Jean-Michel Bodart

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.

Effect of familiarity on the processing of human faces

Most brain imaging studies on face perception have investigated the processing of unknown faces and addressed mainly the question of specific face processing in the human brain. The goal of this study was to highlight the effects of familiarity on the visual processing of faces. Using [15O]water 3D Positron Emission Tomography, regional cerebral blood flow distribution was measured in 11 human subjects performing an identical task (gender categorization) on both unknown and known faces. Subjects also performed two control tasks (a face recognition task and a visual pattern discrimination task). They were scanned after a training phase using videotapes during which they had been familiarized with and learned to recognize a set of faces. Two major results were obtained. On the one hand, we found bilateral activations of the fusiform gyri in the three face conditions, including the so-called fusiform-face area, a region in the right fusiform gyrus specifically devoted to face processing. This common activation suggests that different cognitive tasks performed on known and unknown faces require the involvement of this fusiform region. On the other hand, specific regional cerebral blood flow changes were related to the processing of known and unknown faces. The left amygdala, a structure involved in implicit learning of visual representations, was activated by the categorization task on unknown faces. The same task on known faces induced a relative decrease of activity in early visual areas. These differences between the two categorization tasks reveal that the human brain processes known and unknown faces differently.

Neuronal Mechanisms of Perceptual Learning: Changes in Human Brain Activity with Training in Orientation Discrimination

Using 15O-water 3D positron emission tomography, regional cerebral blood flow was measured twice in six human subjects: before and after extensive training in orientation discrimination. In each session subjects performed two orientation discrimination tasks, during which they discriminated the orientation of a grating at either the trained or untrained reference orientation, and a control task, during which they detected a randomly textured pattern. By comparing the discrimination to the detection tasks, we observed a main effect of task bilaterally in the posterior occipital cortex, extending into the left posterior fusiform gyrus and the right inferior occipital gyrus, bilaterally in the intraparietal sulcus, as well as in the cerebellum, thalamus, and brainstem. When we compared the activation pattern before and after the training period, all the changes observed were activity decreases. The nonspecific changes, which were not related to the orientation used during the training, were situated in the cerebellum and bilaterally in the extrastriate visual cortex. The orientation-specific changes, on the other hand, were restricted to the striate and extrastriate visual cortex, more precisely the right calcarine sulcus, the left lingual gyrus, the left middle occipital, and the right inferior occipital gyrus. These findings confirm our hypothesis concerning the existence of learning related changes at early levels of visual processing in human adults and suggest that mechanisms resulting in neuronal activity decreases might be involved in the present kind of learning.

A pet study of human skill learning: changes in brain activity related to learning an orientation discrimination task.

Using 15O-water 3D positron emission tomography we investigated the effect of training in orientation discrimination upon cerebral activity in healthy human adults. When subjects are trained in this discrimination task, they learn the visuo-motor stimulus-response association required by the task and they increase their perceptual abilities in orientation discrimination. The present study was designed to investigate the rCBF modifications related to both these learning processes induced by training in orientation discrimination. PET data were acquired on two separate days (before and after training). Comparing the activation pattern related to orientation discrimination before and after the training period we observed activity decreases located in the left cerebellar cortex, in the right precentral gyrus and bilaterally in the fusiform gyri. The only region showing an activity increase was located in the body of the right caudate nucleus. These findings confirm the role of the neostriatum in skill learning and highlight the importance of mechanisms resulting in cortical and cerebellar neuronal activity decreases in this type of learning.

Contribution of PET studies in diagnosis of corticobasal degeneration

Five patients with clinical features of corticobasal degeneration (CBD) were studied with PET imaging. The main clinical findings included a unilateral extrapyramidal motor disorder, without significant response to levodopa, as well as clumsiness, dysarthria, apraxia and a clear asymmetry of neurological signs. PET studies with 18F-labeled 2-deoxy-2-fluoro-D-glucose disclosed mainly a significant hypometabolism in the thalamus and motor cortex controlateral to the more affected limbs. Additional relationships between individual clinical signs and PET data were also found. We concluded that PET findings supported the clinical diagnosis of CBD, although the specific pattern related to this condition needs to be more precisely defined. Further studies are especially needed to correlate clinical data and PET results with pathological examination.

Functional imaging of visual semantic processing in the human brain

Previous neuroimaging studies have identified a large network of cortical areas involved in semantic processing in the human brain, which includes left occipito-temporal and inferofrontal areas. Most studies, however, investigated exclusively the associative/functional semantic knowledge by using mainly words and/or language related tasks, and this factor may have contributed to the large left hemisphere superiority found in semantic processing and to the controversial involvement of left prefrontal structures. The present study investigates the neural basis of visual objects knowledge, accessed exclusively through pictorial information. Regional cerebral blood flow (rCBF) was assessed using positron emission tomography (PET) during 3 conditions in right-handed normal volunteers: resting with eyes closed, retrieval of semantic information related to visual properties of objects (real size), and visual categorization based on physical properties of the image. Confirming previous experiments and neuropsychological findings, most activations were found in left occipito-temporal areas during retrieval of visual semantic knowledge. The absence of any activation in the left prefrontal inferior cortex for visual semantic processing confirms recent observations which suggest that this region would not be involved in retrieval of visual semantic knowledge from living entities. Rather, such knowledge about visual properties of objects, situated closely to cortical regions mediating perception of the visual attributes, can be retrieved directly from these regions when visual images are used as entry level stimuli.