Hemispheric asymmetries in visual mental imagery

Visual mental imagery is the faculty whereby we can "visualize" objects that are not in our line of sight. Longstanding evidence dating back over thirty years has shown that unilateral brain lesions, especially in the left temporal lobe, can impair aspects of this ability. Yet, there is currently no attempt to identify analogies between these neuropsychological findings of hemispheric asymmetry and those from other neuroscientific approaches. Here, we present a critical review of the available literature on the hemispheric laterality of visual mental imagery, by looking at cross-method patterns of evidence in the domains of lesion neuropsychology, neuroimaging, and direct cortical stimulation. Results can be summarized under three main axes. First, frontoparietal networks in both hemispheres appear to be associated with visual mental imagery. Second, lateralization patterns emerge in the temporal lobes, with the left inferior temporal lobe being the most common finding in the literature for endogenously generated images, especially, but not exclusively, when orthographic material is used to ignite imagery. Third, an opposite pattern of hemispheric laterality emerges when visual mental images are induced by exogenous stimulation; direct cortical electrical stimulation tends to produce visual imagery experiences predominantly when applied to the right temporal lobe. These patterns of hemispheric asymmetry are difficult to reconcile with the dominant model of visual mental imagery, which emphasizes the implication of early sensory cortices. They suggest instead that visual mental imagery relies on large-scale brain networks, with a crucial participation of high-level visual regions in the temporal lobes.

Damasio's error - Prosopagnosia with intact within-category object recognition

The sudden inability to recognize individual faces following brain damage was first reported in a scientific journal 150 years ago and termed 'prosopagnosia' 70 years ago. While the term originally identified a face-selective neurological condition, it is now obscured by a sequence of imprecisions. First, prosopagnosia is routinely used to define symptoms of individual face recognition (IFR) difficulties in the context of visual object agnosia or other neurological conditions, or even in the normal population. Second, this over-expansive definition has lent support to a long-standing within-category recognition account of prosopagnosia, that is, that the impairment of IFR reflects a general impairment in recognizing within-category objects. However, stringent experimental studies of classical cases of prosopagnosia following brain damage show that their core impairment is not in recognizing physically similar exemplars within non-face object categories. Instead, the impairment presents specifically for recognizing exemplars of the category of faces. Moreover, compared to typical observers, the impairment appears even more severe for recognizing individual faces against physically dissimilar than similar distractors. Here, I argue that we need to limit accordingly our definition of prosopagnosia to a clinical (i.e., neurological) condition in which there is no basic-level object recognition impairment. Other criteria for prosopagnosia are proposed, with the hope that this conservative definition enables the study of human IFR processes in isolation, and supports progress in understanding the nature of these processes.

Why do herpes simplex encephalitis and semantic dementia show a different pattern of semantic impairment in spite of their main common involvement within the anterior temporal lobes?

A very challenging problem in the domain of the cognitive neurosciences is to explain why herpes simplex encephalitis and semantic dementia show, respectively, a category-specific semantic disorder for biological entities and an across-categories semantic disruption, despite highly overlapping areas of anterior temporal lobe damage. The aim of the present review consisted in trying to make a separate survey of anatomo-clinical investigations (single-case studies and group studies) and of activation studies, in order to analyse the factors that could explain these different patterns of semantic disruption. Factors taken into account in this review were laterality of lesions, disease aetiology, kind of brain pathology and locus of damage within the temporal lobes. Locus of damage within the temporal lobes and kind of brain pathology seemed to play the most important role, because in patients with herpes simplex encephalitis and category-specific semantic disorder for biological entities the lesions prevailed in the anteromedial temporal lobes. Furthermore, the neuropathology concerned both the anterior temporal cortices and the white matter pathways connecting these areas with the posterior visual areas, whereas in semantic dementia the inferior longitudinal fasciculus involvement was restricted to the rostral temporal lobe and did not extend into the cortically uninvolved occipital lobe.

Experiments with a Hollow Mask and a Reverspective: Top-down Influences in the Inversion Effect for 3-D Stimuli

Earlier psychophysical and physiological studies, obtained mostly with two-dimensional (2-D) stimuli, provided evidence for the hypothesis that the processing of faces differs from that of scenes. We report on our experiments, employing realistic three-dimensional (3-D) stimuli of a hollow mask and a scene, that offer further evidence for this hypothesis. The stimuli used for both faces and scenes were bistable, namely they could elicit either the veridical or an illusory volumetric percept. Our results indicate that the illusion is weakened when the stimuli are inverted, suggesting the involvement of top-down processes. This inversion effect is statistically significant for the facial stimulus, but the trend did not reach statistical significance for the scene stimulus. These results support the hypothesis that configural processing is stronger for the 3-D perception of faces than it is for scenes, and extend the conclusions of earlier studies on 2-D stimuli.

