Laurence Dricot

Hemispheric Asymmetries for Whole-Based and Part-Based Face Processing in the Human Fusiform Gyrus

Behavioral studies indicate a right hemisphere advantage for processing a face as a whole and a left hemisphere superiority for processing based on face features. The present PET study identifies the anatomical localization of these effects in well-defined regions of the middle fusiform gyri of both hemispheres. The right middle fusiform gyrus, previously described as a face-specific region, was found to be more activated when matching whole faces than face parts whereas this pattern of activity was reversed in the left homologous region. These lateralized differences appeared to be specific to faces since control objects processed either as wholes or parts did not induce any change of activity within these regions. This double dissociation between two modes of face processing brings new evidence regarding the lateralized localization of face individualization mechanisms in the human brain.

Defining face perception areas in the human brain : a large-scale factorial fMRI face localizer analysis

A number of human brain areas showing a larger response to faces than to objects from different categories, or to scrambled faces, have been identified in neuroimaging studies. Depending on the statistical criteria used, the set of areas can be overextended or minimized, both at the local (size of areas) and global (number of areas) levels. Here we analyzed a whole-brain factorial functional localizer obtained in a large sample of right-handed participants (40). Faces (F), objects (O; cars) and their phase-scrambled counterparts (SF, SO) were presented in a block design during a one-back task that was well matched for difficulty across conditions. A conjunction contrast at the group level {(F-SF) and (F-O)} identified six clusters: in the pulvinar, inferior occipital gyrus (so-called OFA), middle fusiform gyrus (so-called FFA), posterior superior temporal sulcus, amygdala, and anterior infero-temporal cortex, which were all strongly right lateralized. While the FFA showed the largest difference between faces and cars, it also showed the least face-selective response, responding more to cars than scrambled cars. Moreover, the FFAs larger response to scrambled faces than scrambled cars suggests that its face-sensitivity is partly due to low-level visual cues. In contrast, the pattern of activation in the OFA points to a higher degree of face-selectivity. A BOLD latency mapping analysis suggests that face-sensitivity emerges first in the right FFA, as compared to all other areas. Individual brain analyses support these observations, but also highlight the large amount of interindividual variability in terms of number, height, extent and localization of the areas responding preferentially to faces in the human ventral occipito-temporal cortex. This observation emphasizes the need to rely on different statistical thresholds across the whole brain and across individuals to define these areas, but also raises some concerns regarding any objective labeling of these areas to make them correspond across individual brains. This large-scale analysis helps understanding the set of face-sensitive areas in the human brain, and encourages in-depth single participant analyses in which the whole set of areas is considered in each individual brain.

Holistic Face Categorization in Higher Order Visual Areas of the Normal and Prosopagnosic Brain: Toward a Non-Hierarchical View of Face Perception

How a visual stimulus is initially categorized as a face in a network of human brain areas remains largely unclear. Hierarchical neuro-computational models of face perception assume that the visual stimulus is first decomposed in local parts in lower order visual areas. These parts would then be combined into a global representation in higher order face-sensitive areas of the occipito-temporal cortex. Here we tested this view in fMRI with visual stimuli that are categorized as faces based on their global configuration rather than their local parts (two-tones Mooney figures and Arcimboldos facelike paintings). Compared to the same inverted visual stimuli that are not categorized as faces, these stimuli activated the right middle fusiform gyrus (“Fusiform face area”) and superior temporal sulcus (pSTS), with no significant activation in the posteriorly located inferior occipital gyrus (i.e., no “occipital face area”). This observation is strengthened by behavioral and neural evidence for normal face categorization of these stimuli in a brain-damaged prosopagnosic patient whose intact right middle fusiform gyrus and superior temporal sulcus are devoid of any potential face-sensitive inputs from the lesioned right inferior occipital cortex. Together, these observations indicate that face-preferential activation may emerge in higher order visual areas of the right hemisphere without any face-preferential inputs from lower order visual areas, supporting a non-hierarchical view of face perception in the visual cortex.

