Face memory deficits in patients deprived of early visual input by bilateral congenital cataracts

Face-specific identification impairments following sight-providing treatment may be alleviated by an initial period of low visual acuity

Identifying faces requires configural processing of visual information. We previously proposed that the poor visual acuity experienced by newborns in their first year of life lays the groundwork for such configural processing by forcing integration over larger spatial fields. This hypothesis predicts that children treated for congenital cataracts late in life will exhibit persistent impairments in face- but not object-identification, because they begin their visual journey with higher than newborn acuity. This would not be the case for patients whose pretreatment condition has allowed for initial low acuity vision, like that of a newborn. Here, we test this prediction by assessing the development of facial identification skill in three groups: patients treated for congenital cataracts whose pretreatment visual acuity was worse than that of a newborn, patients whose pretreatment acuity was better than that of a newborn, and age-matched controls. We find that while both patient groups show significant gains in object-identification, the emergence of face identification is determined by pretreatment acuity: patients with pre-operative acuity worse than a newborn did not show any improvements on face-identification tasks despite years of visual experience, whereas those with pretreatment acuity comparable to a newborn improved on both the object- and face-identification tasks. These findings not only answer our research question but also provide new insights into the role of early visual acuity in facial identification development. We discuss these results in the context of both typical and atypical visual development.

The neuropsychological evaluation of face identity recognition

Facial identity recognition (FIR) is arguably the ultimate form of recognition for the adult human brain. Even if the term prosopagnosia is reserved for exceptionally rare brain-damaged cases with a category-specific abrupt loss of FIR at adulthood, subjective and objective impairments or difficulties of FIR are common in the neuropsychological population. Here we provide a critical overview of the evaluation of FIR both for clinicians and researchers in neuropsychology. FIR impairments occur following many causes that should be identified objectively by both general and specific, behavioral and neural examinations. We refute the commonly used dissociation between perceptual and memory deficits/tests for FIR, since even a task involving the discrimination of unfamiliar face images presented side-by-side relies on cortical memories of faces in the right-lateralized ventral occipito-temporal cortex. Another frequently encountered confusion is between specific deficits of the FIR function and a more general impairment of semantic memory (of people), the latter being most often encountered following anterior temporal lobe damage. Many computerized tests aimed at evaluating FIR have appeared over the last two decades, as reviewed here. However, despite undeniable strengths, they often suffer from ecological limitations, difficulties of instruction, as well as a lack of consideration for processing speed and qualitative information. Taking into account these issues, a recently developed behavioral test with natural images manipulating face familiarity, stimulus inversion, and correct response times as a key variable appears promising. The measurement of electroencephalographic (EEG) activity in the frequency domain from fast periodic visual stimulation also appears as a particularly promising tool to complete and enhance the neuropsychological assessment of FIR.

