Scale invariant adaptation in fusiform face-responsive regions

Top-down activation of fusiform cortex without seeing faces in prosopagnosia

Face processing can be modified by bottom-up and top-down influences, but it is unknown how these processes interact in patients with face-recognition impairments (prosopagnosia). We investigated a prosopagnosic with lesions in right occipital and left fusiform cortex but whose right fusiform gyrus is intact and still activated during face-processing tasks. P.S., a patient with a well-established and selective agnosia for faces, was instructed to detect the presence of either faces or houses in pictures with different amounts of noise. The right fusiform face area (FFA) showed reduced responses to face information when visual images were degraded with noise. However, her right FFA still activated to noise-only images when she was instructed to detect faces. These results reveal that fusiform activation is still selectively modulated by task demands related to the anticipation of a face, despite severe face-recognition deficits and the fact that no reliable stimulus-driven response is evoked by actual facial information. Healthy controls showed stimulus-driven responses to faces in fusiform, and in right but not left occipital cortex, suggesting that the latter area alone might provide insufficient facial information in P.S. These results provide a novel account for residual activation of the FFA and underscore the importance of controlling task demands during functional magnetic resonance imaging.

How spatial frequencies and visual awareness interact during face processing

In vision, high and low spatial frequencies have been dissociated at the cognitive and neural levels. Usually, high spatial frequency (HSF) is associated with slow analysis along the ventral cortical stream, and low spatial frequency (LSF) is associated with fast and automatic processing. These findings suggest a specific relation between spatial-frequency processing and visual awareness. We investigated this issue using masked-face priming with hybrid prime images of variable visibility. We found subliminal priming for both LSF and HSF information, along with a strong interaction between spatial frequency and visibility: HSF-related priming increased with stimulus visibility, whereas LSF influences remained unchanged. We argue that the results limit the validity of the coarse-to-fine model of vision and of models equating ventral-stream activity with perceptual awareness. Interpreting our results in light of the diagnostic approach suggests a close relation between awareness and diagnosticity.

Adaptation in the fusiform face area (FFA): image or person?

Viewing a sequence of faces of two different people results in a greater Blood Oxygenation Level Dependent (BOLD) response in FFA compared to a sequence of identical faces. Changes in identity, however, necessarily involve changes in the image. Is the release from adaptation a result of a change in face identity, per se, or could it be an effect that would arise from any change in the image of a face? Subjects viewed a sequence of two faces that could be of the same or different person, and in the same or different orientation in depth. Critically, the physical similarity of view changes of the same person was scaled, by Gabor-jet differences, to be equivalent to that produced by an identity change. Both person and orientation changes produced equivalent releases from adaptation in FFA (relative to identical faces) suggesting that FFA is sensitive to the physical similarity of faces rather than to the individuals depicted in the images.

Defining the face processing network: optimization of the functional localizer in fMRI

Functional localizers that contrast brain signal when viewing faces versus objects are commonly used in functional magnetic resonance imaging studies of face processing. However, current protocols do not reliably show all regions of the core system for face processing in all subjects when conservative statistical thresholds are used, which is problematic in the study of single subjects. Furthermore, arbitrary variations in the applied thresholds are associated with inconsistent estimates of the size of face-selective regions-of-interest (ROIs). We hypothesized that the use of more natural dynamic facial images in localizers might increase the likelihood of identifying face-selective ROIs in individual subjects, and we also investigated the use of a method to derive the statistically optimal ROI cluster size independent of thresholds. We found that dynamic facial stimuli were more effective than static stimuli, identifying 98% (versus 72% for static) of ROIs in the core face processing system and 69% (versus 39% for static) of ROIs in the extended face processing system. We then determined for each core face processing ROI, the cluster size associated with maximum statistical face-selectivity, which on average was approximately 50 mm(3) for the fusiform face area, the occipital face area, and the posterior superior temporal sulcus. We suggest that the combination of (a) more robust face-related activity induced by a dynamic face localizer and (b) a cluster-size determination based on maximum face-selectivity increases both the sensitivity and the specificity of the characterization of face-related ROIs in individual subjects.