The pupillary response discriminates between subjective and objective familiarity and novelty

The pupil response discriminates between old and new stimuli, with old stimuli characterized by larger pupil dilation patterns than new stimuli. We sought to explore the cause of the pupil old/new effect and discount the effect of targetness, effort, recollection retrieval, and complexity of the recognition decision. Two experiments are reported in which the pupil response and the eye fixation patterns were measured, while participants identified novel and familiar object stimuli, in two separate tasks, emphasizing either novelty or familiarity detection. In Experiment 1, familiarity and novelty decisions were taken using a rating scale, while in Experiment 2 a simpler yes/no decision was used. In both experiments, we found that detection of target familiar stimuli resulted in greater pupil dilation than the detection of target novel stimuli, while the duration of the first fixation discriminated between familiar and novel stimuli as early as within 320 ms after stimulus onset. Importantly, the pupil response distinguished between the objective (during an earlier temporal component) and the subjective (during a later temporal component) status of the stimulus for misses and false alarms. In the light of previous findings, we suggest that the pupil and fixation old/new effects reflect the distinct neural and cognitive mechanisms involved in the familiarity and novelty decisions. The findings also have important implications for the use of pupil dilation and eye movement patterns to explore explicit and implicit memory processes.

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.

Early commitment of neural substrates for face recognition

We present evidence of a striking failure of plasticity in the neural substrates of face recognition, which suggests that the distinction between faces and other objects, and the localisation of faces relative to other objects, is fully determined prior to any postnatal experience. A boy who sustained brain damage at 1 day of age has the classic lesions and behavioural profile of adult-acquired prosopagnosia. He has profoundly impaired face recognition, whereas his recognition of objects is much less impaired. This implies that the human genome contains sufficiently explicit information about faces and nonface objects, or visual features by which they can be distinguished, that experience with these categories is not necessary for their functional delineation and differential brain localisation.

Structural encoding precludes recognition of face parts in prosopagnosia

The extent and the impact of spared processing of facial stimuli in the prosopagnosic patient LH is examined using the inversion effect and the face context effect. Our study asked how the deficit in individual face recognition is related to two perceptual abilities that are spared in this patient but between which there is interference when both are applied to the face stimulus, i.e. structural encoding of the face and parts-based matching procedures. Three experiments studied this relationship with task demands and stimulus properties designed to trigger the parts-based processes. In the first experiment, human and animal faces are presented upright or inverted with good performance only for the inverted condition. In Experiment 2 normals show a clear face context effect (matching of upright faces easier than scrambled or inverted ones) in the full face matching task whereas in the parts matching task the face superiority effect disappears. In contrast, LH shows a face inferiority effect when matching full faces but also when matching an isolated face part to a face part in a full face context. The results show that structural encoding of the face overrules parts-based procedures that could otherwise be helpful to tell individual faces apart.

First-pass selectivity for semantic categories in human anteroventral temporal lobe

How the brain encodes the semantic concepts represented by words is a fundamental question in cognitive neuroscience. Hemodynamic neuroimaging studies have robustly shown that different areas of posteroventral temporal lobe are selectively activated by images of animals versus manmade objects. Selective responses in these areas to words representing animals versus objects are sometimes also seen, but they are task-dependent, suggesting that posteroventral temporal cortex may encode visual categories, while more anterior areas encode semantic categories. Here, using the spatiotemporal resolution provided by intracranial macroelectrode and microelectrode arrays, we report category-selective responses to words representing animals and objects in human anteroventral temporal areas including inferotemporal, perirhinal, and entorhinal cortices. This selectivity generalizes across tasks and sensory modalities, suggesting that it represents abstract lexicosemantic categories. Significant category-specific responses are found in measures sensitive to synaptic activity (local field potentials, high gamma power, current sources and sinks) and unit-firing (multiunit and single-unit activity). Category-selective responses can occur at short latency (as early as 130 ms) in middle cortical layers and thus are extracted in the first pass of activity through the anteroventral temporal lobe. This activation may provide input to posterior areas for iconic representations when required by the task, as well as to the hippocampal formation for categorical encoding and retrieval of memories, and to the amygdala for emotional associations. More generally, these results support models in which the anteroventral temporal lobe plays a primary role in the semantic representation of words.