Prosopagnosia: face blindness and its association with neurological disorders

Loss of facial recognition or prosopagnosia has been well-recognized for over a century. It has been categorized as developmental or acquired depending on whether the onset is in early childhood or beyond, and acquired cases can have degenerative or non-degenerative aetiologies. Prosopagnosia has been linked to involvement of the fusiform gyri, mainly in the right hemisphere. The literature on prosopagnosia comprises case reports and small case series. We aim to assess demographic, clinical and imaging characteristics and neurological and neuropathological disorders associated with a diagnosis of prosopagnosia in a large cohort. Patients were categorized as developmental versus acquired; those with acquired prosopagnosia were further subdivided into degenerative versus non-degenerative, based on neurological aetiology. We assessed regional involvement on [18F] fluorodeoxyglucose-PET and MRI of the right and left frontal, temporal, parietal and occipital lobes. The Intake and Referral Center at the Mayo Clinic identified 487 patients with possible prosopagnosia, of which 336 met study criteria for probable or definite prosopagnosia. Ten patients, 80.0% male, had developmental prosopagnosia including one with Niemann-Pick type C and another with a forkhead box G1 gene mutation. Of the 326 with acquired prosopagnosia, 235 (72.1%) were categorized as degenerative, 91 (27.9%) as non-degenerative. The most common degenerative diagnoses were posterior cortical atrophy, primary prosopagnosia syndrome, Alzheimer's disease dementia and semantic dementia, with each diagnosis accounting for >10% of this group. The most common non-degenerative diagnoses were infarcts (ischaemic and haemorrhagic), epilepsy-related and primary brain tumours, each accounting for >10%. We identified a group of patients with non-degenerative transient prosopagnosia in which facial recognition loss improved or resolved over time. These patients had migraine-related prosopagnosia, posterior reversible encephalopathy syndrome, delirium, hypoxic encephalopathy and ischaemic infarcts. On [18F] fluorodeoxyglucose-PET, the temporal lobes proved to be the most frequently affected regions in 117 patients with degenerative prosopagnosia, while in 82 patients with non-degenerative prosopagnosia, MRI revealed the right temporal and right occipital lobes as most affected by a focal lesion. The most common pathological findings in those with degenerative prosopagnosia were frontotemporal lobar degeneration with hippocampal sclerosis and mixed Alzheimer's and Lewy body disease pathology. In this large case series of patients diagnosed with prosopagnosia, we observed that facial recognition loss occurs across a wide range of acquired degenerative and non-degenerative neurological disorders, most commonly in males with developmental prosopagnosia. The right temporal and occipital lobes, and connecting fusiform gyrus, are key areas. Multiple different pathologies cause degenerative prosopagnosia.

Face memory deficit in visually impaired children who live in a developing country

PURPOSE

The development of face processing abilities is a continuous process reaching maturity in adulthood. To achieve it in plenitude, children must have an adequate visual function. The purpose of this study was to assess how the face memory ability of children with visual impairment living in a developing country compares to those with normal vision in the same setting.

METHODS

This was a case-control study. Children with visual impairment of different causes and age-matched normal controls underwent a complete eye examination and the Cambridge Face Memory Test for Children (CFMT-C). Images were presented on a computer screen and the test results were expressed as a percentage of right answers (%). Children with impaired vision were assorted into binocular and monocular deficiency and the groups were compared with the Kruskal-Wallis test.

RESULTS

The sample comprised 40 children with visual impairment and 31 age-matched controls. The groups did not differ in age and gender distribution. Patients with binocular impairment (18 subjects) had lower mean CFMT-C scores, as compared to monocular patients (22 patients with strabismic amblyopia) and children with normal vision (57.7 ± 18.9, 76.2 ± 15.6, and 71.3 ± 12.7, respectively, p = 0.008).

CONCLUSIONS

Children with binocular visual impairment had diminished face memory ability. Amblyopia due to strabismus did not affect face memory. Attempts should focus on the prevention of visual loss and early rehabilitation so that these children can develop adequate face memory ability.

Sound-encoded faces activate the left fusiform face area in the early blind

Face perception in humans and nonhuman primates is accomplished by a patchwork of specialized cortical regions. How these regions develop has remained controversial. In sighted individuals, facial information is primarily conveyed via the visual modality. Early blind individuals, on the other hand, can recognize shapes using auditory and tactile cues. Here we demonstrate that such individuals can learn to distinguish faces from houses and other shapes by using a sensory substitution device (SSD) presenting schematic faces as sound-encoded stimuli in the auditory modality. Using functional MRI, we then asked whether a face-selective brain region like the fusiform face area (FFA) shows selectivity for faces in the same subjects, and indeed, we found evidence for preferential activation of the left FFA by sound-encoded faces. These results imply that FFA development does not depend on experience with visual faces per se but may instead depend on exposure to the geometry of facial configurations.