Functional MRI reveals compromised neural integrity of the face processing network in congenital prosopagnosia

The summed activity of multiple nodes of a distributed cortical network supports face recognition in humans, including "core" ventral occipitotemporal cortex (VOTC) regions, and "extended" regions outside VOTC. Many individuals with congenital prosopagnosia-an impairment in face processing-exhibit normal blood oxygenation level-dependent (BOLD) activation in the core VOTC regions. These individuals evince a reduction in the structural integrity of the white matter tracts connecting VOTC to anterior temporal and frontal cortices, part of the "extended" face network. The impairment in congenital prosopagnosia may arise, therefore, not from a dysfunction of the core VOTC areas but from a failure to propagate signals between the intact VOTC and the extended nodes of the network. Using the fMR adaptation paradigm with famous and unknown faces, we show that individuals with congenital prosopagnosia evince normal adaptation effects in VOTC, indicating sensitivity to facial identity, but show no differential activation for familiar versus unknown faces outside VOTC, particularly in the precuneus/posterior cingulate cortex and the anterior paracingulate cortex. Normal BOLD activation in VOTC is thus insufficient to subserve intact face recognition, and disrupted information propagation between VOTC and the extended face processing network may explain the functional impairment in congenital prosopagnosia.

An image-dependent representation of familiar and unfamiliar faces in the human ventral stream

People are extremely proficient at recognizing faces that are familiar to them, but are much worse at matching unfamiliar faces. We used fMR-adaptation to ask whether this difference in recognition might be reflected by an image-invariant representation for familiar faces in face-selective regions of the human ventral visual processing stream. Consistent with models of face processing, we found adaptation to repeated images of the same face image in the fusiform face area (FFA), but not in the superior-temporal face region (STS). To establish if the neural representation in the FFA was invariant to changes in view, we presented different images of the same face. Contrary to our hypothesis, we found that the response in the FFA to different images of the same person was the same as the response to images of different people. A group analysis showed a distributed pattern of adaptation to the same image of a face, which extended beyond the face-selective areas, including other regions of the ventral visual stream. However, this analysis failed to reveal any regions showing significant image-invariant adaptation. These results suggest that information about faces is represented in a distributed network using an image-dependent neural code.

Functional magnetic resonance adaptation in visual neuroscience

Functional magnetic resonance imaging (fMRI) is a powerful non-invasive tool to investigate neuronal processing. In the last ten years a new methodological approach in the field of fMRI has been developed: fMRI adaptation. It has been found that the repetition of a stimulus leads to a decrease of the fMRI signal in the brain region that processes this stimulus. The phenomenon has been related to neuronal adaptation effects found in single-cell recordings. Since the first experiments that observed fMRI-adaptation effects, the method has been applied extensively to study various visual phenomena, such as the perception of motion, shape, objects, and orientation. The great advantage of fMRI adaptation is that it allows assessing the functional response profile of a brain region at a subvoxel level. The purpose of the current review is to evaluate the different experimental approaches used to elicit fMRI-adaptation effects. We discuss papers published in the domain of visual neuroscience that made use of fMRI-adaptation paradigms. In doing so, we focus on methodological considerations concerning experimental design, stimulus presentation and influencing factors such as awareness and attention. In the course of this review, we show that different fMRI-adaptation designs capture heterogeneous neuronal adaptation effects. As the picture of the mechanisms underlying neuronal adaptation changes from simple synaptic fatigue to complex network interactions, the concept of fMRI adaptation has to be redefined.

Mechanisms of face perception

Faces are among the most informative stimuli we ever perceive: Even a split-second glimpse of a person's face tells us his identity, sex, mood, age, race, and direction of attention. The specialness of face processing is acknowledged in the artificial vision community, where contests for face-recognition algorithms abound. Neurological evidence strongly implicates a dedicated machinery for face processing in the human brain to explain the double dissociability of face- and object-recognition deficits. Furthermore, recent evidence shows that macaques too have specialized neural machinery for processing faces. Here we propose a unifying hypothesis, deduced from computational, neurological, fMRI, and single-unit experiments: that what makes face processing special is that it is gated by an obligatory detection process. We clarify this idea in concrete algorithmic terms and show how it can explain a variety of phenomena associated with face processing.

A tale of two recognition systems: implications of the fusiform face area and the visual word form area for lateralized object recognition models

Two areas of current intense interest in the neuroimaging literature are that of the visual word form area (VWFA) and of the fusiform face area (FFA) and their roles in word and face perception, respectively. These two areas are of particular relevance to laterality research because visual word identification and face identification have long been shown to be especially lateralized to the left hemisphere and the right hemisphere, respectively. This review therefore seeks to evaluate their significance for the broader understanding of lateralization of object recognition. A multi-level model of lateralized object recognition is proposed based on a combination of behavioral and neuroimaging findings. Rather than seek to characterize hemispheric asymmetries according to a single principle (e.g., serial-parallel), it is suggested that current observations can be understood in terms of three asymmetric levels of processing, using the framework of the Janus model of hemispheric function. It is suggested that the left hemisphere represents features using an abstract-category code whereas the RH utilizes a specific-exemplar code. The relationships between these features are also coded asymmetrically, with the LH relying on associative co-occurrence values and the RH relying on spatial metrics. Finally, the LH controlled selection system focuses on isolating features and the RH focuses on conjoining features. It is suggested that each hemisphere utilizes efficient (apparently parallel) processing when stimuli are congruent with its preferred processing style and inefficient (apparently serial) processing when they are not, resulting in the typical left-lateralization for orthographic analysis and right-lateralization for face analysis.

