Structure and function in acquired prosopagnosia: lessons from a series of 10 patients with brain damage

Evolutionary Psychology as a Metatheory for the Social Sciences: How to Gather Interdisciplinary Evidence for a Psychological Adaptation

Evolutionary psychology has been proposed as a new metatheory for the social sciences ( Buss, 1995 ). Evolutionary psychology is an approach that emphasizes the evolutionary background of psychological phenomena (e.g., cognition, motivation, perception), with the expectation that knowledge about this background enhances our understanding of the working of the present human mind. This proposal has met with both enthusiasm and criticism. An important criticism is that it is hard, if possible at all, to find empirical evidence for a hypothesized psychological adaptation. This criticism has been addressed with the proposal to build a nomological network of evidence around a hypothesized psychological adaptation ( Schmitt & Pilcher, 2004 ). In this article, we show that it is possible to use this nomological network of evidence to support the hypothesis that face recognition is an adaptation. We reviewed the literature on face recognition from different disciplines (psychology, medicine, neuroscience, genetics, primatology, and anthropology) and conclude that there is an extensive network of evidence for the proposed hypothesis. We argue that building a nomological network of evidence is a promising way to address several criticisms of evolutionary psychology, and that such a network can serve as a metatheoretical framework for the social sciences.

The processing of voice identity in developmental prosopagnosia

BACKGROUND

Developmental prosopagnosia is a disorder of face recognition that is believed to reflect impairments of visual mechanisms. However, voice recognition has rarely been evaluated in developmental prosopagnosia to clarify if it is modality-specific or part of a multi-modal person recognition syndrome.

OBJECTIVE

Our goal was to examine whether voice discrimination and/or recognition are impaired in subjects with developmental prosopagnosia.

DESIGN/METHODS

73 healthy controls and 12 subjects with developmental prosopagnosia performed a match-to-sample test of voice discrimination and a test of short-term voice familiarity, as well as a questionnaire about face and voice identification in daily life.

RESULTS

Eleven subjects with developmental prosopagnosia scored within the normal range for voice discrimination and voice recognition. One was impaired on discrimination and borderline for recognition, with equivalent scores for face and voice recognition, despite being unaware of voice processing problems.

CONCLUSIONS

Most subjects with developmental prosopagnosia are not impaired in short-term voice familiarity, providing evidence that developmental prosopagnosia is usually a modality-specific disorder of face recognition. However, there may be heterogeneity, with a minority having additional voice processing deficits. Objective tests of voice recognition should be integrated into the diagnostic evaluation of this disorder to distinguish it from a multi-modal person recognition syndrome.

Imaging of Retrochiasmal and Higher Cortical Visual Disorders

Retrochiasmal visual pathways include optic tracts, lateral geniculate nuclei, optic radiations, and striate cortex (V1). Homonymous hemianopsia and field defect variants with relatively normal visual acuity suggest that the lesions involve retrochiasmal pathways. From V1, visual input is projected to higher visual association areas that are responsible for perception of objects, faces, colors, and orientation. Visual association areas are classified into ventral and dorsal pathways. Damage to the ventral stream results in visual object agnosia, prosopagnosia, and achromatopsia. Balint syndrome, visual inattention, and pure alexia are examples of dorsal stream disorders. Posterior cortical atrophy can involve ventral and dorsal streams, often preceding dementia.

A vision of graded hemispheric specialization

Understanding the process by which the cerebral hemispheres reach their mature functional organization remains challenging. We propose a theoretical account in which, in the domain of vision, faces and words come to be represented adjacent to retinotopic cortex by virtue of the need to discriminate among homogeneous exemplars. Orthographic representations are further constrained to be proximal to typically left-lateralized language-related information to minimize connectivity length between visual and language areas. As reading is acquired, orthography comes to rely more heavily (albeit not exclusively) on the left fusiform region to bridge vision and language. Consequently, due to competition from emerging word representations, face representations that were initially bilateral become lateralized to the right fusiform region (albeit, again, not exclusively). We review recent research that describes constraints that give rise to this graded hemispheric arrangement. We then summarize empirical evidence from a variety of studies (behavioral, evoked response potential, functional imaging) across different populations (children, adolescents, and adults; left handers and individuals with developmental dyslexia) that supports the claims that hemispheric lateralization is graded rather than binary and that this graded organization emerges dynamically over the course of development. Perturbations of this system either during development or in adulthood provide further insights into the principles governing hemispheric organization.

Word and text processing in acquired prosopagnosia

OBJECTIVE

A novel hypothesis of object recognition asserts that multiple regions are engaged in processing an object type, and that cerebral regions participate in processing multiple types of objects. In particular, for high-level expert processing, it proposes shared rather than dedicated resources for word and face perception, and predicts that prosopagnosic subjects would have minor deficits in visual word processing, and alexic subjects would have subtle impairments in face perception. In this study, we evaluated whether prosopagnosic subjects had deficits in processing either the word content or the style of visual text.

