paradigm has been used in more than 60 studies that have provided information about the specificity and nature of perceptual integration between facial parts (“holistic face perception”), the impairment of this process in acquired prosopagnosia, its developmental course, temporal dynamics, and neural basis. Following a review of the main contributions made with the paradigm, I explain its rationale and strengths, and discuss its methodological parameters, making a number of proposals for its optimal use and refinement in order to improve our understanding of holistic face perception. Finally, I explain how this standard composite face paradigm is fundamentally different than the application to facial parts of a congruency/interference paradigm that has a long tradition in experimental psychology since Stroop (1935), and which was originally developed to measure attentional and response interference between different representations rather than perceptual integration. Moreover, a version of this congruency/interference paradigm used extensively over the past years with composite faces lacks a baseline measure and has decisional, attentional, and stimulus confounds, making the findings of these studies impossible to interpret in terms of holistic perception. I conclude by encouraging researchers in this field to concentrate fully on the standard composite face paradigm, gaze contingency, and other behavioural measures that can help us take one of the most important challenges of visual perception research: Understanding the neural mechanisms of holistic face perception.     

Outgroup homogeneity effect in perception: An exploration with Ebbinghaus illusion

An outgroup homogeneity (OH) effect implies that outgroup members are perceived to be more similar than ingroup members. At present, however, it is not clear whether the OH effect is truly perceptual. Here, we used an Ebbinghaus illusion to demonstrate the OH effect in perception. Participants were presented with one central face that was surrounded by four surrounding faces. The central face was judged to be smaller as the size of the surrounding faces increased, thereby demonstrating an Ebbinghaus illusion. As predicted, however, this illusion was significantly greater when the faces allegedly belonged to an outgroup than when they allegedly belonged to an ingroup. This perceptual OH effect bore no significant relationship with cognitive OH measures. The perceptual versus cognitive OH effects might therefore be mediated by separate mechanisms.     

Electrophysiological correlates of the composite face illusion: Disentangling perceptual and decisional components of holistic face processing in the human brain

When the bottom halves of two faces differ, people’s behavioral judgment of the identical top halves of those faces is impaired: they report that the top halves are different, and/or take more time than usual to provide a response. This behavioral measure is known as the composite face effect (CFE) and has traditionally been taken as evidence that faces are perceived holistically. Recently, however, it has been claimed that this effect is driven almost entirely by decisional, rather than perceptual, factors ( Richler, Gauthier, Wenger, & Palmeri, 2008). To disentangle the contribution of perceptual and decisional brain processes, we aimed to obtain an event-related potential (ERP) measure of the CFE at a stage of face encoding ( Jacques & Rossion, 2009) in the absence of a behavioral CFE effect. Sixteen participants performed a go/no-go task in an oddball paradigm, lifting a finger of their right or left hand when the top half of a face changed identity. This change of identity of the top of the face was associated with an increased ERP signal on occipito-temporal electrode sites at the N170 face-sensitive component (∼160 ms), the later decisional P3b component, and the lateralized readiness potential (LRP) starting at ∼350 ms. The N170 effect was observed equally early when only the unattended bottom part of the face changed, indicating that an identity change was perceived across the whole face in this condition. Importantly, there was no behavioral response bias for the bottom change trials, and no evidence of decisional biases from electrophysiological data (no P3b and LRP deflection in no-go trials). These data show that an early CFE can be measured in ERPs in the absence of any decisional response bias, indicating that the CFE reflects primarily the visual perception of the whole face.     

Holistic processing in the composite task depends on face size

Holistic processing is a hallmark of face processing. There is evidence that holistic processing is strongest for faces at identification distance 2–10 metres from the observer. However, this evidence is based on tasks that have been little used in the literature and that are indirect measures of holistic processing. We use the composite task—a well validated and frequently used paradigm—to measure the effect of viewing distance on holistic processing. In line with previous work, we find a congruency x alignment effect that is strongest for faces that are close (2 m equivalent distance) than for faces that are further away (24 m equivalent distance). In contrast, the alignment effect for same trials, used by several authors to measure holistic processing, produced results that are difficult to interpret. We conclude that our results converge with previous findings providing more direct evidence for an effect of size on holistic processing.     

