Prosopagnosia can be associated with damage confined to the right hemisphere--an MRI and PET study and a review of the literature

Interaction of Face and Voice Areas during Speaker Recognition

Face and voice processing contribute to person recognition, but it remains unclear how the segregated specialized cortical modules interact. Using functional neuroimaging, we observed cross-modal responses to voices of familiar persons in the fusiform face area, as localized separately using visual stimuli. Voices of familiar persons only activated the face area during a task that emphasized speaker recognition over recognition of verbal content. Analyses of functional connectivity between cortical territories show that the fusiform face region is coupled with the superior temporal sulcus voice region during familiar speaker recognition, but not with any of the other cortical regions normally active in person recognition or in other tasks involving voices. These findings are relevant for models of the cognitive processes and neural circuitry involved in speaker recognition. They reveal that in the context of speaker recognition, the assessment of person familiarity does not necessarily engage supra-modal cortical substrates but can result from the direct sharing of information between auditory voice and visual face regions.

In the Face of Emotions: Event-Related Potentials in Supraliminal and Subliminal Facial Expression Recognition

Is facial expression recognition marked by specific event-related potentials (ERPs) effects? Are conscious and unconscious elaborations of emotional facial stimuli qualitatively different processes? In Experiment 1, ERPs elicited by supraliminal stimuli were recorded when 21 participants viewed emotional facial expressions of four emotions and a neutral stimulus. Two ERP components (N2 and P3) were analyzed for their peak amplitude and latency measures. First, emotional face-specificity was observed for the negative deflection N2, whereas P3 was not affected by the content of the stimulus (emotional or neutral). A more posterior distribution of ERPs was found for N2. Moreover, a lateralization effect was revealed for negative (right lateralization) and positive (left lateralization) facial expressions. In Experiment 2 (20 participants), 1-ms subliminal stimulation was carried out. Unaware information processing was revealed to be quite similar to aware information processing for peak amplitude but not for latency. In fact, unconscious stimulation produced a more delayed peak variation than conscious stimulation.

Are the perceptual biases found in chimeric face processing reflected in eye-movement patterns?

Studies of patients with focal brain lesions and neuroimaging indicate that face processing is predominantly based on right hemisphere function. Additionally, experiments using chimeric faces, where the left and the right-hand side of the face are different, have shown that observers tend to bias their responses toward the information on the left. Here, we monitored eye-movements during a gender identification task using blended face images for both whole and chimeric (half female, half male) faces [Neuropsychologia 35 (1997) 685]. As expected, we found a left perceptual bias: subjects based their gender decision significantly more frequently on the left side of the chimeric faces. Analysis of the first saccade showed a significantly greater number of left fixations independent of perceptual bias presumably reflecting the tendency to first inspect the side of the face better suited to face analysis (left side of face/right hemisphere). On top of this though, there was a relationship between response and fixation pattern. On trials where participants showed a left perceptual bias they produced significantly more left saccades and fixated for longer on the left. In contrast, for trials where participants showed a right perceptual bias there was no reliable difference between the number, or total fixation duration, on the left or the right. These results demonstrate that on a trial-by-trial basis subtle differences in the extent of left or right side scanning are related to the perceptual response of the participant, although an overall initial fixation bias to the left occurs irrespective of response bias.

Capgras syndrome: a review of the neurophysiological correlates and presenting clinical features in cases involving physical violence

OBJECTIVE

Acts of violence have been frequently reported in cases of Capgras syndrome (CS), a misidentification syndrome characterized by the delusional belief that imposters have replaced people familiar to the individual. CS has been observed in many neuropsychiatric and organic disorders, and neuroimaging studies indicate an association between CS and right hemisphere abnormalities. However, CS has received limited attention from a forensic psychiatric perspective. We propose that elucidating demographic and clinical features noted in cases of violence secondary to CS may highlight important factors in the progression of CS to violence.

METHOD

We review the neurophysiological correlates and clinical factors observed in CS and present characteristics of a series of cases that demonstrate the potential of CS patients for severe physical violence toward the misidentified person.

RESULTS

For patients with CS involving assault, we present and discuss commonly reported demographic and clinical features that may contribute to an increased risk for violence.

CONCLUSIONS

An understanding of the presenting clinical features of CS resulting in aggressive acts may assist clinicians to assess the potential for violence in these patients.

Visual recognition of faces, objects, and words using degraded stimuli: where and when it occurs

We studied time course and cerebral localisation of word, object, and face recognition using event-related potentials (ERPs) and source localisation techniques. To compare activation rates of these three categories, we used degraded images that easily pop out without any change in the physical features of the stimuli, once the meaning is revealed. Comparisons before and after identification show additional periods of activation beginning at 100 msec for faces and at around 200 msec for objects and words. For faces, this activation occurs predominantly in right temporal areas, whereas for objects, the specific time period gives rise to bilateral posterior but right dominant foci. Finally, words show a maximum area of activation in the left temporooccipital area at their specific time period. These results provide unequivocal evidence that when effects of low-level visual features are circumvented, faces, objects, and words are not only distinct in terms of their anatomic routes, but also in terms of their times of processing.