METHODS

Eleven prosopagnosic subjects, 6 with unilateral right lesions and 5 with bilateral lesions, participated. In the first study, we evaluated their word length effect in reading single words. In the second study, we assessed their time and accuracy for sorting text by word content independent of style, and for sorting text by handwriting or font style independent of word content.

RESULTS

Only subjects with bilateral lesions showed mildly elevated word length effects. Subjects were not slowed in sorting text by word content, but were nearly uniformly impaired in accuracy for sorting text by style.

INTERPRETATION

Our results show that prosopagnosic subjects are impaired not only in face recognition but also in perceiving stylistic aspects of text. This supports a modified version of the many-to-many hypothesis that incorporates hemispheric specialization for processing different aspects of visual text.

The independence of expression and identity in face-processing: evidence from neuropsychological case studies

The processing of facial identity and facial expression have traditionally been seen as independent—a hypothesis that has largely been informed by a key double dissociation between neurological patients with a deficit in facial identity recognition but not facial expression recognition, and those with the reverse pattern of impairment. The independence hypothesis is also reflected in more recent anatomical models of face-processing, although these theories permit some interaction between the two processes. Given that much of the traditional patient-based evidence has been criticized, a review of more recent case reports that are accompanied by neuroimaging data is timely. Further, the performance of individuals with developmental face-processing deficits has recently been considered with regard to the independence debate. This paper reviews evidence from both acquired and developmental disorders, identifying methodological and theoretical strengths and caveats in these reports, and highlighting pertinent avenues for future research.

Beyond the core face-processing network: Intracerebral stimulation of a face-selective area in the right anterior fusiform gyrus elicits transient prosopagnosia

According to neuropsychological evidence, a distributed network of regions of the ventral visual pathway - from the lateral occipital cortex to the temporal pole - supports face recognition. However, functional magnetic resonance imaging (fMRI) studies have generally confined ventral face-selective areas to the posterior section of the occipito-temporal cortex, i.e., the inferior occipital gyrus occipital face area (OFA) and the posterior and middle fusiform gyrus fusiform face area (FFA). There is recent evidence that intracranial electrical stimulation of these areas in the right hemisphere elicits face matching and recognition impairments (i.e., prosopagnosia) as well as perceptual face distortions. Here we report a case of transient inability to recognize faces following electrical stimulation of the right anterior fusiform gyrus, in a region located anteriorly to the FFA. There was no perceptual face distortion reported during stimulation. Although no fMRI face-selective responses were found in this region due to a severe signal drop-out as in previous studies, intracerebral face-selective event-related potentials and gamma range electrophysiological responses were found at the critical site of stimulation. These results point to a causal role in face recognition of the right anterior fusiform gyrus and more generally of face-selective areas located beyond the "core" face-processing network in the right ventral temporal cortex. It also illustrates the diagnostic value of intracerebral electrophysiological recordings and stimulation in understanding the neural basis of face recognition and visual recognition in general.

The 20-item prosopagnosia index (PI20): a self-report instrument for identifying developmental prosopagnosia

Self-report plays a key role in the identification of developmental prosopagnosia (DP), providing complementary evidence to computer-based tests of face recognition ability, aiding interpretation of scores. However, the lack of standardized self-report instruments has contributed to heterogeneous reporting standards for self-report evidence in DP research. The lack of standardization prevents comparison across samples and limits investigation of the relationship between objective tests of face processing and self-report measures. To address these issues, this paper introduces the PI20; a 20-item self-report measure for quantifying prosopagnosic traits. The new instrument successfully distinguishes suspected prosopagnosics from typically developed adults. Strong correlations were also observed between PI20 scores and performance on objective tests of familiar and unfamiliar face recognition ability, confirming that people have the necessary insight into their own face recognition ability required by a self-report instrument. Importantly, PI20 scores did not correlate with recognition of non-face objects, indicating that the instrument measures face recognition, and not a general perceptual impairment. These results suggest that the PI20 can play a valuable role in identifying DP. A freely available self-report instrument will permit more effective description of self-report diagnostic evidence, thereby facilitating greater comparison of prosopagnosic samples, and more reliable classification.

Functionally defined white matter reveals segregated pathways in human ventral temporal cortex associated with category-specific processing

It is unknown if the white-matter properties associated with specific visual networks selectively affect category-specific processing. In a novel protocol we combined measurements of white-matter structure, functional selectivity, and behavior in the same subjects. We find two parallel white-matter pathways along the ventral temporal lobe connecting to either face-selective or place-selective regions. Diffusion properties of portions of these tracts adjacent to face- and place-selective regions of ventral temporal cortex correlate with behavioral performance for face or place processing, respectively. Strikingly, adults with developmental prosopagnosia (face blindness) express an atypical structure-behavior relationship near face-selective cortex, suggesting that white-matter atypicalities in this region may have behavioral consequences. These data suggest that examining the interplay between cortical function, anatomical connectivity, and visual behavior is integral to understanding functional networks and their role in producing visual abilities and deficits.

