Right parietal lesions often lead to neglect, in which patients fail to attend to leftward stimuli. Recent models of neglect suggest that, in addition to attentional impairments, patients demonstrate impairments of spatial remapping and/or spatial working memory (SWM). Although spatial remapping could be considered a kind of spatial memory process itself (i.e., updating remembered locations based on anticipated saccade outcomes), the two processes operate on very different time scales (milliseconds versus seconds). In the present study, the influence of saccadic and spatial remapping on SWM was examined in healthy individuals. An initial control condition, in which participants had to respond to a probe stimulus (i.e., “is the probe in the location previously occupied by the target?”) following a 1500 ms delay, was contrasted with conditions in which the fixation point moved (left, right, up, or down) at the onset of the delay. In a second version of the task, participants made covert shifts of attention at delay onset requiring covert spatial, rather than saccadic, remapping. In both tasks SWM performance was best when no remapping was required. Decrements in SWM were largest overall in the spatial remapping task, whereas for both saccadic and spatial remapping, a consistent cost was observed for remapping the target array into right visual space. Results are discussed in terms of hemispheric biases in attention and differences in performance for peripersonal versus extrapersonal space.

Right parietal lesions often lead to neglect, in which patients fail to attend to leftward stimuli. Recent models of neglect suggest that, in addition to attentional impairments, patients demonstrate impairments of spatial remapping and/or spatial working memory (SWM). Although spatial remapping could be considered a kind of spatial memory process itself (i.e., updating remembered locations based on anticipated saccade outcomes), the two processes operate on very different time scales (milliseconds versus seconds). In the present study, the influence of saccadic and spatial remapping on SWM was examined in healthy individuals. An initial control condition, in which participants had to respond to a probe stimulus (i.e., “is the probe in the location previously occupied by the target?”) following a 1500 ms delay, was contrasted with conditions in which the fixation point moved (left, right, up, or down) at the onset of the delay. In a second version of the task, participants made covert shifts of attention at delay onset requiring covert spatial, rather than saccadic, remapping. In both tasks SWM performance was best when no remapping was required. Decrements in SWM were largest overall in the spatial remapping task, whereas for both saccadic and spatial remapping, a consistent cost was observed for remapping the target array into right visual space. Results are discussed in terms of hemispheric biases in attention and differences in performance for peripersonal versus extrapersonal space.

Additional cognitive deficits, including impairments in spatial working memory and/or saccadic remapping processes, have recently been implicated in unilateral neglect – a neurological condition classically characterized as a disorder of attention. The interactions between saccadic remapping and three memory processes (position memory, object memory and object-location binding) were investigated in healthy young (n=27) and elderly (n=20) participants to establish a baseline of comparison for future use with neglect patients and to study the effects of aging on these processes. In a computerized task, participants were instructed to first detect a target, and then hold in memory either its position, identity or both over a delay period. Subsequently, participants were tested on their memory for that particular task. The saccadic remapping component was introduced at the onset of the delay period with the fixation cross shifting either to the left, or right, requiring participants to remap the visual array into either right or left space, or remaining in the centre of the screen (i.e., no remapping condition). In the position memory and object-location binding task, a consistent cost to memory performance was found when remapping right only for the young participants. Overall the elderly did not perform any of the tasks involving a position memory component as well as the young participants and showed spatial asymmetries in the target detection task. The lack of an effect of remapping in the elderly group may be due to a general decrement in performance. These results are discussed in terms of hemispheric asymmetries and cognitive theories of aging.

