Behavioral change and its neural correlates in visual agnosia after expertise training

Differential functional reorganization of ventral and dorsal visual pathways following childhood hemispherectomy

Hemispherectomy is a surgical procedure in which an entire hemisphere of a patient's brain is resected or functionally disconnected to manage seizures in individuals with drug-resistant epilepsy. Despite the extensive loss of both ventral and dorsal visual pathways in one hemisphere, pediatric patients who have undergone hemispherectomy show a remarkably high degree of perceptual function across many domains. In the current study, we sought to understand the extent to which functions of the ventral and dorsal visual pathways reorganize to the contralateral hemisphere following childhood hemispherectomy. To this end, we collected fMRI data from an equal number of left and right hemispherectomy patients who completed tasks that typically elicit lateralized responses from the ventral or the dorsal pathway, namely, word (left ventral), face (right ventral), tool (left dorsal), and global form (right dorsal) perception. Overall, there was greater evidence of functional reorganization in the ventral pathway than in the dorsal pathway. Importantly, because ventral and dorsal reorganization was tested within the very same patients, these results cannot be explained by idiosyncratic factors such as disease etiology, age at the time of surgery, or age at testing. These findings suggest that because the dorsal pathway may mature earlier, it may have a shorter developmental window of plasticity than the ventral pathway and, hence, be less malleable after perturbation.

The rehabilitation of object agnosia and prosopagnosia: A systematic review

BACKGROUND

Agnosia for objects is often overlooked in neuropsychology, especially with respect to rehabilitation. Prosopagnosia has been studied more extensively, yet there have been few attempts at training it. The lack of training protocols may partially be accounted for by their relatively low incidence and specificity to sensory modality. However, finding effective rehabilitations for such deficits may help to reduce their impact on the social and psychological functioning of individuals.

OBJECTIVE

Our aim in this study was to provide clinicians and researchers with useful information with which to conduct new studies on the rehabilitation of object agnosia and prosopagnosia. To accomplish this, we performed a systematic and comprehensive review of the effect of neuropsychological rehabilitation on visual object and prosopagnosia.

METHODS

The Preferred Reporting Items for Systematic reviews and Meta-Analyses guidelines were followed. In addition, the Single-Case Experimental Design (SCED) and the Critical Appraisal Skills Programme (CASP) scales were used to assess the quality of reporting.

RESULTS

Seven articles regarding object agnosia, eight articles describing treatments for prosopagnosia, and two articles describing treatments for both deficits were included.

CONCLUSIONS

In the light of the studies reviewed, treatments based on analysis of parts seem effective for object agnosia, while prosopagnosia appears to benefit most from treatments relying on holistic/configural processing. However, more attempts at rehabilitation of face and object agnosia are needed to clarify the mechanisms of these processes and possible rehabilitations. Moreover, a publication bias could mask a broader attempt to find effective treatments for visual agnosia and leaving out studies that are potentially more informative.

Prosopagnosia and disorders of face processing

Face recognition is a form of expert visual processing. Acquired prosopagnosia is the loss of familiarity for facial identity and has several functional variants, namely apperceptive, amnestic, and associative forms. Acquired forms are usually caused by either occipitotemporal or anterior temporal lesions, right or bilateral in most cases. In addition, there is a developmental form, whose functional and structural origins are still being elucidated. Despite their difficulties with recognizing faces, some of these subjects still show signs of covert recognition, which may have a number of explanations. Other aspects of face perception can be spared in prosopagnosic subjects. Patients with other types of face processing difficulties have been described, including impaired expression processing, impaired lip-reading, false familiarity for faces, and a people-specific amnesia. Recent rehabilitative studies have shown some modest ability to improve face perception in prosopagnosic subjects through perceptual training protocols.

Experience Transforms Conjunctive Object Representations: Neural Evidence for Unitization After Visual Expertise

Certain transformations must occur within the brain to allow rapid processing of familiar experiences. Complex objects are thought to become unitized, whereby multifeature conjunctions are retrieved as rapidly as a single feature. Behavioral studies strongly support unitization theory, but a compelling neural mechanism is lacking. Here, we examined how unitization transforms conjunctive representations to become more “feature-like” by recruiting posterior regions of the ventral visual stream (VVS) whose architecture is specialized for processing single features. We used functional magnetic resonance imaging to scan humans before and after visual training with novel objects. We implemented a novel multivoxel pattern analysis to measure a conjunctive code, which represented a conjunction of object features above and beyond the sum of the parts. Importantly, a multivoxel searchlight showed that the strength of conjunctive coding in posterior VVS increased posttraining. Furthermore, multidimensional scaling revealed representational separation at the level of individual features in parallel to the changes at the level of feature conjunctions. Finally, functional connectivity between anterior and posterior VVS was higher for novel objects than for trained objects, consistent with early involvement of anterior VVS in unitizing feature conjunctions in response to novelty. These data demonstrate that the brain implements unitization as a mechanism to refine complex object representations over the course of multiple learning experiences.

