A presente pesquisa centra-se no estudo das relações entre memória de curto prazo, memória de trabalho, velocidade de processamento, processamento visuoespacial, funções executivas, atenção seletiva e alternada e desempenho matemático, uma vez que essas relações podem esclarecer as diferenças entre os alunos na aprendizagem da matemática. O trabalho teve como objeto de observação crianças da 2ª série do ensino fundamental. Consiste em um estudo com base correlacional e comparativa. A amostra desta pesquisa é composta por 40 alunos, com idade entre 8 e 9 anos. A investigação envolvendo o funcionamento neuropsicológico e o desempenho cognitivo na aprendizagem matemática é relevante, pois para desenvolver habilidades de cálculo matemático é fundamental que o aluno tenha construído o conceito de número e esta construção está vinculada ao desenvolvimento das funções neuropsicológicas. O desempenho matemático é avaliado através da Prova de Aritmética de Capovilla, Montiel e Capovilla (2007) e as funções neuropsicológicas, através de quatro subtestes da Escala Wechsler WISCIII, que são: Memória de Curto Prazo – teste de Dígitos de Ordem Direta; Memória de Trabalho – Dígitos de Ordem Inversa; Velocidade de Processamento - Procurar Símbolos e Códigos; Percepção Visuoespacial – teste de Cubos. A Função Executiva é avaliada através do Teste das Trilhas de Montiel e Capovilla (2007) e a Atenção, através do Teste de Atenção por Cancelamento de Montiel e Capovilla (2007). Observa-se uma correlação significativa entre Desempenho Matemático e Memória de Curto Prazo, Velocidade de Processamento, Função Executiva e Atenção. Neste estudo, não se encontrou uma correlação estatisticamente significativa entre as funções Memória de Trabalho e Processamento Visuoespacial. / This paper approaches the relationship amongst neuropsychological functions –Short Term Memory (STM), Working Memory (WM), Speed Processing (SP), Visual Spatial Perception (VSP), Executive Functions (EF), Attention (A) and Math Performance (MP) in 2nd grade children at a public Elementary School from Porto Alegre. It is a comparative and correlation study. The sample of this research is composed for 40 pupils with are between 8 and 9 years. Research involving neuropsychological functioning and cognitive performance in mathematics is pertinent, for it is essential that the student build the notion of number in order to develop mathematical skills and such construction relies on the expansion of neuropsychological functions. The mathematical performance is measured by the Arthmetic Test of Capovilla, Montiel and Capovilla (2007) and the neuropsychological functions are verified by subtests Wechsler WIS CIII; Short Term Memory is assessed by the Digits in Direct Order, while the Working Memory is established by the Digits in Reverse Order. The speed processing is assessed through the use of Find Symbols and Codes activities, and spatial perception is confirmed by the utilization of cubes. The Executive Function is assessed by the Trail Making of Montiel and Capovilla (2007) and attention is assessed by the Attention Test for Cancellation of Montiel and Capovilla (2007). There is a momentous correlation amid math performance and Short-Term Memory, speed processing, executive function and attention. This study one did not mett, however, the establishment of a statistically significant correlation between the Working Memory functions and the Visual Spatial Processing.

A presente pesquisa centra-se no estudo das relações entre memória de curto prazo, memória de trabalho, velocidade de processamento, processamento visuoespacial, funções executivas, atenção seletiva e alternada e desempenho matemático, uma vez que essas relações podem esclarecer as diferenças entre os alunos na aprendizagem da matemática. O trabalho teve como objeto de observação crianças da 2ª série do ensino fundamental. Consiste em um estudo com base correlacional e comparativa. A amostra desta pesquisa é composta por 40 alunos, com idade entre 8 e 9 anos. A investigação envolvendo o funcionamento neuropsicológico e o desempenho cognitivo na aprendizagem matemática é relevante, pois para desenvolver habilidades de cálculo matemático é fundamental que o aluno tenha construído o conceito de número e esta construção está vinculada ao desenvolvimento das funções neuropsicológicas. O desempenho matemático é avaliado através da Prova de Aritmética de Capovilla, Montiel e Capovilla (2007) e as funções neuropsicológicas, através de quatro subtestes da Escala Wechsler WISCIII, que são: Memória de Curto Prazo – teste de Dígitos de Ordem Direta; Memória de Trabalho – Dígitos de Ordem Inversa; Velocidade de Processamento - Procurar Símbolos e Códigos; Percepção Visuoespacial – teste de Cubos. A Função Executiva é avaliada através do Teste das Trilhas de Montiel e Capovilla (2007) e a Atenção, através do Teste de Atenção por Cancelamento de Montiel e Capovilla (2007). Observa-se uma correlação significativa entre Desempenho Matemático e Memória de Curto Prazo, Velocidade de Processamento, Função Executiva e Atenção. Neste estudo, não se encontrou uma correlação estatisticamente significativa entre as funções Memória de Trabalho e Processamento Visuoespacial. / This paper approaches the relationship amongst neuropsychological functions –Short Term Memory (STM), Working Memory (WM), Speed Processing (SP), Visual Spatial Perception (VSP), Executive Functions (EF), Attention (A) and Math Performance (MP) in 2nd grade children at a public Elementary School from Porto Alegre. It is a comparative and correlation study. The sample of this research is composed for 40 pupils with are between 8 and 9 years. Research involving neuropsychological functioning and cognitive performance in mathematics is pertinent, for it is essential that the student build the notion of number in order to develop mathematical skills and such construction relies on the expansion of neuropsychological functions. The mathematical performance is measured by the Arthmetic Test of Capovilla, Montiel and Capovilla (2007) and the neuropsychological functions are verified by subtests Wechsler WIS CIII; Short Term Memory is assessed by the Digits in Direct Order, while the Working Memory is established by the Digits in Reverse Order. The speed processing is assessed through the use of Find Symbols and Codes activities, and spatial perception is confirmed by the utilization of cubes. The Executive Function is assessed by the Trail Making of Montiel and Capovilla (2007) and attention is assessed by the Attention Test for Cancellation of Montiel and Capovilla (2007). There is a momentous correlation amid math performance and Short-Term Memory, speed processing, executive function and attention. This study one did not mett, however, the establishment of a statistically significant correlation between the Working Memory functions and the Visual Spatial Processing.