The electrophysiological correlates sustaining the retrieval of face-name associations: An ERP study

An ERP study on 9 healthy participants was carried out to temporally constrain the neural network proposed by Campanella et al. (2001) in a PET study investigating the cerebral areas involved in the retrieval of face-name associations. Three learning sessions served to familiarize the participants with 24 face-name associations grouped in 12 male/female couples. During EEG recording, participants were confronted with four experimental conditions, requiring the retrieval of previously learned couples on the basis of the presentation of name-name (NN), face-face (FF), name-face (NF), or face-name (FN) pairs of stimuli. The main analysis of this experiment consisted in the subtraction of the nonmixed conditions (NN and FF) from the mixed conditions (NF and FN). It revealed two main ERP components: a negative wave peaking at left parieto-occipital sites around 285 ms and its positive counterpart recorded at left centro-frontal electrodes around 300 ms. Moreover, a dipole modeling using three dipoles whose localization corresponded to the three cerebral areas observed in the PET study (left inferior frontal gyrus, left medial frontal gyrus, left inferior parietal lobe) explained more than 90% of the variance of the results. The complementarity between anatomical and neurophysiological techniques allowed us to discuss the temporal course of these cerebral activities and to propose an interactive and original anatomo-temporal model of the retrieval of face-name associations.

Is the fusiform face area specialized for faces, individuation, or expert individuation?

Several brain imaging studies have identified a region of fusiform gyrus (FG) that responds more strongly to faces than common objects. The precise functional role of this fusiform face area (FFA) is, however, a matter of dispute. We sought to distinguish among three hypotheses concerning FFA function: face specificity, individuation, and expert individuation. According to the face-specificity hypothesis, the FFA is specialized for face processing. Alternatively, the FFA may be specialized for individuating visually similar items within a category (the individuation hypothesis) or for individuating within categories with which a person has expertise (the expert-individuation hypothesis). Our results from two experiments supported the face-specificity hypothesis. Greater FFA activation to faces than Lepidoptera, another homogeneous object class, occurred during both free viewing and individuation, with similar FFA activation to Lepidoptera and common objects (Experiment 1). Furthermore, during individuation of Lepidoptera, 83% of activated FG voxels were outside the face FG region and only 15% of face FG voxels were activated. This pattern of results suggests that distinct areas may individuate faces and Lepidoptera. In Experiment 2, we tested Lepidoptera experts using the same experimental design. Again, the results supported the face-specificity hypothesis. Activation to faces in the FFA was greater than to both Lepidoptera and objects with little overlap between FG areas activated by faces and Lepidoptera. Our results suggest that distinct populations of neurons in human FG may be tuned to the features needed to individuate the members of different object classes, as has been reported in monkey inferotemporal cortex, and that the FFA contains neurons tuned for individuating faces.

Dissociating person-specific from general semantic knowledge: roles of the left and right temporal lobes

The cognitive architecture and neural underpinnings of different semantic domains remains highly controversial. We report two patients with focal temporal lobe atrophy who presented with contrasting and theoretically informative dissociations of person-specific versus general semantic knowledge. Subject J.P. showed severely impaired person-specific semantics, with relative preservation of knowledge about objects and animals, while subject M.A. exhibited the opposite pattern of performance (good knowledge of people in the context of impoverished general semantics). Voxel-based morphometric analysis of MR images in the two cases established predominantly right temporal atrophy associated with J.P.'s deficit for person knowledge and predominantly left temporal atrophy in M.A. who was impaired in general conceptual knowledge.

Visual agnosia

The visual agnosias are an intriguing class of clinical phenomena that have important implications for current theories of high-level vision. Visual agnosia is defined as impaired object recognition that cannot be attributed to visual loss, language impairment, or a general mental decline. At least in some instances, agnostic patients generate an adequate internal representation of the stimulus but fail to recognize it. In this review, we begin by describing the classic works related to the visual agnosias, followed by a description of the major clinical variants and their occurrence in degenerative disorders. In keeping with the theme of this issue, we then discuss recent contributions to this domain. Finally, we present evidence from functional imaging studies to support the clinical distinction between the various types of visual agnosias.

Cognitive and Behavioral Profile in a Case of Right Anterior Temporal Lobe Neurodegeneration

Semantic dementia (SD) is a clinical variant of frontotemporal lobar degeneration (FTLD) characterized by progressive deterioration of semantic memory with relative sparing of other cognitive functions. It is associated with mainly left anterior temporal atrophy, and is also referred to as "left-temporal lobe variant" of FTLD. Recently, patients with mainly right-sided atrophy, or "right-temporal lobe variant"(RTLV), have been described. While some authors have reported that the initial and most significant deficit in these right-sided cases is a difficulty in recognizing famous people, others have observed that major behavioral abnormalities are the presenting symptoms. Here we report a detailed neuropsychological, language, behavioral and neuroimaging assessment of JT, a case of right temporal lobe variant of FTLD. JT showed early and prominent behavioral changes accompanied by a severe impairment in recognizing foods by their look, flavor or name. Later she also developed a difficulty in recognizing familiar people and objects. Standardized caregiver questionnaires of JT's pre- and post-morbid personality and interpersonal functioning showed that she went from being a flexible, dominant, extraverted, person to showing rigid, submissive and introverted behaviors. Her levels of neuroticism significantly increased, while her scores on agreeableness and cognitive and emotional empathy dropped. Voxel-based morphometry (VBM) showed most significant atrophy in the right amygdala/anterior hippocampal complex and collateral sulcus, extending to the right insula. We discuss the atypical cognitive and behavioral features of this case of RTLV of FTLD and stress the importance of behavioral changes and atypical semantic deficits for early diagnosis.