Face processing in the infant brain after pandemic lockdown

The role of visual experience in the development of face processing has long been debated. We present a new angle on this question through a serendipitous study that cannot easily be repeated. Infants viewed short blocks of faces during fMRI in a repetition suppression task. The same identity was presented multiple times in half of the blocks (repeat condition) and different identities were presented once each in the other half (novel condition). In adults, the fusiform face area (FFA) tends to show greater neural activity for novel versus repeat blocks in such designs, suggesting that it can distinguish same versus different face identities. As part of an ongoing study, we collected data before the COVID-19 pandemic and after an initial local lockdown was lifted. The resulting sample of 12 infants (9-24 months) divided equally into pre- and post-lockdown groups with matching ages and data quantity/quality. The groups had strikingly different FFA responses: pre-lockdown infants showed repetition suppression (novel > repeat), whereas post-lockdown infants showed the opposite (repeat > novel), often referred to as repetition enhancement. These findings provide speculative evidence that altered visual experience during the lockdown, or other correlated environmental changes, may have affected face processing in the infant brain.

Calibrating Vision: Concepts and Questions

The idea that visual coding and perception are shaped by experience and adjust to changes in the environment or the observer is universally recognized as a cornerstone of visual processing, yet the functions and processes mediating these calibrations remain in many ways poorly understood. In this article we review a number of facets and issues surrounding the general notion of calibration, with a focus on plasticity within the encoding and representational stages of visual processing. These include how many types of calibrations there are - and how we decide; how plasticity for encoding is intertwined with other principles of sensory coding; how it is instantiated at the level of the dynamic networks mediating vision; how it varies with development or between individuals; and the factors that may limit the form or degree of the adjustments. Our goal is to give a small glimpse of an enormous and fundamental dimension of vision, and to point to some of the unresolved questions in our understanding of how and why ongoing calibrations are a pervasive and essential element of vision.

Brain-like functional specialization emerges spontaneously in deep neural networks

The human brain contains multiple regions with distinct, often highly specialized functions, from recognizing faces to understanding language to thinking about what others are thinking. However, it remains unclear why the cortex exhibits this high degree of functional specialization in the first place. Here, we consider the case of face perception using artificial neural networks to test the hypothesis that functional segregation of face recognition in the brain reflects a computational optimization for the broader problem of visual recognition of faces and other visual categories. We find that networks trained on object recognition perform poorly on face recognition and vice versa and that networks optimized for both tasks spontaneously segregate themselves into separate systems for faces and objects. We then show functional segregation to varying degrees for other visual categories, revealing a widespread tendency for optimization (without built-in task-specific inductive biases) to lead to functional specialization in machines and, we conjecture, also brains.

Neural mechanisms of visual sensitive periods in humans

Sensitive periods in brain development are phases of enhanced susceptibility to experience. Here we discuss research from human and non-human neuroscience studies which have demonstrated a) differences in the way infants vs. adults learn; b) how the brain adapts to atypical conditions, in particular a congenital vs. a late onset blindness (sensitive periods for atypical brain development); and c) the extent to which neural systems are capable of acquiring a typical brain organization after sight restoration following a congenital vs. late phase of pattern vision deprivation (sensitive periods for typical brain development). By integrating these three lines of research, we propose neural mechanisms characteristic of sensitive periods vs. adult neuroplasticity and learning.

Warped rhythms: Epileptic activity during critical periods disrupts the development of neural networks for human communication

It is well established that temporal lobe epilepsy-the most common and well-studied form of epilepsy-can impair communication by disrupting social-emotional and language functions. In pediatric epilepsy, where seizures co-occur with the development of critical brain networks, age of onset matters: The earlier in life seizures begin, the worse the disruption in network establishment, resulting in academic hardship and social isolation. Yet, little is known about the processes by which epileptic activity disrupts developing human brain networks. Here we take a synthetic perspective-reviewing a range of research spanning studies on molecular and oscillatory processes to those on the development of large-scale functional networks-in support of a novel model of how such networks can be disrupted by epilepsy. We seek to bridge the gap between research on molecular processes, on the development of human brain circuitry, and on clinical outcomes to propose a model of how epileptic activity disrupts brain development.