Object recognition in acquired and developmental prosopagnosia

Whether face and object recognition are dissociated in prosopagnosia continues to be debated: a recent review highlighted deficiencies in prior studies regarding the evidence for such a dissociation. Our goal was to study cohorts with acquired and developmental prosopagnosia with a complementary battery of tests of object recognition that address prior limitations, as well as evaluating for residual effects of object expertise. We studied 15 subjects with acquired and 12 subjects with developmental prosopagnosia on three tests: the Old/New Tests, the Cambridge Bicycle Memory Test, and the Expertise-adjusted Test of Car Recognition. Most subjects with developmental prosopagnosia were normal on the Old/New Tests: for acquired prosopagnosia, subjects with occipitotemporal lesions often showed impairments while those with anterior temporal lesions did not. Ten subjects showed a putative classical dissociation between the Cambridge Face and Bicycle Memory Tests, seven of whom had normal reaction times. Both developmental and acquired groups showed reduced car recognition on the expertise-adjusted test, though residual effects of expertise were still evident. Two subjects with developmental prosopagnosia met criteria for normal object recognition across all tests. We conclude that strong evidence for intact object recognition can be found in a few subjects but the majority show deficits, particularly those with the acquired form. Both acquired and developmental forms show residual but reduced object expertise effects.

Domain-general and domain-specific neural changes underlying visual expertise

Visual expertise induces changes in neural processing for many different domains of expertise. However, it is unclear how expertise effects for different domains of expertise are related. In the present fMRI study, we combine large-scale univariate and multi-voxel analyses to contrast the expertise-related neural changes associated with two different domains of expertise, bird expertise (ornithology) and mineral expertise (mineralogy). Results indicated distributed expertise-related neural changes, with effects for both domains of expertise in high-level visual cortex and effects for bird expertise even extending to low-level visual regions and the frontal lobe. Importantly, a multivariate generalization analysis showed that effects in high-level visual cortex were specific to the domain of expertise. In contrast, the neural changes in the frontal lobe relating to expertise showed significant generalization, signaling the presence of domain-independent expertise effects. In conclusion, expertise is related to a combination of domain-specific and domain-general changes in neural processing.

Preserved Expert Object Recognition in a Case of Visual Hemiagnosia

We examined a stroke patient (HWS) with a unilateral lesion of the right medial ventral visual stream, involving the right fusiform and parahippocampal gyri. In a number of object recognition tests with lateralized presentations of target stimuli, HWS showed significant symptoms of hemiagnosia with contralesional recognition deficits for everyday objects. We further explored the patient's capacities of visual expertise that were acquired before the current perceptual impairment became effective. We confronted him with objects he was an expert for already before stroke onset and compared this performance with the recognition of familiar everyday objects. HWS was able to identify significantly more of the specific (“expert”) than of the everyday objects on the affected contralesional side. This observation of better expert object recognition in visual hemiagnosia allows for several interpretations. The results may be caused by enhanced information processing for expert objects in the ventral system in the affected or the intact hemisphere. Expert knowledge could trigger top–down mechanisms supporting object recognition despite of impaired basic functions of object processing. More importantly, the current work demonstrates that top–down mechanisms of visual expertise influence object recognition at an early stage, probably before visual object information propagates to modules of higher object recognition. Because HWS showed a lesion to the fusiform gyrus and spared capacities of expert object recognition, the current study emphasizes possible contributions of areas outside the ventral stream to visual expertise.

Developmental visual perception deficits with no indications of prosopagnosia in a child with abnormal eye movements

Visual categories are associated with eccentricity biases in high-order visual cortex: Faces and reading with foveally-biased regions, while common objects and space with mid- and peripherally-biased regions. As face perception and reading are among the most challenging human visual skills, and are often regarded as the peak achievements of a distributed neural network supporting common objects perception, it is unclear why objects, which also rely on foveal vision to be processed, are associated with mid-peripheral rather than with a foveal bias. Here, we studied BN, a 9 y.o. boy who has normal basic-level vision, abnormal (limited) oculomotor pursuit and saccades, and shows developmental object and contour integration deficits but with no indication of prosopagnosia. Although we cannot infer causation from the data presented here, we suggest that normal pursuit and saccades could be critical for the development of contour integration and object perception. While faces and perhaps reading, when fixated upon, take up a small portion of central visual field and require only small eye movements to be properly processed, common objects typically prevail in mid-peripheral visual field and rely on longer-distance voluntary eye movements as saccades to be brought to fixation. While retinal information feeds into early visual cortex in an eccentricity orderly manner, we hypothesize that propagation of non-foveal information to mid and high-order visual cortex critically relies on circuitry involving eye movements. Limited or atypical eye movements, as in the case of BN, may hinder normal information flow to mid-eccentricity biased high-order visual cortex, adversely affecting its development and consequently inducing visual perceptual deficits predominantly for categories associated with these regions.