Simulation of talking faces in the human brain improves auditory speech recognition

Human face-to-face communication is essentially audiovisual. Typically, people talk to us face-to-face, providing concurrent auditory and visual input. Understanding someone is easier when there is visual input, because visual cues like mouth and tongue movements provide complementary information about speech content. Here, we hypothesized that, even in the absence of visual input, the brain optimizes both auditory-only speech and speaker recognition by harvesting speaker-specific predictions and constraints from distinct visual face-processing areas. To test this hypothesis, we performed behavioral and neuroimaging experiments in two groups: subjects with a face recognition deficit (prosopagnosia) and matched controls. The results show that observing a specific person talking for 2 min improves subsequent auditory-only speech and speaker recognition for this person. In both prosopagnosics and controls, behavioral improvement in auditory-only speech recognition was based on an area typically involved in face-movement processing. Improvement in speaker recognition was only present in controls and was based on an area involved in face-identity processing. These findings challenge current unisensory models of speech processing, because they show that, in auditory-only speech, the brain exploits previously encoded audiovisual correlations to optimize communication. We suggest that this optimization is based on speaker-specific audiovisual internal models, which are used to simulate a talking face.

The engagement of mid-ventrolateral prefrontal cortex and posterior brain regions in intentional cognitive activity

It is now widely recognized that cognitive processes are carried out by a distributed network of brain areas, some of which are involved in perceptual processing of a stimulus, whilst others are involved in cognitive control processes required to carry out certain tasks. In this study, differential contributions of higher visual areas and of an area involved in cognitive control processes were investigated in a task requiring participants to simply look at a stimulus or to look with the intention of remembering. Varying the extent to which intentional cognitive processes were required and the stimulus material in this task allowed the analysis of "top-down" and "bottom-up" influences on these areas, respectively. Significant increases in the mid-ventrolateral prefrontal cortex (mid-VLPFC) were only observed when the stimuli were viewed with an intention in mind, irrespective of the stimulus type. In contrast, activity in the parahippocampal place area and the fusiform face area, was only modulated in conditions requiring intentional control when stimuli were presented that also elicited activity in these regions during passive viewing. These findings help to clarify the complimentary role that the mid-VLPFC and posterior higher visual areas play in controlled and relatively automatic memory processing.

Interpreting fMRI data: maps, modules and dimensions

Neuroimaging research over the past decade has revealed a detailed picture of the functional organization of the human brain. Here we focus on two fundamental questions that are raised by the detailed mapping of sensory and cognitive functions and illustrate these questions with findings from the object-vision pathway. First, are functionally specific regions that are located close together best understood as distinct cortical modules or as parts of a larger-scale cortical map? Second, what functional properties define each cortical map or module? We propose a model in which overlapping continuous maps of simple features give rise to discrete modules that are selective for complex stimuli.

Role of features and second-order spatial relations in face discrimination, face recognition, and individual face skills: behavioral and functional magnetic resonance imaging data

We compared the contribution of featural information and second-order spatial relations (spacing between features) in face processing. A fully factorial design has the same or different "features" (eyes, mouth, and nose) across two successive displays, whereas, orthogonally, the second-order spatial relations between those features were the same or different. The range of such changes matched the possibilities within the population of natural face images. Behaviorally, we found that judging whether two successive faces depicted the same person was dominated by features, although second-order spatial relations also contributed. This influence of spatial relations correlated, for individual subjects, with their skill at recognition of faces (as famous, or as previously exposed) in separate behavioral tests. Using the same repetition design in functional magnetic resonance imaging, we found feature-dependent effects in the lateral occipital and right fusiform regions. In addition, there were spatial relation effects in the bilateral inferior occipital gyrus and right fusiform that correlated with individual differences in (separately measured) behavioral sensitivity to those changes. The results suggest that featural and second-order spatial relation aspects of faces make distinct contributions to behavioral discrimination and recognition, with features contributing most to face discrimination and second-order spatial relational aspects correlating best with recognition skills. Distinct neural responses to these aspects were found with functional magnetic resonance imaging, particularly when individual skills were taken into account for the impact of second-order spatial relations.

