Myelin facilitates efficient neural signal transmission along axons. While predominantly present in white matter, myelin is also found in smaller amounts in the outermost layer of gray matter, known as the cortex, as well as deep subcortical structures. Quantitative magnetic resonance imaging (qMRI) metrics, including longitudinal relaxation rate (R1), can report myelin levels in vivo, although these measures are often estimated and can vary depending on pulse sequences and acquisition parameters used to collect the data. Multi-site approaches can increase the statistical power and ecological validity of studies by recruiting participants from larger and more diverse samples, but considerable variability has previously been reported in qMRI estimates of myelin computed using data from different MRI scanners. Here, we assessed intra- and inter-vendor variability in R1 maps acquired at three sites using scanners from two manufacturers, General Electric (GE) and Siemens. Two ‘traveling heads’ scans, where a participant visits and completes a scan at each site, were acquired to directly assess site effects independent of biological variability. Histogram-based site-specific scaling factors determined from the ‘traveling heads’ scans were then applied to harmonize the main dataset. Before site correction, comparable inter- and intra-vendor differences were observed in both the main dataset and the ‘traveling heads’ scans. Our method of harmonization reduced the inter-site variability considerably, while remaining sensitive to age effects in R1. We then characterized age trajectories of R1 across the whole brain using linear regressions. In the age range of 16-36, age was significantly associated with mean R1s of bilateral putamen, left pallidum, and left amygdala, in line with past research. Cortical analyses also replicated earlier findings, with higher age associations in regions of the motor and lateral frontal cortices. The findings contribute to the growing research in multi-site harmonization, while providing normative estimates and trajectories of whole-brain R1s that can be referred to in future clinical studies. / Thesis / Master of Science (MSc) / Myelin is crucial for efficient signal transmission in nerve cells, and advancements in magnetic resonance imaging (MRI) have allowed us to indirectly estimate myelin levels in the brain. Collaborative multi-site investigations have become increasingly popular due to their ability to recruit larger and more diverse samples, making their findings more generalizable to the public. However, differences in scanner hardware and configurations across sites can introduce site-dependent biases to MRI scans. In this thesis, we found considerable inter-site variability in our whole-brain MRI-based estimate of myelin based on longitudinal relaxation rate (R1). We thus developed a method that can remove unwanted site-dependent differences from images by comparing repeated scans across sites. Using the corrected data, we showed how myelin amounts in different brain structures changes with age during early adulthood. Overall, we highlight the importance of carefully considering site-specific differences in multi-site MRI data and characterized how myelin changes with age in young adults.

No description available.