Conscious awareness is required for holistic face processing

Investigating the limits of unconscious processing is essential to understand the function of consciousness. Here, we explored whether holistic face processing, a mechanism believed to be important for face processing in general, can be accomplished unconsciously. Using a novel “eyes-face” stimulus we tested whether discrimination of pairs of eyes was influenced by the surrounding face context. While the eyes were fully visible, the faces that provided context could be rendered invisible through continuous flash suppression. Two experiments with three different sets of face stimuli and a subliminal learning procedure converged to show that invisible faces did not influence perception of visible eyes. In contrast, surrounding faces, when they were clearly visible, strongly influenced perception of the eyes. Thus, we conclude that conscious awareness might be a prerequisite for holistic face processing.     

Holistic processing is tuned for in-group faces

Past research has found that mere in-group/out-group categorizations are sufficient to elicit biases in face memory. The current research yields novel evidence that mere social categorization is also sufficient to modulate processes underlying face perception, even for faces for which we have strong perceptual expertise: same-race (SR) faces. Using the composite face paradigm, we find that SR faces categorized as in-group members (i.e., fellow university students) are processed more holistically than are SR faces categorized as out-group members (i.e., students at another university). Hence, holding perceptual expertise with faces constant, categorizing an SR target as an out-group member debilitates the strong holistic processing typically observed for SR faces.     

The electrophysiological correlates of integrated face and body-part perception

Previous studies have suggested that the human visual system processes faces and bodies holistically—that is, the different body parts are integrated into a unified representation. However, the time course of this integrative process is less known. In the present study, we investigated this issue by recording event-related potentials evoked by a face and two hands presented simultaneously and in different configurations. When the hands were rotated to obtain a biologically implausible configuration, a reduction of the P2 amplitude was observed relative to the condition in which the face and hands were retained in their veridical configuration and were supplemented with visual cues to highlight further the overall body posture. Our results show that the P2 component is sensitive to manipulations affecting the configuration of face and hand stimuli and suggest that the P2 reflects the operation of perceptual mechanisms responsible for the integrated processing of visually presented body parts.     

Holistic face encoding is modulated by perceived face race: Evidence from perceptual adaptation

Human expertise at processing faces relies on how facial features are encoded: as a whole template rather than as a sum of independent features. This holistic encoding is less prominent for other-race faces, possibly accounting for the difficulty one encounters in recognizing these faces (the ‘other-race effect’). Here, we tested the hypothesis that the magnitude of holistic face encoding can be modulated by racial categorization of the face. Caucasian participants performed a face-composite task with ‘racially-ambiguous’ face-stimuli (cross-race morphed faces, equally categorized as Asian or Caucasian faces in an independent task). The perceived race of the ambiguous faces was manipulated using adaptation. Experiment 1 showed that identical morphed face-stimuli were processed more holistically when perceived as ‘same-race’ than as ‘other-race’, i.e., following adaptation to ‘other-race’ versus ‘same-race’, respectively. Experiment 2 ascertained that the determining factor in the observed holistic processing modulation was the race of the racially-ambiguous face as perceived, rather than expected, by the participants, which supports the idea that the holistic processing of the face-stimuli was modulated by their race-categorization at the perceptual level.     

Perceptual coupling of multiple point-light figures

When confronted with multiple bistable stimuli at the same time, the visual system tends to generate a common interpretation for all stimuli. We exploit this perceptual-coupling phenomenon to investigate the perception of depth in bistable point-light figures. Observers indicate the global depth orientation of simultaneously presented point-light figures while the similarity between the stimuli is manipulated. In a first experiment, a higher occurrence of coupled percepts is found for identical figures, but coupling breaks down when either the movement pattern or both the viewpoint and the phase-relation are changed. A second experiment confirms these results, but also demonstrates that different point-light actions can be subject to perceptual coupling as long as they share the same viewpoint and exhibit equivalent degrees of perceptual ambiguity. The data are consistent with an explanation in terms of differential contributions of stored, view-dependent object and action representations and of an interaction between stages processing local stimulus features. The consequences of these results are discussed in the framework of an explicit model for the perception of depth in biological motion.     