Neuronal response to texture- and contrast-defined boundaries in early visual cortex

Natural scenes contain a variety of visual cues that facilitate boundary perception (e.g., luminance, contrast, and texture). Here we explore whether single neurons in early visual cortex can process both contrast and texture cues. We recorded neural responses in cat A18 to both illusory contours formed by abutting gratings (ICs, texture-defined) and contrast-modulated gratings (CMs, contrast-defined). We found that if a neuron responded to one of the two stimuli, it also responded to the other. These neurons signaled similar contour orientation, spatial frequency, and movement direction of the two stimuli. A given neuron also exhibited similar selectivity for spatial frequency of the fine, stationary grating components (carriers) of the stimuli. These results suggest that the cue-invariance of early cortical neurons extends to different kinds of texture or contrast cues, and might arise from a common nonlinear mechanism.

TMS Evidence for the Involvement of the Right Occipital Face Area in Early Face Processing

Extensive research has demonstrated that several specialized cortical regions respond preferentially to faces. One such region, located in the inferior occipital gyrus, has been dubbed the occipital face area (OFA). The OFA is the first stage in two influential face-processing models, both of which suggest that it constructs an initial representation of a face, but how and when it does so remains unclear. The present study revealed that repetitive transcranial magnetic stimulation (rTMS) targeted at the right OFA (rOFA) disrupted accurate discrimination of face parts but had no effect on the discrimination of spacing between these parts. rTMS to left OFA had no effect. A matched part and spacing discrimination task that used house stimuli showed no impairment. In a second experiment, rTMS to rOFA replicated the face-part impairment but did not produce the same effect in an adjacent area, the lateral occipital cortex. A third experiment delivered double pulses of TMS separated by 40 ms at six periods after stimulus presentation during face-part discrimination. Accuracy dropped when pulses were delivered at 60 and 100 ms only. These findings indicate that the rOFA processes face-part information at an early stage in the face-processing stream.

Event-related potential correlates of repetition priming for ignored faces

An attentional capacity limit was recently suggested for faces, such that only one face can be processed at a time. We measured interference and repetition priming caused by irrelevant distractor faces. Participants initially performed male/female judgments for central faces or symbols flanked by distractor faces. Interference (slower responses for sex-incongruent target-distractor pairs) occurred for central symbols but was absent for central faces. In subsequent fame judgements, previously presented distractor faces had no repetition priming effect on response times. Relative to new faces, event-related brain potentials revealed a right occipitotemporal negativity approximately 400-600 ms for faces previously shown as distractors flanking central symbols (but not distractors flanking faces). These findings support a face-specific attentional capacity limit, showing that event-related brain potential priming effects can reveal covert distractor processing.

A preference for contralateral stimuli in human object- and face-selective cortex

Visual input from the left and right visual fields is processed predominantly in the contralateral hemisphere. Here we investigated whether this preference for contralateral over ipsilateral stimuli is also found in high-level visual areas that are important for the recognition of objects and faces. Human subjects were scanned with functional magnetic resonance imaging (fMRI) while they viewed and attended faces, objects, scenes, and scrambled images in the left or right visual field. With our stimulation protocol, primary visual cortex responded only to contralateral stimuli. The contralateral preference was smaller in object- and face-selective regions, and it was smallest in the fusiform gyrus. Nevertheless, each region showed a significant preference for contralateral stimuli. These results indicate that sensitivity to stimulus position is present even in high-level ventral visual cortex.

Distinct and convergent visual processing of high and low spatial frequency information in faces

We tested for differential brain response to distinct spatial frequency (SF) components in faces. During a functional magnetic resonance imaging experiment, participants were presented with "hybrid" faces containing superimposed low and high SF information from different identities. We used a repetition paradigm where faces at either SF range were independently repeated or changed across consecutive trials. In addition, we manipulated which SF band was attended. Our results suggest that repetition and attention affected partly overlapping occipitotemporal regions but did not interact. Changes of high SF faces increased responses of the right inferior occipital gyrus (IOG) and left inferior temporal gyrus (ITG), with the latter response being also modulated additively by attention. In contrast, the bilateral middle occipital gyrus (MOG) responded to repetition and attention manipulations of low SF. A common effect of high and low SF repetition was observed in the right fusiform gyrus (FFG). Follow-up connectivity analyses suggested direct influence of the MOG (low SF), IOG, and ITG (high SF) on the FFG responses. Our results reveal that different regions within occipitotemporal cortex extract distinct visual cues at different SF ranges in faces and that the outputs from these separate processes project forward to the right FFG, where the different visual cues may converge.