Les capacités attentionnelles sont nécessaires à la plupart des tâches de la vie quotidienne. Au cours du vieillissement normal, ces habiletés se modifient. De même, les études suggèrent que l’activité neurofonctionnelle du réseau fronto-pariétal qui sous-tend les capacités attentionnelles diffère entre les individus âgés et de jeunes adultes. Par contre, les changements en contexte du vieillissement du réseau fronto-pariétal ventral, aussi appelé le réseau attentionnel ventral, ont été peu investigués. Une telle question doit être soulevée dans le contexte où les plus récents modèles décrivant les changements fonctionnels associés au vieillissement rapportent que des possibles transformations neurofonctionnelles peuvent survenir au niveau intrahémisphérique et interhémisphérique. Le but de cet ouvrage est de déterminer comment le vieillissement normal affecte le réseau attentionnel ventral et de décrire la nature des changements qui peuvent survenir sur les axes intra et interhémisphériques. Pour y parvenir, la méthode de connectivité fonctionnelle fut privilégiée puisqu’elle permet de quantifier l’interaction neurofonctionnelle entre diverses régions composant un réseau fonctionnel. La première étude de cette thèse a permis de décrire les modifications de connectivité fonctionelle intrahémisphériques du réseau attentionnel ventral en comparant des adultes jeunes et âgés lorsqu’ils réalisent une tâche d’attention sélective en imagerie par résonance magnétique. Sur le plan comportemental, les individus âgés répondaient significativement plus lentement et commettaient davantage d’erreurs que le groupe composé de jeunes adultes. Les résultats de connectivité fonctionnelle montrent que le degré d’intégration de la connectivité fonctionnelle intrahémisphérique est globalement plus élevé chez les individus âgés dans l’ensemble des régions fronto-pariétales composant ce réseau. De plus, il semble que les aires antérieures du réseau, soit les aires préfrontales et insulaires, sont moins intégrées chez les individus âgés, alors que les zones pariétales, temporales et cérébelleuses le sont davantage. Le degré d’intégration de la connectivité est également plus élevé chez les adultes âgés entre les régions postérieures et antérieures. Ainsi, les résultats de cette étude suggèrent que la dynamique des régions antérieures et postérieures du réseau attentionnel ventral est modifiée au cours du vieillissement normal et que les régions postérieures occupent au sein de ce réseau un rôle plus important avec l’âge. Cette hyperconnectivité des aires pariétales pourrait représenter une stratégie de compensation intrahémisphérique (i.e. recrutement de régions additionnelles en postérieur) qui aurait cependant atteint un certain plateau puisque bien que les âgés réussissent à réaliser la tâche, ils performent significativement plus faiblement que de jeunes adultes. La seconde étude s’est intéressée aux modifications de connectivité interhémisphériques du même réseau fonctionnel en comparant le degré de connectivité fonctionnelle entre des individus jeunes et âgés. De manière similaire à l’étude 1, sur le plan comportemental les individus âgés répondaient significativement plus lentement et commettaient plus d’erreurs que les jeunes adultes. En ce qui concerne la dimension inter-hémisphérique du réseau, les résultats des analyses de connectivité montrent que le degré d’intégration des régions hémisphériques gauches fronto-pariétales et temporales est plus faible pour les participants âgés que pour les participants jeunes. Au contraire, les régions frontales, pariétales, temporales et sous-corticales de l’hémisphère droit sont plus intégrées. Par ailleurs, les résultats montrent également que le degré d’intégration interhémisphérique est plus élevé chez les individus âgés. Ainsi, cette étude suggère que le degré de connectivité fonctionnelle entre les régions hémisphériques droites du réseau attentionnel ventral augmente au cours du vieillissement, suggérant ainsi une amplification de la latéralisation de ce réseau vers l’hémisphère droit avec l’âge. Cette étude montre également que malgré une augmentation de la latéralisation du VAN à droite, celle-ci s’accompagne d’une augmentation du degré de connectivité fonctionnelle interhémisphérique qui pourrait être envisagée comme une tentative de compensation interhémisphérique (i.e. recrutement des régions homologues) qui aurait atteint toutefois un certain plateau car même si les âgés réussissent à réaliser la tâche, leur niveau de performance reste significativement plus faible que les jeunes. En somme, ce travail a permis de contribuer à notre compréhension de l’impact du vieillissement sur le réseau attentionnel ventral sur l’axe intrahémisphérique et interhémisphérique. Cet ouvrage lance de nouvelles pistes d’investigation dans ce domaine et pourrait éventuellement mener à l’élaboration d’interventions susceptibles de promouvoir une santé cognitive optimale lors du vieillissement. / Attention is necessary for most of daily life’s tasks. During aging, these cognitive abilities are changed. Studies suggest that the neurofunctional activity of the frontoparietal network, which upholds the attentional capacities, differ between young and older adults. However, age-related changes of the ventral frontoparietal network, also called the ventral attention network, have been less investigated. Such question has to be raised in context of recent models of neurofunctional changes in aging, who report possible functional transformation that could occur both at the intrahemispheric and interhemispheric levels. The goal of the present thesis is to determine how aging affects the ventral attention network and describe the nature of such changes that can occur on the intrahemispheric and interhemispheric axis. To do so, functional connectivity methods were favoured because of their capacity to measure the neurofunctional interaction between the regions of a network. The first study of the present thesis has allowed describing the age-related intrahemispheric modifications of functional connectivity in this network by comparing young and older adults while they respond on a selective attention task during a