Le vieillissement dit "naturel", affecte physiologiquement les structures auditives périphériques; il en va de même du collicule supérieur (CS) lors du traitement des signaux auditifs et visuels. Chez le rat âgé, la sensibilité auditive périphérique diminue et l’extraction des attributs des signaux auditifs est modifiée, et ce, dès les noyaux cochléaires (premiers relais centraux de la voie auditive ascendante). De plus, les propriétés spectrales, temporelles et directionnelles des neurones auditifs du CS sont altérées lors du vieillissement. Ceci se manifeste aussi au niveau de l’organisation topographique de la sensibilité à la direction qui est abolie au sein des couches profondes du CS. Dans la même optique, l’extraction des attributs des fréquences spatiales concentriques mobiles (en présence ou en absence d’objet sonore modulé en amplitude) est altérée aussi au niveau des neurones audiovisuels du CS lors du vieillissement. En effet, au niveau spatial, chez l’animal âgé, la présence de déficits visuels est particulièrement marquée par une diminution de la sensibilité aux stimuli visuels et audiovisuels mobiles et rapides lors du déplacement de l’organisme dans son environnement. Compte tenu de l’ampleur des changements qui s’installent avec le vieillissement au niveau des structures périphériques et centrales, inévitablement, les mécanismes nerveux de la plasticité audiovisuelle de bas niveau, i.e. au niveau des neurones du CS, sont perturbés. En outre, chez l’animal âgé, le gain audiovisuel induit par l’activité des neurones du CS ne produit pas d’amélioration notable par rapport à la modalité unisensorielle. Dans l’ensemble, ces résultats montrent que le vieillissement perturbe l’activité neuronale du CS permettant de détecter les informations sensorielles pertinentes dans un environnement audiovisuel complexe. / Age-related physiological changes affect the peripheral auditory structures; this also applies to the superior colliculus (SC) auditory and visual processes. In aged rats, the peripheral hearing sensitivity decreases and at more central regions, particularly the first central node, in the ascending auditory projections, auditory processing of sounds is altered. Furthermore, at the level of the SC, spectral, temporal and directional properties of auditory neurons are also altered during aging. In addition, no systematic directional spatial arrangement is encountered among the neurons of aged rats, implying that the topographical organization of the auditory directional map is abolished in the deep layers of the SC. Moreover, in a condition where SC visual neurons are stimulated with looming concentric sinusoidal gratings (in the presence or in the absence of modulated audio signals), visual deficits in aged animals are particularly marked by a decrease in sensitivity to fast moving visual and audiovisual stimuli during self-motion. Given the links of age-related changes in the peripheral and in the central structures, inevitably, the mechanisms underlying the neuronal audiovisual plasticity, in the low-level (SC), are somehow disrupted. Specifically, in aged animals, the presence of the auditory stimulus when coupled with the visual signal did not enhance the response activity of the visual neurons. This seems to suggest that the mechanism that may serve to amplify the visual signal under weak or ambiguous and noisy conditions thus improving greater behavioral relevance of detecting rapidly approaching visual and audiovisual moving objects during self-motion is altered with aging. Overall, these results show that aging disrupts the SC neuronal activity that enables detection of relevant sensory information in a complex audiovisual environment.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal.

La fovéa est la seule zone de la rétine où l’acuité visuelle est maximale. Les mécanismes qui visent à déterminer à quels endroits vont avoir lieu les prochaines fixations restent débattus. Certains auteurs suggèrent que les yeux sont principalement attirés par les zones les plus saillantes de la scène. D’autres suggèrent qu’ils sont orientés vers les objets. Les propriétés du système oculomoteur contraignent également les mouvements oculaires. L’objectif de cette thèse était de départager ces différents points de vue en examinant la contribution respective et le décours temporel des facteurs visuo-moteurs de bas niveau et des facteurs cognitifs de plus haut niveau dans la détermination des mouvements oculaires. Nous nous sommes focalisés sur trois phénomènes comportementaux clés : la tendance à bouger les yeux à proximité du centre des objets (Prefered-Viewing Location), les répercussions qu’ont ces positions d’atterrissage sur les durées initiales des fixations (Inverted Optimal-Viewing-Position) et les probabilités de refixation (Optimal-Viewing-Position). Nous avons observé que ces trois effets émergeaient, et ce, quel que soit le type de stimulus testé: des objets, des mots ou des objets sans signification. Cela suggère que ces effets reflètent les propriétés de bas niveau des systèmes visuel et oculomoteur. Par ailleurs, nous avons observé que l’endroit où les yeux atterrissaient au sein de l’objet était également influencé au fil du temps par les traitements de plus haut-niveau des propriétés du stimulus. Les saccades initiées plus tardivement atterrissaient plus proches de la zone la plus informative de l’objet. / High-quality vision is restricted to the fovea - a small region at the center of gaze. The mechanisms that determine which locations in a scene are selected for fixation remain debated. Some suggest that eye movements are mainly driven by the salient features in a scene. Others suggest that eye guidance is object based. The properties of the oculomotor system also strongly constrain eye behavior, but these have been neglected in most existing models. The purpose of this thesis was to disentangle between these different views, by investigating how low-level visuomotor factors versus higher-level cognitive factors contribute to eye movements towards and within isolated objects, and with which time course. We focused on three viewing-position effects: the tendency to move the eyes near the centers of objects (the PVL effect), and the repercussions these initial landing positions have on initial fixation durations (the I-OVP effect) and refixation probabilities (the OVP effect). We found that these three viewing-position effects emerged, and were comparable, in all stimulus types that we tested in this thesis: objects, words and even meaningless non-objects. This suggests that the effects reflect low-level properties of the visual and oculomotor systems. Furthermore, we found that where the eye moved within objects became influenced by ongoing processing of higher-level stimulus properties (e.g., object affordances) over time. Later- compared to early-triggered initial saccades, and even more so within-object refixations, were biased towards the most informative part of the objects, and away from their center of gravity.