Thèse de doctorat présenté en vue de l'obtention du doctorat en psychologie - recherche intervention, option neuropsychologie clinique (Ph.D) / Selon les dernières estimations mondiales, plus de 43,3 millions d’individus seraient atteints d’une privation visuelle. Autrement dit, l’aptitude de ces individus à percevoir visuellement le monde et les détails qui le composent est presque ou totalement nulle. Bien que privés de l’un des sens les plus importants, ces derniers parviennent à s’adapter et interagir, de façon remarquable, avec l’environnement, et ce, en s’appuyant principalement sur leurs sens préservés. La navigation dans l’espace, la localisation et l’identification d’objets ou de personnes ainsi que la lecture ne sont que quelques exemples permettant d’illustrer la compensation comportementale réalisée par l’entremise des autres canaux sensoriels chez les individus atteints de cécité. L’accomplissement fructueux de telles activités nécessite certes un apprentissage chez les individus aveugles, mais permet également de mettre en lumière qu’au-delà d’une différence de perception visuelle, l’adaptation à une existence vécue dans l’obscurité la plus complète occasionne des changements dans le traitement de l’information en provenance des autres modalités sensorielles. Ces transformations comportementales, issues principalement de la réorganisation cérébrale occasionnée par un déficit perceptif, constitue un sujet d’intérêt pour la communauté scientifique depuis de nombreuses années. Bien que l’apparition des techniques d’imagerie cérébrale non-invasives a permis d’élucider jusqu’à présent de nombreux mystères concernant l’incroyable capacité du cerveau à se modifier sous l’effet de l’expérience, de nombreuses questions demeurent sans réponse. Ainsi, les articles composant cette thèse ont pour objectif principal de contribuer à l’état des connaissances actuelles concernant la réorganisation cérébrale chez les individus atteints de cécité, tant au niveau fonctionnel que structurel, et ce, afin de mieux saisir les répercussions de ces changements cérébraux sur le comportement des non-voyants. Plus particulièrement, nous avons souhaité investiguer l’impact de la privation visuelle précoce sur les régions cérébrales impliquées dans traitement des informations chimiosensorielles (olfactives et trigéminales). Pour ce faire, nous avons d’abord souhaité préciser les mécanismes de réorganisation cérébrale de type structurel prenant place au sein des régions traitant l’information de nature olfactive auprès d’aveugles congénitaux, et ce en comparaison à leurs pairs voyants. L’article 1 de cette thèse révèle que les individus aveugles présentent de multiples altérations cérébrales dans les régions composant le système olfactif. Bien que les aveugles congénitaux inclus dans notre étude présentent des volumes significativement réduits au niveau des bulbes olfactifs, du cortex orbitofrontal et du complexe parahippocampique, leurs performances olfactives, mesurées à l’aide d’épreuves standardisées, demeurent comparables à celles obtenues chez leurs pairs voyants. Nos résultats supportent ainsi la présence d’une réorganisation intramodale au sein du système olfactif chez les aveugles congénitaux, laquelle n’ayant aucune incidence mesurable sur leurs performances olfactive. D’autre part, nous avons souhaité explorer, par le biais de l’imagerie par résonnance magnétique fonctionnelle, si, tout comme pour le traitement de stimuli auditifs et tactiles, les régions habituellement dévolues au traitement visuel participaient, chez l’individu atteint cécité au traitement particulier d’une odeur. Plus précisément, l’article 2 de cette thèse révèle que les aveugles congénitaux sollicitent davantage leur cortex occipital que leurs pairs voyants lors d’une tâche de localisation d’odeurs. Toutefois, lorsque ces mêmes odeurs doivent être identifiées, aveugles et voyants présentent des niveaux d’activité cérébrales comparable dans les régions occipitales. Nos résultats supportent ainsi la présence d’un recrutement intermodal lors du traitement chimiosensoriel de différentes odeurs. Mis ensemble, ces résultats contribuent à une meilleure appréciation des changements cérébraux à la suite d’une privation visuelle, notamment en ce qui concerne les régions impliquées dans le traitement des informations de nature chimiosensorielle. Cette thèse doctorale s’inscrit de façon intéressante au sein des théories de la compensation comportementale présente chez l’individu aveugles, de même que les mécanismes sous-tendant la neuroplasticité structurelle et fonctionnelle des systèmes sensorielles. / According to the latest global estimates, more than 43.3 million people suffer from visual deprivation. In other words, the ability of these individuals to visually perceive the world and the details that compose it is almost or totally absent. Although deprived of one of the most important senses, they manage to adapt and interact with their environment, and this, by relying mainly on their remaining senses. Navigating in space, locating and identifying objects or people, and reading are just few examples that illustrate the behavioral compensation achieved through other sensory channels among individuals living with blindness. The successful accomplishment of such activities certainly requires learning or adaptation. However, it also highlights that beyond a difference in visual perception, adaptation to an existence lived in complete darkness causes changes in the processing of information from other sensory modalities. These behavioral transformations, resulting from the cerebral reorganization caused by a perceptual deficit, have been a subject of interest in the scientific community for many years. Although the emergence of non-invasive brain imaging techniques has, so far, elucidated many mysteries concerning the incredible capacity of the brain to change under the effect of experience, many questions remain unanswered. Thus, the articles included in this thesis have the main objective of contributing to the current knowledge of cerebral reorganization found in individuals living with blindness. More specifically, we wanted to investigate the impact of early visual deprivation on brain regions involved in the processing of chemosensory information (olfactory and trigeminal). To do this, we first evaluated the structural cerebral reorganization taking place among congenitally blind individuals within the olfactory regions. The first study of this thesis reveals that blind individuals present multiple cerebral alterations in the regions composing the olfactory system. More precisely, we found that congenitally blind individuals present significantly reduced olfactory bulb volume, as well as reduction in grey matter density in the orbitofrontal cortex and the parahippocampal complex. Despite this, their olfactory performances, measured using standardized tests, remain comparable to those obtained by their seeing counterparts. Therefore, our results support the presence in congenitally blind individuals of an intramodal reorganization within the olfactory system, which has no measurable impact on their olfactory performance. Through functional magnetic resonance imaging, we wanted to explore if regions that are usually devoted to visual processing are in congenitally blind individuals engaged for the processing of different components of an odor (i.e. olfactory and trigeminal components). More specifically, compared to their sighted counterparts, we found stronger activation in the occipital cortex of blind individuals during our odor localization task. However, when identifying the same odorants, blind and sighted individuals show comparable levels of brain activity in the occipital regions. Our results thus support the presence of cross-modal recruitment during the chemosensory processing of odors. Taken together, these results contribute to a better appreciation of cerebral changes following visual deprivation, particularly in regions involved for the processing of chemosensory information. In an interesting way, the results of this doctoral thesis fit some of the theories stated for blind individuals, such as the structural and functional neuroplasticity of sensory systems.

No description available.

No description available.

No description available.

No description available.

