Mécanismes cognitifs dans la catégorisation d'objets visuels

La catégorisation est un processus fondamental de la reconnaissance d'objets. Pour comprendre ses mécanismes sous-jacents, cette thèse interroge le rôle du niveau de catégorisation, de l'attention, de la mémoire, et de la relation entre les catégories d'objets, dans la catégorisation de scènes naturelles. Les résultats de la première étude indiquent que les performances de catégorisation sont influencées par les caractéristiques diagnostiques de la tâche. Une seconde étude montre que des objets naturels peuvent être catégorisés en quasi-absence d'attention. Les résultats de la troisième étude indiquent que les scènes sont encodées en mémoire à long-terme sans instruction explicite et catégorisées automatiquement. La dernière étude explore les interactions entre deux représentations d'objets actives simultanément. Plus le degré de relation entre deux objets est élevé, plus le traitement du second objet est affecté. Pour expliquer ces résultats un modèle, inspiré de la physiologie, est proposé qui postule que le niveau d'interaction entre des catégories d'objet actives simultanément dépend du niveau de chevauchement entre les patterns d'activité du cortex inféro-temporal produits par chacun des objets. Les résultats de cette thèse sont compatibles avec l'idée que les caractéristiques visuelles des objets sont traitées automatiquement (étude 3) en quasi-absence d'attention (étude 2) et représentées dans la voie visuelle ventrale de façon distribuée et continue. Les performances de catégorisation dépendraient de la similarité des catégories cibles et distracteurs (étude 1) ou de la similarité entre les représentations actives de deux objets (étude 4).Categorization is a fundamental process of object recognition. To determine its underlying mechanisms, a series of experiments examined the roles of stimulus properties, categorization level, attention, memory, and category-relatedness in natural scene categorization tasks. The results of the first study suggest that categorization performance is driven by characteristics that are diagnostic for the task. A second study shows that visual objects embedded in complex natural scenes can be categorized in the near-absence of attention. The results of a third study suggest that long-term encoding of complex scenes happens without any explicit instruction, and information about object categories is processed automatically. The final study explores the interaction between two concurrently active category representations by presenting two objects in a rapid sequence. The greater the degree of relatedness between two objects, the more affected the processing of the second object is. To explain these results a physiologically inspired model is proposed, which posits that the extent of interaction between concurrently active objects depends on the extent of overlap between the activity patterns in the infero-temporal cortex elicited by the two objects. The results of this thesis support the idea that visual object characteristics are processed automatically (study 3) in the near-absence of attention (study 2) and represented in the ventral stream in a distributed and continuous manner. Categorization performance would depend on the similarity between and within the target and the distractor categories (study 1) or on the similarity between two active object representations (study 4)

Mucus and ciliated cells of human lung : splitting strategies for particle methods and 3D stokes flows

Lung walls are covered by a film of mucus, whose motility is fundamental for a healthy behavior. Indeed, mucus traps inhaled aerosols (bacteria, dust, ...), and moves from smallest to largest airways, until it reaches esophagus where is it swallowed or expectorated. A lot of biological parameters are responsible for mucus motion [6], such as the vibrations of ciliated cells covering lung walls (cilia height, frequency, ...), mucus/air interaction, water saturation in mucin network, mucus thickness

Numerical and experimental investigation of mucociliary clearance breakdown in cystic fibrosis

The human tracheobronchial tree surface is covered with mucus. A healthy mucus is a heterogeneous material flowing toward the esophagus and a major defense actor against local pathogen proliferation and pollutant deposition. An alteration of mucus or its environment such as in cystic fibrosis dramatically impacts the mucociliary clearance. In the present study, we investigate the mechanical organization and the physics of such mucus in human lungs by means of a joint experimental and numerical work. In particular, we focus on the influence of the shear-thinning mucus mobilized by a ciliated epithelium for mucociliary clearance. The proposed robust numerical method is able to manage variations of more than 5 orders of magnitude in the shear rate and viscosity. It leads to a cartography that allows to discuss major issues on defective mucociliary clearance in cystic fibrosis. Furthermore, the computational rheological analysis based on measurements shows that cystic fibrosis shear-thinning mucus tends to aggregate in regions of lower clearance. Yet, a rarefaction of periciliary fluid has a greater impact than the mucus shear-thinning effects

Briefly Flashed Scenes Can Be Stored in Long-Term Memory

The capacity of human memory is impressive. Previous reports have shown that when asked to memorize images, participants can recognize several thousands of visual objects in great details even with a single viewing of a few seconds per image. In this experiment, we tested recognition performance for natural scenes that participants saw for 20 ms only once (untrained group) or 22 times over many days (trained group) in an unrelated task. 400 images (200 previously viewed and 200 novel images) were flashed one at a time and participants were asked to lift their finger from a pad whenever they thought they had already seen the image (go/no-go paradigm). Compared to previous reports of excellent recognition performance with only single presentations of a few seconds, untrained participants were able to recognize only 64% of the 200 images they had seen few minutes before. On the other hand, trained participants, who had processed the flashed images (20 ms) several times, could correctly recognize 89% of them. EEG recordings confirmed these behavioral results. As early as 230 ms after stimulus onset, a significant event-related-potential (ERP) difference between familiar and new images was observed for the trained but not for the untrained group. These results show that briefly flashed unmasked scenes can be incidentally stored in long-term memory when repeated