Hemisphere-dependent holistic processing of familiar faces

In two behavioral experiments involving lateralized stimulus presentation, we tested whether one of the most commonly used measures of holistic face processing—the composite face effect—would be more pronounced for stimuli presented to the right as compared to the left hemisphere. In experiment 1, we investigated the composite face effect in a verbal identification task, similar to its original report (Young, Hellawell, & Hay, 1987). Aligning top and bottom halves of composite face stimuli led to performance decreases irrespective of hemifield, indicating holistic processing of comparable magnitude for inputs provided separately to either hemisphere. However, when matching of the same top parts was required in experiment 2, an alignment-dependent performance decrease was found for stimuli presented in the left, but not right visual field. These observations suggest that the right hemisphere dominates in early stages of holistic processing, as indexed by the composite face effect, but that later processes such as face identification and naming are based on unified representations that are independent of input lateralization. Moreover, the composite face effect may not rely on the exact same representation(s) when measured in matching and identification tasks.     

Yang's iris illusion: External contour causes length‐assimilation illusions1

The Delboeuf illusion and the Ebbinghaus illusion (also known as the Titchener illusion) demonstrate that an external contour can lead to size‐assimilation and size‐contrast perception. This paper explores a novel illusion, revealing that neighboring external contours can also lead to a distortion in length perception. The illusion was originally discovered from a face stimulus (Experiment 1) in which a face was depicted alongside its mirror image so as to make the four irises absolutely equidistant. The distance between the middle two irises was underestimated in Asian faces, but overestimated in Caucasian faces. The illusion was also maintained when the facial stimuli were replaced by line drawings of eyes (Experiment 2). However, the illusion vanished when the irises were presented alone. Further scrutiny of the differences in facial characteristics between Asian and Caucasian faces reveals that the illusion might be elicited by the relative position of the eye shapes. This hypothesis was confirmed in Experiment 3, in which the distances between the eye shapes and the irises were manipulated.     

The influence of divided attention on holistic face perception.

There is evidence that upright, but not inverted, faces are encoded holistically. The holistic coding of faces was examined in four experiments by manipulating the attention allocated to target faces. In Experiment 1, participants in a divided attention condition were asked to match two upright flanker faces while encoding a centrally presented upright target face. Although holistic coding was evident in the full attention conditions, dividing attention disrupted holistic coding of target faces. In Experiment 2, we found that while matching upright flanker faces disrupted holistic coding, matching inverted flanker faces did not. Experiment 3 demonstrated that the differential effects of flanker orientation were not due to participants taking longer to match upright, than inverted, flanker faces. In Experiment 4, we found that matching fractured faces had an intermediate effect to that of matching upright and inverted flankers, on the holistic coding of the target faces. The findings emphasize the differences in processing of upright, fractured and inverted faces and suggest that there are limitations in the number of faces that can be holistically coded in a brief time.     

Holistic face processing is penetrable … depending on the composite design

Holistic processing (HP) of faces is usually measured by the composite effect. While Weston and Perfect [2005. Effects of processing bias on the recognition of composite face halves. Psychonomic Bulletin & Review, 12, 1038–1042. doi:10.3758/BF03206440] found that priming at the local level speeded recognition of components of faces, Gao et al. [2011. Priming global and local processing of composite faces: Revisiting the processing-bias effect on face perception. Attention Perception & Psychophysics, 73, 1477–1486. doi:10.3758/s13414-011-0109-7] found that only global priming had an effect on HP of faces. The two studies used different versions of the composite task (the partial design, which is considered to be prone on bias, and the complete design). However, the two studies also differed in other respects and it is difficult to know to what extent issues with the partial design contributed to the differing conclusions. In the present study, the HP indexed by the complete design measure was augmented by global priming. In contrast, no effect was observed in the partial design index. We claim that the partial design index reflects other factors besides HP, including response bias, and conclude that HP can be understood within the context of domain-general attentional processes.     