Biological Action Identification Does Not Require Early Visual Input for Development

Visual input during the first years of life is vital for the development of numerous visual functions. While normal development of global motion perception seems to require visual input during an early sensitive period, the detection of biological motion (BM) does not seem to do so. A more complex form of BM processing is the identification of human actions. Here, we tested whether identification rather than detection of BM is experience dependent. A group of human participants who had been treated for congenital cataracts (CC; of up to 18 years in duration, CC group) had to identify ten actions performed by human line figures. In addition, they performed a coherent motion (CM) detection task, which required identifying the direction of CM amid the movement of random dots. As controls, developmental cataract (DC) reversal individuals (DC group) who had undergone the same surgical treatment as CC group were included. Moreover, normally sighted controls were tested both with vision blurred to match the visual acuity (VA) of CC individuals [vision matched (VM) group] and with full sight [sighted control (SC) group]. The CC group identified biological actions with an extraordinary high accuracy (on average ∼85% correct) and was indistinguishable from the VM control group. By contrast, CM processing impairments of the CC group persisted even after controlling for VA. These results in the same individuals demonstrate an impressive resilience of BM processing to aberrant early visual experience and at the same time a sensitive period for the development of CM processing.

Birthweight predicts individual differences in adult face recognition ability

It has long been known that premature birth and/or low birthweight can lead to general difficulties in cognitive and emotional functioning throughout childhood. However, the influence of these factors on more specific processes has seldom been addressed, despite their potential to account for wide individual differences in performance that often appear innate. Here, we examined the influence of gestation and birthweight on adults' face perception and face memory skills. Performance on both sub-processes was predicted by birthweight and birthweight-for-gestation, but not gestation alone. Evidence was also found for the domain-specificity of these effects: No perinatal measure correlated with performance on object perception or memory tasks, but they were related to the size of the face inversion effect on the perceptual test. This evidence indicates a novel, very early influence on individual differences in face recognition ability, which persists into adulthood, influences face-processing strategy itself, and may be domain-specific.

The Detrimental Effect of Noisy Visual Input on the Visual Development of Human Infants

We followed visual development in a rare yet large sample of patients with congenital bilateral cataract for 4 years. We divided the patients into two groups: a complete deprivation group with no response to a flashlight pointing to either of their eyes and otherwise an incomplete deprivation group. All the patients received cataract surgery at age of 3 months. From 27 months onward, the complete deprivation group showed better developmental outcomes in acuity and eyeball growth than the incomplete deprivation group. Such a seemingly counterintuitive finding is consistent with research on visually deprived animals. Plasticity is better preserved in animals receiving a short period of complete visual deprivation from birth than in animals who saw diffuse light. The current finding that plasticity in visual development is better preserved in human infants with complete visual deprivation than in those who can see diffuse light but not patterned visual input has important clinical implications.

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Infants with complete deprivation have developed better acuity than incomplete ones

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Plasticity is better preserved in complete deprivation infants than in incomplete ones

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Infants with complete deprivation have less myopic shift than incompletes ones

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Early noisy input has the detrimental effect on human visual development

Self-reported face recognition is highly valid, but alone is not highly discriminative of prosopagnosia-level performance on objective assessments

Severe developmental deficits in face recognition ability (developmental prosopagnosia, or DP) have been vigorously studied over the past decade, yet many questions remain unanswered about their origins, nature, and social consequences. A rate-limiting factor in answering such questions is the challenge of recruiting rare DP participants. Although self-reported experiences have long played a role in efforts to identify DPs, much remains unknown about how such self-reports can or should contribute to screening or diagnosis. Here, in a large, population-based web sample, we investigated the effectiveness of self-report, used on its own, as a screen to identify individuals who will ultimately fail, at a conventional cutoff, the two types of objective tests that are most commonly used to confirm DP diagnoses: the Cambridge Face Memory Test (CFMT) and the famous faces memory test (FFMT). We used a highly reliable questionnaire (alpha = .91), the Cambridge Face Memory Questionnaire (CFMQ), and revealed strong validity via high correlations of .44 with the CFMT and .52 with the FFMT. However, cutoff analyses revealed that no CFMQ score yielded a clinical-grade combination of sensitivity and positive predictive value in enough individuals to support using it alone as a DP diagnostic or screening tool. This result was replicated in an analysis of data from the widely used PI20 questionnaire, a 20-question self-assessment of facial recognition similar in form to the CFMQ. We therefore recommend that screens for DP should, wherever possible, include objective as well as subjective assessment tools.