The study of brain nuclei in neuroimaging poses challenges owing to its small size. Many neuroimaging studies have been reported for effectively locating these nuclei and characterizing their functional connectivity with other regions of the brain. Hypothalamus, Locus Coeruleus, and Ventral Tegmental area are such nuclei found in the human brain, which are challenging to visualize owing to their size and lack of tissue contrast with surrounding regions. Resting-state functional magnetic resonance imaging (rsfMRI) analysis on these nuclei enabled researchers to characterize their connectivity with other regions of the brain. An automated method to successfully isolate voxels belonging to these nuclei is still a great challenge in the field of neuroimaging. Atlas-based segmentation is the most common method used to study the anatomy and the functional connectivity of these brain nuclei. However, atlas-based segmentation has shown inconsistency due to variation in brain atlases owing to different population studies. Therefore, in this study, we try to address the research problem of brain nuclei imaging using a clustering-based approach. Clustering-based methods separate of voxels utilizing their structural and functional homogeneity to each other. This type of method can help locate and cluster the voxels belonging to the nuclei. Elimination of erroneous voxels by the use of clustering methods would significantly improve the structural and functional analysis of the nuclei in the human brain. Since several clustering methods are available in neuroimaging studies, the goal of this study is to find a robust model that has less variability across different subjects. Non-parametrical statistical analysis was performed as functional magnetic resonance imaging (fMRI) based studies are corrupted with noise and artefact. Statistical investigation on the fMRI data helps to assess the significant experimental effects.

<p>The aim of this research study was to examine the clinical, behavioural and neurobiological characteristics of children who are considered to be at increased risk for developing anxiety disorders. The study population included high-risk children who have at least one parent with social phobia and normal-risk control subjects. The first objective of the study was to examine the prevalence of anxiety disorders in high-risk children. We determined the proportion of high-risk children who met criteria for a psychiatric disorder using structured clinical interviews and assessed symptom severity using measures of anxiety and depression. We found the prevalence of anxiety disorders to be elevated in high-risk children with 77% meeting criteria for a lifetime psychiatric disorder. High-risk subjects also had significantly higher levels of anxiety symptoms relative to normal-risk subjects. The second objective of the study was to examine threat-related attention processing in high-risk and normal-risk children using the dot-probe attention orienting task. We compared probe detection reaction times of high-risk children and normal-risk control children when they were exposed to emotional facial stimuli. We did not find any significant within-group or between-group differences in reaction times in our high-risk and normal-risk subjects. However, we did observe a trend towards longer reaction times in high-risk subjects for all trial types relative to normal-risk subjects, which could indicate general processing deficits in the high-risk group. The third objective of this study was to examine the activity of emotion processing brain regions using functional magnetic resonance imaging (<em>f</em>MRI) in children who are at increased risk for developing anxiety disorders. We compared the blood oxygenation level dependant (BOLD) response while high-risk and normal-risk subjects were engaged in the dot probe attention orienting task. Using <em>f</em>MRI, the BOLD response was measured while subjects were exposed to masked emotional (angry, happy or neutral) facial stimuli. We found increased activation of several frontal, temporal and limbic regions in high-risk subjects relative to normal-risk subjects during the presentation of emotional facial stimuli. These regions included the prefrontal cortex, anterior cingulate, hippocampus, insula, basal ganglia and temporal regions. To our knowledge this is the first study to characterize a sample of children at-risk for anxiety disorders using clinical, behavioural and neuroimaging data. The findings from this study demonstrate that high-risk children experience heightened anxiety symptoms and that they also present with functional abnormalities of brain regions involved in emotion processing. These results highlight the need for early identification and intervention for children at-risk for anxiety disorders. Future studies should aim for longitudinal study designs combined with neuroimaging techniques to examine changes in anxiety symptoms over time and to study the effects of treatment on the function of limbic and prefrontal structures in children at-risk for anxiety disorders.</p> / Doctor of Philosophy (PhD)

Life satisfaction is a key aspect of subjective well-being (SWB) and is often referred to as an individual’s cognitive assessment of their overall life. Measured by tools such as the Satisfaction with Life Scale (SWLS), life satisfaction is crucial for reliably assessing SWB. It serves as a stable measure, reflecting long-term judgements of SWB - unlike affect measures that capture more short-term judgements influenced by situational variability. While the SWLS demonstrates internal consistency, the neural correlates of life satisfaction remain largely unknown, limiting our comprehension of SWB’s cognitive dimension. This systematic review aims to bridge the gap by comprehensively examining relevant literature, emphasizing resting-state neuroimaging studies. Despite methodological differences, the authors discovered relationships between brain structures and life satisfaction, revealing a spectrum of associations ranging from positive to negative, alongside correlations with varying strengths. These structures include the left SFG, bilateral MFG, frontoinsular cortex, and other areas correlated with microstructural connections, including the SPL, IPL and TPJ. Considering the novelty and limited establishment of this subject, ongoing research is crucial for uncovering the precise neural correlates of life satisfaction.