Normal Body Perception despite the Loss of Right Fusiform Gyrus

Human extrastriate cortex contains functional regions that are selective for particular categories such as faces, bodies, and places, but it is unclear whether these category-selective regions are necessary for normal perception of their preferred stimuli. One of these regions is the right fusiform body area (FBA), which is selectively involved in body perception. Do loss of the right fusiform gyrus and the absence of the right FBA necessarily lead to deficits in body perception? Here we report the performance of Galen, a brain-damaged patient who lost the right fusiform gyrus and has no right FBA, on eight tasks of body perception. Despite his lesion, Galen showed normal performance on all tasks. Galen's results demonstrate that damage to the right fusiform gyrus and the lack of the right FBA do not necessarily lead to persisting deficits in body perception.

Electrical stimulation over bilateral occipito-temporal regions reduces N170 in the right hemisphere and the composite face effect

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation technique that can modulate cortical excitability. Although the clinical value of tDCS has been advocated, the potential of tDCS in cognitive rehabilitation of face processing deficits is less understood. Face processing has been associated with the occipito-temporal cortex (OT). The present study investigated whether face processing in healthy adults can be modulated by applying tDCS over the OT. Experiment 1 investigated whether tDCS can affect N170, a face-sensitive ERP component, with a face orientation judgment task. The N170 in the right hemisphere was reduced in active stimulation conditions compared with the sham stimulation condition for both upright faces and inverted faces. Experiment 2 further demonstrated that tDCS can modulate the composite face effect, a type of holistic processing that reflects the obligatory attention to all parts of a face. The composite face effect was reduced in active stimulation conditions compared with the sham stimulation condition. Additionally, the current polarity did not modulate the effect of tDCS in the two experiments. The present study demonstrates that N170 can be causally manipulated by stimulating the OT with weak currents. Furthermore, our study provides evidence that obligatory attention to all parts of a face can be affected by the commonly used tDCS parameter setting.

Multi-voxel pattern analysis (MVPA) reveals abnormal fMRI activity in both the "core" and "extended" face network in congenital prosopagnosia

The ability to identify faces is mediated by a network of cortical and subcortical brain regions in humans. It is still a matter of debate which regions represent the functional substrate of congenital prosopagnosia (CP), a condition characterized by a lifelong impairment in face recognition, and affecting around 2.5% of the general population. Here, we used functional Magnetic Resonance Imaging (fMRI) to measure neural responses to faces, objects, bodies, and body-parts in a group of seven CPs and ten healthy control participants. Using multi-voxel pattern analysis (MVPA) of the fMRI data we demonstrate that neural activity within the “core” (i.e., occipital face area and fusiform face area) and “extended” (i.e., anterior temporal cortex) face regions in CPs showed reduced discriminability between faces and objects. Reduced differentiation between faces and objects in CP was also seen in the right parahippocampal cortex. In contrast, discriminability between faces and bodies/body-parts and objects and bodies/body-parts across the ventral visual system was typical in CPs. In addition to MVPA analysis, we also ran traditional mass-univariate analysis, which failed to show any group differences in face and object discriminability. In sum, these findings demonstrate (i) face-object representations impairments in CP which encompass both the “core” and “extended” face regions, and (ii) superior power of MVPA in detecting group differences.

Electrical stimulation of the left and right human fusiform gyrus causes different effects in conscious face perception

Neuroimaging and electrophysiological studies across species have confirmed bilateral face-selective responses in the ventral temporal cortex (VTC) and prosopagnosia is reported in patients with lesions in the VTC including the fusiform gyrus (FG). As imaging and electrophysiological studies provide correlative evidence, and brain lesions often comprise both white and gray matter structures beyond the FG, we designed the current study to explore the link between face-related electrophysiological responses in the FG and the causal effects of electrical stimulation of the left or right FG in face perception. We used a combination of electrocorticography (ECoG) and electrical brain stimulation (EBS) in 10 human subjects implanted with intracranial electrodes in either the left (5 participants, 30 FG sites) or right (5 participants, 26 FG sites) hemispheres. We identified FG sites with face-selective ECoG responses, and recorded perceptual reports during EBS of these sites. In line with existing literature, face-selective ECoG responses were present in both left and right FG sites. However, when the same sites were stimulated, we observed a striking difference between hemispheres. Only EBS of the right FG caused changes in the conscious perception of faces, whereas EBS of strongly face-selective regions in the left FG produced non-face-related visual changes, such as phosphenes. This study examines the relationship between correlative versus causal nature of ECoG and EBS, respectively, and provides important insight into the differential roles of the right versus left FG in conscious face perception.

