Categorical Face Perception Research Articles

Aging, pattern separation, and categorical perception of faces

Categorical perception (CP) is the phenomenon by which observers view linear changes that occur across a continuum as distinct categories. Although categorical perception is a perceptual phenomenon, it may be subserved by mnemonic processes such as pattern separation. To examine this hypothesis, following standard CP tasks, we assessed younger and older participants’ abilities to identify and discriminate between members of pairs of famous or non-famous faces. We hypothesized that if CP is dependent upon neural pattern separation, which declines with aging, discrimination ability as indexed by CP would be compromised in older adults, as was found in our study. Since familiarity promotes pattern separation, CP should be enhanced for famous, as compared to non-famous faces, even in older adults. We found that all participants benefited from familiarity, but younger adults outperformed older adults overall. We next examined the effects of face inversion on CP for both famous and non-famous faces. If pattern separation, and CP, is determined solely by the similarity across physical features, then CP should be similar for upright and inverted faces since these features are perceptually invariant across orientation. If, however, pattern separation, and CP, depends on how stimuli are represented, then orientation may matter as upright and inverted faces are represented holistically or part-based, respectively. We found that inversion disrupted CP in younger adults whereas older adults performed similarly across both conditions, suggesting that face-representation is more part-based in older adults.

Facial disfigurement, categorical perception, and the influence of Disgust Sensitivity

ABSTRACT Previous research supports the categorical perception of faces on dimensions including emotion, identity, and gender. Two experiments using standard paradigms investigated whether facial disfigurement forms another perceptual category. In the Identification task, faces were presented in varying degrees of disfigurement for a simple disfigured / non-disfigured decision. As degree of disfigurement increased, the percentage of participants defining each image as disfigured increased non-linearly such that a cubic curve provided the best fit to the data, consistent with categorical perception (Experiment 1 and 2). In the ABX task, participants displayed superior discrimination between two faces when they crossed the category boundary between non-disfigured and disfigured (Experiment 1 and participants low in Disgust Sensitivity in Experiment 2). Participants high in Disgust Sensitivity (Experiment 2) showed a pattern that suggested the category boundary was shifted towards earlier perception of disfigurement. Overall, the results suggest categorical perception of facial disfigurement.

Categorical face perception in fish: How a fish brain warps reality to dissociate "same" from "different".

Categorical perception (CP) is the phenomenon by which a smoothly varying stimulus property undergoes a nonlinear transformation during processing in the brain. Consequently, the stimuli are perceived as belonging to distinct categories separated by a sharp boundary. Originally thought to be largely innate, the discovery of CP in tasks such as novel image discrimination has piqued the interest of cognitive scientists because it provides compelling evidence that learning can shape a category's perceptual boundaries. CP has been particularly closely studied in human face perception. In nonprimates, there is evidence for CP for sound and color discrimination, but not for image or face discrimination. Here, we investigate the potential for learned CP in a lower vertebrate, the damselfish Pomacentrus amboinensis. Specifically, we tested whether the ability of these fish to discriminate complex facial patterns tracked categorical rather than metric differences in the stimuli. We first trained the fish to discriminate sets of two facial patterns. Next, we morphed between these patterns and determined the just noticeable difference (JND) between a morph and original image. Finally, we tested for CP by analyzing the discrimination ability of the fish for pairs of JND stimuli along the spectrum of morphs between two original images. Discrimination performance was significant for the image pair straddling the boundary between categories, and chance for equivalent stimulus pairs on either side, thus producing the classic "category boundary" effect. Our results reveal how perception can be influenced in a top-down manner even in the absence of a visual cortex.