Modeling Cystic Fibrosis and Mucociliary Clearance

International audienceThis chapter considers the investigation of airway clearance efficiency and dysfunction using rheological measurements, modeling and simulations of mucus flows. The work is mainly dedicated to the respiratory epithelium dysfunction subsequently to the consumption of cilia-inhibiting drugs (e.g. nicotine) or viral and/or bacterial infections, and cystic fibrosis, although many respiratory diseases are associated with an altered mucus transport

Theta-phase dependent neuronal coding during sequence learning in human single neurons

International audienceThe ability to maintain a sequence of items in short-term memory (STM) is a fundamental cognitive function. In the rodent hippocampus, the representation of sequentially organized spatial locations is reflected by the phase of action potentials relative to the theta oscillation (phase precession). We investigated whether the timing of neuronal activity relative to the theta brain oscillation also reflects sequence order in the medial temporal lobe of humans. We used a task in which human subjects learned sequences of pictures and recorded single neuron and local field potential activity with implanted electrodes. We report that spikes for three consecutive items in the sequence (the preferred stimulus for each cell, as well as the stimuli immediately preceding and following it) were phase-locked at distinct phases of the theta oscillation. Consistent with phase precession, spikes were fired at progressively earlier phases as the sequence advanced. These findings generalize previous findings in the rodent hippocampus to the human temporal lobe and suggest that encoding stimulus information at distinct oscillatory phases may play a role in maintaining their sequential order in STM.The ability to maintain a sequence of items in short-term memory (STM) is a fundamental cognitive function. In the rodent hippocampus, the representation of sequentially organized spatial locations is 25 reflected by the phase of action potentials relative to the theta oscillation (phase precession). We investigated whether the timing of neuronal activity relative to the theta brain oscillation also reflects sequence order in the medial temporal lobe of humans. We used a task in which human subjects learned sequences of pictures and recorded single neuron and local field potential activity with implanted electrodes. We report that spikes for three consecutive items in the sequence (the preferred stimulus for 30 each cell, as well as the stimuli immediately preceding and following it) were phase-locked at distinct phases of the theta oscillation. Consistent with phase precession, spikes were fired at progressively earlier phases as the sequence advanced. These findings generalize previous findings in the rodent hippocampus to the human temporal lobe and suggest that encoding stimulus information at distinct oscillatory phases may play a role in maintaining their sequential order in STM

Theta-phase dependent neuronal coding during sequence learning in human single neurons

The ability to maintain a sequence of items in memory is a fundamental cognitive function. In the rodent hippocampus, the representation of sequentially organized spatial locations is reflected by the phase of action potentials relative to the theta oscillation (phase precession). We investigated whether the timing of neuronal activity relative to the theta brain oscillation also reflects sequence order in the medial temporal lobe of humans. We used a task in which human participants learned a fixed sequence of pictures and recorded single neuron and local field potential activity with implanted electrodes. We report that spikes for three consecutive items in the sequence (the preferred stimulus for each cell, as well as the stimuli immediately preceding and following it) were phase-locked at distinct phases of the theta oscillation. Consistent with phase precession, spikes were fired at progressively earlier phases as the sequence advanced. These findings generalize previous findings in the rodent hippocampus to the human temporal lobe and suggest that encoding stimulus information at distinct oscillatory phases may play a role in maintaining sequential order in memory

Nouvelle-Calédonie

Avec 40000 km2 de récifs et de lagons et plus de 15 000 espèces, la Nouvelle-Calédonie abrite la deuxième plus grande barrière corallienne du monde. À l'heure où les récifs coralliens figurent parmi les écosystèmes les plus menacés de la planète, face aux activités humaines, au réchauffement climatique et à l'acidification des océans, il est devenu impératif de préserver cet exceptionnel héritage environnemental et culturel inscrit au Patrimoine mondial de l'Unesco. Associant des chercheurs de diverses disciplines (sciences de la nature, sciences humaines et sociales) et des acteurs en charge de la gestion des récifs et lagons néo-calédoniens, cet ouvrage présente l'état des connaissances les plus actuelles sur ces espaces. Il permet d'appréhender l'extraordinaire diversité de ces milieux en lien avec l'histoire de l'environnement marin, ainsi que la complexité des relations entre les différents organismes qui les composent. Il accorde également une large place à la manière dont ces écosystèmes offrent aux populations des ressources essentielles et constituent l'un des socles de la culture kanak. Enfin, il interroge la capacité de résilience de ces milieux très vulnérables face aux changements environnementaux globaux et présente les dispositifs mis en place pour leur protection. Rédigé dans un style accessible à tous et très richement illustré, cet ouvrage s'adresse à tout lecteur intéressé par ce patrimoine exceptionnel et, au-delà, il sensibilisera le large public aux enjeux de conservation de la biodiversité, de l'environnement et des cultures