Perceptual Learning of Faces: A Rehabilitative Study of Acquired Prosopagnosia

Despite many studies of acquired prosopagnosia, there have been only a few attempts at its rehabilitation, all in single cases, with a variety of mnemonic or perceptual approaches, and of variable efficacy. In a cohort with acquired prosopagnosia, we evaluated a perceptual learning program that incorporated variations in view and expression, which was aimed at training perceptual stages of face processing with an emphasis on ecological validity. Ten patients undertook an 11-week face training program and an 11-week control task. Training required shape discrimination between morphed facial images, whose similarity was manipulated by a staircase procedure to keep training near a perceptual threshold. Training progressed from blocks of neutral faces in frontal view through increasing variations in view and expression. Whereas the control task did not change perception, training improved perceptual sensitivity for the trained faces and generalized to new untrained expressions and views of those faces. There was also a significant transfer to new faces. Benefits were maintained over a 3-month period. Training efficacy was greater for those with more perceptual deficits at baseline. We conclude that perceptual learning can lead to persistent improvements in face discrimination in acquired prosopagnosia. This reflects both acquisition of new skills that can be applied to new faces as well as a degree of overlearning of the stimulus set at the level of 3-D expression-invariant representations.

Functional Neuroimaging in Stroke Recovery and Neurorehabilitation: Conceptual Issues and Perspectives

Background

In stroke, functional neuroimaging has become a potent diagnostic tool; opened new insights into the pathophysiology of ischaemic damage in the human brain; and made possible the assessment of functional–structural relationships in postlesion recovery.

Summary of review

Here, we give a critical account on the potential and limitation of functional neuroimaging and discuss concepts related to the use of neuroimaging for exploring the neurobiological and neuroanatomical mechanisms of poststroke recovery and neurorehabilitation. We identify and provide evidence for five hypotheses that functional neuroimaging can provide new insights into: adaptation occurs at the level of functional brain systems; the brain–behaviour relationship varies with recovery and over time; functional neuroimaging can improve our ability to predict recovery and select individuals for rehabilitation; mechanisms of recovery reflect different pathophysiological phases; and brain adaptation may be modulated by experience and specific rehabilitation. The significance and application of this new evidence is discussed, and recommendations made for investigations in the field.

Conclusion

Functional neuroimaging is an important tool to explore the mechanisms underlying brain plasticity and, thereby, to guide clinical research in neurorehabilitation.

The relationship between visual word and face processing lateralization in the fusiform gyri: A cross-sectional study

Visual words and faces activate similar networks but with complementary hemispheric asymmetries, faces being lateralized to the right and words to the left. A recent theory proposes that this reflects developmental competition between visual word and face processing. We investigated whether this results in an inverse correlation between the degree of lateralization of visual word and face activation in the fusiform gyri. 26 literate right-handed healthy adults underwent functional MRI with face and word localizers. We derived lateralization indices for cluster size and peak responses for word and face activity in left and right fusiform gyri, and correlated these across subjects. A secondary analysis examined all face- and word-selective voxels in the inferior occipitotemporal cortex. No negative correlations were found. There were positive correlations for the peak MR response between word and face activity within the left hemisphere, and between word activity in the left visual word form area and face activity in the right fusiform face area. The face lateralization index was positively rather than negatively correlated with the word index. In summary, we do not find a complementary relationship between visual word and face lateralization across subjects. The significance of the positive correlations is unclear: some may reflect the influences of general factors such as attention, but others may point to other factors that influence lateralization of function.

What is special about expertise? Visual expertise reveals the interactive nature of real-world object recognition

Ever since Diamond and Carey (1986. J. Exp. Psychol.: Gen., vol. 115, pp. 107-117) seminal work, the main model for studying expertise in visual object recognition ("visual expertise") has been face perception. The underlying assumption was that since faces may be considered the ultimate domain of visual expertise, any face-processing signature might actually be a general characteristic of visual expertise. However, while humans are clearly experts in face recognition, visual expertise is not restricted to faces and can be observed in a variety of domains. This raises the question of whether face recognition is in fact the right model to study visual expertise, and if not, what are the common cognitive and neural characteristics of visual expertise. The current perspective article addresses this question by revisiting past and recent neuroimaging and behavioural works on visual expertise. The view of visual expertise that emerges from these works is that expertise is a unique phenomenon, with distinctive neural and cognitive characteristics. Specifically, visual expertise is a controlled, interactive process that develops from the reciprocal interactions between the visual system and multiple top-down factors, including semantic knowledge, top-down attentional control, and task relevance. These interactions enable the ability to flexibly access domain-specific information at multiple scales and levels guided by multiple recognition goals. Extensive visual experience with a given object category culminates in the recruitment of these multiple systems, and is reflected in widespread neural activity, extending well beyond visual cortex, to include higher-level cortical areas.