My left brain and me: a dissociation in the perception of self and others

We investigated hemispheric asymmetries in face processing using a task in which participants judged the likeness of chimeric faces to their own face and to the face of a close friend based on their memory for those faces. When asked to choose which of two mirror-symmetric images (one made from the left half and one from the right half of a photograph of their face) looked more like themselves as remembered, participants showed a significant bias for the composite corresponding to the half face that lies in their right visual field when they look at themselves in the mirror. They showed the opposite bias when asked to make the same choice for images of a close friend, that is, they showed a significant bias for the composite corresponding to the half face that lies in their left visual field when they look at their friend. This result shows that in the case of these highly familiar faces--self and friend--the perceptual asymmetry is preserved in the memory representation. Assuming that people remember their own face as a mirror-image, the data also suggest a dissociation in face processing such that the left brain is dominant for the recognition of self and the right brain is dominant for the recognition of others.

Processing faces and facial expressions

This paper reviews processing of facial identity and expressions. The issue of independence of these two systems for these tasks has been addressed from different approaches over the past 25 years. More recently, neuroimaging techniques have provided researchers with new tools to investigate how facial information is processed in the brain. First, findings from "traditional" approaches to identity and expression processing are summarized. The review then covers findings from neuroimaging studies on face perception, recognition, and encoding. Processing of the basic facial expressions is detailed in light of behavioral and neuroimaging data. Whereas data from experimental and neuropsychological studies support the existence of two systems, the neuroimaging literature yields a less clear picture because it shows considerable overlap in activation patterns in response to the different face-processing tasks. Further, activation patterns in response to facial expressions support the notion of involved neural substrates for processing different facial expressions.

Development of face-sensitive event-related potentials during infancy: a review

Event-related potential (ERP) studies in adults have identified a number of components related to encoding and recognition memory of faces. Although behavioural studies indicate that even very young infants are able to detect faces and recognise familiar individuals, very few ERP studies document the neural correlates of these early abilities. In this article, we review four components (P1, N290, P400, Nc) and slow wave activity that are elicited while infants view faces. Where possible we draw links between these components and their possible equivalents to those observed in children and adults, and we highlight areas where further investigation is required. The theoretical importance of ERP studies of face processing in infants for debates about the origins and domain specificity of the adult cortical face processing system are discussed.

Face memory and its disorders

Face recognition is an essential biologic and social skill. Accurate recognition depends on the ability to encode, store, and retrieve distinct memory representations for the faces of countless individuals encountered in everyday life. In addition, face memory records must be integrated with specific biographic and name information in order to allow the recognition of each person's unique identity. Converging evidence from functional imaging, cortical electrical recording, and neuropsychologic studies suggests that face memory operations in the human brain are mediated by a distributed neural system. Components of this network include specialized memory storage sites within temporal neocortex that interact with medial temporal lobe and prefrontal cortical areas during face memory encoding and retrieval. Selective damage to these neuroanatomic regions gives rise to face recognition disorders characterized by memory loss or memory distortion.

Expert face processing requires visual input to the right hemisphere during infancy

Adult expertise in face processing is mediated largely by neural networks in the right hemisphere. Here we evaluate the contribution of early visual input in establishing this neural substrate. We compared visually normal individuals to patients for whom visual input had been restricted mainly to one hemisphere during infancy. We show that early deprivation of visual input to the right hemisphere severely impairs the development of expert face processing, whereas deprivation restricted mainly to the left hemisphere does not. Our results indicate that the neural circuitry responsible for adults' face expertise is not pre-specified, but requires early visual experience. However, the two hemispheres are not equipotent: only the right hemisphere is capable of using the early input to develop expertise at face processing.

Transient postoperative prosopagnosia

A 23-year-old right-handed woman developed isolated transient prosopagnosia following surgical resection of a right posterior temporal seizure focus. At 18 years of age she had developed secondarily generalized tonic-clonic seizures. Preoperative neuropsychological evaluation was normal, and neurological examination revealed only a left superior quadrant achromatopsia. MRI revealed a circumscribed lesion in the right inferolateral temporo-occipital junction. Following surgery she was agitated for 36 hours, and afterward, when her attention and orientation improved, she was unable to recognize familiar faces. She could, however, recognize familiar voices. Her prosopagnosia resolved over the next 6-7 days. This case demonstrates that isolated prosopagnosia can occur in patients with lesions restricted to the right inferior posterior temporal-anterior occipital region. The temporary nature of the prosopagnosia may result from postsurgical tissue injury, including focal cerebral edema, with compensation by ipsilateral or contralateral areas.

Implicit face perception in a patient with visual agnosia? Evidence from behavioural and eye-tracking analyses

This paper investigates face perception in a visual agnosic and prosopagnosic patient (SB). Despite very extensive lesions of visual areas, SB remains capable of some visual processing [Brain 125 (2002) 58]. However, in everyday situations SB does not exhibit signs of specific face recognition. To investigate how SB may process faces, we tested two hypotheses. According to the 'spared module hypothesis,' SBs abilities come from spared modules of implicit face processing. According to the 'general strategy hypothesis,' SB may have developed some deliberate compensatory strategies. A two-session experimental design was constructed. In both sessions, face and non-face pictures were shown to participants. In Session 1 (implicit condition), participants had to decide whether each picture was a vegetable. In Session 2 (explicit condition), participants had to decide whether each picture was a face. Verbal reports showed that SB was not aware of faces in Session 1. However, behavioural results showed that (1). SB could process faces; (2). even when SB was not aware of faces, he processed them differently than non-faces; (3). when knowing the presence of faces, he did not process faces better. In addition, eye-tracking data suggested that SB did not change the nature of his processing from Sessions 1 to 2. Pupil diameters showed that fixated facial features were processed similarly as in control participants. Together, these results are not compatible with a general compensatory strategy hypothesis and suggest sparing of an implicit face processing module in SB.