The role of holistic face processing in acquired prosopagnosia: evidence from the composite face effect

Faces are processed more holistically than other objects, and it has been suggested that the loss of holistic face processing causes acquired prosopagnosia. Support for this hypothesis comes from several cases who failed to show holistic face effects as well as the absence of reports of prosopagnosics with unequivocally normal holistic face perception. The current study examines the relationship between holistic face processing and prosopagnosia by testing seven acquired prosopagnosics with the face composite task, a classic measure of holistic face processing. To enhance the robustness of the findings, each prosopagnosic was tested with two versions of the composite task showing upright faces. We also tested an inverted condition to exclude the possibility that more general factors account for composite effects for upright faces. Four of the seven acquired prosopagnosic participants showed consistent upright face composite effects with minimal inverted face composite effects. We conclude that severe face processing deficits can co-occur with intact holistic face processing and that factors other than a loss of holistic processing contribute to the perceptual and recognition deficits in acquired prosopagnosia.     

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.     

The perception of (naked only) bodies and faceless heads relies on holistic processing: Evidence from the inversion effect

Faces and bodies are more difficult to perceive when presented inverted than when presented upright (i.e., stimulus inversion effect), an effect that has been attributed to the disruption of holistic processing. The features that can trigger holistic processing in faces and bodies, however, still remain elusive. In this study, using a sequential matching task, we tested whether stimulus inversion affects various categories of visual stimuli: faces, faceless heads, faceless heads in body context, headless bodies naked, whole bodies naked, headless bodies clothed, and whole bodies clothed. Both accuracy and inversion efficiency score results show inversion effects for all categories but for clothed bodies (with and without heads). In addition, the magnitude of the inversion effect for face, naked body, and faceless heads was similar. Our findings demonstrate that the perception of faces, faceless heads, and naked bodies relies on holistic processing. Clothed bodies (with and without heads), on the other side, may trigger clothes-sensitive rather than body-sensitive perceptual mechanisms.     

The other-race effect: Holistic coding differences and beyond

We evaluate claims that the other-race effect in face memory reflects stronger holistic coding of own-race than other-race faces. Considering evidence from a range of paradigms, including the inversion effect, part–whole effect, composite effect, and the scrambled/blurred task, we find considerable inconsistency, both between paradigms and between participant ethnicities. At the same time, however, studies that isolate configural and component feature processing consistently show better featural, as well as better configural, processing of own-race faces, for both Caucasian and Asian participants. These results raise the possibility that the key feature of own-race face processing is not stronger holistic processing per se, but rather more effective processing of all types of face information (featural as well as holistic).     

Impaired holistic and analytic face processing in congenital prosopagnosia: Evidence from the eye-contingent mask/window paradigm

There is abundant evidence that face recognition, in comparison to the recognition of other objects, is based on holistic processing rather than analytic processing. One line of research that provides evidence for this hypothesis is based on the study of people who experience pronounced difficulties in visually identifying conspecifics on the basis of their face. Earlier, we developed a behavioural paradigm to directly test analytic vs. holistic face processing. In comparison to a to be remembered reference face stimulus, one of two test stimuli was either presented in full view, with an eye-contingently moving window (only showing the fixated face feature, and therefore only affording analytic processing), or with an eye-contingently moving mask or scotoma (masking the fixated face feature, but still allowing holistic processing). In the present study we use this paradigm (that we used earlier in acquired prosopagnosia) to study face perception in congenital prosopagnosia (people having difficulties recognizing faces from birth on, without demonstrable brain damage). We observe both holistic and analytic face processing deficits in people with congenital prosopagnosia. Implications for a better understanding, both of congenital prosopagnosia and of normal face perception, are discussed.