Eye-Search: A web-based therapy that improves visual search in hemianopia

Persisting hemianopia frequently complicates lesions of the posterior cerebral hemispheres, leaving patients impaired on a range of key activities of daily living. Practice-based therapies designed to induce compensatory eye movements can improve hemianopic patients' visual function, but are not readily available. We used a web-based therapy (Eye-Search) that retrains visual search saccades into patients' blind hemifield. A group of 78 suitable hemianopic patients took part. After therapy (800 trials over 11 days), search times into their impaired hemifield improved by an average of 24%. Patients also reported improvements in a subset of visually guided everyday activities, suggesting that Eye-Search therapy affects real-world outcomes.

Robust expertise effects in right FFA

The fusiform face area (FFA) is one of several areas in occipito-temporal cortex whose activity is correlated with perceptual expertise for objects. Here, we investigate the robustness of expertise effects in FFA and other areas to a strong task manipulation that increases both perceptual and attentional demands. With high-resolution fMRI at 7T, we measured responses to images of cars, faces and a category globally visually similar to cars (sofas) in 26 subjects who varied in expertise with cars, in (a) a low load 1-back task with a single object category and (b) a high load task in which objects from two categories were rapidly alternated and attention was required to both categories. The low load condition revealed several areas more active as a function of expertise, including both posterior and anterior portions of FFA bilaterally (FFA1/FFA2, respectively). Under high load, fewer areas were positively correlated with expertise and several areas were even negatively correlated, but the expertise effect in face-selective voxels in the anterior portion of FFA (FFA2) remained robust. Finally, we found that behavioral car expertise also predicted increased responses to sofa images but no behavioral advantages in sofa discrimination, suggesting that global shape similarity to a category of expertise is enough to elicit a response in FFA and other areas sensitive to experience, even when the category itself is not of special interest. The robustness of expertise effects in right FFA2 and the expertise effects driven by visual similarity both argue against attention being the sole determinant of expertise effects in extrastriate areas.

Neural correlates of cognitive intervention in persons at risk of developing Alzheimer's disease

Cognitive training is an emergent approach that has begun to receive increased attention in recent years as a non-pharmacological, cost-effective intervention for Alzheimer’s disease (AD). There has been increasing behavioral evidence regarding training-related improvement in cognitive performance in early stages of AD. Although these studies provide important insight about the efficacy of cognitive training, neuroimaging studies are crucial to pinpoint changes in brain structure and function associated with training and to examine their overlap with pathology in AD. In this study, we reviewed the existing neuroimaging studies on cognitive training in persons at risk of developing AD to provide an overview of the overlap between neural networks rehabilitated by the current training methods and those affected in AD. The data suggest a consistent training-related increase in brain activity in medial temporal, prefrontal, and posterior default mode networks, as well as increase in gray matter structure in frontoparietal and entorhinal regions. This pattern differs from the observed pattern in healthy older adults that shows a combination of increased and decreased activity in response to training. Detailed investigation of the data suggests that training in persons at risk of developing AD mainly improves compensatory mechanisms and partly restores the affected functions. While current neuroimaging studies are quite helpful in identifying the mechanisms underlying cognitive training, the data calls for future multi-modal neuroimaging studies with focus on multi-domain cognitive training, network level connectivity, and individual differences in response to training.

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

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

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.

A meta-analysis and review of holistic face processing

The concept of holistic processing is a cornerstone of face recognition research, yet central questions related to holistic processing remain unanswered, and debates have thus far failed to reach a resolution despite accumulating empirical evidence. We argue that a considerable source of confusion in this literature stems from a methodological problem. Specifically, 2 measures of holistic processing based on the composite paradigm (complete design and partial design) are used in the literature, but they often lead to qualitatively different results. First, we present a comprehensive review of the work that directly compares the 2 designs, and which clearly favors the complete design over the partial design. Second, we report a meta-analysis of holistic face processing according to both designs and use this as further evidence for one design over the other. The meta-analysis effect size of holistic processing in the complete design is nearly 3 times that of the partial design. Effect sizes were not correlated between measures, consistent with the suggestion that they do not measure the same thing. Our meta-analysis also examines the correlation between conditions in the complete design of the composite task, and suggests that in an individual differences context, little is gained by including a misaligned baseline. Finally, we offer a comprehensive review of the state of knowledge about holistic processing based on evidence gathered from the measure we favor based on the 1st sections of our review-the complete design-and outline outstanding research questions in that new context.