Transient inability to distinguish between faces: electrophysiologic studies

It is not known with certainty at which level of face processing by the cortex the distinction between a familiar and an unfamiliar face is made. Subdural electrodes were implanted under the fusiform gyrus of the right temporal lobe in a patient who developed an unusual inability to distinguish differences between faces as part of the epileptic aura ("all faces looked the same"). A cortical region located posterior to the epileptic focus was identified that exhibited a maximum evoked response to the presentation of facial images (N165), but not to objects, scenes, or character strings. Evoked potentials elicited by a variety of visual images indicated that any perturbation away from novel whole-face stimuli produced submaximal responses from this region of the right temporal lobe. Electrical stimulation of this region resulted in an impairment of face discrimination. It was found that presentation of familiar faces (grandmother, treating physician) produced a different response from that observed for novel faces. These observations demonstrate that within 165 msec of face presentation, and before the conscious precept of face familiarity has formed, this cortical region has already begun to distinguish between a familiar and an unfamiliar face.

Covert recognition and the neural system for face processing

In this viewpoint, we discuss the new evidence on covert face recognition in prosopagnosia presented by Bobes et al. (2003, this issue) and by Sperber and Spinnler (2003, this issue). Contrary to earlier hypotheses, both papers agree that covert and overt face recognition are based on the same mechanism. In line with this suggestion, an analysis of reported cases with prosopagnosia indicates that a degree of successful encoding of facial representations is a prerequisite for covert recognition to occur. While we agree with this general conclusion as far as Bobes et al.'s and Sperber and Spinnler's data are concerned, we also discuss evidence for a dissociation between different measures of covert recognition. Specifically, studies in patients with Capgras delusion and patients with prosopagnosia suggest that skin conductance and behavioural indexes of covert face recognition are mediated by partially different mechanisms. We also discuss implications of the new data for models of normal face recognition that have been successful in simulating covert recognition phenomena (e.g., Young and Burton, 1999, and O'Reilly et al., 1999). Finally, in reviewing recent neurophysiological and brain imaging evidence concerning the neural system for face processing, we argue that the relationship between ERP components (specifically, N170, N250r, and N400) and different cognitive processes in face recognition is beginning to emerge.

The emergence of delusional companions in Alzheimer's disease: an unusual misidentification syndrome

INTRODUCTION

Misidentifications in the course of organic diseases, including Alzheimer's disease (AD), have been associated with right sided brain dysfunction. The reasons for this association might be clarified by investigating the contribution of neurobiological, cognitive and emotional factors to distinctive types of misidentification.

METHODS

This study reports the cases of three patients with AD presenting a novel misidentification delusion in which objects with intrinsically comforting associations, soft toys, were experienced as sentient and triggered associated behaviours. Investigations included regional cerebral blood flow (rCBF) measurements with HMPAO SPECT and a comprehensive clinical and neuropsychological assessment.

RESULTS

All three patients showed a distinctive pattern of rCBF with dysfunction centred in the right parietal area and severe visuospatial and visuoperceptive processing deficits, with relatively preserved memory and language abilities.

CONCLUSION

These findings support the hypothesis that this type of misidentification delusion may be facilitated by right sided regional brain dysfunction involving visuoperceptual processing. Other historical and cognitive data from these patients suggests that the appearance of the symptom may be determined by the coexistence of a more widespread disorder of reality processing, which interacts with cognitive and psychogenic factors.

Visual scanning of faces in autism

The visual scanpaths of five high-functioning adult autistic males and five adult male controls were recorded using an infrared corneal reflection technique as they viewed photographs of human faces. Analyses of the scanpath data revealed marked differences in the scanpaths of the two groups. The autistic participants viewed nonfeature areas of the faces significantly more often and core feature areas of the faces (i.e., eyes, nose, and mouth) significantly less often than did control participants. Across both groups of participants, scanpaths generally did not differ as a function of the instructions given to the participants (i.e., "Please look at the faces in any manner you wish." vs. "Please identify the emotions portrayed in these faces."). Autistic participants showed a deficit in emotion recognition, but this effect was driven primarily by deficits in the recognition of fear. Collectively, these results indicate disorganized processing of face stimuli in autistic individuals and suggest a mechanism that may subserve the social information processing deficits that characterize autism spectrum disorders.

