Sylvane Faure

New questions on the hemispheric encoding/retrieval asymmetry (HERA) model assessed by divided visual-field tachistoscopy in normal subjects.

According to the hemispheric encoding/retrieval asymmetry (HERA) model, based on data obtained through functional neuroimaging, the left and right prefrontal cortices are preferentially, and, respectively, involved in long-term episodic memory encoding and retrieval. In this study, the HERA model was tested from a behavioral perspective using divided visual-field tachistoscopy. A recognition paradigm with both verbal and visuospatial materials was devised to differentiate memory-related effects (encoding vs. retrieval) from effects linked to the materials. The paradigm used lists of 12 and four items to assess long-term episodic memory and short-term memory, respectively. The aim of the latter condition was to test whether the HERA model is applicable in short-term memory. For long-term episodic memory, the data obtained validated the HERA model; the direction of the hemispheric asymmetry was found to depend on the type of materials used, whereas its magnitude was determined by the type of memory process. For verbal short-term memory, the HERA model seems to be confirmed. The pre-existing representations of the material could take into account the similarity of the hemispheric asymmetry pattern between short-term memory and long-term memory. In contrast, for visuospatial short-term memory, Baddeleys working memory model seems to better explain our results insofar as the asymmetries were essentially linked to the material in encoding but not in retrieval. This latter difference between short-term memory and long-term indicates that processes involved in LTM depend on episodic processes per se, hence, lending more support for the HERA model. Accordingly, these two memory systems seem to bring into play two different modes of hemisphere specialization.

Brain Functional Profiles in Formal and Semantic Fluency Tasks: A SPECT Study in Normals

SPECT method is used to analyze changes in regional cerebral blood flow in a group of 19 normal subjects during a baseline task (repetition of two words) and two verbal fluency tasks, a semantic fluency and a formal fluency. The semantic fluency task was associated with a relative CBF increase in the right dorso-lateral and medial frontal region when compared with that seen in the baseline condition. No specific activation was found for the formal fluency task compared to that seen in the baseline task. We suggest that the activation of the right frontal region reflects semantic categorization strategies in semantic fluency. The lack of activation of the left frontal region may be due to an activation induced by the nature of the baseline task (i.e., a self-paced repetition task).

Load-dependent posterior-anterior shift in aging in complex visual selective attention situations.

The cognitive reserve hypothesis proposes that the brain actively attempts to cope with age-related changes by using pre-existing cognitive networks (neural reserve) or enlisting compensatory processes (neural compensation). In a context of visual selective attention, the current study compared task-related activation with BOLD fMRI signals in younger (N=16) and older (N=16) adults using a letter-name-matching task with two attentional load levels. In the low-load condition, the target letter might share the same identity (e.g., a/A) with one of two probes in the display, while in the high-load condition the display included four probes. The results suggest that there is an age-related change within the frontoparietal network that underlies visual selective attention processing. In the low-load condition, the older group needed to recruit more bilateral frontal regions to successfully perform the task, while the younger participants recruited more bilateral occipital regions, in agreement with the PASA (Posterior-Anterior Shift in Aging) phenomenon and the neural compensation hypothesis of cognitive reserve. In addition, in the high-load condition, we found a load-dependent posterior-anterior shift in the older participants, which was not present in the younger ones, involving the anterior part of the cingulate cortex. By showing a load-dependent PASA, our study indicates that the PASA phenomenon is supported more by the compensation mechanism (solicited exclusively in older participants) than by the reserve.

Semantic consistency versus perceptual salience in visual scenes: findings from change detection.