functional magnetic resonance imagery scan. On the task, aged adults performed significantly slower and made more errors than the young adults. At the functional connectivity level, the results show higher level of the functional connectivity between all frontoparietal regions of this network for the older group. Further, the integration level of functional connectivity in anterior regions of the network seems to be less integrated for the older participants, while posterior regions have more neurofunctional signal dependency. Also, the level of integration of functional connectivity is higher in older adults between anterior and posterior regions. Thus, results from this study suggest that the anterior and posterior regions of the ventral attention network interact differently during aging and that the posterior regions play a more important role with age in this network. This hyperconnectivity in the parietal regions could represent an unsuccessful intrahemispheric compensation attempt (i.e. recruitment of additional regions in posterior part of the brain) since older adults perform significantly less well than younger adults. The second study has investigated interhemispheric alterations of functional connectivity in the same functional network by comparing young and older adults. Like in the first study, younger adults were faster to respond on task and were more accurate. Regarding the neurofunctional lateralization of the network, the degree of functional connectivity is lower in older adults for the left hemisphere’s frontoparietal and temporal regions. However, older adults have a higher degree of functional connectivity in the right frontal, parietal, temporal and subcortical regions of the same network. Also, the results also show that the interhemispheric integration level is superior for the older adults. Thus, this study suggests that the level of functional connectivity with the right hemisphere’s regions of the ventral attention network increases with age, which could suggest an age-related lateralization of this network towards the right hemisphere. In this context, increased interhemispheric functional connectivity could be interpreted as a failed interhemispheric compensation attempt (i.e. recruitment of homologous regions) since the performance of older adults on task was significantly lower than younger adults. In short, this work has allowed contributing to our understanding of the impact of aging on the ventral attention network both on the intrahemispheric and interhemispheric axis. These various results bring up new hypothesis that needs to be investigated in further studies and eventually that could lead to the establishment of intervention that promote an optimal healthy cognitive aging.

In the human visual system, one of the most prominent functions of the extensive feedback from the higher brain areas within and outside of the visual cortex is attentional modulation. The feedback helps the brain to concentrate its resources on visual features that are relevant for recognition, i. e. it iteratively selects certain aspects of the visual scene for refined processing by the lower areas until the inference process in the higher areas converges to a single hypothesis about this scene. In order to minimize a number of required selection-refinement iterations, one has to find a short sequence of maximally informative portions of the visual input. Since the feedback is not static, the selection process is adapted to a scene that should be recognized. To find a scene-specific subset of informative features, the adaptive selection process on every iteration utilizes results of previous processing in order to reduce the remaining uncertainty about the visual scene. This phenomenon inspired us to develop a computational algorithm solving a visual classification task that would incorporate such principle, adaptive feature selection. It is especially interesting because usually feature selection methods are not adaptive as they define a unique set of informative features for a task and use them for classifying all objects. However, an adaptive algorithm selects features that are the most informative for the particular input. Thus, the selection process should be driven by statistics of the environment concerning the current task and the object to be classified. Applied to a classification task, our adaptive feature selection algorithm favors features that maximally reduce the current class uncertainty, which is iteratively updated with values of the previously selected features that are observed on the testing sample. In information-theoretical terms, the selection criterion is the mutual information of a class variable and a feature-candidate conditioned on the already selected features, which take values observed on the current testing sample. Then, the main question investigated in this thesis is whether the proposed adaptive way of selecting features is advantageous over the conventional feature selection and in which situations. Further, we studied whether the proposed adaptive information-theoretical selection scheme, which is a computationally complex algorithm, is utilized by humans while they perform a visual classification task. For this, we constructed a psychophysical experiment where people had to select image parts that as they think are relevant for classification of these images. We present the analysis of behavioral data where we investigate whether human strategies of task-dependent selective attention can be explained by a simple ranker based on the mutual information, a more complex feature selection algorithm based on the conventional static mutual information and the proposed here adaptive feature selector that mimics a mechanism of the iterative hypothesis refinement. Hereby, the main contribution of this work is the adaptive feature selection criterion based on the conditional mutual information. Also it is shown that such adaptive selection strategy is indeed used by people while performing visual classification.:1. Introduction 2. Conventional feature selection 3. Adaptive feature selection 4. Experimental investigations of ACMIFS 5. Information-theoretical strategies of selective attention 6. Discussion Appendix Bibliography