Differential contribution of right and left temporo-occipital and anterior temporal lesions to face recognition disorders

In the study of prosopagnosia, several issues (such as the specific or non-specific manifestations of prosopagnosia, the unitary or non-unitary nature of this syndrome and the mechanisms underlying face recognition disorders) are still controversial. Two main sources of variance partially accounting for these controversies could be the qualitative differences between the face recognition disorders observed in patients with prevalent lesions of the right or left hemisphere and in those with lesions encroaching upon the temporo-occipital (TO) or the (right) anterior temporal cortex. Results of our review seem to confirm these suggestions. Indeed, they show that (a) the most specific forms of prosopagnosia are due to lesions of a right posterior network including the occipital face area and the fusiform face area, whereas (b) the face identification defects observed in patients with left TO lesions seem due to a semantic defect impeding access to person-specific semantic information from the visual modality. Furthermore, face recognition defects resulting from right anterior temporal lesions can usually be considered as part of a multimodal people recognition disorder. The implications of our review are, therefore, the following: (1) to consider the components of visual agnosia often observed in prosopagnosic patients with bilateral TO lesions as part of a semantic defect, resulting from left-sided lesions (and not from prosopagnosia proper); (2) to systematically investigate voice recognition disorders in patients with right anterior temporal lesions to determine whether the face recognition defect should be considered a form of "associative prosopagnosia" or a form of the "multimodal people recognition disorder."

Holistic perception of the individual face is specific and necessary: evidence from an extensive case study of acquired prosopagnosia

We present an extensive investigation (24 experiments) of a new case of prosopagnosia following right unilateral damage, GG, with the aim of addressing two classical issues: (1) Can a visual recognition impairment truly be specific to faces? (2) What is the nature of acquired prosopagnosia? We show that GG recognizes nonface objects perfectly and quickly, even when it requires fine-grained analysis to individualize these objects. He is also capable of perceiving objects and faces as integrated wholes, as indicated by normal Navon effect, 3D-figures perception and perception of Mooney and Arcimboldo face stimuli. However, the patient could not perceive individual faces holistically, showing no inversion, composite, or whole-part advantage effects for faces. We conclude that an occipito-temporal right hemisphere lesion may lead to a specific impairment of holistic perception of individual items, a function that appears critical for normal face recognition but not for object recognition.

[Presentation of an assessment battery for visual mental imagery and visual perception]

INTRODUCTION

The relationship between visual perception and visual mental imagery are at the center of a lively theoretical debate between those postulating common neurocognitive processes between perception and imagery and those who emphasize the differences between these two entities. Neuropsychology can make an important contribution to this debate, by assessing associations and dissociations between perceptual and imaginal deficits in patients with brain damage. However, currently there is no standardized test battery available for such assessments.

MATERIAL AND METHODS

Here we present a battery of paper-and-pencil tests assessing different domains of visual mental imagery and visual perception abilities: object form and color, animals, orthographic material, numbers, faces, and space. We also explored the effects of age, educational level and gender on performance on a group of 103 participants free of neurological damage.

RESULTS

The battery includes two parts: one composed of 14 tests assessing mental imagery and the second part composed of eight tests assessing the abilities of visual perception. We calculated the correlations between the tests, and found that, with the exception of orthographic material, there were generally poor correlations between imagery and perceptual tests.

CONCLUSION

This result seems inconsistent with hypotheses postulating a strict correspondence between perceptual and imagery abilities.

Category-specificity in visual object recognition

Are all categories of objects recognized in the same manner visually? Evidence from neuropsychology suggests they are not: some brain damaged patients are more impaired in recognizing natural objects than artefacts whereas others show the opposite impairment. Category-effects have also been demonstrated in neurologically intact subjects, but the findings are contradictory and there is no agreement as to why category-effects arise. This article presents a pre-semantic account of category-effects (PACE) in visual object recognition. PACE assumes two processing stages: shape configuration (the binding of shape elements into elaborate shape descriptions) and selection (among competing representations in visual long-term memory), which are held to be differentially affected by the structural similarity between objects. Drawing on evidence from clinical studies, experimental studies with neurologically intact subjects and functional imaging studies, it is argued that PACE can account for category-effects at both behavioural and neural levels in patients and neurologically intact subjects. The theory also accounts for the way in which category-effects are affected by different task parameters (the degree of perceptual differentiation called for), stimulus characteristics (whether stimuli are presented as silhouettes, full line-drawings, or fragmented forms), stimulus presentation (stimulus exposure duration and position) as well as interactions between these parameters.