Extensive childhood experience with Pokémon suggests eccentricity drives organization of visual cortex

The functional organization of human high-level visual cortex, such as the face- and place-selective regions, is strikingly consistent across individuals. An unanswered question in neuroscience concerns which dimensions of visual information constrain the development and topography of this shared brain organization. To answer this question, we used functional magnetic resonance imaging to scan a unique group of adults who, as children, had extensive visual experience with Pokémon. These animal-like, pixelated characters are dissimilar from other ecological categories, such as faces and places, along critical dimensions (foveal bias, rectilinearity, size, animacy). We show not only that adults who have Pokémon experience demonstrate distinct distributed cortical responses to Pokémon, but also that the experienced retinal eccentricity during childhood can predict the locus of Pokémon responses in adulthood. These data demonstrate that inherent functional representations in the visual cortex-retinal eccentricity-combined with consistent viewing behaviour of particular stimuli during childhood result in a shared functional topography in adulthood.

Tests of whole upright face processing in prosopagnosia: A literature review

Prosopagnosia refers to an acquired or developmental deficit in face recognition. This neuropsychological impairment has received increasing attention over the last decade, in particular because of an increased scientific interest in developmental prosopagnosia. Studies investigating prosopagnosia have used a variety of different clinical and experimental tests to assess face processing abilities. With such a large variety of assessment methods available, test selection can be challenging. Some previous works have aimed to provide an overview of tests used to diagnose prosopagnosia. However, no overview that is based on a structured review of the literature is available. We review the literature to identify tests that have been used to assess the processing of whole upright faces in acquired and developmental prosopagnosia over the last five years (2013-2017). We not only review tests that have been used for diagnostic purposes, but also tests that have been used for experimental purposes. Tests are categorised according to i) their experimental designs and, ii) the stage of face processing that they assess. On this basis, we discuss considerations regarding test designs for future studies. A visual illustration providing a structured overview of paradigms available for testing the processing of whole upright faces is provided. This visual illustration can be used to inform test selection when designing a study and to apply a structured approach to interpreting findings from the literature. The different approaches to assessment of face processing in prosopagnosia have been necessary and fruitful in generating data and hypotheses about the cause of face processing deficits. However, impairments at different levels of face processing have often been interpreted as reflecting a deficit in the recognition stage of face processing. Based on the data now available on prosopagnosia, we advocate for a more structured approach to assessment, which may facilitate a better understanding of the key deficits in prosopagnosia and of the level(s) of face processing that are impaired.

Potential downside of high initial visual acuity

Children who are treated for congenital cataracts later exhibit impairments in configural face analysis. This has been explained in terms of a critical period for the acquisition of normal face processing. Here, we consider a more parsimonious account according to which deficits in configural analysis result from the abnormally high initial retinal acuity that children treated for cataracts experience, relative to typical newborns. According to this proposal, the initial period of low retinal acuity characteristic of normal visual development induces extended spatial processing in the cortex that is important for configural face judgments. As a computational test of this hypothesis, we examined the effects of training with high-resolution or blurred images, and staged combinations, on the receptive fields and performance of a convolutional neural network. The results show that commencing training with blurred images creates receptive fields that integrate information across larger image areas and leads to improved performance and better generalization across a range of resolutions. These findings offer an explanation for the observed face recognition impairments after late treatment of congenital blindness, suggest an adaptive function for the acuity trajectory in normal development, and provide a scheme for improving the performance of computational face recognition systems.