Réalisée en cotutelle avec l'Unité de Formation à la Recherche Lettres Arts et Sciences Humaines - Université Nice-Sophia Antipolis. / La survenue de modifications cérébrales issues du vieillissement normal ou provoquées par la maladie d’Alzheimer entraîne un certain bouleversement de l’attention visuelle sélective, soit la capacité à centrer volontairement les mécanismes de perception sur un stimulus particulier en négligeant les stimuli non pertinents. A ces modifications viennent s’ajouter des phénomènes de réorganisations cérébrales qui peuvent s’illustrer sur l’axe inter-hémisphérique par une réduction de la latéralisation cérébrale dans le vieillissement normal et sur l’axe intra-hémisphérique par un accroissement de l’engagement des régions frontales et préfrontales dans le vieillissement normal et la maladie d’Alzheimer. Toutefois, les mécanismes sur lesquels reposent ces phénomènes de réorganisations cérébrales dans le vieillissement normal et la maladie d’Alzheimer dans le contexte de l’attention visuelle restent peu compris. Ce travail de thèse s’intéresse à la nature de la réorganisation cérébrale dans le vieillissement normal (inter-hémisphérique vs. intra-hémisphérique) en condition d’attention visuelle sélective et s’inscrit dans le modèle de la réserve cognitive afin de préciser la nature des mécanismes sous-jacents de la réorganisation cérébrale (réserve neurale vs. compensation neurale). Concernant la maladie d’Alzheimer, ce travail met l’emphase sur la dynamique inter-hémisphérique et son principal substrat anatomique, le corps calleux, afin d’étudier l’efficience des mécanismes inter-hémisphériques. Dans l’ensemble de cette thèse, le substrat (fonctionnel et anatomique) de la réorganisation cérébrale est étudié en manipulant la complexité, ce qui permet d’identifier le mode d’adaptation face à l’accroissement de la demande cognitive soit, les mécanismes qui permettent de faire face au vieillissement normal et aux troubles de la cognition mais aussi d’étudier l’efficience de ces mécanismes. Le premier chapitre présente l’attention sélective, ses modifications neuro-fonctionnelles et les divers phénomènes de réorganisations cérébrales connus au cours du vieillissement normal et dans la maladie d’Alzheimer. Le cadre théorique dans lequel s’inscrit cette thèse, celui de la réserve cognitive, et la problématique du travail de recherche sont également décrits en fin de chapitre. La recherche effectuée est ensuite rapportée sous forme de 5 chapitres distincts. Enfin, les concepts théoriques évoqués par la revue de la littérature et les résultats des expérimentations font l’objet d’une discussion générale. Le deuxième chapitre de cette thèse (Article 1), visant à effectuer une revue de la littérature concernant les aspects inter-hémisphériques, permet de préciser, pour la première fois dans le domaine, le rôle du couplage inter-hémisphérique des ressources cérébrales dans la réorganisation au cours du vieillissement normal, mais aussi de considérer l’effet du découplage dans la maladie d’Alzheimer. Le troisième chapitre (Article 2) explore les mécanismes de réorganisation cérébrale dans le vieillissement normal face à l’accroissement de la charge attentionnelle au niveau perceptif de l’attention sélective (i.e., appariement perceptuel, A-A). Les résultats montrent un patron de réorganisation intra-hémisphérique chez les âgés, sous-tendu en situation de faible charge attentionnelle (i.e., 3 lettres) par la compensation neurale mais qui se voit associée à l’implication concomitante de la réserve neurale en situation de charge attentionnelle élevée (i.e., 5 lettres). Ce travail montre, par conséquent, une certaine flexibilité dans le déploiement de ces mécanismes qui apparaît modulé par la demande cognitive de la tâche. Le quatrième chapitre (Article 3) examine ces mêmes mécanismes de réorganisation dans le vieillissement normal, dans le même contexte attentionnel en ayant recours à un niveau de charge attentionnelle faible (i.e., 3 lettres) et à un niveau de charge attentionnelle élevé (i.e., 5 lettres), mais à un niveau plus complexe de l’attention sélective du fait qu’il implique une opération d’appariement basée sur le nom de la lettre (i.e., appariement nominatif, a-A). Les résultats montrent une amplification du renversement postéro-antérieur avec l’âge sur l’axe intra-hémisphérique face à l’accroissement de la charge attentionnelle (i.e., 5 lettres), suggérant ainsi que ce renversement est exclusivement lié à l’âge et qu’il repose principalement sur le mécanisme de compensation neurale. Le cinquième chapitre (Article 4) présente deux études dans lesquelles la présentation d’une tâche d’appariement de lettres en champ visuel divisé et l’IRM morphologique du corps calleux ont été couplées. L’objectif de cette étude était d’examiner la relation entre les variations de volume du corps calleux et la dynamique inter-hémisphérique chez des participants âgés atteints de la Maladie d’Alzheimer, de troubles cognitifs légers ou qui connaissent un vieillissement normal. La première étude se situait au niveau perceptuel et concernait 3 groupes de participants (Maladie d’Alzheimer vs trouble cognitif léger vs vieillissement normal). La deuxième étude concernait les participants atteints de trouble cognitif léger et des participants âgés contrôles sains et comportait deux tâches avec deux conditions de complexité chacune, une tâche dans laquelle la complexité varie selon le type de jugement - appariement perceptuel (e.g., A-A) vs. nominatif (e.g., a-A) - et une dans laquelle la complexité varie selon la charge attentionnelle (i.e., 3 lettres vs. 5 lettres). Les mesures IRM correspondent au volume total du corps calleux et à cinq volumes régionaux : C1-Rostrum, Genou et partie antérieure du tronc, C2-partie médiane, C3-partie caudale, C4-Isthme et C5-Splenium. Ces deux études montrent que le volume du corps calleux semble affecter la mise en jeu de l’effet facilitateur du traitement inter-hémisphérique selon l’état cognitif des participants et la nature de la demande cognitive mise en jeu. Cet impact modulateur semble partiellement déterminé dans la maladie d’Alzheimer par les portions