The fusiform face area: a cortical region specialized for the perception of faces

Faces are among the most important visual stimuli we perceive, informing us not only about a person's identity, but also about their mood, sex, age and direction of gaze. The ability to extract this information within a fraction of a second of viewing a face is important for normal social interactions and has probably played a critical role in the survival of our primate ancestors. Considerable evidence from behavioural, neuropsychological and neurophysiological investigations supports the hypothesis that humans have specialized cognitive and neural mechanisms dedicated to the perception of faces (the face-specificity hypothesis). Here, we review the literature on a region of the human brain that appears to play a key role in face perception, known as the fusiform face area (FFA). Section 1 outlines the theoretical background for much of this work. The face-specificity hypothesis falls squarely on one side of a longstanding debate in the fields of cognitive science and cognitive neuroscience concerning the extent to which the mind/brain is composed of: (i) special-purpose ('domain-specific') mechanisms, each dedicated to processing a specific kind of information (e.g. faces, according to the face-specificity hypothesis), versus (ii) general-purpose ('domain-general') mechanisms, each capable of operating on any kind of information. Face perception has long served both as one of the prime candidates of a domain-specific process and as a key target for attack by proponents of domain-general theories of brain and mind. Section 2 briefly reviews the prior literature on face perception from behaviour and neurophysiology. This work supports the face-specificity hypothesis and argues against its domain-general alternatives (the individuation hypothesis, the expertise hypothesis and others). Section 3 outlines the more recent evidence on this debate from brain imaging, focusing particularly on the FFA. We review the evidence that the FFA is selectively engaged in face perception, by addressing (and rebutting) five of the most widely discussed alternatives to this hypothesis. In section 4, we consider recent findings that are beginning to provide clues into the computations conducted in the FFA and the nature of the representations the FFA extracts from faces. We argue that the FFA is engaged both in detecting faces and in extracting the necessary perceptual information to recognize them, and that the properties of the FFA mirror previously identified behavioural signatures of face-specific processing (e.g. the face-inversion effect). Section 5 asks how the computations and representations in the FFA differ from those occurring in other nearby regions of cortex that respond strongly to faces and objects. The evidence indicates clear functional dissociations between these regions, demonstrating that the FFA shows not only functional specificity but also area specificity. We end by speculating in section 6 on some of the broader questions raised by current research on the FFA, including the developmental origins of this region and the question of whether faces are unique versus whether similarly specialized mechanisms also exist for other domains of high-level perception and cognition.

Channel surfing in the visual brain

Vision provides us with an ever-changing neural representation of the world from which we must extract stable object categorizations. We argue that visual analysis involves a fundamental interaction between the observer's top-down categorization goals and the incoming stimulation. Specifically, we discuss the information available for categorization from an analysis of different spatial scales by a bank of flexible, interacting spatial-frequency (SF) channels. We contend that the activity of these channels is not determined simply bottom-up by the stimulus. Instead, we argue that, following perceptual learning a specification of the diagnostic, object-based, SF information dynamically influences the top-down processing of retina-based SF information by these channels. Our analysis of SF processing provides a case study that emphasizes the continuity between higher-level cognition and lower-level perception.

The neural bases of prosopagnosia and pure alexia: recent insights from functional neuroimaging

PURPOSE OF REVIEW

To discuss whether recent functional neuroimaging results can account for clinical phenomenology in visual associative agnosias.

RECENT FINDINGS

Functional neuroimaging studies in healthy human subjects have identified only two regions of ventral occipitotemporal cortex that invariantly respond to individual faces and visual words, respectively. The signature of face identity coding in the fusiform neural response was shown to be missing in a patient with prosopagnosia. Another case study established that a surgical lesion close to the region sensitive to visual words can result in pure alexia.

SUMMARY

Evidence is increasing that functional specialization for processing face identity and visual word forms is restricted to two specialized sensory modules in the occipitotemporal cortex. A structural or functional lesion to face-sensitive and word-sensitive regions in the ventral occipitotemporal cortex can provide the most parsimonious account for the clinical syndromes of prosopagnosia and agnosic alexia. This review suggests that functional specialization should be considered in terms of whether exclusively one brain region (instead of many) underpins a defined function and not as whether this brain region underpins exclusively one cognitive function. Such functional specialization seems to exist for at least two higher-order visual perceptual functions, face and word identification.

