Biomedical subjects

Galia Avidan

Publications and source records attributed to Galia Avidan.

11 recordsLinked to original sources

PubMed2007

Structural imaging reveals anatomical alterations in inferotemporal cortex in congenital prosopagnosia.

Congenital prosopagnosia (CP) refers to the lifelong impairment in face recognition in individuals who have intact low-level visual processing, normal cognitive abilities, and no known neurological disorder. Although the face recognition impairment is profound and debilitating, its neural basis remains elusive. To investigate this, we conducted detailed morphometric and volumetric analyses of the occipitotemporal (OT) cortex in a group of CP individuals and matched control subjects using high-spatial resolution magnetic resonance imaging. Although there were no significant group differences in the depth or deviation from the midline of the OT or collateral sulci, the CP individuals evince a larger anterior and posterior middle temporal gyrus and a significantly smaller anterior fusiform (aF) gyrus. Interestingly, this volumetric reduction in the aF gyrus is correlated with the behavioral decrement in face recognition. These findings implicate a specific cortical structure as the neural basis of CP and, in light of the familial history of CP, target the aF gyrus as a potential site for further, focused genetic investigation.

Adult↗

PubMed2007

A detailed investigation of facial expression processing in congenital prosopagnosia as compared to acquired prosopagnosia.

Whether the ability to recognize facial expression can be preserved in the absence of the recognition of facial identity remains controversial. The current study reports the results of a detailed investigation of facial expression recognition in three congenital prosopagnosic (CP) participants, in comparison with two patients with acquired prosopagnosia (AP) and a large group of 30 neurologically normal participants, including individually age- and gender-matched controls. Participants completed a fine-grained expression recognition paradigm requiring a six-alternative forced-choice response to continua of morphs of six different basic facial expressions (e.g. happiness and surprise). Accuracy, sensitivity and reaction times were measured. The performance of all three CP individuals was indistinguishable from that of controls, even for the most subtle expressions. In contrast, both individuals with AP displayed pronounced difficulties with the majority of expressions. The results from the CP participants attest to the dissociability of the processing of facial identity and of facial expression. Whether this remarkably good expression recognition is achieved through normal, or compensatory, mechanisms remains to be determined. Either way, this normal level of performance does not extend to include facial identity.

Adult↗

PubMed2006

Configural processing in autism and its relationship to face processing.

Studies of the perceptual performance of individuals with autism have focused, to a large extent, on two domains of visual behavior, one associated with face processing and the other associated with global or holistic processing. Whether autistic individuals differ from neurotypical individuals in these domains is debatable and, moreover, the relationship between the behaviors in these two domains remains unclear. We first compared the face processing ability of 14 adult individuals with autism with that of neurotypical controls and showed that the autistic individuals were slowed in their speed of face discrimination. We then showed that the two groups differed in their ability to derive the global whole in two different tasks, one using hierarchical compound letters and the other using a microgenetic primed matching task with geometric shapes, with the autistic group showing a bias in favor of local information. A significant correlation was also observed between performance on the face task and the configural tasks. We then confirmed the prediction that the ability to derive the global whole is not only critical for faces but also for other objects as well, as the autistic individuals performed more slowly than the control group in discriminating between objects. Taken together, the results suggest that the bias for local processing seen in autistic individuals might have an adverse impact on their ability to process faces and objects.

Adult↗

PubMed2005

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.

Brain↗

PubMed2005

Detailed exploration of face-related processing in congenital prosopagnosia: 2. Functional neuroimaging findings.

Specific regions of the human occipito-temporal cortex are consistently activated in functional imaging studies of face processing. To understand the contribution of these regions to face processing, we examined the pattern of fMRI activation in four congenital prosopagnosic (CP) individuals who are markedly impaired at face processing despite normal vision and intelligence, and with no evidence of brain damage. These individuals evinced a normal pattern of fMRI activation in the fusiform gyrus (FFA) and in other ventral occipito-temporal areas, in response to faces, buildings, and other objects, shown both as line drawings in detection and discrimination tasks and under more naturalistic testing conditions when no task was required. CP individuals also showed normal adaptation levels in a block-design adaptation experiment and, like control subjects, exhibited evidence of global face representation in the FFA. The absence of a BOLD-behavioral correlation (profound behavioral deficit, normal face-related activation in the ventral occipito-temporal cortex) challenges existing accounts of face representation, and suggests that activation in these cortical regions per se is not sufficient to ensure intact face processing.

Adult↗

PubMed2005

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.

Adult↗

PubMed2003

Face-selective activation in a congenital prosopagnosic subject.

Congenital prosopagnosia is a severe impairment in face identification manifested from early childhood in the absence of any evident brain lesion. In this study, we used fMRI to compare the brain activity elicited by faces in a congenital prosopagnosic subject (YT) relative to a control group of 12 subjects in an attempt to shed more light on the nature of the brain mechanisms subserving face identification. The face-related activation pattern of YT in the ventral occipito-temporal cortex was similar to that observed in the control group on several parameters: anatomical location, activation profiles, and hemispheric laterality. In addition, using a modified vase-face illusion, we found that YT's brain activity in the face-related regions manifested global grouping processes. However, subtle differences in the degree of selectivity between objects and faces were observed in the lateral occipital cortex. These data suggest that face-related activation in the ventral occipito-temporal cortex, although necessary, might not be sufficient by itself for normal face identification.