plus antérieures du corps calleux (C2) et, via l’ensemble du corps calleux dans le vieillissement normal (C2, C3, C4). En revanche cette modulation semble déficitaire chez les participants atteints de trouble cognitif léger et reliée à une atrophie du corps calleux chez les participants atteints de trouble cognitif léger. Enfin, le chapitre 6 constitue la discussion générale de la thèse. L’ensemble des résultats est résumé et discuté en rapport aux différents modèles de la réorganisation cérébrale, de la réserve cognitive et de la dynamique hémisphérique. / Normal aging and disrupt selective visual attention – the ability to focus perceptual mechanisms on target stimuli by neglecting irrelevant stimuli. These modifications are accompanied by brain reorganization on both interhemispheric (hemispheric asymmetry reduction) and intrahemispheric (posterior-anterior shift) dimensions. However, the mechanisms underlying this brain reorganization in the context of visual attention in normal aging and Alzheimer’s disease remain largely unknown. The goal of this dissertation is to better understand the nature of brain reorganization for visual selective attention, as well as the functional neural changes and the anatomical modification in normal aging and in cognitive impairment in nthe ederly. In normal aging, this research focuses on the nature of the brain reorganization (interhemispheric vs. intrahemispheric) in the context of visual selective attention and based on the cognitive reserve model to differentiate the underlying mechanisms (neural reserve vs. neural compensation). It also focuses on Alzheimer’s disease in the context of interhemispheric dynamics and its main anatomical substrate, the corpus callosum. Throughout this dissertation, the substrate (functional and anatomical) of brain reorganization is examined by manipulating the cognitive demand associated with the tasks to better understand the mechanisms applied to cope with normal aging and cognitive impairment. The first chapter introduces the concepts of selective attention, neurofunctional changes in normal aging and cognitive impairment, and the cognitive reserve model before introducing the present research topic. The second chapter (Article 1) reviews the contribution of hemispheric coupling to the preservation of cognitive abilities with age and its uncoupling in Alzheimer’s disease. In this review, the variable contribution of the adaptive callosal mechanism is discussed with reference to successful aging and Alzheimer’s disease. The third (Article 2) and fourth chapters (Article 3) report studies conducted with functional magnetic resonance imaging (fMRI). The purpose of these studies was to investigate to what extent and how the neural reserve and neural compensation mechanisms contribute to coping with normal aging in two contexts of visual selective attention: simple perceptual processing and more complex naming processing. In both studies, the complexity of processes was also manipulated by varying the attentional load related to the number of stimuli to be processed (low: 3 letters vs. high: 5 letters). The results for the perceptual matching study, reported in the third chapter, support the neural compensation hypothesis of cognitive reserve, as the activations regions underlying task performance differed in the younger and older groups. The older group recruited bilateral frontal superior gyri more than the younger one, as in the PASA (posterior-anterior shift in aging) phenomenon, from the lowest attentional load level. In addition, the elderly were found to use compensation and neural reserve concurrently to cope with increasing attentional load for perceptual processing. The results for the naming matching study, reported in the fourth chapter, indicate a load-dependant PASA, supporting the hypothesis that an enhancement of compensatory mechanism is required for the most complex processing. The fifth chapter (Article 4) presents studies investigating the interhemispheric dynamic, using variants of the letter matching paradigm, where cognitive demand is parametrically varied, along with 3D callosal total and regional volume measured, in individuals with Alzheimer disease and amnestic-mild cognitive impairment as well as age-matched healthy individuals. The results show that the relationship between the corpus callosum volume and the across-field advantage emerges for the lowest cognitive demand level in participants with Alzheimer disease but exclusively at high cognitive demand levels in normal aging; this suggests the existence of an interhemispheric compensation mechanism in Alzheimer disease based on the integrity of the corpus callosum and similar processes to those that allow the normal aging brain to cope with cognitive demand. These mechanisms are deficient in amnestic-mild cognitive impairment individuals in the high attentional load condition, due to callosal atrophy. In chapter six the nature of mechanisms to cope with aging and cognitive impairment is discussed, addressing the two main dimensions of brain reorganization: intrahemispheric and interhemispheric. In normal aging, cerebral reorganization of visual selective attention implies an intrahemispheric PASA phenomenon based on neural compensation. To cope with increased cognitive demand, neural reserve can also be recruited in basic perceptual processing, while the recruitment of compensation mechanisms increases in more complex processing of visual selective attention. Our study suggests that compensation and reserve are flexible, adaptive and deployed according to the cognitive demand and the type of processing required. In cognitive impairment, our study suggests the existence of an interhemispheric compensation mechanism in Alzheimer’s disease based on integrity of the corpus callosum and similar processes to those that allow the normal aging brain to cope with cognitive demand. This study provides evidence that interhemispheric interactions, supported by the corpus callosum, constitute a flexible mechanism that can improve the brain’s ability to handle processing demands and thus may compensate for the neural decline in Alzheimer’s disease in low cognitive demand situations.