Face categorization in visual scenes may start in a higher order area of the right fusiform gyrus: evidence from dynamic visual stimulation in neuroimaging

How a visual stimulus is initially categorized as a face by the cortical face-processing network remains largely unclear. In this study we used functional MRI to study the dynamics of face detection in visual scenes by using a paradigm in which scenes containing faces or cars are revealed progressively as they emerge from visual noise. Participants were asked to respond as soon as they detected a face or car during the noise sequence. Among the face-sensitive regions identified based on a standard localizer, a high-level face-sensitive area, the right fusiform face area (FFA), showed the earliest difference between face and car activation. Critically, differential activation in FFA was observed before differential activation in the more posteriorly located occipital face area (OFA). A whole brain analysis confirmed these findings, with a face-sensitive cluster in the right fusiform gyrus being the only cluster showing face preference before successful behavioral detection. Overall, these findings indicate that following generic low-level visual analysis, a face stimulus presented in a gradually revealed visual scene is first detected in the right middle fusiform gyrus, only after which further processing spreads to a network of cortical and subcortical face-sensitive areas (including the posteriorly located OFA). These results provide further evidence for a nonhierarchical organization of the cortical face-processing network.

Right anterior temporal lobe atrophy and person-based semantic defect: A detailed case study

We report a new case of a right temporal pole variant of frontotemporal dementia (Rtv-FTLD), MD, who presented a slowly progressive deterioration of the recognition of familiar and famous people. We thoroughly investigated MDs face processing and semantic abilities, including a neuroimaging investigation. This analysis revealed a cross-modal person-based deficit together with a more general semantic alteration. However, there was no evidence of impairment in face perception, including holistic processing, or of an abnormal pattern of brain activation in face-sensitive cortical areas. We discuss the nature of face processing in the Rtv-FTLD and the context of a person-based semantic defect.

Neural correlates of shape and surface reflectance information in individual faces

Faces are recognized by means of both shape and surface reflectance information. However, it is unclear how these two types of diagnostic information are represented in the human brain. To clarify this issue, we tested 14 participants in an event-related functional magnetic resonance adaptation paradigm, with four conditions created by using a 3D morphable model: (1) repetition of the same adapting face; (2) variation in shape only; (3) variation in surface reflectance only; (4) variation in both shape and surface reflectance. Change in face shape alone was the dominant driving force of the adaptation release in functionally defined face-sensitive areas in the right hemisphere (fusiform face area [FFA], occipital face area [OFA]). In contrast, homologous areas of the left hemisphere showed comparable adaptation release to changes in face shape and surface reflectance. When both changes in shape and reflectance were combined, there was no further increased release from adaptation in face-sensitive areas. Overall, these observations indicate that the two main sources of information in individual faces, shape and reflectance, contribute to individual face sensitivity found in the cortical face network. Moreover, the sensitivity to shape cues is more dominant in the right hemisphere, possibly reflecting a privileged mode of global (holistic) face processing.

Associative encoding deficits in amnestic mild cognitive impairment : a volumetric and functional MRI study.

BACKGROUND Previous functional MRI studies have shown increased hippocampus activation in response to item encoding in amnestic mild cognitive impairment (aMCI). Recent behavioral studies suggested that associative memory could be more impaired than item memory in aMCI. So far, associative encoding has not been evaluated separately from item encoding in functional MRI studies. METHODS We conducted a volumetric and functional MRI study investigating associative encoding in 16 aMCI and 16 elderly controls while controlling for item encoding. RESULTS We confirmed the presence of associative memory impairment in aMCI even after controlling for item memory differences between groups. Associative memory but not item memory correlated with hippocampus volume in aMCI. Such a correlation was not observed in elderly controls. The left anterior hippocampus activation in response to successful associative encoding was decreased in aMCI, even after correction for hippocampus atrophy. CONCLUSION Associative memory impairment in aMCI appears to be related to hippocampus atrophy and left anterior hippocampus hypoactivation.