Spatio-temporal dynamics and laterality effects of face inversion, feature presence and configuration, and face outline

Although a crucial role of the fusiform gyrus (FG) in face processing has been demonstrated with a variety of methods, converging evidence suggests that face processing involves an interactive and overlapping processing cascade in distributed brain areas. Here we examine the spatio-temporal stages and their functional tuning to face inversion, presence and configuration of inner features, and face contour in healthy subjects during passive viewing. Anatomically-constrained magnetoencephalography (aMEG) combines high-density whole-head MEG recordings and distributed source modeling with high-resolution structural MRI. Each person's reconstructed cortical surface served to constrain noise-normalized minimum norm inverse source estimates. The earliest activity was estimated to the occipital cortex at ~100 ms after stimulus onset and was sensitive to an initial coarse level visual analysis. Activity in the right-lateralized ventral temporal area (inclusive of the FG) peaked at ~160 ms and was largest to inverted faces. Images containing facial features in the veridical and rearranged configuration irrespective of the facial outline elicited intermediate level activity. The M160 stage may provide structural representations necessary for downstream distributed areas to process identity and emotional expression. However, inverted faces additionally engaged the left ventral temporal area at ~180 ms and were uniquely subserved by bilateral processing. This observation is consistent with the dual route model and spared processing of inverted faces in prosopagnosia. The subsequent deflection, peaking at ~240 ms in the anterior temporal areas bilaterally, was largest to normal, upright faces. It may reflect initial engagement of the distributed network subserving individuation and familiarity. These results support dynamic models suggesting that processing of unfamiliar faces in the absence of a cognitive task is subserved by a distributed and interactive neural circuit.

Voice Recognition in Face-Blind Patients

Right or bilateral anterior temporal damage can impair face recognition, but whether this is an associative variant of prosopagnosia or part of a multimodal disorder of person recognition is an unsettled question, with implications for cognitive and neuroanatomic models of person recognition. We assessed voice perception and short-term recognition of recently heard voices in 10 subjects with impaired face recognition acquired after cerebral lesions. All 4 subjects with apperceptive prosopagnosia due to lesions limited to fusiform cortex had intact voice discrimination and recognition. One subject with bilateral fusiform and anterior temporal lesions had a combined apperceptive prosopagnosia and apperceptive phonagnosia, the first such described case. Deficits indicating a multimodal syndrome of person recognition were found only in 2 subjects with bilateral anterior temporal lesions. All 3 subjects with right anterior temporal lesions had normal voice perception and recognition, 2 of whom performed normally on perceptual discrimination of faces. This confirms that such lesions can cause a modality-specific associative prosopagnosia.

Face processing improvements in prosopagnosia: successes and failures over the last 50 years

Clinicians and researchers have widely believed that face processing cannot be improved in prosopagnosia. Though more than a dozen reported studies have attempted to enhance face processing in prosopagnosics over the last 50 years, evidence for effective treatment approaches has only begun to emerge. Here, we review the current literature on spontaneous recovery in acquired prosopagnosia (AP), as well as treatment attempts in acquired and developmental prosopagnosia (DP), differentiating between compensatory and remedial approaches. We find that for AP, rather than remedial methods, strategic compensatory training such as verbalizing distinctive facial features has shown to be the most effective approach (despite limited evidence of generalization). In children with DP, compensatory training has also shown some effectiveness. In adults with DP, two recent larger-scale studies, one using remedial training and another administering oxytocin, have demonstrated group-level improvements and evidence of generalization. These results suggest that DPs, perhaps because of their more intact face processing infrastructure, may benefit more from treatments targeting face processing than APs.

Dissociation between face perception and face memory in adults, but not children, with developmental prosopagnosia

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Individuals with developmental prosopagnosia (DP) have impaired face recognition.

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In theory, DP could involve impaired face memory, impaired face perception, or both.

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Memory deficits were present in all of our child and adult DPs.

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All children, but less than half of the adults had impaired face perception.

Cognitive models propose that face recognition is accomplished through a series of discrete stages, including perceptual representation of facial structure, and encoding and retrieval of facial information. This implies that impaired face recognition can result from failures of face perception, face memory, or both. Studies of acquired prosopagnosia, autism spectrum disorders, and the development of normal face recognition support the idea that face perception and face memory are distinct processes, yet this distinction has received little attention in developmental prosopagnosia (DP). To address this issue, we tested the face perception and face memory of children and adults with DP. By definition, face memory is impaired in DP, so memory deficits were present in all participants. However, we found that all children, but only half of the adults had impaired face perception. Thus, results from adults indicate that face perception and face memory are dissociable, while the results from children provide no evidence for this division. Importantly, our findings raise the possibility that DP is qualitatively different in childhood versus adulthood. We discuss theoretical explanations for this developmental pattern and conclude that longitudinal studies are necessary to better understand the developmental trajectory of face perception and face memory deficits in DP.

Higher cortical visual deficits

PURPOSE OF REVIEW

This article reviews the various types of visual dysfunction that can result from lesions of the cerebral regions beyond the striate cortex.