Adaptive Coding and Face Recognition

Face adaptation generates striking face aftereffects, but is this adaptation useful? The answer appears to be yes, with several lines of evidence suggesting that it contributes to our face-recognition ability. Adaptation to face identity is reduced in a variety of clinical populations with impaired face recognition. In addition, individual differences in face adaptation are linked to face-recognition ability in typical adults. People who adapt more readily to new faces are better at recognizing faces. This link between adaptation and recognition holds for both identity and expression recognition. Adaptation updates face norms, which represent the typical or average properties of the faces we experience. By using these norms to code how faces differ from average, the visual system can make explicit the distinctive information that we need to recognize faces. Thus, adaptive norm-based coding may help us to discriminate and recognize faces despite their similarity as visual patterns.

Hometown size affects the processing of naturalistic face variability

Face exposure during development determines adults' abilities to recognize faces and the information they use to process them. Individual differences in the face categories represented in the visual environment can lead to category-specific deficits for recognizing faces that are atypical of observer's experience (e.g. the other-race effect). But what happens when observers have limited opportunities to learn about faces in general? In previous work, we found that observers from depopulated areas have poorer face recognition performance than observers from larger communities, suggesting that impoverished face experience limits face processing broadly. Here, we further investigate this phenomenon by examining how hometown size impacts the ability to assess appearance variability in natural images of faces and bodies. We asked individuals from small and large communities to complete (1) an unconstrained card-sorting task designed to test observers' ability to categorize within-person and between-person appearance variability properly, and (2) the Cambridge Face Memory Test. For both tasks, we examined the direct comparison between groups as well as the relationship between CFMT scores and sorting performance as a function of face experience. We find that small-town observers perform more poorly on the CFMT, but exhibit both better and worse performance than large-town observers on different aspects of the card-sorting task. Further, we also examine the relationship between CFMT performance and card-sorting errors. Our results suggest that individual differences in lifetime face exposure induce important variation in face processing abilities.

The Influence of Averageness on Adults' Perceptions of Attractiveness: The Effect of Early Visual Deprivation

Adults who missed early visual input because of congenital cataracts later have deficits in many aspects of face processing. Here we investigated whether they make normal judgments of facial attractiveness. In particular, we studied whether their perceptions are affected normally by a face's proximity to the population mean, as is true of typically developing adults, who find average faces to be more attractive than most other faces. We compared the judgments of facial attractiveness of 12 cataract-reversal patients to norms established from 36 adults with normal vision. Participants viewed pairs of adult male and adult female faces that had been transformed 50% toward and 50% away from their respective group averages, and selected which face was more attractive. Averageness influenced patients' judgments of attractiveness, but to a lesser extent than controls. The results suggest that cataract-reversal patients are able to develop a system for representing faces with a privileged position for an average face, consistent with evidence from identity aftereffects. However, early visual experience is necessary to set up the neural architecture necessary for averageness to influence perceptions of attractiveness with its normal potency.

Reduced adaptability, but no fundamental disruption, of norm-based face coding following early visual deprivation from congenital cataracts

Faces are adaptively coded relative to visual norms that are updated by experience, and this adaptive coding is linked to face recognition ability. Here we investigated whether adaptive coding of faces is disrupted in individuals (adolescents and adults) who experience face recognition difficulties following visual deprivation from congenital cataracts in infancy. We measured adaptive coding using face identity aftereffects, where smaller aftereffects indicate less adaptive updating of face-coding mechanisms by experience. We also examined whether the aftereffects increase with adaptor identity strength, consistent with norm-based coding of identity, as in typical populations, or whether they show a different pattern indicating some more fundamental disruption of face-coding mechanisms. Cataract-reversal patients showed significantly smaller face identity aftereffects than did controls (Experiments 1 and 2). However, their aftereffects increased significantly with adaptor strength, consistent with norm-based coding (Experiment 2). Thus we found reduced adaptability but no fundamental disruption of norm-based face-coding mechanisms in cataract-reversal patients. Our results suggest that early visual experience is important for the normal development of adaptive face-coding mechanisms.