In a one-shot change detection task, we investigated the relationship between semantic properties (high consistency, i.e., diagnosticity, versus inconsistency with regard to gist) and perceptual properties (high versus low salience) of objects in guiding attention in visual scenes and in constructing scene representations. To produce the change an object was added or deleted in either the right or left half of coloured drawings of daily-life events. Diagnostic object deletions were more accurately detected than inconsistent ones, indicating rapid inclusion into early scene representation for the most predictable objects. Detection was faster and more accurate for high salience than for low salience changes. An advantage was found for diagnostic object changes in the high salience condition, although it was limited to additions when considering response speed. For inconsistent objects of high salience, deletions were detected faster than additions. These findings may indicate that objects are primarily selected on a perceptual basis with subsequent and supplementary effect of semantic consistency, in the sense of facilitation due to object diagnosticity or lengthening of processing time due to inconsistency.

Change Detection in Complex Scenes: Hemispheric Contribution and the Role of Perceptual and Semantic Factors

The perceptual salience and semantic relevance of objects for the meaning of a scene were evaluated with multiple criteria and then manipulated in a change-detection experiment that used an original combination of one-shot and tachistoscopic divided-visual-field paradigms to study behavioural hemispheric asymmetry. Coloured drawings that depicted meaningful situations were presented centrally and very briefly (120 ms) and only the changes were lateralised by adding an object in the right or in the left visual hemifield. High salience and high relevance improved both response times (RTs) and accuracy, although the overall contribution of salience was greater than that of relevance. Moreover, only for low-salience changes did relevance affect speed. RTs were shorter when a change occurred in the left visual hemifield, suggesting a right-hemisphere advantage for detection of visual change. Also, men responded faster than women. The theoretical and methodological implications are discussed.

Coping with task demand in aging using neural compensation and neural reserve triggers primarily intra-hemispheric-based neurofunctional reorganization

It has been proposed that cognitive reserve is supported by two neural mechanisms: neural compensation and neural reserve. The purpose of this study was to test how these neural mechanisms are solicited in aging in the context of visual selective attention processing and whether they are inter- or intra-hemispheric. Younger and older participants were scanned using fMRI during a visual letter-matching task with two attentional load levels. The results show that in the low-load condition, the older participants activated frontal superior gyri bilaterally; these regions were not activated in the younger participants, in accordance with the compensation mechanism and the Posterior-Anterior Shift in Aging (PASA) phenomenon. However, when task demand increased, the older participants recruited the same regions (parietal) as the younger ones, showing the involvement of a similar neural reserve mechanism. This result suggests that successful cognitive aging relies on the concurrent use of both neural compensation and neural reserve in high-demand tasks, calling on the frontoparietal network. In addition, the finding of intra-hemispheric-based neurofunctional reorganization with a PASA phenomenon for all attentional load levels suggests that the PASA phenomenon is a function more of compensation than of reserve.

Right Hemisphere Semantic Performance and Competence in a Case of Partial Interhemispheric Disconnection

Various forms of right hemisphere (RH)-spontaneous or provoked--performance changes were observed in a partial split-brain patient. Variations in hemispheric activation levels and their interhemispheric balance were retained as the most plausible mechanisms for accounting for these facts. This article presents our attempts to obtain higher RH lexical semantic performance than that observed in a preliminary study where RH performance was at chance level: two forms of the dual-task paradigm were used in order to improve the expression of an assumed RH potential for lexical semantics. Experiment 1 was aimed at activating the RH with a visuo-spatial task as a prior activity before each semantic decision trial. Experiment 2 was intended to overload LH processing systems with a concurrent verbal memory task and induce RH release from LH inhibition. As expected, better RH lexical semantic performance was obtained in these two conditions. Using pictures as stimuli, the aim of Experiment 3 was to observe the spontaneous temporal course of RH implementation in a semantic decision task devoid of written words. These results are discussed along with the issues of performance/competence and the dynamics of neurocognitive function.