Personally familiar faces are perceived categorically in face-selective regions other than the fusiform face area.

Neuroimaging studies of humans have provided inconsistent evidence with respect to the response properties of the fusiform face area (FFA). It has been claimed that neural populations within this region are sensitive to subtle differences between individual faces only when they are perceived as distinct identities [P. Rotshtein et al. (2005)Nature Neuroscience, 8, 107–113]. However, sensitivity to subtle changes of identity was found in previous studies using unfamiliar faces, for which categorical perception is less pronounced. Using functional magnetic resonance adaptation and morph continua of personally familiar faces, we investigated sensitivity to subtle changes between faces that were located either on the same or opposite sides of a categorical perceptual boundary. We found no evidence for categorical perception within the FFA, which exhibited reliable sensitivity to subtle changes of face identity whether these were perceived as distinct identities, or not. On the contrary, both the posterior superior temporal sulcus and prefrontal cortex exhibited categorical perception, as subtle changes between faces perceived as different identities yielded larger release from adaptation than those perceived as the same identity. These observations suggest that, whereas the FFA discriminates subtle physical changes of personally familiar faces, other regions encode faces in a categorical fashion.

Classification of Non-Demented Patients Attending a Memory Clinic using the New Diagnostic Criteria for Alzheimer's Disease with Disease-Related Biomarkers

BACKGROUND New diagnostic criteria for predemential Alzheimers disease (AD) advocate the use of biomarkers. However, the benefit of using biomarkers has not been clearly demonstrated in clinical practice. OBJECTIVE To investigate whether a combination of biomarkers may be helpful in classifying a population of non-demented patients attending a Memory Clinic. METHODS Sixty non-demented patients were compared with 31 healthy elderly subjects. All subjects underwent a neuropsychological examination, brain 3T magnetic resonance imaging, [F18]-fluorodeoxyglucose and [F18]-flutemetamol positron emission tomography. According to their performance on memory, language, executive, and visuo-spatial domains, the patients were classified as mild cognitive impairment (amnestic, non-amnestic, single, or multiple domain) or subjective cognitive impairment. Patients were then classified according to the National Institute on Aging-Alzheimers Association (NIA-AA) criteria, using the normalized mean hippocampal volume (Freesurfer), [F18]-FDG PALZAD, and [F18]-flutemetamol standard uptake value ratio (SUVr) (cut-off at the 10th percentile of controls). The standard of truth was the clinical status at study entry (patient versus control). RESULTS The sensitivity/specificity of the clinical classification was 65/84%. The NIA-AA criteria were applicable in 85% of patients and 87% of controls. For biomarkers the best sensitivity (72%) at a fixed specificity of 84% was achieved by a combination of the three biomarkers. The clinical diagnosis was reconsidered in more than one third of the patients (42%) as a result of including the biomarker results. CONCLUSIONS Application of the new NIA-AA AD diagnostic criteria based on biomarkers in an unselected sample of non-demented patients attending a Memory Clinic was useful in allowing for a better classification of the subjects.

Right occipital cortex activation correlates with superior odor processing performance in the early blind.

Using functional magnetic resonance imaging (fMRI) in ten early blind humans, we found robust occipital activation during two odor-processing tasks (discrimination or categorization of fruit and flower odors), as well as during control auditory-verbal conditions (discrimination or categorization of fruit and flower names). We also found evidence for reorganization and specialization of the ventral part of the occipital cortex, with dissociation according to stimulus modality: the right fusiform gyrus was most activated during olfactory conditions while part of the left ventral lateral occipital complex showed a preference for auditory-verbal processing. Only little occipital activation was found in sighted subjects, but the same right-olfactory/left-auditory-verbal hemispheric lateralization was found overall in their brain. This difference between the groups was mirrored by superior performance of the blind in various odor-processing tasks. Moreover, the level of right fusiform gyrus activation during the olfactory conditions was highly correlated with individual scores in a variety of odor recognition tests, indicating that the additional occipital activation may play a functional role in odor processing.