Mémoire numérisé par la Division de la gestion de documents et des archives de l'Université de Montréal

Made available in DSpace on 2014-12-17T15:36:35Z (GMT). No. of bitstreams: 1 SebastiaoFS.pdf: 2551917 bytes, checksum: 3538250a323b39c32585d41d316bcb6a (MD5) Previous issue date: 2009-09-08 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / The primary and accessory optic systems comprise two set of retinorecipient neural clusters. In this study, these visual related centers in the rock cavy were evaluated by using the retinal innervations pattern and Nissl staining cytoarchigtecture. After unilateral intraocular injection of cholera toxin B subunit and immunohistochemical reaction of coronal and sagittal sections from the diencephalon and midbrain region of rock cavy. Three subcortical centres of primary visual system were identified, superior colliculus, lateral geniculate complex and pretectal complex. The lateral geniculate complex is formed by a series of nuclei receiving direct visual information from the retina, dorsal lateral geniculate nucleus, intergeniculate leaflet and ventral lateral geniculate nucleus. The pretectal complex is formed by series of pretectal nuclei, medial pretectal nucleus, olivary pretectal nucleus, posterior pretectal nucleus, nucleus of the optic tract and anterior pretectal nucleus. In the accessory optic system, retinal terminals were observed in the dorsal terminal, lateral terminal and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. All retinorecipient nuclei received bilateral input, with a contralateral predominance. This is the first study of this nature in the rock cavy and the results are compared with the data obtained for other species. The investigation represents a contribution to the knowledge regarding the organization of visual optic systems in relation to the biology of species. / Os sistemas visual prim?rio e ?ptico acess?rio compreendem dois conjuntos de grupamentos neurais, que recebem proje??o direta da retina. Nesse estudo, estes dois sistemas foram avaliados com rela??o a citoarquitetura e padr?o de inerva??o retiniana, usando como tra?ador neural anter?grado, a subunidade B da toxina col?rica revelada por imunoistoqu?mica para detectar este tra?ador em terminais sobre grupamentos neuronais (alvos) no enc?falo do moc? (Kerodon rupestris), um roedor nativo do Nordeste Brasileiro. Os resultados permitiram identificar os componentes do sistema visual prim?rio o complexo geniculado lateral, o complexo pr?-tectal e o col?culo superior. O complexo geniculado lateral cont?m o n?cleo geniculado lateral dorsal, o n?cleo geniculado lateral ventral e o folheto intergeniculado. Todos recebem fibras da retina com predomin?ncia contralateral, menos intensa para o folheto intergeniculado. O complexo pr?-tectal cont?m os n?cleos pr?-tectal anterior, pr?-tectal medial, pr?-tectal posterior, olivar pr?-tectal e n?cleo do trato ?ptico, os quais recebem proje??o retiniana predominantemente contralateral. Do mesmo modo, o col?culo superior recebe fibras da retina contralateral nas camadas superficiais. Tamb?m foi identificado o sistema ?ptico acess?rio completo no moc?, constitu?do pelos n?cleos terminal medial, terminal lateral, terminal dorsal e intersticial do fasc?culo superior posterior. Esses n?cleos recebem inerva??o retiniana com forte predomin?ncia contralateral, sendo que o n?cleo terminal medial, embora preserve a predomin?ncia contralateral, exibe uma evidente inerva??o ipsolateral.