Interhemispheric integration at different spatial scales: the evidence from EEG coherence and FMRI

The early visual system processes different spatial frequencies (SFs) separately. To examine where in the brain the scale-specific information is integrated, we mapped the neural assemblies engaged in interhemispheric coupling with electroencephalographic (EEG) coherence and blood-oxygen-level dependent (BOLD) signal. During similar EEG and functional magnetic resonance imaging (fMRI) experiments, our subjects viewed centrally presented bilateral gratings of different SF (0.25-8.0 cpd), which either obeyed Gestalt grouping rules (iso-oriented, IG) or violated them (orthogonally oriented, OG). The IG stimuli (0.5-4.0 cpd) synchronized EEG at discrete beta frequencies (beta1, beta2) and increased BOLD (0.5 and 2.0 cpd tested) in ventral (around collateral sulcus) and dorsal (parieto-occipital fissure) regions compared with OG. At both SF, the beta1 coherence correlated with the ventral activations, whereas the beta2 coherence correlated with the dorsal ones. Thus distributed neural substrates mediated interhemispheric integration at single SF. The relative impact of the ventral versus dorsal networks was modulated by the SF of the stimulus.

Faces are "spatial"--holistic face perception is supported by low spatial frequencies

Faces are perceived holistically, a phenomenon best illustrated when the processing of a face feature is affected by the other features. Here, the authors tested the hypothesis that the holistic perception of a face mainly relies on its low spatial frequencies. Holistic face perception was tested in two classical paradigms: the whole-part advantage (Experiment 1) and the composite face effect (Experiments 2-4). Holistic effects were equally large or larger for low-pass filtered faces as compared to full-spectrum faces and significantly larger than for high-pass filtered faces. The disproportionate composite effect found for low-pass filtered faces was not observed when holistic perception was disrupted by inversion (Experiment 3). Experiment 4 showed that the composite face effect was enhanced only for low spatial frequencies, but not for intermediate spatial frequencies known be critical for face recognition. These findings indicate that holistic face perception is largely supported by low spatial frequencies. They also suggest that holistic processing precedes the analysis of local features during face perception.

Individual differences in FFA activity suggest independent processing at different spatial scales

The brain processes images at different spatial scales, but it is unclear how far into the visual stream different scales remain segregated. Using functional magnetic resonance imaging, we found evidence that BOLD activity in the fusiform face area (FFA) reflects computations based on separate spatial frequency inputs. When subjects perform different tasks (attend location vs. identity; attend whole vs. parts) or the same task with different stimuli (upright or inverted) with high- and low-pass images of cars and faces, individual differences in the FFA in one condition are correlated with those in the other condition. However, FFA activity in response to low-pass stimuli is independent of its response to high-pass stimuli. These results suggest that spatial scales are not integrated before the FFA and that processing in this area could support the flexible use of different sources of information present in broad-pass images.

Voice Recognition and Cross-Modal Responses to Familiar Speakers' Voices in Prosopagnosia

Recognizing the voices of people we know does not only activate "voice areas" in the temporal lobe but also extraauditory areas including the fusiform "face area" (FFA). This cross-modal effect could reflect that individual face and voice information become specifically associated when becoming acquainted with a person. Here, we addressed whether the ability to have individual face representations 1) plays a role in voice recognition and 2) is required to observe cross-modal responses to voices in face areas. We compared speaker recognition performance and neuroimaging responses during the processing of familiar and nonfamiliar speakers' voices in a developmental prosopagnosic subject (SO) with the respective findings obtained in a group of 9 control subjects. Despite scoring worse than controls on recognition of familiar speakers' voices, SO had normal cross-modal responses in the FFA and normal connectivity between FFA and the voice regions. However, she had reduced activations in areas that usually respond to familiarity with people. An indication for the malfunctioning of her FFA was reduced connectivity of the FFA to a subset of these supramodal areas. In combination these data suggest that 1) voice recognition benefits from the ability to process faces at an individual level and 2) cross-modal association of voices and faces in the brain is achieved by a sensory binding and does not depend on a top-down mechanism subsequent to successful person recognition.