RECENT FINDINGS

Patients with dyschromatopsia can exhibit problems with color constancy. The apperceptive form of prosopagnosia is associated with damage to posterior occipital and fusiform gyri, and an associative/amnestic form is linked to damage to more anterior temporal regions. Pure alexia can be accompanied by a surface dysgraphia. New word-length effect criteria distinguish pure alexia from hemianopic dyslexia. Subtler problems with perception of numbers and faces can be seen in patients with pure alexia as well. Also, a developmental form of topographic disorientation, which is due to problems with forming cognitive maps of the environment, has been discovered. In Balint syndrome, added features of decreased flexibility of attention in simultanagnosia include local and global capture. Balint syndrome can affect not just localization in space, but also in time, as manifest in sequence agnosia.

SUMMARY

Lesions at intermediate levels of a processing hierarchy can cause difficulty with color perception or motion perception. At a higher level, ventral lesions of the occipitotemporal lobes can lead to a variety of problems with object recognition. Dorsal lesions of the occipitoparietal lobes can cause difficulty with spatial localization and guidance.

The rehabilitation of face recognition impairments: a critical review and future directions

While much research has investigated the neural and cognitive characteristics of face recognition impairments (prosopagnosia), much less work has examined their rehabilitation. In this paper, we present a critical analysis of the studies that have attempted to improve face-processing skills in acquired and developmental prosopagnosia, and place them in the context of the wider neurorehabilitation literature. First, we examine whether neuroplasticity within the typical face-processing system varies across the lifespan, in order to examine whether timing of intervention may be crucial. Second, we examine reports of interventions in acquired prosopagnosia, where training in compensatory strategies has had some success. Third, we examine reports of interventions in developmental prosopagnosia, where compensatory training in children and remedial training in adults have both been successful. However, the gains are somewhat limited-compensatory strategies have resulted in labored recognition techniques and limited generalization to untrained faces, and remedial techniques require longer periods of training and result in limited maintenance of gains. Critically, intervention suitability and outcome in both forms of the condition likely depends on a complex interaction of factors, including prosopagnosia severity, the precise functional locus of the impairment, and individual differences such as age. Finally, we discuss future directions in the rehabilitation of prosopagnosia, and the possibility of boosting the effects of cognitive training programmes by simultaneous administration of oxytocin or non-invasive brain stimulation. We conclude that future work using more systematic methods and larger participant groups is clearly required, and in the case of developmental prosopagnosia, there is an urgent need to develop early detection and remediation tools for children, in order to optimize intervention outcome.

Predictive codes of familiarity and context during the perceptual learning of facial identities

Face recognition is a key component of successful social behaviour. However, the computational processes that underpin perceptual learning and recognition as faces transition from unfamiliar to familiar are poorly understood. In predictive coding, learning occurs through prediction errors that update stimulus familiarity, but recognition is a function of both stimulus and contextual familiarity. Here we show that behavioural responses on a two-option face recognition task can be predicted by the level of contextual and facial familiarity in a computational model derived from predictive-coding principles. Using fMRI, we show that activity in the superior temporal sulcus varies with the contextual familiarity in the model, whereas activity in the fusiform face area covaries with the prediction error parameter that updated facial familiarity. Our results characterize the key computations underpinning the perceptual learning of faces, highlighting that the functional properties of face-processing areas conform to the principles of predictive coding.

Intracerebral electrical stimulation of a face-selective area in the right inferior occipital cortex impairs individual face discrimination

During intracerebral stimulation of the right inferior occipital cortex, a patient with refractory epilepsy was transiently impaired at discriminating two simultaneously presented photographs of unfamiliar faces. The critical electrode contact was located in the most posterior face-selective brain area of the human brain (right "occipital face area", rOFA) as shown both by low- (ERP) and high-frequency (gamma) electrophysiological responses as well as a face localizer in fMRI. At this electrode contact, periodic visual presentation of 6 different faces by second evoked a larger electrophysiological periodic response at 6 Hz than when the same face identity was repeated at the same rate. This intracerebral EEG repetition suppression effect was markedly reduced when face stimuli were presented upside-down, a manipulation that impairs individual face discrimination. These findings provide original evidence for a causal relationship between the face-selective right inferior occipital cortex and individual face discrimination, independently of long-term memory representations. More generally, they support the functional value of electrophysiological repetition suppression effects, indicating that these effects can be used as an index of a necessary neural representation of the changing stimulus property.

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.

General holistic impairment in congenital prosopagnosia: evidence from Garner's speeded-classification task

Congenital prosopagnosia (CP), a lifelong impairment in face processing in the absence of brain damage, is often ascribed to impairment in holistic processing. It is still debated whether such difficulties are restricted to faces or whether they can also be observed for nonfacial stimuli. Here, we investigate this issue by examining CP individuals and their controls on two variations of the Garner speeded classification task tailored to assess holistic processing of nonfacial stimuli. In Experiment 1, participants were asked to judge the width of visually presented rectangles while ignoring their irrelevant height, or to judge changes in width while height remained constant. Critically, while controls exhibited the expected Garner interference, no such interference was observed for the CPs, indicating impaired holistic processing of integral, nonfacial shape dimensions. Experiment 2, utilized the same Garner paradigm, but here participants were asked to judge integral dimensions that are unrelated to shape (colour). Importantly, both CPs and controls exhibited the same level of Garner interference, indicating intact integral processing of colour dimensions. This dissociation between the performance on the two Garner tasks indicates that CPs do not exhibit a general local processing bias or impaired integration of any perceptual dimensions, but rather a deficit that is restricted to tasks requiring holistic integral perception of shape dimensions. Taken together, these results provide evidence for the existence of a general impairment in holistic shape perception in CP, which may be related to the mechanisms underlying this disorder.