The aim of this thesis was to help elucidate the mechanisms that underlie age-related decline in visual selective attention and working memory (WM). Older and younger adults completed a behavioural WM task, after which electroencephalogram (EEG) was recorded as participants perform a localized attentional interference (LAI) task – competition/attentional interference was manipulated by systematically altering the distance between targets and distractors. Older adults showed impaired accuracy and reaction time on the WM and LAI tasks. Two event-related-potentials, indexing spatial attention (N2pc) and target processing (Ptc), displayed attenuated amplitude and increased latency in older adults. Thus, spatial selection, target enhancement and processing speed deficits may contribute to age-related attentional impairments. Furthermore, an unexpected component was found between the N2pc and Ptc in the older adult waveforms. Preliminary analyses suggest this may be the PD, implicated in distractor suppression, which may be differentially contributing to older and younger adults’ electrophysiology and attentional processing.

La survenue de modifications cérébrales issues du vieillissement normal ou provoquées par la maladie d’Alzheimer entraîne un certain bouleversement de l’attention visuelle sélective, soit la capacité à centrer volontairement les mécanismes de perception sur un stimulus particulier en négligeant les stimuli non pertinents. A ces modifications viennent s’ajouter des phénomènes de réorganisations cérébrales qui peuvent s’illustrer sur l’axe inter-hémisphérique par une réduction de la latéralisation cérébrale dans le vieillissement normal et sur l’axe intra-hémisphérique par un accroissement de l’engagement des régions frontales et préfrontales dans le vieillissement normal et la maladie d’Alzheimer. Toutefois, les mécanismes sur lesquels reposent ces phénomènes de réorganisations cérébrales dans le vieillissement normal et la maladie d’Alzheimer dans le contexte de l’attention visuelle restent peu compris. Ce travail de thèse s’intéresse à la nature de la réorganisation cérébrale dans le vieillissement normal (inter-hémisphérique vs. intra-hémisphérique) en condition d’attention visuelle sélective et s’inscrit dans le modèle de la réserve cognitive afin de préciser la nature des mécanismes sous-jacents de la réorganisation cérébrale (réserve neurale vs. compensation neurale). Concernant la maladie d’Alzheimer, ce travail met l’emphase sur la dynamique inter-hémisphérique et son principal substrat anatomique, le corps calleux, afin d’étudier l’efficience des mécanismes inter-hémisphériques. Dans l’ensemble de cette thèse, le substrat (fonctionnel et anatomique) de la réorganisation cérébrale est étudié en manipulant la complexité, ce qui permet d’identifier le mode d’adaptation face à l’accroissement de la demande cognitive soit, les mécanismes qui permettent de faire face au vieillissement normal et aux troubles de la cognition mais aussi d’étudier l’efficience de ces mécanismes. Le premier chapitre présente l’attention sélective, ses modifications neuro-fonctionnelles et les divers phénomènes de réorganisations cérébrales connus au cours du vieillissement normal et dans la maladie d’Alzheimer. Le cadre théorique dans lequel s’inscrit cette thèse, celui de la réserve cognitive, et la problématique du travail de recherche sont également décrits en fin de chapitre. La recherche effectuée est ensuite rapportée sous forme de 5 chapitres distincts. Enfin, les concepts théoriques évoqués par la revue de la littérature et les résultats des expérimentations font l’objet d’une discussion générale. Le deuxième chapitre de cette thèse (Article 1), visant à effectuer une revue de la littérature concernant les aspects inter-hémisphériques, permet de préciser, pour la première fois dans le domaine, le rôle du couplage inter-hémisphérique des ressources cérébrales dans la réorganisation au cours du vieillissement normal, mais aussi de considérer l’effet du découplage dans la maladie d’Alzheimer. Le troisième chapitre (Article 2) explore les mécanismes de réorganisation cérébrale dans le vieillissement normal face à l’accroissement de la charge attentionnelle au niveau perceptif de l’attention sélective (i.e., appariement perceptuel, A-A). Les résultats montrent un patron de réorganisation intra-hémisphérique chez les âgés, sous-tendu en situation de faible charge attentionnelle (i.e., 3 lettres) par la compensation neurale mais qui se voit associée à l’implication concomitante de la réserve neurale en situation de charge attentionnelle élevée (i.e., 5 lettres). Ce travail montre, par conséquent, une certaine flexibilité dans le déploiement de ces mécanismes qui apparaît modulé par la demande cognitive de la tâche. Le quatrième chapitre (Article 3) examine ces mêmes mécanismes de réorganisation dans le vieillissement normal, dans le même contexte attentionnel en ayant recours à un niveau de charge attentionnelle faible (i.e., 3 lettres) et à un niveau de charge attentionnelle élevé (i.e., 5 lettres), mais à un niveau plus complexe de l’attention sélective du fait qu’il implique une opération d’appariement basée sur le nom de la lettre (i.e., appariement nominatif, a-A). Les résultats montrent une amplification du renversement postéro-antérieur avec l’âge sur l’axe intra-hémisphérique face à l’accroissement de la charge attentionnelle (i.e., 5 lettres), suggérant ainsi que ce renversement est exclusivement lié à l’âge et qu’il repose principalement sur le mécanisme de compensation neurale. Le cinquième chapitre (Article 