Expertise Effects in Face-Selective Areas are Robust to Clutter and Diverted Attention, but not to Competition

Expertise effects for nonface objects in face-selective brain areas may reflect stable aspects of neuronal selectivity that determine how observers perceive objects. However, bottom-up (e.g., clutter from irrelevant objects) and top-down manipulations (e.g., attentional selection) can influence activity, affecting the link between category selectivity and individual performance. We test the prediction that individual differences expressed as neural expertise effects for cars in face-selective areas are sufficiently stable to survive clutter and manipulations of attention. Additionally, behavioral work and work using event related potentials suggest that expertise effects may not survive competition; we investigate this using functional magnetic resonance imaging. Subjects varying in expertise with cars made 1-back decisions about cars, faces, and objects in displays containing one or 2 objects, with only one category attended. Univariate analyses suggest car expertise effects are robust to clutter, dampened by reducing attention to cars, but nonetheless more robust to manipulations of attention than competition. While univariate expertise effects are severely abolished by competition between cars and faces, multivariate analyses reveal new information related to car expertise. These results demonstrate that signals in face-selective areas predict expertise effects for nonface objects in a variety of conditions, although individual differences may be expressed in different dependent measures depending on task and instructions.

Normal acquisition of expertise with greebles in two cases of acquired prosopagnosia

Face recognition is generally thought to rely on different neurocognitive mechanisms than most types of objects, but the specificity of these mechanisms is debated. One account suggests the mechanisms are specific to upright faces, whereas the expertise view proposes the mechanisms operate on objects of high within-class similarity with which an observer has become proficient at rapid individuation. Much of the evidence cited in support of the expertise view comes from laboratory-based training experiments involving computer-generated objects called greebles that are designed to place face-like demands on recognition mechanisms. A fundamental prediction of the expertise hypothesis is that recognition deficits with faces will be accompanied by deficits with objects of expertise. Here we present two cases of acquired prosopagnosia, Herschel and Florence, who violate this prediction: Both show normal performance in a standard greeble training procedure, along with severe deficits on a matched face training procedure. Herschel and Florence also meet several response time criteria that advocates of the expertise view suggest signal successful acquisition of greeble expertise. Furthermore, Herschel's results show that greeble learning can occur without normal functioning of the right fusiform face area, an area proposed to mediate greeble expertise. The marked dissociation between face and greeble expertise undermines greeble-based claims challenging face-specificity and indicates face recognition mechanisms are not necessary for object recognition after laboratory-based training.

Can training enhance face cognition abilities in middle-aged adults?

Face cognition is a crucial skill for social interaction and shows large individual differences in healthy adults, suggesting a possibility for improvement in some. We developed and tested specific training procedures for the accuracy of face memory and the speed of face cognition. Two groups each of 20 healthy middle-aged trainees practiced for 29 daily sessions of 15 minutes duration with different computerized home-based training procedures. In addition, 20 matched and 59 non-matched controls were included. Face cognition speed training enhanced performance during the training and transferred to the latent factor level as measured in a pre-post comparison. Persistence of the training effect was evidenced at the manifest level after three months. However, the training procedure influenced the speed of processing object stimuli to the same extent as face stimuli and therefore seems to have affected a more general ability of processing complex visual stimuli and not only faces. No effects of training on the accuracy of face memory were found. This study demonstrates that face-specific abilities may be hard to improve but also shows the plasticity of the speed of processing complex visual stimuli – for the first time in middle-aged, normal adults.

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

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

High-resolution imaging of expertise reveals reliable object selectivity in the fusiform face area related to perceptual performance

The fusiform face area (FFA) is a region of human cortex that responds selectively to faces, but whether it supports a more general function relevant for perceptual expertise is debated. Although both faces and objects of expertise engage many brain areas, the FFA remains the focus of the strongest modular claims and the clearest predictions about expertise. Functional MRI studies at standard-resolution (SR-fMRI) have found responses in the FFA for nonface objects of expertise, but high-resolution fMRI (HR-fMRI) in the FFA [Grill-Spector K, et al. (2006) Nat Neurosci 9:1177-1185] and neurophysiology in face patches in the monkey brain [Tsao DY, et al. (2006) Science 311:670-674] reveal no reliable selectivity for objects. It is thus possible that FFA responses to objects with SR-fMRI are a result of spatial blurring of responses from nonface-selective areas, potentially driven by attention to objects of expertise. Using HR-fMRI in two experiments, we provide evidence of reliable responses to cars in the FFA that correlate with behavioral car expertise. Effects of expertise in the FFA for nonface objects cannot be attributed to spatial blurring beyond the scale at which modular claims have been made, and within the lateral fusiform gyrus, they are restricted to a small area (200 mm(2) on the right and 50 mm(2) on the left) centered on the peak of face selectivity. Experience with a category may be sufficient to explain the spatially clustered face selectivity observed in this region.

Does acquisition of Greeble expertise in prosopagnosia rule out a domain-general deficit?