Residual fMRI sensitivity for identity changes in acquired prosopagnosia

While a network of cortical regions contribute to face processing, the lesions in acquired prosopagnosia are highly variable, and likely result in different combinations of spared and affected regions of this network. To assess the residual functional sensitivities of spared regions in prosopagnosia, we designed a rapid event-related functional magnetic resonance imaging (fMRI) experiment that included pairs of faces with same or different identities and same or different expressions. By measuring the release from adaptation to these facial changes we determined the residual sensitivity of face-selective regions-of-interest. We tested three patients with acquired prosopagnosia, and all three of these patients demonstrated residual sensitivity for facial identity changes in surviving fusiform and occipital face areas of either the right or left hemisphere, but not in the right posterior superior temporal sulcus. The patients also showed some residual capabilities for facial discrimination with normal performance on the Benton Facial Recognition Test, but impaired performance on more complex tasks of facial discrimination. We conclude that fMRI can demonstrate residual processing of facial identity in acquired prosopagnosia, that this adaptation can occur in the same structures that show similar processing in healthy subjects, and further, that this adaptation may be related to behavioral indices of face perception.

Individual differences in posterior cortical volume correlate with proneness to pride and gratitude

Proneness to specific moral sentiments (e.g. pride, gratitude, guilt, indignation) has been linked with individual variations in functional MRI (fMRI) response within anterior brain regions whose lesion leads to inappropriate behaviour. However, the role of structural anatomical differences in rendering individuals prone to particular moral sentiments relative to others is unknown. Here, we investigated grey matter volumes (VBM8) and proneness to specific moral sentiments on a well-controlled experimental task in healthy individuals. Individuals with smaller cuneus, and precuneus volumes were more pride-prone, whereas those with larger right inferior temporal volumes experienced gratitude more readily. Although the primary analysis detected no associations with guilt- or indignation-proneness, subgenual cingulate fMRI responses to guilt were negatively correlated with grey matter volumes in the left superior temporal sulcus and anterior dorsolateral prefrontal cortices (right >left). This shows that individual variations in functional activations within critical areas for moral sentiments were not due to grey matter volume differences in the same areas. Grey matter volume differences between healthy individuals may nevertheless play an important role by affecting posterior cortical brain systems that are non-critical but supportive for the experience of specific moral sentiments. This may be of particular relevance when their experience depends on visuo-spatial elaboration.

Decoding facial blends of emotion: visual field, attentional and hemispheric biases

Most clinical research assumes that modulation of facial expressions is lateralized predominantly across the right-left hemiface. However, social psychological research suggests that facial expressions are organized predominantly across the upper-lower face. Because humans learn to cognitively control facial expression for social purposes, the lower face may display a false emotion, typically a smile, to enable approach behavior. In contrast, the upper face may leak a person's true feeling state by producing a brief facial blend of emotion, i.e. a different emotion on the upper versus lower face. Previous studies from our laboratory have shown that upper facial emotions are processed preferentially by the right hemisphere under conditions of directed attention if facial blends of emotion are presented tachistoscopically to the mid left and right visual fields. This paper explores how facial blends are processed within the four visual quadrants. The results, combined with our previous research, demonstrate that lower more so than upper facial emotions are perceived best when presented to the viewer's left and right visual fields just above the horizontal axis. Upper facial emotions are perceived best when presented to the viewer's left visual field just above the horizontal axis under conditions of directed attention. Thus, by gazing at a person's left ear, which also avoids the social stigma of eye-to-eye contact, one's ability to decode facial expressions should be enhanced.

Plasticity in unimodal and multimodal brain areas reflects multisensory changes in self-face identification

Nothing provides as strong a sense of self as seeing one's face. Nevertheless, it remains unknown how the brain processes the sense of self during the multisensory experience of looking at one's face in a mirror. Synchronized visuo-tactile stimulation on one's own and another's face, an experience that is akin to looking in the mirror but seeing another's face, causes the illusory experience of ownership over the other person's face and changes in self-recognition. Here, we investigate the neural correlates of this enfacement illusion using fMRI. We examine activity in the human brain as participants experience tactile stimulation delivered to their face, while observing either temporally synchronous or asynchronous tactile stimulation delivered to another's face on either a specularly congruent or incongruent location. Activity in the multisensory right temporo-parietal junction, intraparietal sulcus, and the unimodal inferior occipital gyrus showed an interaction between the synchronicity and the congruency of the stimulation and varied with the self-reported strength of the illusory experience, which was recorded after each stimulation block. Our results highlight the important interplay between unimodal and multimodal information processing for self-face recognition, and elucidate the neurobiological basis for the plasticity required for identifying with our continuously changing visual appearance.