4) présente deux études dans lesquelles la présentation d’une tâche d’appariement de lettres en champ visuel divisé et l’IRM morphologique du corps calleux ont été couplées. L’objectif de cette étude était d’examiner la relation entre les variations de volume du corps calleux et la dynamique inter-hémisphérique chez des participants âgés atteints de la Maladie d’Alzheimer, de troubles cognitifs légers ou qui connaissent un vieillissement normal. La première étude se situait au niveau perceptuel et concernait 3 groupes de participants (Maladie d’Alzheimer vs trouble cognitif léger vs vieillissement normal). La deuxième étude concernait les participants atteints de trouble cognitif léger et des participants âgés contrôles sains et comportait deux tâches avec deux conditions de complexité chacune, une tâche dans laquelle la complexité varie selon le type de jugement - appariement perceptuel (e.g., A-A) vs. nominatif (e.g., a-A) - et une dans laquelle la complexité varie selon la charge attentionnelle (i.e., 3 lettres vs. 5 lettres). Les mesures IRM correspondent au volume total du corps calleux et à cinq volumes régionaux : C1-Rostrum, Genou et partie antérieure du tronc, C2-partie médiane, C3-partie caudale, C4-Isthme et C5-Splenium. Ces deux études montrent que le volume du corps calleux semble affecter la mise en jeu de l’effet facilitateur du traitement inter-hémisphérique selon l’état cognitif des participants et la nature de la demande cognitive mise en jeu. Cet impact modulateur semble partiellement déterminé dans la maladie d’Alzheimer par les portions plus antérieures du corps calleux (C2) et, via l’ensemble du corps calleux dans le vieillissement normal (C2, C3, C4). En revanche cette modulation semble déficitaire chez les participants atteints de trouble cognitif léger et reliée à une atrophie du corps calleux chez les participants atteints de trouble cognitif léger. Enfin, le chapitre 6 constitue la discussion générale de la thèse. L’ensemble des résultats est résumé et discuté en rapport aux différents modèles de la réorganisation cérébrale, de la réserve cognitive et de la dynamique hémisphérique. / Normal aging and disrupt selective visual attention – the ability to focus perceptual mechanisms on target stimuli by neglecting irrelevant stimuli. These modifications are accompanied by brain reorganization on both interhemispheric (hemispheric asymmetry reduction) and intrahemispheric (posterior-anterior shift) dimensions. However, the mechanisms underlying this brain reorganization in the context of visual attention in normal aging and Alzheimer’s disease remain largely unknown. The goal of this dissertation is to better understand the nature of brain reorganization for visual selective attention, as well as the functional neural changes and the anatomical modification in normal aging and in cognitive impairment in nthe ederly. In normal aging, this research focuses on the nature of the brain reorganization (interhemispheric vs. intrahemispheric) in the context of visual selective attention and based on the cognitive reserve model to differentiate the underlying mechanisms (neural reserve vs. neural compensation). It also focuses on Alzheimer’s disease in the context of interhemispheric dynamics and its main anatomical substrate, the corpus callosum. Throughout this dissertation, the substrate (functional and anatomical) of brain reorganization is examined by manipulating the cognitive demand associated with the tasks to better understand the mechanisms applied to cope with normal aging and cognitive impairment. The first chapter introduces the concepts of selective attention, neurofunctional changes in normal aging and cognitive impairment, and the cognitive reserve model before introducing the present research topic. The second chapter (Article 1) reviews the contribution of hemispheric coupling to the preservation of cognitive abilities with age and its uncoupling in Alzheimer’s disease. In this review, the variable contribution of the adaptive callosal mechanism is discussed with reference to successful aging and Alzheimer’s disease. The third (Article 2) and fourth chapters (Article 3) report studies conducted with functional magnetic resonance imaging (fMRI). The purpose of these studies was to investigate to what extent and how the neural reserve and neural compensation mechanisms contribute to coping with normal aging in two contexts of visual selective attention: simple perceptual processing and more complex naming processing. In both studies, the complexity of processes was also manipulated by varying the attentional load related to the number of stimuli to be processed (low: 3 letters vs. high: 5 letters). The results for the perceptual matching study, reported in the third chapter, support the neural compensation hypothesis of cognitive reserve, as the activations regions underlying task performance differed in the younger and older groups. The older group recruited bilateral frontal superior gyri more than the younger one, as in the PASA (posterior-anterior shift in aging) phenomenon, from the lowest attentional load level. In addition, the elderly were found to use compensation and neural reserve concurrently to cope with increasing attentional load for perceptual processing. The results for the naming matching study, reported in the fourth chapter, indicate a load-dependant PASA, supporting the hypothesis that an enhancement of compensatory mechanism is required for the most complex processing. The fifth chapter (Article 4) presents studies investigating the interhemispheric dynamic, using variants