The contribution of the fusiform gyrus and superior temporal sulcus in processing facial attractiveness: neuropsychological and neuroimaging evidence

Current cognitive models suggest that the processing of dynamic facial attributes, including social signals such as gaze direction and facial expression, involves the superior temporal sulcus, whereas the processing of invariant facial structure such as the individuals' identity involves the fusiform face area. Where facial attractiveness, a social signal that may emerge from invariant facial structure, is processed within this dual-route model of face perception is uncertain. Here, we present two studies. First, we investigated the explicit judgments of facial attractiveness and attractiveness-motivated behavior in patients with acquired prosopagnosia, a deficit in familiar face recognition usually associated with damage to medial occipitotemporal cortex. We found that both abilities were impaired in these patients, with some weak residual ability for attractiveness judgments found only in those patients with unilateral right occipitotemporal or bilateral anterior temporal lesions. Importantly, deficits in attractiveness perception correlated with the severity of the face recognition deficit. Second, we performed a functional magnetic resonance imaging study in healthy subjects that included an implicit and explicit processing of facial attractiveness. We found increased neural activity when explicitly judging facial attractiveness within a number of cortical regions including the fusiform face area, but not the superior temporal sulcus, indicating a potential contribution of the fusiform face area to this judgment. Thus, converging neuropsychological and neuroimaging evidence points to a critical role of the inferior occipitotemporal cortex in the processing of facial attractiveness.

Haptic face identification activates ventral occipital and temporal areas: An fMRI study

Many studies in visual face recognition have supported a special role for the right fusiform gyrus. Despite the fact that faces can also be recognized haptically, little is known about the neural correlates of haptic face recognition. In the current fMRI study, neurologically intact participants were intensively trained to identify specific facemasks (molded from live faces) and specific control objects. When these stimuli were presented in the scanner, facemasks activated left fusiform and right hippocampal/parahippocampal areas (and other regions) more than control objects, whereas the latter produced no activity greater than the facemasks. We conclude that these ventral occipital and temporal areas may play an important role in the haptic identification of faces at the subordinate level. We further speculate that left fusiform gyrus may be recruited more for facemasks than for control objects because of the increased need for sequential processing by the haptic system.

Detailed Exploration of Face-related Processing in Congenital Prosopagnosia: 1. Behavioral Findings

We show that five individuals with congenital prosopagnosia (CP) are impaired at face recognition and discrimination and do not exhibit the normal superiority for upright over inverted faces despite intact visual acuity, low-level vision and intelligence, and in the absence of any obvious neural concomitant. Interestingly, the deficit is not limited to faces: The CP individuals were also impaired at discriminating common objects and novel objects although to a lesser extent than discriminating faces. The perceptual deficit may be attributable to a more fundamental visual processing disorder; the CP individuals exhibited difficulty in deriving global configurations from simple visual stimuli, even with extended exposure duration and considerable perceptual support in the image. Deriving a global configuration from local components is more critical for faces than for other objects, perhaps accounting for the exaggerated deficit in face processing. These findings elucidate the psychological mechanisms underlying CP and support the link between configural and face processing.

Congenital prosopagnosia: face-blind from birth

Congenital prosopagnosia refers to the deficit in face processing that is apparent from early childhood in the absence of any underlying neurological basis and in the presence of intact sensory and intellectual function. Several such cases have been described recently and elucidating the mechanisms giving rise to this impairment should aid our understanding of the psychological and neural mechanisms mediating face processing. Fundamental questions include: What is the nature and extent of the face-processing deficit in congenital prosopagnosia? Is the deficit related to a more general perceptual deficit such as the failure to process configural information? Are any neural alterations detectable using fMRI, ERP or structural analyses of the anatomy of the ventral visual cortex? We discuss these issues in relation to the existing literature and suggest directions for future research.