Selective attention modulates neural substrates of repetition priming and "implicit" visual memory: suppressions and enhancements revealed by FMRI

Attention can enhance processing for relevant information and suppress this for ignored stimuli. However, some residual processing may still arise without attention. Here we presented overlapping outline objects at study, with subjects attending to those in one color but not the other. Attended objects were subsequently recognized on a surprise memory test, whereas there was complete amnesia for ignored items on such direct explicit testing; yet reliable behavioral priming effects were found on indirect testing. Event-related fMRI examined neural responses to previously attended or ignored objects, now shown alone in the same or mirror-reversed orientation as before, intermixed with new items. Repetition-related decreases in fMRI responses for objects previously attended and repeated in the same orientation were found in the right posterior fusiform, lateral occipital, and left inferior frontal cortex. More anterior fusiform regions also showed some repetition decreases for ignored objects, irrespective of orientation. View-specific repetition decreases were found in the striate cortex, particularly for previously attended items. In addition, previously ignored objects produced some fMRI response increases in the bilateral lingual gyri, relative to new objects. Selective attention at exposure can thus produce several distinct long-term effects on processing of stimuli repeated later, with neural response suppression stronger for previously attended objects, and some response enhancement for previously ignored objects, with these effects arising in different brain areas. Although repetition decreases may relate to positive priming phenomena, the repetition increases for ignored objects shown here for the first time might relate to processes that can produce "negative priming" in some behavioral studies. These results reveal quantitative and qualitative differences between neural substrates of long-term repetition effects for attended versus unattended objects.

Recovery from adaptation to facial identity is larger for upright than inverted faces in the human occipito-temporal cortex

Human faces look more similar to each other when they are presented upside-down, leading to an increase of error rates and response times during individual face discrimination tasks. Here we used functional magnetic resonance imaging (fMRI) to test the hypothesis that this perceived similarity leads to a lower recovery from identity adaptation for inverted faces than for upright faces in face-sensitive areas of the occipito-temporal cortex. Ten subjects were presented with blocks of upright and inverted faces, with the same face identity repeated consecutively in half of the blocks, and different facial identities repeated in the other blocks. When face stimuli were presented upright, the percent signal change in the bilateral middle fusiform gyrus (MFG) was larger for different faces as compared to same faces, replicating previous observations of a recovery from facial identity adaptation in this region. However, there was no significant recovery from adaptation when different inverted faces were presented. Most interestingly, the difference in activation between upright and inverted faces increased progressively during a block when different facial identities were presented. A similar pattern of activation was found in the left middle fusiform gyrus, but was less clear-cut in bilateral face-sensitive areas of the inferior occipital cortex. These findings show that the differential level of activation to upright and inverted faces in the fusiform gyrus is mainly due to a difference in recovery from adaptation, and they explain the discrepancies in the results reported in previous fMRI studies which compared the processing of upright and inverted faces. The lack of recovery from adaptation for inverted faces in the fusiform gyrus may underlie the face inversion effect (FIE), which takes place during perceptual encoding of individual face representations.

Impaired face discrimination in acquired prosopagnosia is associated with abnormal response to individual faces in the right middle fusiform gyrus

The middle fusiform gyrus (MFG) and the inferior occipital gyrus (IOG) are activated by both detection and identification of faces. Paradoxically, patients with acquired prosopagnosia following lesions to either of these regions in the right hemisphere cannot identify faces, but can still detect faces. Here we acquired functional magnetic resonance imaging (fMRI) data during face processing in a patient presenting a specific deficit in individual face recognition, following lesions encompassing the right IOG. Using an adaptation paradigm we show that the fMRI signal in the rMFG of the patient, while being larger in response to faces as compared to objects, does not differ between conditions presenting identical and distinct faces, in contrast to the larger response to distinct faces observed in controls. These results suggest that individual discrimination of faces critically depends on the integrity of both the rMFG and the rIOG, which may interact through re-entrant cortical connections in the normal brain.

Portraits or People? Distinct Representations of Face Identity in the Human Visual Cortex

Humans can identify individual faces under different viewpoints, even after a single encounter. We determined brain regions responsible for processing face identity across view changes after variable delays with several intervening stimuli, using event-related functional magnetic resonance imaging during a long-term repetition priming paradigm. Unfamiliar faces were presented sequentially either in a frontal or three-quarter view. Each face identity was repeated once after an unpredictable lag, with either the same or another viewpoint. Behavioral data showed significant priming in response time, irrespective of view changes. Brain imaging results revealed a reduced response in the lateral occipital and fusiform cortex with face repetition. Bilateral face-selective fusiform areas showed view-sensitive repetition effects, generalizing only from three-quarter to front-views. More medial regions in the left (but not in the right) fusiform showed repetition effects across all types of viewpoint changes. These results reveal that distinct regions within the fusiform cortex hold view-sensitive or view-invariant traces of novel faces, and that face identity is represented in a view-sensitive manner in the functionally defined face-selective areas of both hemispheres. In addition, our finding of a better generalization after exposure to a 3/4-view than to a front-view demonstrates for the first time a neural substrate in the fusiform cortex for the common recognition advantage of three-quarter faces. This pattern provides new insights into the nature of face representation in the human visual system.