of the letter matching paradigm, where cognitive demand is parametrically varied, along with 3D callosal total and regional volume measured, in individuals with Alzheimer disease and amnestic-mild cognitive impairment as well as age-matched healthy individuals. The results show that the relationship between the corpus callosum volume and the across-field advantage emerges for the lowest cognitive demand level in participants with Alzheimer disease but exclusively at high cognitive demand levels in normal aging; this suggests the existence of an interhemispheric compensation mechanism in Alzheimer disease based on the integrity of the corpus callosum and similar processes to those that allow the normal aging brain to cope with cognitive demand. These mechanisms are deficient in amnestic-mild cognitive impairment individuals in the high attentional load condition, due to callosal atrophy. In chapter six the nature of mechanisms to cope with aging and cognitive impairment is discussed, addressing the two main dimensions of brain reorganization: intrahemispheric and interhemispheric. In normal aging, cerebral reorganization of visual selective attention implies an intrahemispheric PASA phenomenon based on neural compensation. To cope with increased cognitive demand, neural reserve can also be recruited in basic perceptual processing, while the recruitment of compensation mechanisms increases in more complex processing of visual selective attention. Our study suggests that compensation and reserve are flexible, adaptive and deployed according to the cognitive demand and the type of processing required. In cognitive impairment, our study suggests the existence of an interhemispheric compensation mechanism in Alzheimer’s disease based on integrity of the corpus callosum and similar processes to those that allow the normal aging brain to cope with cognitive demand. This study provides evidence that interhemispheric interactions, supported by the corpus callosum, constitute a flexible mechanism that can improve the brain’s ability to handle processing demands and thus may compensate for the neural decline in Alzheimer’s disease in low cognitive demand situations. / Réalisée en cotutelle avec l'Unité de Formation à la Recherche Lettres Arts et Sciences Humaines - Université Nice-Sophia Antipolis.

The aim of this thesis was to help elucidate the mechanisms that underlie age-related decline in visual selective attention and working memory (WM). Older and younger adults completed a behavioural WM task, after which electroencephalogram (EEG) was recorded as participants perform a localized attentional interference (LAI) task – competition/attentional interference was manipulated by systematically altering the distance between targets and distractors. Older adults showed impaired accuracy and reaction time on the WM and LAI tasks. Two event-related-potentials, indexing spatial attention (N2pc) and target processing (Ptc), displayed attenuated amplitude and increased latency in older adults. Thus, spatial selection, target enhancement and processing speed deficits may contribute to age-related attentional impairments. Furthermore, an unexpected component was found between the N2pc and Ptc in the older adult waveforms. Preliminary analyses suggest this may be the PD, implicated in distractor suppression, which may be differentially contributing to older and younger adults’ electrophysiology and attentional processing.

<pre><p>Difficulty understanding speech in background noise is the most common hearing complaint. Elucidating the neurophysiological mechanisms underlying speech intelligibility in everyday environments with multiple sound sources and distortions is hence important for any technology that aims to improve real-world listening. Using a combination of behavioral, electroencephalography (EEG), and computational modeling experiments, this dissertation provides insight into how the brain analyzes such complex scenes, and what roles different acoustic cues play in facilitating this process and in conveying phonetic content. Experiment #1 showed that brain oscillations selectively track the temporal envelopes (i.e., modulations) of attended speech in a mixture of competing talkers, and that the strength and pattern of this attention effect differs between individuals. Experiment #2 showed that the fidelity of neural tracking of attended-speech envelopes is strongly shaped by the modulations in interfering sounds as well as the temporal fine structure (TFS) conveyed by the cochlea, and predicts speech intelligibility in diverse listening environments. Results from Experiments #1 and #2 support the theory that temporal coherence of sound elements across envelopes and/or TFS shapes scene analysis and speech intelligibility. Experiment #3 tested this theory further by measuring and computationally modeling consonant categorization behavior in a range of background noises and distortions. We found that a physiologically plausible model that incorporated temporal-coherence effects predicted consonant confusions better than conventional speech-intelligibility models, providing independent evidence that temporal coherence influences scene analysis. Finally, results from Experiment #3 also showed that TFS is used to extract speech content (voicing) for consonant categorization even when intact envelope cues are available. Together, the novel insights provided by our results can guide future models of speech intelligibility and scene analysis, clinical diagnostics, improved assistive listening devices, and other audio technologies.</p></pre>