Category-related activation for written words in the posterior fusiform is task specific

Category-related brain activations have been reported in the posterior fusiform gyri when people view pictures of tools and animals, but only a single study has observed this pattern when the stimuli were words, rather than pictures. Here we replicate these category effects with words and provide evidence that distinctive patterns of activation are task specific. The results suggest that category-related activation in the posterior fusiform gyri can be driven either "bottom-up" by visual processing of images or "top-down" by word processing.

The influence of gender and lesion location on naming disorders for animals, plants and artefacts

The aim of this systematic review of single case studies of patients showing a category-specific disorder was to evaluate the influence of gender and lesion location on category-specific disorders for biological versus artefact categories and, within the former, for animals versus plant life categories. Two complementary studies were made, taking into account all the available single case reports of category-specific disorders found in the literature. The first study consisted of an overall statistical evaluation of the influence that gender and lesion location can have upon naming scores obtained with these different categories in patients selected only because they showed some kind of category-specific disorder. The second study assessed the influence of these variables on more selected groups of patients, contrasting those showing a categorical impairment for living things versus artefacts and, respectively, for animals versus plant life categories. Results of these studies consistently showed that: (a) Lesion location has a strong influence on the distinction between biological and artefacts categories, but not on that between animals and plant life domains. In patients with a prevalent impairment either for animals or for plant life items, lesions usually encroach upon the anterior or the posterior parts of the ventral stream of visual processing, whereas in patients with a prevalent impairment for artefacts they are located elsewhere (usually on more dorsal structures of the brain). (b) Gender, on the contrary, does not influence the distinction between living and non-living things, but, within the living entities, has a strong influence on the distinction between animals and plant life. Consistent with data obtained in normal people, which show that men are more familiar with animals and women with fruit and vegetables, men were, indeed, more impaired with plant life categories, whereas women were more impaired with animals.

Structural similarity and category-specificity: a refined account

It has been suggested that category-specific recognition disorders for natural objects may reflect that natural objects are more structurally (visually) similar than artefacts and therefore more difficult to recognize following brain damage. On this account one might expect a positive relationship between blood flow and structural similarity in areas involved in visual object recognition. Contrary to this expectation we report a negative relationship in that identification of articles of clothing cause more extensive activation than identification of vegetables/fruit and animals even though items from the categories of animals and vegetables/fruit are rated as more structurally similar than items from the category of articles of clothing. Given that this pattern cannot be explained in terms of a tradeoff between activation and accuracy, we interpret these findings within a model where the matching of visual forms to memory incorporates two operations: (i) the integration of stored object features into whole object representations (integral units), and (ii) the competition between activated integral units for selection (i.e. identification). In addition, we suggest that these operations are differentially affected by structural similarity in that high structural similarity may be beneficial for the integration of stored features into integral units, thus explaining the greater activation found with articles of clothing, whereas it may be harmful for the selection process proper because a greater range of candidate integral units will be activated and compete for selection, thus explaining the higher error rate associated with animals. We evaluate the model based on previous evidence from both normal subjects and patients with category-specific disorders and argue that this model can help reconcile otherwise conflicting data.

Processing of normal, inverted, and scrambled faces in a patient with prosopagnosia: behavioural and eye tracking data

In this study, we addressed the issue of a spared processing of faces in a patient (SB) with severe prosopagnosia. We designed an experiment comprising of two parts. In Part I, normal upright faces were entwined with scrambled faces, while in Part II normal upright faces were mixed with inverted faces, under unlimited time exposure. Performance, decision times, and eye movements were measured in both parts. The results indicated that SB categorised the normal faces better in the context of inverted faces than in the context of scrambled faces. Furthermore, SB's performance was better for the inverted faces than for the scrambled faces. Overall, SB performed better on the abnormal faces than on the normal faces, as did the control participants. Eye-tracking data showed that the pattern observed for the number of fixations and for exploration order was similar in SB and in controls. In the discussion, we propose that, despite his severe prosopagnosia, SB might have retained some kind of processing specific to face perception. Further investigations will be required, using limited time exposure, to determine the nature of this spared processing.

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.

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.