Spécialisation hémisphérique versus coopération inter-hémisphérique

Resume La premiere partie de cet article retrace les principales decouvertes ayant abouti a la notion de specialisation hemispherique, depuis l’antiquite egyptienne jusqu’a nos jours. Dans un deuxieme temps, cette conception tres dichotomique du fonctionnement cerebral, attribuant a chaque hemisphere une fonction donnee, transmise ensuite par l’intermediaire du corps calleux (CC) a l’hemisphere controlateral, est remise en question a la lumiere des etudes recentes portant sur la comprehension du langage. A une approche structuraliste de specialisation hemispherique se substituent actuellement les idees plus dynamiques de « complementarite », de « division du travail » et de « cooperation », a travers des reseaux neuronaux trans-corticaux. Enfin, le role du corps calleux dans les echanges inter-hemispheriques est brievement evoque. L’accent est mis sur la grande diversite de cette structure (taille des fibres, mode de connexion homo versus heterotopique) a l’origine de fonctions tres differentes. A l’image d’un CC fonctionnant comme un canal de transmission d’informations est superposee celle d’un faisceau de fibres co-activant l’hemisphere non engage, donc sous-active, afin de le preparer a repondre a une eventuelle stimulation. En ce sens, le CC minimise les disparites dans la distribution de l’attention entre les deux hemispheres.

Intérêt des potentiels évoqués cognitifs dans la détection des troubles cognitifs précoces dans la sclérose en plaques

Resume Introduction Les troubles cognitifs ont longtemps ete peu etudies dans la sclerose en plaques (SEP), une des plus frequentes pathologies neurologiques de l’adulte jeune, et ce bien qu’ils puissent evoluer vers une demence fronto-sous-corticale et qu’ils soient presents chez plus de la moitie des patients. Meme si les potentiels evoques (PE) sensoriels sont classiquement utilises dans la SEP, les PE cognitifs ne sont pas proposes en clinique dans cette pathologie. Patients et methode Nous rapportons deux etudes visant a evaluer l’interet des PE cognitifs pour detecter precocement des troubles cognitifs dans la SEP. Tous les patients presentaient une SEP de type remittente depuis moins de 5 ans avec un handicap physique modere et exprimaient une plainte mnesique. Ils ont beneficie de tests neuropsychologiques et de l’enregistrement des PE lors du paradigme de l’ oddball , en modalites visuelle et auditive. Dans la premiere etude, 10 patients sans anomalie du bilan neuropsychologique ont ete inclus, 10 patients ayant une perturbation du bilan attentionnel et 10 sujets controles. Dix patients avec une perturbation du bilan mnesique et 10 controles ont participe a la seconde etude. Resultats Nos donnees sont en faveur d’une modification plus precoce des PE cognitifs dans la modalite visuelle qu’en auditif, avant meme la perturbation du bilan neuropsychologique. Dans les deux etudes, les parametres de la P300 etaient correles aux performances cognitives (attentionnelles dans la premiere etude et mnesiques dans la seconde) dans les deux modalites. Discussion Nos resultats montrent l’interet d’enregistrer les PE cognitifs non seulement en modalite auditive, mais aussi visuelle afin de rechercher des perturbations electrophysiologiques precoces alors meme que le bilan neuropsychologique n’est que peu contributif. Cette serie souligne l’apport clinique des PE cognitifs justifiant leur inclusion dans le bilan et le suivi de la SEP.

Mental rotation and simulation of a reaching and grasping manual movement.

This paper deals with the kind of manual movement subjects mentally simulate when solving a left-right judgment task that requires rotating images of hands. 50 female students were asked to judge the laterality of drawings of rotated hands presented successively to the right and left visual hemifields by clicking on a mouse using either the right or left hand. Reaction times and accuracy of judgment were recorded. Analysis showed performances varied with the rotation angle at which the stimulus was presented, indicating that the subjects mentally simulated a rotation process. An interaction occurred between the visually presented hand and the responding hand, which suggests that the mental rotation process involved the simulation of a hand movement. Performance improved when the drawing of a hand was presented in the ‘palm-up’ position, and to the visual hemifield opposite with respect to the hand the subject moved mentally. The latter two findings suggest that the subjects performed a simulated reaching and grasping movement rather than a simulated positioning movement.