According to the expertise account of face specialization, a deficit that affects general expertise mechanisms should similarly impair the expert individuation of both faces and other visually homogeneous object classes. To test this possibility, we attempted to train a prosopagnosic patient, LR, to become a Greeble expert using the standard Greeble expertise-training paradigm (Gauthier & Tarr, 2002). Previous research demonstrated that LR's prosopagnosia was related to an inability to simultaneously use multiple features in a speeded face recognition task (Bukach, Bub, Gauthier, & Tarr, 2006). We hypothesized that LR's inability to use multiple face features would manifest in his acquisition of Greeble expertise, even though his basic object recognition is unimpaired according to standard neuropsychological testing. Although LR was eventually able to reach expertise criterion, he took many more training sessions than controls, suggesting use of an abnormal strategy. To further explore LR's Greeble processing strategies, we assessed his ability to use multiple Greeble features both before and after Greeble training. LR's performance in two versions of this task demonstrates that, even after training, he relies heavily on a single feature to identify Greebles. This correspondence between LR's face recognition and post-training Greeble recognition supports the idea that impaired face recognition is simply the most visible symptom of a more general object recognition impairment in acquired prosopagnosia.

The functional neuroanatomy of object agnosia: a case study

Cortical reorganization of visual and object representations following neural injury was examined using fMRI and behavioral investigations. We probed the visual responsivity of the ventral visual cortex of an agnosic patient who was impaired at object recognition following a lesion to the right lateral fusiform gyrus. In both hemispheres, retinotopic mapping revealed typical topographic organization and visual activation of early visual cortex. However, visual responses, object-related, and -selective responses were reduced in regions immediately surrounding the lesion in the right hemisphere, and also, surprisingly, in corresponding locations in the structurally intact left hemisphere. In contrast, hV4 of the right hemisphere showed expanded response properties. These findings indicate that the right lateral fusiform gyrus is critically involved in object recognition and that an impairment to this region has widespread consequences for remote parts of cortex. Finally, functional neural plasticity is possible even when a cortical lesion is sustained in adulthood.

Irrelevant objects of expertise compete with faces during visual search

Prior work suggests that nonface objects of expertise can interfere with the perception of faces when the two categories are alternately presented, suggesting competition for shared perceptual resources. Here, we ask whether task-irrelevant distractors from a category of expertise compete when faces are presented in a standard visual search task. Participants searched for a target (face or sofa) in an array containing both relevant and irrelevant distractors. The number of distractors from the target category (face or sofa) remained constant, whereas the number of distractors from the irrelevant category (cars) varied. Search slopes, calculated as a function of the number of irrelevant cars, were correlated with car expertise. The effect was not due to car distractors grabbing attention, because they did not compete with sofa targets. Objects of expertise interfere with face perception even when they are task irrelevant, visually distinct, and separated in space from faces.

Probing the face-space of individuals with prosopagnosia

A useful framework for understanding the mental representation of facial identity is face-space (Valentine, 1991), a multi-dimensional cognitive map in which individual faces are coded relative to the average of previously encountered faces, and in which the distance among faces represents their perceived similarity. We examined whether individuals with prosopagnosia, a disorder characterized by an inability to recognize familiar faces despite normal visual acuity and intellectual abilities, evince behavior consistent with this underlying representational schema. To do so, we compared the performance of 6 individuals with congenital prosopagnosia (CP), with a group of age- and gender-matched control participants in a series of experiments involving judgments of facial identity. We used digital images of male and female faces and morphed them to varying degrees relative to an average face, to create caricatures, anti-caricatures, and anti-faces (i.e. faces of the opposite identity). Across 5 behavioral tasks, CP individuals' performance was similar to that of the control group and consistent with the face-space framework. As a test of the sensitivity of our measures in revealing face processing abnormalities, we also tested a single acquired prosopagnosic (AP) individual, whose performance on the same tasks deviated significantly from the control and CP groups. The findings suggest that, despite an inability to recognize individual identities, CPs perceive faces in a manner consistent with norm-based coding of facial identity, although their representation is likely supported by a feature-based strategy. We suggest that the apparently normal posterior cortical regions, including the fusiform face area, serve as the neural substrate for at least a coarse, feature-based face-space map in CP and that their face recognition impairment arises from the disconnection between these regions and more anterior cortical sites.

Top-down engagement modulates the neural expressions of visual expertise

Perceptual expertise is traditionally associated with enhanced brain activity in response to objects of expertise in category-selective visual cortex, primarily face-selective regions. We reevaluated this view by investigating whether the brain activity associated with expertise in object recognition is limited to category-selective cortex and specifically whether the extent of expertise-related activity manifests automatically or whether it can be top-down modulated. We conducted 2 functional magnetic resonance imaging studies comparing changes in hemodynamic activity associated with car expertise in a conventional 1-back task (Experiment 1) and when the task relevance of cars was explicitly manipulated (Experiment 2). Whole-brain analysis unveiled extensive expertise-related activity throughout the visual cortex, starting as early as V1 and extending into nonvisual areas. However, when the cars were task irrelevant, the expertise-related activity drastically diminished, indeed, becoming similar to the activity elicited by cars in novices. We suggest that expertise entails voluntary top-down engagement of multiple neural networks in addition to stimulus-driven activation associated with perceptual mechanisms.