Dynamics of visual feature analysis and object-level processing in face versus letter-string perception

Neurones in the human inferior occipitotemporal cortex respond to specific categories of images, such as numbers, letters and faces, within 150-200 ms. Here we identify the locus in time when stimulus-specific analysis emerges by comparing the dynamics of face and letter-string perception in the same 10 individuals. An ideal paradigm was provided by our previous study on letter-strings, in which noise-masking of stimuli revealed putative visual feature processing at 100 ms around the occipital midline followed by letter-string-specific activation at 150 ms in the left inferior occipitotemporal cortex. In the present study, noise-masking of cartoon-like faces revealed that the response at 100 ms increased linearly with the visual complexity of the images, a result that was similar for faces and letter-strings. By 150 ms, faces and letter-strings had entered their own stimulus-specific processing routes in the inferior occipitotemporal cortex, with identical timing and large spatial overlap. However, letter-string analysis lateralized to the left hemisphere, whereas face processing occurred more bilaterally or with right-hemisphere preponderance. The inferior occipitotemporal activations at approximately 150 ms, which take place after the visual feature analysis at approximately 100 ms, are likely to represent a general object-level analysis stage that acts as a rapid gateway to higher cognitive processing.

Do I know you? Face perception and memory in patients with selective amygdalo-hippocampectomy

In 1968, Milner (Neuropsychologia 6 (1968) 191) demonstrated a face-memory impairment in patients with right, but not left, temporal-lobe excisions. Because all the removals included lateral and inferior temporal neocortex together with amygdala, parahippocampal gyrus and varying amounts of hippocampus, a combined-lesion effect could not be ruled out. We therefore examined the contribution of right temporal structures to recognition of previously unfamiliar faces by repeating Milner's original study, testing patients who had undergone selective amygdalo-hippocampectomy (AH), in addition to those with anterior temporal-lobectomy (TL). The paradigm involved selecting 12 previously studied faces from an array of 25 photographs. The Mooney Closure Faces Test was also administered. Subjects included 29 AH patients (14 left (LAH) and 15 right (RAH)) and 59 TL patients (30 L and 29 R) who were categorized further based on extensive (18 LTH and 21 RTH) or minimal (12 LTh and 8 RTh) hippocampal encroachment. Twenty age- and education-matched normal control subjects (NC) were also tested. For the face-memory task, one-way ANOVA revealed a strong group effect (P<0.001), and post-hoc tests confirmed that both the RTH and RAH groups recognized fewer faces than the NC and LAH groups; the RAH group also differed from the LTh, LTH and RTh groups. No group differences were found for the closure test. Our findings suggest that right medial temporal-lobe structures are critically involved in the retention, but probably not in the perception, of new faces.

The delusions of Capgras and intermetamorphosis in a patient with right-hemisphere white-matter pathology

Previous neuropsychological studies have demonstrated an association between person misidentification and right-hemisphere dysfunction. In the study reported here, we explore the contribution of facial and visual recognition impairments in a patient with right-hemisphere subcortical white-matter pathology in the frontal and parietal lobes and a diagnosis of vascular cognitive impairment. The patient displayed false recognition of unfamiliar faces and deficient retrieval of key biographic detail for famous faces. These results are discussed in the context of the contribution of deficiencies in the visual system and subcortical white-matter lesions to the development of Capgras delusion.

Face learning and memory: the twins test

The clinical utility of current face recognition tests has been questioned. To evaluate if a new paradigm may measure this type of memory more accurately, the authors created a novel test to examine face learning (previously uninvestigated) and short- and long-term retention. For this initial investigation of test sensitivity to hemisphere of dysfunction, patients with surgical resection from a temporal lobe and healthy subjects were tested. Recognition was evaluated on 3 trials: after a single exposure, after 4 exposures (for learning), and after a 24-hr delay interval. Patients with a right resection performed significantly worse than healthy controls and patients with left resection. There was no difference between patients with a left resection and controls. Classification of individual patients to side of resection based on test results showed higher sensitivity (82%) than published for other tests and maintained good specificity (79%).

Structural encoding of human and schematic faces: holistic and part-based processes

The range of specificity and the response properties of the extrastriate face area were investigated by comparing the N170 event-related potential (ERP) component elicited by photographs of natural faces, realistically painted portraits, sketches of faces, schematic faces, and by nonface meaningful and meaningless visual stimuli. Results showed that the N170 distinguished between faces and nonface stimuli when the concept of a face was clearly rendered by the visual stimulus, but it did not distinguish among different face types: Even a schematic face made from simple line fragments triggered the N170. However, in a second experiment, inversion seemed to have a different effect on natural faces in which face components were available and on the pure gestalt-based schematic faces: The N170 amplitude was enhanced when natural faces were presented upside down but reduced when schematic faces were inverted. Inversion delayed the N170 peak latency for both natural and schematic faces. Together, these results suggest that early face processing in the human brain is subserved by a multiple-component neural system in which both whole-face configurations and face parts are processed. The relative involvement of the two perceptual processes is probably determined by whether the physiognomic value of the stimuli depends upon holistic configuration, or whether the individual components can be associated with faces even when presented outside the face context.

Association of the distinct visual representations of faces and names: a PET activation study

A PET study of seven normal individuals was carried out to investigate the neural populations involved in the retrieval of the visual representation of a face when presented with an associated name, and conversely. Face-name associations were studied by means of four experimental matching conditions, including the retrieval of previously learned (1) name-name (NN), (2) face-face (FF), (3) name-face (NF), and (4) face-name (FN) associations, as well as a resting scan with eyes closed. Before PET images acquisition, subjects were presented with 24 unknown face-name associations to encode in 12 male/female couples. During PET scanning, their task was to decide whether the presented pair was a previously learned association. The right fusiform gyrus was strongly activated in FF condition as compared to NN and Rest conditions. However, no specific activations were found for NN condition relative to FF condition. A network of three areas distributed in the left hemisphere, both active in (NF-FF) and (FN-NN) comparisons, was interpreted as the locus of the integration of visual faces and names representations. These three regions were localized in the inferior frontal gyrus (BA 45), the medial frontal gyrus (BA 6) and the supramarginal gyrus of the inferior parietal lobe (BA 40). An interactive model accounting for these results, with BA 40 seen as an amodal binding region, is proposed.