Covert person recognition: its fadeout in a case of temporal lobe degeneration

Covert person recognition was investigated longitudinally over a three-year period in a patient suffering from "Crossmodal Familiar Person Agnosia", possibly due to a fronto-temporal dementia in its right temporal variant (Gentileschi et al., 2001). The progressive neuronal degeneration in the cortical regions critical for face recognition (viz., right infero-temporal areas) presented us with the opportunity to check Burton et al.'s (1991) and Farah et al.'s (1993) hypothesis on the dissociation between overt and covert face recognition in a neuropsychological condition which, however, is neurologically and cognitively different from that of focal "associative prosopagnosia". Covert person recognition starting from overtly unrecognised faces was assessed by means of learning tasks of face/name association involving celebrities. It was assumed that some unconsciously spared information would selectively enhance the relearning rates when famous faces were paired with their true names. In fact, the true-name advantage (i.e., selective saving for experimental relearning of true name pairings) reached significance at first assessment, carried out five years from clinical onset. Effect faded away two and three years later on, thus abolishing the overt/covert dissociation in face recognition. These findings support Burton et al.'s (1991) and Farah et al.'s (1993) hypothesis of covert face recognition as the consequence of partial and incomplete activation of person semantics, due, in the present case, to the impoverishment of Gentileschi et al.'s (2001) "exemplar semantics" storehouse. Moreover, it turned out that covert recognition does not imply a different learning slope, but an overall different level of the learning profile.

Covert matching of unfamiliar faces in a case of prosopagnosia: an ERP study

In addition to their deficit in overt face recognition, patients with prosopagnosia also have difficulties in matching sequentially presented unfamiliar faces. Here we assessed the possibility that covert matching of faces was present in a case with prosopagnosia using event-related potentials (ERPs). The participants (patient FE and normal controls) were challenged with a face-identity matching task, in which they decided whether two sequentially presented photographs of unfamiliar faces represented the same person. Only internal face features were used and the two faces in a pair differed in emotional expression. FE failed to overtly match these stimuli. In contrast, the ERPs revealed evidence of covert matching. If the two faces within a pair of stimuli depicted different posers, then the response to the second face contained an enhanced N300 compared to the situation where the identity of the faces was the same. The latency of the N300 was the same as a similar component found in controls. These results suggest that some cases with prosopagnosia have a covert ability to match unfamiliar faces, with similar temporal dynamics as controls, which in contrast with the idea that a generalized slowing of face processing occurs in all cases of prosopagnosia.

The relationship between visual perception and visual mental imagery: a reappraisal of the neuropsychological evidence

Visual perception and visual mental imagery, the faculty whereby we can revisualise a visual item from memory, have often been regarded as cognitive functions subserved by common mechanisms. Thus, the leading cognitive model of visual mental imagery holds that visual perception and visual imagery share a number of mental operations, and rely upon common neural structures, including early visual cortices. In particular, a single visual buffer would be used "bottom-up" to display visual percepts and "top-down" to display internally generated images. The proposed neural substrate for this buffer consists of some cortical visual areas organised retinotopically, that is, the striate and extrastriate occipital areas. Empirical support for this model came from the report of brain-damaged patients showing an imagery deficit which parallels a perceptual impairment in the same cognitive domain. However, recent reports of patients showing double dissociations between perception and imagery abilities challenged the perception-imagery equivalence hypothesis from the functional point of view. From the anatomical point of view, the available evidence suggests that occipital damage is neither necessary nor sufficient to produce imagery deficits. On the other hand, extensive left temporal damage often accompanies imagery deficits for object form or colour. Thus, visual mental imagery abilities might require the integrity of brain areas related to vision, but at an higher level of integration than previously proposed.

How well do you know the back of your hand? Toward evidence-based knowledge

Familiarity with the back of one's hand has long been used as a reference criterion for knowledge despite a lack of supporting evidence. The present study prospectively tests normal subjects' knowledge of dorsal hand features. Sixty surgical, medical, and allied hospital employees (30 men, 30 women) were asked 5 questions with binary answers about features on the dorsum of their dominant hands while their hands were concealed. The proportion of correct answers to each question ranged from 0.45 to 0.65, and none was significantly different from 0.50. Similarly, the mean percentage of correct answers for all subjects and all questions was 54%, which was not significantly different from 50%. Thus, the accuracy of the answers approximated random guesses. Hand specialists scored significantly higher (75%) than other occupation groups. Men and women scored equally as a whole. These data refute the use of the hand idiom as a reference criterion for knowledge.