Impaired holistic processing of unfamiliar individual faces in acquired prosopagnosia

Prosopagnosia is an impairment at individualizing faces that classically follows brain damage. Several studies have reported observations supporting an impairment of holistic/configural face processing in acquired prosopagnosia. However, this issue may require more compelling evidence as the cases reported were generally patients suffering from integrative visual agnosia, and the sensitivity of the paradigms used to measure holistic/configural face processing in normal individuals remains unclear. Here we tested a well-characterized case of acquired prosopagnosia (PS) with no object recognition impairment, in five behavioral experiments (whole/part and composite face paradigms with unfamiliar faces). In all experiments, for normal observers we found that processing of a given facial feature was affected by the location and identity of the other features in a whole face configuration. In contrast, the patient's results over these experiments indicate that she encodes local facial information independently of the other features embedded in the whole facial context. These observations and a survey of the literature indicate that abnormal holistic processing of the individual face may be a characteristic hallmark of prosopagnosia following brain damage, perhaps with various degrees of severity.

I can't recognize your face but I can recognize its movement

Idiosyncratic facial movements can provide a route to facial identity (review in Roark, Barrett, Spence, Abdi, & O'Toole, 2003). However, it is unclear whether recognizing a face in this way involves the same cognitive or neural mechanisms that are involved in recognizing a static face. Three studies on a developmental prosopagnosic (C.S.) showed that although he is impaired at recognizing static faces, he can discriminate between dynamic identities (Experiments 1a and 1b) and can learn to name individuals on the basis of their idiosyncratic facial movements (Experiment 2), at levels that are comparable to those of matched and undergraduate control groups. These results suggest a possible cognitive dissociation between mechanisms involved in dynamic compared to static face recognition. However, future work is needed to fully understand this dissociation.

Becoming a face expert: a computerized face-training program for high-functioning individuals with autism spectrum disorders

Typically developing individuals process faces using strategies that differ from those used for processing objects, and which tend to be holistic and based on configural information. Behavioral and neuroimaging studies suggest that individuals with autism may not utilize the same specialized strategies for face processing. The present study was designed to investigate whether computerized face-specific training, based on a modified version of Gauthier and Tarr's (1997) expertise protocol, can influence the face processing strategies and abilities of adolescents and young adults with Autism Spectrum Disorder (ASD). Ten individuals with ASD were assigned to either a training protocol designed to improve face processing (N = 5) or a control condition (N = 5). Outcomes assessed holistic processing and configural sensitivity. All trained individuals achieved a behavioral criterion of developing expertise in face recognition established in the literature. Outcome assessments indicated that the trained group showed significantly greater sensitivity to second-order configural relations compared with untrained controls, but did not differ on the measure of holistic processing. These findings suggest that face processing ability and strategies in autism can be significantly improved through training.

Are faces special? A case of pure prosopagnosia

The ability to recognize individual faces is of crucial social importance for humans and evolutionarily necessary for survival. Consequently, faces may be "special" stimuli, for which we have developed unique modular perceptual and recognition processes. Some of the strongest evidence for face processing being modular comes from cases of prosopagnosia, where patients are unable to recognize faces whilst retaining the ability to recognize other objects. Here we present the case of an acquired prosopagnosic whose poor recognition was linked to a perceptual impairment in face processing. Despite this, she had intact object recognition, even at a subordinate level. She also showed a normal ability to learn and to generalize learning of nonfacial exemplars differing in the nature and arrangement of their parts, along with impaired learning and generalization of facial exemplars. The case provides evidence for modular perceptual processes for faces.

Impairments in part-whole representations of objects in two cases of integrative visual agnosia

How complex multipart visual objects are represented perceptually remains a subject of ongoing investigation. One source of evidence that has been used to shed light on this issue comes from the study of individuals who fail to integrate disparate parts of visual objects. This study reports a series of experiments that examine the ability of two such patients with this form of agnosia (integrative agnosia; IA), S.M. and C.R., to discriminate and categorize exemplars of a rich set of novel objects, "Fribbles", whose visual similarity (number of shared parts) and category membership (shared overall shape) can be manipulated. Both patients performed increasingly poorly as the number of parts required for differentiating one Fribble from another increased. Both patients were also impaired at determining when two Fribbles belonged in the same category, a process that relies on abstracting spatial relations between parts. C.R., the less impaired of the two, but not S.M., eventually learned to categorize the Fribbles but required substantially more training than normal perceivers. S.M.'s failure is not attributable to a problem in learning to use a label for identification nor is it obviously attributable to a visual memory deficit. Rather, the findings indicate that, although the patients may be able to represent a small number of parts independently, in order to represent multipart images, the parts need to be integrated or chunked into a coherent whole. It is this integrative process that is impaired in IA and appears to play a critical role in the normal object recognition of complex images.