Familiarity enhances invariance of face representations in human ventral visual cortex: fMRI evidence

Face recognition across different viewing conditions is strongly improved by familiarity. In the present study, we tested the hypothesis that the neural basis of this effect is a less view-dependent representation of familiar faces in ventral visual cortex by assessing priming-related fMRI repetition effects. 15 healthy volunteers made male/female judgements on familiar (famous) and unfamiliar (novel) faces preceded by the same image, a different image of the same face, or another (unprimed) face. Reaction times revealed priming by same and different images independent of familiarity and more pronounced for same than different images. In the imaging data, a main effect of prime condition was found in bilateral fusiform and orbitofrontal regions. A right anterior fusiform region expressed stronger response decreases to repetition of familiar than unfamiliar faces. Bilateral mid-fusiform areas showed stronger response decreases to repetition of same than different images. A regions-of-interest analysis focussing specifically on face responsive regions suggested differences in the degree of image dependency across fusiform cortex. Collapsing across familiarity, there was greater image dependency of repetition effects in right than left anterior fusiform, replicating previous imaging findings obtained with common objects. For familiar faces alone, there was greater generalisation of repetition effects over different images in anterior than middle fusiform. This suggests a role of anterior fusiform cortex in coding image-independent representations of familiar faces.

Recapitulating emotional context: activity of amygdala, hippocampus and fusiform cortex during recollection and familiarity

The amygdala is thought to enhance long-term memory for emotionally arousing events by modulating memory formation and storage in the hippocampus and in neocortical areas. Recent animal studies have raised the possibility that cooperativity between amygdala and hippocampus contributes to the retrieval of fear memories. The functional contributions of the amygdala to the retrieval of emotional memories in humans are less well known. Here, in a functional magnetic resonance imaging experiment, 20 healthy subjects studied neutral words in the context of a fearful or a neutral human face. In a subsequent test, they made 'remember' (conscious recollection of the study context), 'know' (familiarity in the absence of conscious recollection) and 'new' judgements on the studied and newly presented neutral words, in the absence of face stimuli. At test, bilateral amygdala, hippocampus and fusiform face area (FFA) were more strongly activated during recollection than during familiarity. Higher activity for fearful than for neutral study context was found in bilateral FFA during recollection but not during familiarity. This difference recapitulated higher activity for fearful than for neutral context in the FFA during study. These data suggest that the amygdalae and hippocampi contribute to the retrieval of emotion-laden context memories by coordinating the reactivation of stored representations in neocortical areas, such as the FFA. However, there also was a recapitulation of emotional study context in the right amygdala during familiarity only, which might therefore be related to affective implicit memory.

View-independent coding of face identity in frontal and temporal cortices is modulated by familiarity: an event-related fMRI study

Face recognition is a unique visual skill enabling us to recognize a large number of person identities, despite many differences in the visual image from one exposure to another due to changes in viewpoint, illumination, or simply passage of time. Previous familiarity with a face may facilitate recognition when visual changes are important. Using event-related fMRI in 13 healthy observers, we studied the brain systems involved in extracting face identity independent of modifications in visual appearance during a repetition priming paradigm in which two different photographs of the same face (either famous or unfamiliar) were repeated at varying delays. We found that functionally defined face-selective areas in the lateral fusiform cortex showed no repetition effects for faces across changes in image views, irrespective of pre-existing familiarity, suggesting that face representations formed in this region do not generalize across different visual images, even for well-known faces. Repetition of different but easily recognizable views of an unfamiliar face produced selective repetition decreases in a medial portion of the right fusiform gyrus, whereas distinct views of a famous face produced repetition decreases in left middle temporal and left inferior frontal cortex selectively, but no decreases in fusiform cortex. These findings reveal that different views of the same familiar face may not be integrated within a single representation at initial perceptual stages subserved by the fusiform face areas, but rather involve later processing stages where more abstract identity information is accessed.

fMRI-adaptation reveals dissociable neural representations of identity and expression in face perception

The distributed model of face processing proposes an anatomical dissociation between brain regions that encode invariant aspects of faces, such as identity, and those that encode changeable aspects of faces, such as expression. We tested for a neuroanatomical dissociation for identity and expression in face perception using a functional MRI (fMRI) adaptation paradigm. Repeating identity across face pairs led to reduced fMRI signal in fusiform cortex and posterior superior temporal sulcus (STS), whereas repeating emotional expression across pairs led to reduced signal in a more anterior region of STS. These results provide neuroanatomical evidence for the distributed model of face processing and highlight a dissociation within right STS between a caudal segment coding identity and a more rostral region coding emotional expression.