Adult↗

PubMed2003

Spatial vs. object specific attention in high-order visual areas.

Recently we reported that the topographic organization of visual field eccentricity in human visual cortex extends into high-order, ventral occipitotemporal (VOT) cortex. Within this cortex, regions that respond preferentially to faces and buildings have specific eccentricity biases, suggesting that this category-eccentricity association may reflect differential needs of recognition processes. However, it is still not clear to what extent this center/periphery differentiation within high-order occipitotemporal cortex depends on immediate, moment-to-moment, task demands. Previous attention studies were confined either to exploring the visual field topography (spatial attention) or to object identity (object-based attention). Here, we combined the investigation of these two different attentional mechanisms in the same study. We found that the main source of attentional modulation in occipitotemporal cortex was object-based attention. Shifting attention to different object categories (buildings, faces, and arrows) substantially modulated the object-related activations. The differential activation to each object category in occipitotemporal object areas was maintained, albeit at a reduced level, even when attention was focused on different spatial locations. A slight eccentricity-related attentional differentiation was observed in the more dorsal lateral occipital region, but not in the VOT cortex. These results argue against the possibility that the source of the eccentricity differentiation in VOT cortex is due solely to moment-to-moment shifts in spatial attention mechanism and supports the notion that the eccentricity-biased maps found in this region are due to built-in shape selectivity established over long-term processes.

Adult↗

PubMed2002

Analysis of the neuronal selectivity underlying low fMRI signals.

BACKGROUND: A prevailing assumption in neuroimaging studies is that relatively low fMRI signals are due to weak neuronal activation, and, therefore, they are commonly ignored. However, lower fMRI signals may also result from intense activation by highly selective, albeit small, subsets of neurons in the imaged voxel. We report on an approach that could form a basis for resolving this ambiguity imposed by the low (mm range) spatial resolution of fMRI. Our approach employs fMR-adaptation as an indicator for highly active neuronal populations even when the measured fMRI signal is low. RESULTS: In this study, we first showed that fMRI-adaptation is diminished when overall neuronal activity is lowered substantially by reducing image contrast. We then applied the same adaptation paradigm, but this time we lowered the fMRI signal by changing object shape. While the overall fMRI signal in category-related regions such as the face-related pFs was drastically reduced for non-face stimuli, the adaptation level obtained for these stimuli remained high. We hypothesize that the relatively greater adaptation level following exposure to "nonoptimal" object shapes is indicative of small subsets of neurons responding vigorously to these "nonoptimal" objects even when the overall fMRI activity is low. CONCLUSIONS: Our results show that fMR-adaptation can be used to differentiate between neuronal activation patterns that appear similar in the overall fMRI signal. The results suggest that it may be possible to employ fMR-adaptation to reveal functionally heterogeneous islands of activity, which are too small to image using conventional imaging methods.

Adult↗

PubMed2002

Correlations between the fMRI BOLD signal and visual perception.

Using fMRI and a psychophysical task involving letter identification, Kleinschmidt et al. (2002) (this issue of Neuron) delineate two patterns of neural activation, which manifest in different cortical regions: a transient activation, correlated with the change of a percept, and a longer-term hysteresis, correlated with the maintenance of the percept. These findings are provocative and suggest that neural hysteresis is mediated by visual structures that interact with higher-order regions to support longer-term maintenance of a percept.

Brain Mapping↗

PubMed2002

Contrast sensitivity in human visual areas and its relationship to object recognition.

An important characteristic of visual perception is the fact that object recognition is largely immune to changes in viewing conditions. This invariance is obtained within a sequence of ventral stream visual areas beginning in area V1 and ending in high order occipito-temporal object areas (the lateral occipital complex, LOC). Here we studied whether this transformation could be observed in the contrast response of these areas. Subjects were presented with line drawings of common objects and faces in five different contrast levels (0, 4, 6, 10, and 100%). Our results show that indeed there was a gradual trend of increasing contrast invariance moving from area V1, which manifested high sensitivity to contrast changes, to the LOC, which showed a significantly higher degree of invariance at suprathreshold contrasts (from 10 to 100%). The trend toward increased invariance could be observed for both face and object images; however, it was more complete for the face images, while object images still manifested substantial sensitivity to contrast changes. Control experiments ruled out the involvement of attention effects or hemodynamic "ceiling" in producing the contrast invariance. The transition from V1 to LOC was gradual with areas along the ventral stream becoming increasingly contrast-invariant. These results further stress the hierarchical and gradual nature of the transition from early retinotopic areas to high order ones, in the build-up of abstract object representations.

Adult↗