The role of early visual input in the development of contour interpolation: the case of subjective contours

We tested the effect of early monocular and binocular deprivation of normal visual input on the development of contour interpolation. Patients deprived from birth by dense central cataracts in one or both eyes, and age-matched controls, discriminated between fat and thin shapes formed by either illusory or luminance-defined contours. Thresholds indicated the minimum amount of curvature (the fatness or thinness) required for discrimination of the illusory shape, providing a measure of the precision of interpolation. The results show that individuals deprived of visual input in one eye, but not those deprived in both eyes, later show deficits in perceptual interpolation. The deficits were shown mostly for weakly supported contours in which interpolation of contours between the inducers was over a large distance relative to the size of the inducers. Deficits shown for the unilateral but not for the bilateral patients point to the detrimental effect of unequal competition between the eyes for cortical connections on the later development of the mechanisms underlying contour interpolation.

Guidelines for studying developmental prosopagnosia in adults and children

Developmental prosopagnosia (DP) is a neurodevelopmental condition characterized by severe face identity recognition problems that results from a failure to develop the mechanisms necessary for adequate face processing (Duchaine BC, Nakayama K. Developmental prosopagnosia: a window to content-specific face processing. Curr Opin Neurobiol 2006, 16:166-173.). It occurs in children and adults with normal visual acuity, and without intellectual impairments or known brain injuries. Given the importance of face recognition in daily life, and the detrimental effects of impaired face recognition, DP is an important area of study. Yet conventions for classifying individuals as DP for research purposes are poorly defined. In this focus paper, we discuss: (1) criteria for an operational definition of DP; 2) tests of face recognition and conventions for classifying individuals as DP; and 3) important considerations regarding common associations and dissociations, and cognitive heterogeneity in DP. We also highlight issues unique to studying DP in children, a relatively new endeavor that is proving to be an important complement to the work with adults. Ultimately, we hope to identify challenges researchers face when studying DP, and offer guidelines for others to consider when embarking on their own research pursuits on the topic. For further resources related to this article, please visit the WIREs website.

Global processing in amblyopia: a review

Amblyopia is a neurodevelopmental disorder of the visual system that is associated with disrupted binocular vision during early childhood. There is evidence that the effects of amblyopia extend beyond the primary visual cortex to regions of the dorsal and ventral extra-striate visual cortex involved in visual integration. Here, we review the current literature on global processing deficits in observers with either strabismic, anisometropic, or deprivation amblyopia. A range of global processing tasks have been used to investigate the extent of the cortical deficit in amblyopia including: global motion perception, global form perception, face perception, and biological motion. These tasks appear to be differentially affected by amblyopia. In general, observers with unilateral amblyopia appear to show deficits for local spatial processing and global tasks that require the segregation of signal from noise. In bilateral cases, the global processing deficits are exaggerated, and appear to extend to specialized perceptual systems such as those involved in face processing.

Understanding individual face discrimination by means of fast periodic visual stimulation

This paper reviews a fast periodic visual stimulation (FPVS) approach developed recently to make significant progress in understanding visual discrimination of individual faces. Displaying pictures of faces at a periodic frequency rate leads to a high signal-to-noise ratio (SNR) response in the human electroencephalogram, at the exact frequency of stimulation, a so-called steady-state visual evoked potential (SSVEP, Regan in Electroencephalogr Clin Neurophysiol 20:238-248, 1966). For fast periodic frequency rates, i.e., between 3 and 9 Hz, this response is reduced if the exact same face identity is repeated compared to the presentation of different face identities, the largest difference being observed over the right occipito-temporal cortex. A 6-Hz stimulation rate (cycle duration of ~170 ms) provides the largest difference between different and repeated faces, as also evidenced in face-selective areas of the ventral occipito-temporal cortex in functional magnetic resonance imaging. This high-level discrimination response is reduced following inversion and contrast-reversal of the faces and can be isolated without subtraction thanks to a fast periodic oddball paradigm. Overall, FPVS provides a response that is objective (i.e., at an experimentally defined frequency), implicit, has a high SNR and is directly quantifiable in a short amount of time. Although the approach is particularly appealing for understanding face perception, it can be generalized to study visual discrimination of complex visual patterns such as objects and visual scenes. The advantages of the approach make it also particularly well-suited to investigate these functions in populations who cannot provide overt behavioral responses and can only be tested for short durations, such as infants, young children and clinical populations.