Neural substrates for the recognition of newly learned faces: a functional MRI study

Face recognition is critical to the appreciation of our social and physical relations. Functional magnetic resonance imaging (fMRI) was used to identify brain regions involved in the recognition of newly learned faces. Two experiments were conducted. Experiment 1 contrasted a fixation control task with a face recognition task in which subjects were exposed solely to previously viewed faces (all-target). Experiment 2 compared a fixation control with another face recognition task in which subjects were presented with both novel and viewed faces (half-target). Compared to the fixation control, the all-target face recognition was associated with activation in the bilateral occipital and occipitotemporal regions, whereas the half-target face recognition produced activation in the right parietal and prefrontal regions, in addition to the occipital and occipitotemporal. The all-target minus half-target comparison revealed significant activation in the bilateral fusiform gyrus, suggesting stronger fusiform activity during the all-target than the half-target face recognition. The half-target minus all-target comparison showed significant activation in the superior and inferior parietal lobules and several regions in the right frontal lobe. These findings demonstrated that the bilateral fusiform gyrus is involved, not only in face perception, but in a certain aspect of face recognition memory and that this aspect is related to the actual recognition of previously viewed faces rather than the processing of novel ones, which results are consistent with previous lesion work. The right parietal and frontal regions, in contrast, are differentially more associated with the processes related to the detection of novel faces or retrieval effort.

Role of individual neurons and neural networks in cognitive functioning of the brain: a new insight

The prevailing concept in modern neuroscience is that neuron networks play a dominant role in the functioning of the nervous system, whereas the role of individual neurons is rather insignificant. This concept suggests that "individuality" of single neurons is primarily determined by their place in a network rather than their intrinsic properties. Here I argue that individual neurons may play an important, if not decisive, role in performing cognitive functions of the brain. This tentative viewpoint is supported by experimental and clinical insights into disorders of cognitive functions and by genetic studies of cognitive abilities and disabilities. The results obtained in these studies indicate that many specific cognitive functions are carried out by groups of highly specialized neurons whose roles in performing these functions are genetically predetermined and their activity could not be substituted by the activity of other neurons. In this context, the main role of neural networks and intercellular interactions is to form dynamic ensembles of neurons involved in performing a given cognitive function.

Accordance between EEG alpha power and dual task performance for different visual cognitive tasks

Evidence from clinical and experimental studies suggests that the left hemisphere is preferentially engaged in language processing and mathematical-analytic tasks while the right is involved with spatial relations and synthetic tasks. The purpose of the present study was to investigate accordance between dual task paradigm (effects of concurrent cognitive tasks on right- and left-hand finger tapping frequency) and EEG alpha power changes, which are used to conduct research in lateralization of cognitive function. Subjects performed reading, face recognition and line orientation tasks. First, the EEG was recorded during the tasks and, after six months, the dual task was carried out using the same cognitive tasks. Statistical analysis yielded no significant main effect for the hemispheres, performing hand and three cognitive tasks separately for the dual task and the EEG alpha power changes. However, significant correlation between the two methods was found, indicating the left parietal activation for the reading task, the right temporal activation for the line orientation task, and both hemispheric activation for the face recognition task. Results first suggest a significant accordance between dual task performance and EEG alpha asymmetry studies.

Learning face-name associations and the effect of age and performance: a PET activation study

Learning face-name associations is a complex task to be mastered in every day life that approaches the limits of cognitive capacity in most normal humans. We studied brain activation during face-name learning using positron emission tomography (PET) in 11 normal volunteers. The most intense activation was seen in occipital association cortex (BA 18) bilaterally, also involving lingual and fusiform gyrus (BA 37). In the left hemisphere additional activation were located in inferior temporal gyrus, the inferior part of pre- and postcentral gyrus, and orbitofrontal cortex (BA 11), whereas in the right hemisphere only a region in the precuneus (BA 19) was activated additionally. There was considerable interindividual variation of encoding success, which was significantly related to activation of BA 18 bilaterally. Subject ages covered a range of 26-72 years, but - in contrast to the effect of encoding success - there was no significant age effect on activations. Task-independent habituation effects were seen in cerebellum and left middle temporal gyrus. These results indicate that the intensity of information processing in ventral occipital association cortex is most important for success of face-name encoding. Learning is further mediated by a predominantly left-hemispheric network including inferior temporal and orbitofrontal cortex.