Additional cognitive deficits, including impairments in spatial working memory and/or saccadic remapping processes, have recently been implicated in unilateral neglect – a neurological condition classically characterized as a disorder of attention. The interactions between saccadic remapping and three memory processes (position memory, object memory and object-location binding) were investigated in healthy young (n=27) and elderly (n=20) participants to establish a baseline of comparison for future use with neglect patients and to study the effects of aging on these processes. In a computerized task, participants were instructed to first detect a target, and then hold in memory either its position, identity or both over a delay period. Subsequently, participants were tested on their memory for that particular task. The saccadic remapping component was introduced at the onset of the delay period with the fixation cross shifting either to the left, or right, requiring participants to remap the visual array into either right or left space, or remaining in the centre of the screen (i.e., no remapping condition). In the position memory and object-location binding task, a consistent cost to memory performance was found when remapping right only for the young participants. Overall the elderly did not perform any of the tasks involving a position memory component as well as the young participants and showed spatial asymmetries in the target detection task. The lack of an effect of remapping in the elderly group may be due to a general decrement in performance. These results are discussed in terms of hemispheric asymmetries and cognitive theories of aging.

The hippocampus supports the initial formation and recall of episodic memories, as well as the consolidation of short-term into long-term memories. The ability of hippocampal neurons to rapidly change their connection strengths during learning and maintain these changes over long time-scales may provide a mechanism supporting memory. However, little evidence currently exists concerning the long-term stability of information contained in hippocampal neuronal activity, likely due to limitations in recording extracellular activity in vivo from the same neurons across days. In this thesis I employ calcium imaging in freely moving mice to longitudinally track the activity of large ensembles of hippocampal neurons. Using this technology, I explore the proposal that long-term stability of hippocampal information provides a substrate for episodic memory in three different ways. First, I tested the hypothesis that hippocampal activity should remain stable across days in the absence of learning. I found that place cells – hippocampal neurons containing information about a mouse’s position – maintain a coherent map relative to each other across long time-scales but exhibit instability in how they anchor to the external world. Furthermore, I found that coherent maps were frequently used to represent a different environment and incorporated learning via changes in a subset of neurons. Next, I examined how learning a spatial alternation task impacts neuron stability. I found that splitter neurons whose activity patterns reflected an animal’s future or past trajectory emerged relatively slowly when compared to place cells. However, splitter neurons remained more consistently active and relayed more consistent spatial information across days than did place cells, suggesting that the utility of information provided by a neuron influences its long term stability. Last, I investigated how protein synthesis, known to be necessary for long-term maintenance of changes in hippocampal neuron connection strengths and for proper memory consolidation, influences their activity patterns across days. I found that along with blocking memory consolidation, inhibiting protein synthesis induced a profound, long-lasting decrease in neuronal activity up to two days later. These results combined demonstrate the importance of rapid, lasting changes in the hippocampal neuronal code to supporting long-term memory.

Nos entrées sensorielles, comme nos mouvements, sont entachés d’incertitudes. Pourtant, notre système nerveux central semble être aussi précis que possible compte tenu de ces incertitudes: il les gère de manière optimale, par exemple en pondérant des informations sensorielles redondantes en fonction de leur fiabilité, ou en prenant en compte ses incertitudes motrices lors de la réalisation de mouvements. Si les modalités des combinaisons d’informations redondantes sont bien connues lors de tâches statiques, elles le sont moins en conditions dynamiques, lors de mouvements. La partie expérimentale de cette thèse a permis de confirmer l’existence de mécanismes d’estimation et de contrôle optimaux des mouvements chez l’humain. En effet, nous avons mis en évidence l’intégration optimale d’information visuelle lors de la réalisation de saccades à la lumière: lors de séquences de saccades, le système visuomoteur est capable d’utiliser l’information visuelle pour mettre à jour ses estimations internes de la position de l’œil. Une étude complémentaire des sources de variabilité des saccades suggère un rôle similaire pour la proprioception extra-oculaire. Par une troisième expérience, novatrice, nous avons montré que le toucher est pris en compte en temps réel lors de mouvements de la main en contact avec une surface. Nous avons également inféré une mesure de la variance de l'information tactile. Enfin, à partir des connaissances sur la représentation des variables sensorimotrices dans le système nerveux, nous avons construit plusieurs réseaux de neurones qui implémentent de manière proche de l'optimum statistique la planification et le contrôle de mouvements / Our sensory inputs, as well as our movements, are uncertain. Nevertheless, our central nervous systems appears to be as accurate as possible: these uncertainties are handled in an optimal fashion. For example, redundant sensory signals are weighted according to their accuracy, and motor uncertainties are taken in account when movements are made. The characteristics of the combination of redundant sensory signals are well known for static tasks. However, they are less known in dynamic conditions. The experimental part of this thesis allowed to confirm the use of statistically optimal sensorimotor processes during movements. We showed that visual information can be integrated during sequences of saccades, the oculomotor system being able to use visual information to update its internal estimate of eye position. A complementary study on the sources of variability for saccadic eye movements suggests a similar role for extra-ocular proprioception. In a third original experiment, we showed that tactile input is optimally taken in account for the on-line control of arm movements during which fingertips are in contact with a textured surface. Last, we built several neuronal networks models simulating optimal movement planning. These networks were based on current knowledge about probabilistic representations in the nervous system