Newly learned categories induce pre-attentive categorical perception of faces

Face perception is modulated by categorical information in faces, which is known as categorical perception (CP) of faces. However, it remains unknown whether CP of faces is humans’ inborn capability or the result of acquired categories. Here, we examined whether and when newly learned categories affect face perception. A short-term training method was employed in which participants learned new categories of face stimuli. Behaviorally, using an AB-X discrimination task, we found that the discrimination accuracy of face pairs from different learned categories was significantly higher than that of faces from the same category. Neurally, using a visual oddball task, we found that deviant stimuli whose category differed from standard stimuli evoked a larger N170. Importantly, the visual mismatch negativity (vMMN), starting from 140 ms after stimuli onset, was stronger with the between-category deviants than with the within-category deviants under the unattended condition. Altogether, our study provides empirical evidence indicating that CP of faces could be induced by newly learned categories, and this effect occurs automatically during an early stage of processing.

Emergence of categorical face perception after extended early-onset blindness

It is unknown whether the ability to visually distinguish between faces and nonfaces is subject to a critical period during development. Would a congenitally blind child who gains sight several years after birth be able to acquire this skill? This question has remained unanswered because of the rarity of cases of late sight onset. We had the opportunity to work with five early-blind individuals who gained sight late in childhood after treatment for dense bilateral cataracts. We tested their ability to categorize patterns as faces, using natural images that spanned a spectrum of face semblance. The results show that newly sighted individuals are unable to distinguish between faces and nonfaces immediately after sight onset, but improve markedly in the following months. These results demonstrate preserved plasticity for acquiring face/nonface categorization ability even late in life, and set the stage for investigating the informational and neural basis of this skill acquisition.

Broadly tuned face representation in older adults assessed by categorical perception.

Studies of face recognition in older adults (60 years of age and older) report increases in false alarms over younger adults (usually 18-30 years of age), but no age differences in hits. To examine this phenomenon, we compared older and younger adults in categorical perception of faces. We hypothesized that face representations in older adults would be broadly tuned, resulting in overlapping representations, manifested by a shallower slope in identity categorization than in younger adults, and age-related reductions in the advantage for between-categories, as compared with within-category, face discrimination. We morphed faces to change linearly from one identity to another. We used familiar or unfamiliar faces in separate conditions to examine the role of familiarity. Categorical perception was assessed in an identity-classification task and a discrimination task. Older adults showed a shallower slope and poorer discrimination compared with younger adults, and both groups exhibited better performance with familiar than unfamiliar faces. Enhanced discriminability for between-categories as compared with within-category faces was seen for both familiar and unfamiliar faces in younger adults, but only for familiar faces in older adults. The more broadly tuned representations of unfamiliar faces in older adults may lead to misidentification and greater false alarms for unfamiliar faces, but not for familiar faces.

Emotion categorization does not depend on explicit face categorization

Face perception and emotion recognition have been extensively studied in the past decade; however, the relation between them is still poorly understood. A traditional view is that successful emotional categorization requires categorization of the stimulus as a 'face', at least at the basic level. Here we tested whether emotional information could still be recognized accurately without explicit categorization of a stimulus as a face. For this purpose we created a stimulus set in which facial stimuli expressing a range of happy-to-fear emotions were morphed into another object category (shoe). Interestingly, participants categorized emotions with great accuracy in stimuli that contained so little face information that they were explicitly categorized as shoes. Hence, our results show that accurate emotion categorization can take place in stimuli that contain surprisingly little face information. This finding raises interesting questions about the extent to which processes leading to emotion recognition and categorical face perception might be separable.

Face‐specificity is robust across diverse stimuli and individual people, even when interstimulus variance is zero

The N170 is a brain electrical potential proposed to index the earliest time of categorical perception of faces in occipitotemporal visual areas implicated in face cognition, being more negative for faces than nonface objects between 120 and 200 ms. The interstimulus variance (ISV) account instead explained N170 face-specificity as an artifact due to objects varying more visually than faces. Ganis, Smith, and Schendan (2012) tested this account directly, finding that N170 face-specificity remains even when ISV is eliminated. Here, N170 peak amplitude and face-specificity is quantified for individual stimuli and participants, revealing that the right hemisphere N170 is especially sensitive to stimulus variability. Further, ISV contributes 0 to 37% to N170 face-specificity. These findings provide evidence for optimizing face processing science. The paradigm can apply to any research in which ISV may be uncontrolled (e.g., category comparisons).