Sex-differences of face coding: evidence from larger right hemispheric M170 in men and dipole source modelling

The processing of faces relies on a specialized neural system comprising bilateral cortical structures with a dominance of the right hemisphere. However, due to inconsistencies of earlier findings as well as more recent results such functional lateralization has become a topic of discussion. In particular, studies employing behavioural tasks and electrophysiological methods indicate a dominance of the right hemisphere during face perception only in men whereas women exhibit symmetric and bilateral face processing. The aim of this study was to further investigate such sex differences in hemispheric processing of personally familiar and opposite-sex faces using whole-head magnetoencephalography (MEG). We found a right-lateralized M170-component in occipito-temporal sensor clusters in men as opposed to a bilateral response in women. Furthermore, the same pattern was obtained in performing dipole localization and determining dipole strength in the M170-timewindow. These results suggest asymmetric involvement of face-responsive neural structures in men and allow to ascribe this asymmetry to the fusiform gyrus. This specifies findings from previous investigations employing event-related potentials (ERP) and LORETA reconstruction methods yielding rather extended bilateral activations showing left asymmetry in women and right lateralization in men. We discuss our finding of an asymmetric fusiform activation pattern in men in terms of holistic face processing during face evaluation and sex differences with regard to visual strategies in general and interest for opposite faces in special. Taken together the pattern of hemispheric specialization observed here yields new insights into sex differences in face perception and entails further questions about interactions between biological sex, psychological gender and influences that might be stimulus-driven or task dependent.

A unified coding strategy for processing faces and voices

Both faces and voices are rich in socially-relevant information, which humans are remarkably adept at extracting, including a person's identity, age, gender, affective state, personality, etc. Here, we review accumulating evidence from behavioral, neuropsychological, electrophysiological, and neuroimaging studies which suggest that the cognitive and neural processing mechanisms engaged by perceiving faces or voices are highly similar, despite the very different nature of their sensory input. The similarity between the two mechanisms likely facilitates the multi-modal integration of facial and vocal information during everyday social interactions. These findings emphasize a parsimonious principle of cerebral organization, where similar computational problems in different modalities are solved using similar solutions.

Acquired prosopagnosia with spared within-class object recognition but impaired recognition of degraded basic-level objects

We present a new case of acquired prosopagnosia resulting from extensive lesions predominantly in the right occipitotemporal cortex. Functional brain imaging revealed atypical activation of all core face areas in the right hemisphere, with reduced signal difference between faces and objects compared to controls. In contrast, Herschel's lateral occipital complex showed normal activation to objects. Behaviourally, Herschel is severely impaired with the recognition of familiar faces, discrimination between unfamiliar identities, and the perception of facial expression and gender. Notably, his visual recognition deficits are largely restricted to faces, suggesting that the damaged mechanisms are face-specific. He showed normal recognition memory for a wide variety of object classes in several paradigms, normal ability to discriminate between highly similar items within a novel object category, and intact ability to name basic objects (except four-legged animals). Furthermore, Herschel displayed a normal face composite effect and typical global advantage and global interference effects in the Navon task, suggesting spared integration of both face and nonface information. Nevertheless, he failed visual closure tests requiring recognition of basic objects from degraded images. This abnormality in basic object recognition is at odds with his spared within-class recognition and presents a challenge to hierarchical models of object perception.

Distributed circuits, not circumscribed centers, mediate visual recognition

Increasingly, the neural mechanisms that support visual cognition are being conceptualized as a distributed but integrated system, as opposed to a set of individual, specialized regions that each subserve a particular visual behavior. Consequently, there is an emerging emphasis on characterizing the functional, structural,and computational properties of these broad networks [corrected]. We present a novel theoretical perspective, which elucidates the developmental emergence, computational properties, and vulnerabilities of integrated circuits using face and word recognition as model domains. Additionally, we suggest that, rather than being disparate and independent, these neural circuits are overlapping and subject to the same computational constraints. Specifically, we argue that both word and face recognition rely on fine-grained visual representations but, by virtue of pressure to couple visual and language areas and to keep connection length short, the left hemisphere becomes more finely tuned for word recognition and, consequently, the right hemisphere becomes more finely tuned for face recognition. Thus, both hemispheres ultimately participate in both forms of visual recognition, but their respective contributions are asymmetrically weighted.