Functional Plasticity in Ventral Temporal Cortex following Cognitive Rehabilitation of a Congenital Prosopagnosic

We used functional magnetic resonance imaging (fMRI) and electroencephalography (EEG) to measure neural changes associated with training configural processing in congenital prosopagnosia, a condition in which face identification abilities are not properly developed in the absence of brain injury or visual problems. We designed a task that required discriminating faces by their spatial configuration and, after extensive training, prosopagnosic MZ significantly improved at face identification. Event-related potential results revealed that although the N170 was not selective for faces before training, its selectivity after training was normal. fMRI demonstrated increased functional connectivity between ventral occipital temporal face-selective regions (right occipital face area and right fusiform face area) that accompanied improvement in face recognition. Several other regions showed fMRI activity changes with training; the majority of these regions increased connectivity with face-selective regions. Together, the neural mechanisms associated with face recognition improvements involved strengthening early face-selective mechanisms and increased coordination between face-selective and nonselective regions, particularly in the right hemisphere.

No face-like processing for objects-of-expertise in three behavioural tasks

In the debate between expertise and domain-specific explanations of "special" processing for faces, a common belief is that behavioural studies support the expertise hypothesis. The present article refutes this view, via a combination of new data and review. We tested dog experts with confirmed good individuation of exemplars of their breed-of-expertise. In all experiments, standard results were confirmed for faces. However, dog experts showed no face-like processing for dogs on three behavioural tasks (inversion; the composite paradigm; and sensitivity to contrast reversal). The lack of holistic/configural processing, indicated in the first two of these tests, is shown by review to be consistent rather than inconsistent with previous studies of objects-of-expertise.

Patterns of Normal Human Brain Plasticity After Practice and Their Implications for Neurorehabilitation

OBJECTIVES

To illustrate how our knowledge about normal patterns of experience-induced plasticity can provide insights into the mechanisms of neurorehabilitation; to provide an overview of the practice-effects literature in order to simplify and amalgamate a large number of heterogeneous findings and identify typical patterns of practice effects and their determining factors; and to concentrate on the impact of practice on higher cognitive functions, such as working memory, and present some preliminary but promising behavioral data that show how practice on a complex cognitive task can benefit cognitive functioning more generally.

DATA SOURCES

We performed a systematic search for peer-reviewed journal articles using computerized databases (PubMed, ISI Web of Science, PsycINFO).

DATA SELECTION

Neuroimaging studies using functional magnetic resonance imaging (fMRI) or positron-emission tomography (PET) to examine functional activation changes as a result of practice on sensory, motor, or cognitive tasks in normal (healthy) populations were included in the review. Further studies were identified that examined the effects of rehabilitative training on functional activations in clinical populations using fMRI or PET.

DATA EXTRACTION

Important characteristics of the selected studies were summarized in a systematic manner so to enable the extraction of specific factors impacting on the pattern of practice effects observed.

DATA SYNTHESIS

We identified a number of factors that impact on the patterns of practice effects observed and discuss how the insights gained from the study of healthy populations can by applied to rehabilitation of cognitive deficits in clinical populations.

CONCLUSIONS

Progress in our understanding of neurorehabilitative plasticity will be enabled by neuroimaging examinations of cognitive rehabilitation training grounded in a knowledge of normal (healthy) patterns of brain activation and practice-induced plasticity.

Acquiring long-term representations of visual classes following extensive extrastriate damage

Different areas of human visual cortex are thought to play different roles in the learning of visual information: whereas in low/intermediate cortical areas, plasticity may be manifested by enhanced selectivity to learned visual features, in higher-level areas, plasticity may result in generalization and development of tolerance to degraded versions of the learned stimuli. The most effective tolerance to degraded information is presumably achieved in the case of cooperation between the different forms of plasticity. Whether this tolerance to degraded information also applies when the visual input is degraded as a result of a lesion to lower levels of the visual system remains an open question. To address this, we studied visual classification learning in a patient with an extensive bilateral lesion affecting intermediate/low-level visual areas but sparing higher-level areas. Despite difficulty in perceiving the stimuli, the patient learned to classify them, albeit not as quickly as control participants. Moreover, the patient's learning was maintained over the long term and was accompanied by improved discrimination of individual stimuli. These findings demonstrate that degraded output from lesioned, lower areas can be exploited in the service of a new visual task and the results likely implicate a combination of bottom-up and top-down processing during visual learning.

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.