Beyond localisation: a dynamical dual route account of face recognition

After decades of research the notion that faces are special is still at the heart of heated debates. New techniques like brain imaging have advanced some of the arguments but empirical data from brain-damaged patients like paradoxical recognition effects have required more complex explanations aside from localisation of the face area in normal adults. In this paper we focus on configural face processes and discuss configural processes in prosopagnosics in the light of findings obtained in brain-imaging studies. In order to account for data like paradoxical face recognition effects we propose a dual route model of face recognition. The model is based on the distinction between two separate aspects of face recognition, detection and identification, considered as dynamical and interrelated. In this perspective the face detection system appears as the stronger candidate for face-specific processes. The face identification system on the other hand is part of the object recognition system but derives its specificity in part from interaction with the face-specific detection system. The fact that face detection appears intact in some patients provides us with a possible explanation for the interference of configural processes on feature-based identification.

Paradoxical configuration effects for faces and objects in prosopagnosia

Selective impairment in recognition of faces (prosopagnosia) has been advanced as an argument for a brain module dedicated to face processing and focusing on the specific configural properties of faces. Loss of the inversion effect supposedly strengthened the argument ([10]: de Gelder B, Bachoud-Levi AC, Degos JD. Inversion superiority in visual agnosia may be common to a variety of orientation polarised objects besides faces. Vision Research, 1998;38:2855-61; [20]: Farah MJ, Wilson K, Drain H, Tanaka J. The inverted face inversion effect in prosopagnosia: Evidence for mandatory, face-specific perceptual mechanisms. Vision Research 1995b;35:2089-93). The present study of prosopagnosic patient LH reports that he has lost the normal pattern of superior performance with upright faces and objects and shows instead paradoxical inversion effect for faces but also for objects. Experiment 2 investigated whether LH's use of features based route for processing upright objects would be hindered by the whole-based encoding when processing upright objects. The data show the same context effect for objects as was found for faces. Therefore the inversion effect does not present decisive evidence for the existence of a face module. Moreover, the importance of configuration-based recognition known to be crucial for face processing, must also be taken seriously for object recognition.

Perceptual differentiation and category effects in normal object recognition: a PET study

The purpose of the present PET study was (i) to investigate the neural correlates of object recognition, i.e. the matching of visual forms to memory, and (ii) to test the hypothesis that this process is more difficult for natural objects than for artefacts. This was done by using object decision tasks where subjects decided whether pictures represented real objects or non-objects. The object decision tasks differed in their difficulty (the degree of perceptual differentiation needed to perform them) and in the category of the real objects used (natural objects versus artefacts). A clear effect of task difficulty was found in both the behavioural and in the PET data. In the PET data, the increase in task difficulty was associated with increased regional cerebral blood flow in the posterior part of the right inferior temporal gyrus and in the anterior part of the right fusiform gyrus. This may be the neural correlate of matching visual forms to memory, and the amount of activation in these regions may correspond to the degree of perceptual differentiation required for recognition to occur. With respect to behaviour, it took significantly longer to make object decisions on natural objects than on artefacts in the difficult object decision tasks. Natural objects also recruited larger parts of the right inferior temporal and anterior fusiform gyri compared with artefacts as task difficulty increased. Differences in the amount of activation in these regions may reflect the greater perceptual differentiation required for recognizing natural objects. These findings are discussed in relation to category-specific impairments after neural damage.

Multiple-domain dissociation between impaired visual perception and preserved mental imagery in a patient with bilateral extrastriate lesions

A brain-damaged patient is described whose pattern of performance provides insight into both the functional mechanisms and the neural structures involved in visual mental imagery. The patient became severely agnosic, alexic, achromatopsic and prosopagnosic following bilateral brain lesions in the temporo-occipital cortex. However, her mental imagery for the same visual entities that she could not perceive was perfectly preserved. This clear-cut dissociation held across all the major domains of high-level vision: object recognition, reading, colour and face processing. Our findings, together with other reports on domain-specific dissociations and functional brain imaging studies, provide evidence to support the view that visual perception and visual mental imagery are subserved by independent functional mechanisms, which do not share the same cortical implementation. In particular, our results suggest that mental imagery abilities need not be mediated by early visual cortices.