Canonical views of faces and the cerebral hemispheres

Evidence is given for a special, canonical, status of one specific view in the identification of familiar faces. In the first experiment, subjects identified by name the fully frontal or profile poses of briefly familiarised individuals less efficiently than an intermediate pose. In addition, in a matching experiment using faces seen in different poses, it was found that one specific intermediate pose (corresponding to 22.5 degrees of angle from the full frontal view) was matched more efficiently in the right visual field (RVF) than in the left visual field (LVF). This finding supports the hypothesis of a superiority of the left hemisphere (LH) over the right hemisphere (RH) in processing a familiar face's canonical view. The other tested "noncanonical" views (i.e., full frontal, 45 degrees, and profile) of these same familiar faces were better matched in the LVF (i.e., the RH); especially at low levels of familiarity. We conclude that, for each familiar face, a viewer-centred representation of the canonical (22.5 degrees ) view is stored in the LH's memory system, whereas multiple views of familiar faces are stored in a memory system of the RH. With increasing levels of familiarity other views are increasingly more efficiently encoded by the LH, and in fact for facial self-recognition the full-front view is superior to any of the other tested views. These findings taken together suggest that complementary lateralised memory subsystems in the two cerebral hemispheres store different sets, only partially overlapping, of view-centred face representations.

Prosopagnosia as a deficit in encoding curved surface

RP is a case of "developmental" prosopagnosia who, according to brain-imaging segmentation data, shows reduction in volume of a limited set of structures of the right hemisphere. RP is as accurate as control subjects in tasks requiring the perception of nonface objects (e.g., matching subordinate labels to exemplars, naming two-tone images), with the exception of one perceptual task: The matching of different perspectives of amoebae-like stimuli (i.e., volumes made of a single smooth surface). In terms of speed ("efficiency") of responses, RP's performance falls clearly outside the normal limits also in other tasks that include "natural" but nonface stimuli (i.e., animals, artia facts). Specifically, RP is slow in perceptual judgments made at very low (subordinate) levels of semantic categorization and for objects and artifacts whose geometry present much curved features and surface information. We conclude from these analyses that prosopagnosia can be the result of a deficit in the representation of basic geometric volumes made of curved surface. In turn, this points to the importance (necessity) for the normal visual system of such curved and volumetric information in the identification of human faces.

Dissociating memory processes involved in direct and indirect tests with ERPs to unfamiliar faces

Event-related potentials (ERPs) were recorded during indirect and direct memory tests for unfamiliar faces. In both tests, ERPs displayed the usual positive shift known as the ERP repetition effect. In the indirect test, this effect includes parietal effect (the usual N400 effect) and a right fronto-central effect. Both effects are also present in the direct test. Two additional effects are present only in the direct test. These effects are an early fronto-polar effect and a late posterior effect (the usual P600 effect). These findings are taken as support for the distinction between 'associative' processes elicited in both the direct and indirect tests, and 'episodic' processes elicited only in the direct test. This task dissociation could well provide a scalp correlate of the distinction between the neocortical and cortico-limbic systems that have been shown to contribute respectively to associative and episodic processing. In addition, it is proposed that the dissociation between the two frontal effects could be accounted for by a distinction between the processing of intrinsic vs. extrinsic contextual attributes as a function of the task requirements.

Neuropsychological impairments in the recognition of faces, voices, and personal names

In order to determine the dissociability of face, voice, and personal name recognition, we studied the performance of 36 brain-lesioned patients and 20 control subjects. Participants performed familiarity decisions for portraits, voice samples, and written names of celebrities and unfamiliar people. In those patients who displayed significant impairments in any of these tests, the specificity of these impairments was tested using corresponding object recognition tests (with pictures of objects, environmental sounds, or written common words as stimuli). The results showed that 58% of the patients were significantly impaired in at least one test of person recognition. Moreover, 28% of the patients showed impairments that appeared to be specific for people (i.e., performance was preserved in the corresponding object recognition test). Three patients showed a deficit that appeared to be confined to the recognition of familiar voices, a pattern that was not described previously. Results were generally consistent with the assumption that impairments in face, voice, and name recognition are dissociable from one another. In contrast, there was no clear evidence for a dissociability between deficits in face and voice naming. The results further suggest that (a) impairments in person recognition after brain lesions may be more common than was thought previously and (b) the patterns of impairment that were observed can be interpreted using current cognitive models of person recognition (Bruce & Young, 1986; Burton, Bruce, & Johnston, 1990).

Configurational coding, familiarity and the right hemisphere advantage for face recognition in sheep

This study examined characteristics of visual recognition of familiar and unfamiliar faces in sheep using a 2-way discrimination task. Of particular interest were effects of lateralisation and the differential use of internal (configurational) vs external features of the stimuli. Animals were trained in a Y-maze to identify target faces from pairs, both of which were familiar (same flock as the subjects) or both of which were unfamiliar (different flock). Having been trained to identify the rewarded face a series of stimuli were presented to the sheep, designed to test for the use of each visual hemifield in the discriminations and the use of internal and external facial cues. The first experiment showed that there was a left visual hemifield (LVF) advantage in the identification of 'hemifaces', and 'mirrored hemifaces' and 'chimeric' faces and that this effect was strongest with familiar faces. This represents the first evidence for visual field bias outside the primate literature. Results from the second experiment showed that, whilst both familiar and unfamiliar faces could be identified by the external features alone, only the familiar faces could be recognised by the internal features alone. Overall the results suggest separate recognition methods for socially familiar and unfamiliar faces, with the former being coded more by internal, configurational cues and showing a lateral bias to the left visual field.