Early visual deprivation from congenital cataracts disrupts activity and functional connectivity in the face network

The development of the face-processing network has been examined with functional neuroimaging, but the effect of visual deprivation early in life on this network is not known. We examined this question in a group of young adults who had been born with dense, central cataracts in both eyes that blocked all visual input to the retina until the cataracts were removed during infancy. We used functional magnetic resonance imaging to examine regions in the "core" and "extended" face networks as participants viewed faces and other objects, and performed a face discrimination task. This task required matching faces on the basis of facial features or on the spacing between the facial features. The Cataract group (a) had reduced discrimination performance on the Spacing task relative to Controls; (b) used the same brain regions as Controls when passively viewing faces or making judgments about faces, but showed reduced activation during passive viewing of faces, especially in extended face-network regions; and (c) unlike Controls, showed activation in face-network regions for objects. In addition, the functional connections of the fusiform gyri with the rest of the face network were altered, and these brain changes were related to Cataract participants' performance on the face discrimination task. These results provide evidence that early visual input is necessary to set up or preserve activity and functional connectivity in the face-processing network that will later mediate expert face processing.

Face perception is whole or none: disentangling the role of spatial contiguity and interfeature distances in the composite face illusion

Compelling evidence that faces are perceived holistically or configurally comes from the composite face illusion: identical top halves of a face are perceived as being different if they are aligned with different bottom halves. The visual illusion disappears when the top and bottom face halves are spatially misaligned. Whether this is because the two halves no longer form a whole face (ie they form two segmented parts), or because of an increase in interfeatures distance in the misaligned condition (eg eyes-mouth distance) remains unclear. Here, thirty-four participants performed a delayed matching composite task in which the amount of spatial misalignment between face halves varied parametrically (from 8.33% of face width to 100%). The difference in performance between aligned and misaligned faces (ie the composite face effect) was already of full magnitude at the smallest level of misalignment. These results imply that a small spatial misalignment is sufficient to measure the composite face effect. From a theoretical standpoint, they indicate that it is the breaking of a whole configuration rather than the increase in relative distance between the face parts that explains the presence or absence of the composite face effect, clarifying an outstanding issue concerning the nature of holistic face perception.

Altered representation of facial expressions after early visual deprivation

We investigated the effects of early visual deprivation on the underlying representation of the six basic emotions. Using multi-dimensional scaling (MDS), we compared the similarity judgments of adults who had missed early visual input because of bilateral congenital cataracts to control adults with normal vision. Participants made similarity judgments of the six basic emotional expressions, plus neutral, at three different intensities. Consistent with previous studies, the similarity judgments of typical adults could be modeled with four underlying dimensions, which can be interpreted as representing pleasure, arousal, potency and intensity of expressions. As a group, cataract-reversal patients showed a systematic structure with dimensions representing pleasure, potency, and intensity. However, an arousal dimension was not obvious in the patient group's judgments. Hierarchical clustering analysis revealed a pattern in patients seen in typical 7-year-olds but not typical 14-year-olds or adults. There was also more variability among the patients than among the controls, as evidenced by higher stress values for the MDS fit to the patients' data and more dispersed weightings on the four dimensions. The findings suggest an important role for early visual experience in shaping the later development of the representations of emotions. Since the normal underlying structure for emotion emerges postnatally and continues to be refined until late childhood, the altered representation of emotion in adult patients suggests a sleeper effect.