Dissecting the computational architecture of social inference mechanisms

Scientists have been dissecting the neural architecture of the human mind for several centuries. Dissecting its computational architecture has proven more difficult, however. Within the cognitive sciences, for example, there is a debate about the extent to which human reasoning is generated by computational machinery that is domain specific and functionally specialized. While some claim that the same set of cognitive processes accounts for reasoning across all domains (e.g. Rips 1994, Johnson-Laird & Byrne 1991), others argue that reasoning is generated by several different mechanisms, each designed to operate over a different class of content (e.g. Baron-Cohen 1995, Cheng & Holyoak 1985, Cosmides & Tooby 1992, Leslie 1987). Indeed, it has recently been proposed that the human cognitive architecture contains a faculty of social cognition; a suite of integrated mechanisms, each of which is specialized for reasoning and making decisions about a different aspect of the social world. Candidate devices include a theory of mind mechanism, an eye direction detector, social contract algorithms, permission schemas, obligation schemas, precaution rules, threat detection procedures and others (e.g. Baron-Cohen 1995, Cheng & Holyoak 1985, Cosmides 1985, 1989, Cosmides & Tooby 1989, 1992, 1994, Fiddick et al 1995, Fiske 1991, Jackendoff 1992, Leslie 1987, K. Manktelow & D. Over, unpublished paper, 1st Int Conf on Thinking, Plymouth, UK 1988, Manktelow & Over 1990, M.Rutherford, J. Tooby, L. Cosmides, unpublished paper, 8th Annual Meeting Human Behav Evol Society, Northwestern Univ, IL 1996, J. Tooby & L. Cosmides, unpublished paper, 2nd Annual Meeting Human Behav Evol Society, Evanston, IL 1989). To decide among these sometimes competing proposals, psychologists need empirical methods and theoretical standards that let us carve social inference mechanisms at the joints. We will argue that the theoretical standards needed are those of the 'adaptationist programme' developed in evolutionary biology. To show how these standards can be applied in dissecting the computational architecture of the human mind, we will discuss some recent empirical methods and results.

Principal Component and Neural Network Analyses of Face Images: What Can Be Generalized in Gender Classification?

We present an overview of the major findings of the principal component analysis (pca) approach to facial analysis. In a neural network or connectionist framework, this approach is known as the linear autoassociator approach. Faces are represented as a weighted sum of macrofeatures (eigenvectors or eigenfaces) extracted from a cross-product matrix of face images. Using gender categorization as an illustration, we analyze the robustness of this type of facial representation. We show that eigenvectors representing general categorical information can be estimated using a very small set of faces and that the information they convey is generalizable to new faces of the same population and to a lesser extent to new faces of a different population. Copyright 1997 Academic Press. Copyright 1997 Academic Press

Can we learn from the clinically significant face processing deficits, prosopagnosia and Capgras delusion?

This review describes two clinically significant face processing deficits, prosopagnosia and Capgras delusion, and provides new knowledge about the face recognition process by a convergence of empirical findings. These empirical findings are structured around two questions that are reviewed from the perspectives of the two deficits. First is the question of hemispheric specificity, which inquires into the degree of each hemisphere's contribution to the face recognition process. Second is the question of dual neural pathways, which addresses the possibility that the face recognition process proceeds along two parallel pathways in the brain. Findings from the hemispheric specificity studies reinforce the current view that right hemispheric involvement is necessary for face recognition while left hemispheric involvement is minimal. Findings from the dual neural pathways studies reinforce the plausible but yet unproven hypothesis that two neural pathways pass information from the visual association cortex in the occipital lobe toward the temporal lobes and limbic system when faces are seen and recognized. These findings, which also indicate that each of the dual neural pathways carries different, nonredundant information, could be instrumental in showing that the pathways play different roles in the manifestations of the clinically significant face processing deficits, prosopagnosia and Capgras delusion.

Facial expression processing after amygdalotomy

Facial expression processing impairments were studied in D.R., a 51-year-old woman with a partial bilateral amygdalotomy. D.R. was poor at recognising emotional facial expressions, both in static and moving stimuli. In identity matching tasks (same vs different person) and expression matching tasks (same vs different expression), D.R. was impaired at expression matching with simultaneously or successively presented faces; she experienced difficulties whenever the faces' identities were discrepant with their expressions. For identity matching, she also had problems when simultaneously presented images showed the same face with two different expressions; her deficit in interpreting facial expressions could lead her to mistake differences in expression for a difference in identity. In imagery tasks, D.R. was able to answer questions about the appearance of familiar people, yet she was very poor at imaging facial expressions of emotion. We suggest that her problems in processing facial expressions included impaired knowledge of the patterning of facial features in each emotion; without this, remembering or reconstructing what emotional facial expressions look like is as impaired as perceptual recognition. These findings are consistent with the hypothesis of a role for the amygdala in social behaviour.