Electrical stimulation of human fusiform face-selective regions distorts face perception

Face-selective neural responses in the human fusiform gyrus have been widely examined. However, their causal role in human face perception is largely unknown. Here, we used a multimodal approach of electrocorticography (ECoG), high-resolution functional magnetic resonance imaging (fMRI), and electrical brain stimulation (EBS) to directly investigate the causal role of face-selective neural responses of the fusiform gyrus (FG) in face perception in a patient implanted with subdural electrodes in the right inferior temporal lobe. High-resolution fMRI identified two distinct FG face-selective regions [mFus-faces and pFus-faces (mid and posterior fusiform, respectively)]. ECoG revealed a striking anatomical and functional correspondence with fMRI data where a pair of face-selective electrodes, positioned 1 cm apart, overlapped mFus-faces and pFus-faces, respectively. Moreover, electrical charge delivered to this pair of electrodes induced a profound face-specific perceptual distortion during viewing of real faces. Specifically, the subject reported a "metamorphosed" appearance of faces of people in the room. Several controls illustrate the specificity of the effect to the perception of faces. EBS of mFus-faces and pFus-faces neither produced a significant deficit in naming pictures of famous faces on the computer, nor did it affect the appearance of nonface objects. Further, the appearance of faces remained unaffected during both sham stimulation and stimulation of a pair of nearby electrodes that were not face-selective. Overall, our findings reveal a striking convergence of fMRI, ECoG, and EBS, which together offer a rare causal link between functional subsets of the human FG network and face perception.

Bilateral hemispheric processing of words and faces: evidence from word impairments in prosopagnosia and face impairments in pure alexia

Considerable research has supported the view that faces and words are subserved by independent neural mechanisms located in the ventral visual cortex in opposite hemispheres. On this view, right hemisphere ventral lesions that impair face recognition (prosopagnosia) should leave word recognition unaffected, and left hemisphere ventral lesions that impair word recognition (pure alexia) should leave face recognition unaffected. The current study shows that neither of these predictions was upheld. A series of experiments characterizing speed and accuracy of word and face recognition were conducted in 7 patients (4 pure alexic, 3 prosopagnosic) and matched controls. Prosopagnosic patients revealed mild but reliable word recognition deficits, and pure alexic patients demonstrated mild but reliable face recognition deficits. The apparent comingling of face and word mechanisms is unexpected from a domain-specific perspective, but follows naturally as a consequence of an interactive, learning-based account in which neural processes for both faces and words are the result of an optimization procedure embodying specific computational principles and constraints.

Electrophysiological studies of face processing in developmental prosopagnosia: neuropsychological and neurodevelopmental perspectives

People with developmental prosopagnosia (DP) show severe face-recognition deficits that typically emerge during childhood without history of neurological damage. We review findings from recent event-related brain potential (ERP) studies of face perception and face recognition in DP. The generic face-sensitivity of the N170 component is present in most DPs, suggesting rapid category-selective streaming of facial information. In contrast, DPs show atypical N170 face inversion effects, indicative of impaired structural encoding, specifically for upright faces. In line with neurodevelopmental accounts of DP, these effects are similar to those observed for other developmental disorders, as well as for younger children and older adults. Identity-sensitive ERP components (N250, P600f) triggered during successful face recognition are similar for DPs and control participants, indicating that the same mechanisms are active in both groups. The presence of covert face-recognition effects for the N250 component suggests that visual face memory and semantic memory can become disconnected in some individuals with DP. The implications of these results for neuropsychological and neurodevelopmental perspectives on DP are discussed.

Structural connectivity in a single case of progressive prosopagnosia: the role of the right inferior longitudinal fasciculus

Progressive prosopagnosia (PP) is a clinical syndrome characterized by a progressive and selective inability to recognize and identify faces of familiar people. Here we report a patient (G.S.) with PP, mainly related to a prominent deficit in recognition of familiar faces, without a semantic (cross-modal) impairment. An in-depth evaluation showed that his deficit extended to other classes of objects, both living and non-living. A follow-up neuropsychological assessment did not reveal substantial changes after about 1 year. Structural MRI showed predominant right temporal lobe atrophy. Diffusion tensor imaging was performed to elucidate structural connectivity of the inferior longitudinal fasciculus (ILF) and the inferior fronto-occipital fasciculus (IFOF), the two major tracts that project through the core fusiform region to the anterior temporal and frontal cortices, respectively. Right ILF was markedly reduced in G.S., while left ILF and IFOFs were apparently preserved. These data are in favour of a crucial role of the neural circuit subserved by right ILF in the pathogenesis of PP.

Complementary neural representations for faces and words: a computational exploration

A key issue that continues to generate controversy concerns the nature of the psychological, computational, and neural mechanisms that support the visual recognition of objects such as faces and words. While some researchers claim that visual recognition is accomplished by category-specific modules dedicated to processing distinct object classes, other researchers have argued for a more distributed system with only partially specialized cortical regions. Considerable evidence from both functional neuroimaging and neuropsychology would seem to favour the modular view, and yet close examination of those data reveals rather graded patterns of specialization that support a more distributed account. This paper explores a theoretical middle ground in which the functional specialization of brain regions arises from general principles and constraints on neural representation and learning that operate throughout cortex but that nonetheless have distinct implications for different classes of stimuli. The account is supported by a computational simulation, in the form of an artificial neural network, that illustrates how cooperative and competitive interactions in the formation of neural representations for faces and words account for both their shared and distinctive properties. We set out a series of empirical predictions, which are also examined, and consider the further implications of this account.