Faces call for attention: evidence from patients with visual extinction

Three patients with left spatial neglect and visual extinction from right brain damage were studied to determine whether faces are privileged in summoning attention. In a first experiment, either a face, a name, or a meaningless shape were briefly presented in the right, left or both visual hemifields. On bilateral trials, all patients extinguished a left-side face much less often than a left-side name or a left-side shape. Conversely, they extinguished a left-side shape more often when it was accompanied by a right-side face rather than a right-side name. In a second experiment, either a face or a scrambled face could appear in the right, left or both hemifields. Again, on bilateral trials, a left-side face was less likely to be missed than a scrambled one. These results suggest an advantage of faces in capturing attention and overcoming extinction, which may be related to their special biological and social value, or to the very efficient and automatic operation of specific perceptual processses that extract facial organization in extrastriate visual areas. These findings also demonstrate that the distribution of spatial attention and extinction can be modulated by the relevance of visual stimuli. This implies that substantial analysis and categorization may take place in the visual system before information from the contralesional field is selected for, or excluded from, attentive vision.

Is right hemisphere specialization for face discrimination specific to humans?

Patterns of neural activation during face recognition were investigated in sheep by quantifying altered c-fos mRNA expression in situations where faces (sheep vs. human) can (faces upright) and cannot (faces inverted) be discriminated. Exposure to upright faces selectively increased expression significantly more in the right inferior temporal cortex than in the left, and active choice between upright faces additionally increased expression bilaterally in basal amygdala and hippocampus (CA1-4). Exposure to inverted faces did not lead to enhanced activation in the right inferior temporal cortex, amygdala or hippocampus but instead increased expression levels in the diagonal band of Broca, parietal and cingulate cortices. These results show that discrimination of upright faces in sheep preferentially engages the right temporal cortex, as it does in humans, and that performance of active choices between such faces may additionally involve the basal amygdala and hippocampus.

Human face perception traced by magneto- and electro-encephalography

The temporal and spatial processing of face perception in normal subjects was traced by magnetoencephalography (MEG) and electroencephalography (EEG). We used 5 different visual stimuli: (1) face with opened eyes, (2) face with closed eyes, (3) eyes, (4) scrambled face, and (5) hand, and they were shown in random order. Subjects were asked to count the number of hand stimuli. To analyze the complicated brain responses to visual stimuli, we used brain electric source analysis (BESA) as the spatio-temporal multiple source model. In MEG recording, the 1M and 2M components were identified in all subjects. The 1M component was recorded to all kinds of stimuli. The 2M component was clearly identified only to face stimulation in all subjects, but to eyes stimulation in only 3 subjects with a small amplitude. The 2M component was not identified to scrambled face nor hand stimulation. The 2M component was recorded from the right hemisphere in all subjects, but in only 5 of 10 subjects from the left hemisphere. The mean peak latencies of the 1M and 2M components were approximately 132 and 179 ms, respectively. The interpeak latency between 1M and 2M was approximately 47 ms on average but the interindividual difference was large. There was no significant difference of the 2M latency between face with opened eyes and face with closed eyes. The 1M component was generated in the primary visual cortex in the bilateral hemispheres, and the 2M component was generated in the inferior temporal cortex, around the fusiform gyrus. In the EEG recording, face-specific components, positive at the vertex, P200 (Cz), and the negative at the temporal areas, N190 (T5') and N190 (T6'), were clearly recorded. The EEG results were fundamentally compatible with the MEG results. The amplitude of the component recorded from the right hemisphere was significantly larger than that from the left hemisphere. These findings suggest that the fusiform gyrus is considered to play an important role in face perception in humans, and that the right hemisphere is more dominant. Face perception takes place approximately 47 ms after the primary response to visual stimulation in the primary visual cortex, but the period of information transfer to the fusiform gyrus is variable among subjects. Detailed temporal and spatial analyses of the processing of face perception can be achieved with MEG.

Contrast polarity and face recognition in the human fusiform gyrus

Functional imaging has revealed face-responsive visual areas in the human fusiform gyrus, but their role in recognizing familiar individuals remains controversial. Face recognition is particularly impaired by reversing contrast polarity of the image, even though this preserves all edges and spatial frequencies. Here, combined influences of familiarity and priming on face processing were examined as contrast polarity was manipulated. Our fMRI results show that bilateral posterior areas in fusiform gyrus responded more strongly for faces with positive than with negative contrast polarity. An anterior, right-lateralized fusiform region is activated when a given face stimulus becomes recognizable as a well-known individual.

Imaging visual recognition: PET and fMRI studies of the functional anatomy of human visual recognition

Until recently, the neural bases of visual object recognition in humans could be studied only by the use of brain-damaged subjects with naturally occurring lesions. Functional neuroimaging has given us the capability of studying visual recognition in the normal human brain. In the past ten years a number of PET and fMRI studies have attempted to isolate the neural substrates of human visual recognition. We have reviewed these studies and compared their conclusions regarding the anatomical locations of visual recognition processing in the human brain. The outcome was disappointing, revealing a wide range of locations. Our attempts to reduce the scatter by subgrouping the studies according to different task and stimulus properties were not successful. We discuss possible reasons for the lack of agreement among studies, including differences in the kinds of information yielded by lesion and imaging studies, and issues in the design and analysis of functional neuroimaging experiments. We conclude with a review of a more recent approach to the neuroimaging of human visual recognition, in which the effects of recognizing different types of visual stimuli are compared directly. With these experimental designs neuroimaging yields more replicable results, which also accord better with the known effects of lesions.