The N170, not the P1, indexes the earliest time for categorical perception of faces, regardless of interstimulus variance

A negative event-related potential (ERP) at occipito-temporal sites peaking around 150-170 ms after stimulus onset (N170) is typically larger for faces than other object categories. Most theories interpret this finding as due to face-selective processing in occipito-temporal and temporal cortex. However, a controversial account recently attributed the N170 effect to differences in interstimulus variance (ISV) among the images typically used for face and object conditions and proposed that the earlier P1 instead indexes the categorical processes generally attributed to the N170. This ERP study aimed to test this account definitively by using conditions in which the same face and object were shown repeatedly, eliminating both physical and perceptual ISV. Fourier amplitude spectra of faces and objects were matched to equate basic low-level visual properties that may affect early ERPs such as the P1. Results demonstrate that i) face selectivity of the N170 is largely preserved across many object categories after abolishing ISV, and ii) stimulus category does not modulate the P1. This conclusively refutes the ISV account while strongly supporting category as a critical factor driving N170 face selectivity.

Fine-grained face discrimination and categorical face perception as evidenced by steady-state visual evoked potentials

Fine-grained face discrimination and categorical face perception as evidenced by steady-state visual evoked potentials

Neural basis of contextual modulation on categorical face perception

The ability to categorize patterns is crucial for making sense of the visual world as an orderly collection of discrete objects. Contextual information often facilitates perceptual categorization to disambiguate and organize visual input. In last year's VSS, we demonstrated neural correlates of categorization in the domain of faces. Our experimental strategy involved compiling 300 images that spanned a range of facial similarity from non-faces to genuine faces. Using fMRI, we found that the pattern of activity in the left fusiform changed in a graded fashion as the image stimuli became increasingly face-like, while the pattern of activity in the right fusiform showed a step-like response corresponding to a categorical difference between faces and non-faces. Here we investigated how contextual information can modulate brain activation patterns that are evoked by these stimuli. Behaviorally, we verified that subjects were more accurate at the face/non-face classification task when the 300 images were shown with surrounding context. Consistent with our previous finding, brain activation patterns in the right fusiform correlated with the perceived face categoricity. More interestingly, the pattern of activity in the left fusiform and right inferior occipital gyrus also became categorical in correspondence to the behavioral step-like difference between faces and non-faces. Particularly in the left fusiform, the surrounding context on one hand enhanced activation patterns induced by genuine faces, and on the other hand suppressed the activation patterns induced by the non-face stimuli that happened to look face-like. By contrast, the effect of contextual modulation was not significant in the left inferior occipital gyrus and the calcarine sulcus, ruling out activation change induced by bottom-up effects. These results suggest a categorically selective neural mechanism of contextual modulation and have important implications for models of real-world face-processing.

Using computer vision to probe the neural correlates of categorical face perception

Using computer vision to probe the neural correlates of categorical face perception

The Neural Basis of Categorical Face Perception: Graded Representations of Face Gender in Fusiform and Orbitofrontal Cortices

Face gender, like many other things, is perceived categorically: Subjective perceptions are distorted toward the categories, male or female, and the objective gradiency inherent across faces is partially lost. The neural basis of such categorical face perception remains virtually unknown. Participants passively viewed faces whose sexually dimorphic content was morphed monotonically from male to female while neural activity was measured using functional magnetic resonance imaging. Subjective perceptions revealed strong nonlinearity despite monotonic linear changes in face gender, consistent with categorical perception. Neuroimaging results indicated that the lateral fusiform gyrus, bilaterally, and the fusiform face area linearly encoded graded parameters of objective face gender, but these regions correlated substantially less with subjective perceptions (which were nonlinear and affected by categorical perception effects). Such subjective perceptions, however, were represented in the orbitofrontal cortex, but this region correlated substantially less with objective parameters. The attention-independent graded representations of face gender in fusiform and orbitofrontal cortices reveal how objective face parameters are encoded and transformed into subjective categorically warped perceptions in the human brain.