Une façon d'explorer visuellement notre monde consiste à déplacer très rapidement nos yeux pour en analyser le contenu. Dans certaines circonstances, ces mouvements oculaires, appelés saccades, peuvent perdre de leur précision. Fort heureusement notre cerveau est capable de corriger cette imprécision en ajustant progressivement la taille de ces saccades grâce à des mécanismes de plasticité cérébrale : c'est l'adaptation saccadique. L'adaptation saccadique est souvent utilisée comme modèle d'étude des mécanismes de plasticité visuo-motrice. Nous faisons également l'hypothèse qu'elle puisse servir d'outil thérapeutique. Dans cette thèse, nous nous sommes essentiellement intéressés à l'adaptation des saccades volontaires en augmentation d'amplitude chez des sujets contrôles et chez des patients souffrant d'une amputation homonyme du champ visuel (hémianopsie latérale homonyme). Les expériences 1 à 3, réalisées chez le sujet contrôle, ont permis de découvrir les repères utilisés par le cerveau pour réaliser cette adaptation sensori-motrice et coder l' environnement visuel (expérience 1 ), et les mécanismes d'adaptation saccadique quand la cible visuelle n'est pas directement codée (remapping dans l'expérience 2 et cible virtuelle dans l'expérience 3). Enfin l'expérience 4 propose une application clinique du protocole d'adaptation des anti-saccades utilisé lors de l'expérience 3, dans un but thérapeutique de rééducation comportementale de patients hémianopsiques / One way to visually explore our environment consists to rapidly displace our eyes to analyze its content. Under certain circumstances, these ocular movements called saccades can become inaccurate. Fortunately our brain is able to correct this inaccuracy by progressively adjusting the size of saccades thanks to plasticity mechanisms called saccadic adaptation. Saccadic adaptation is often used as a model to study visuo-motor plasticity. We also suggest that it could be used as a rehabilitation tool. ln this thesis, we were interested in outward adaptation of voluntary saccades in healthy subjects and patients suffering from a half visual field loss for both eyes (lateral homonymous hemianopia). Experiments 1 to 3, performed on healthy subjects, gave us opportunity to acquired complementary knowledge on the reference frame used by the brain to code its environment (experience 1) and on adaptation mechanisms when a visual target is not directly coded (remapping in experiment 2 virtual target in experiment 3). Finally, experiment 4 suggests to clinically implement the anti-saccades adaptation protocol used in experiment 3, as a new rehabilitation tool tested in hemianopie patients

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The distribution of the cliff chipmunk in Idaho was previously considered to include only the Raft River Valley and the Goose Creek Basin. A pilot study was conducted in 2003 and 2004. Thirty-five cliff chipmunk presence locations and 124 absence locations were recorded. Habitat variables of elevation, slope, deviation from south, distance to water, and vegetation type were extracted for all of the absence and presence points by means of GIS analysis. The data were analyzed by implementing a classification tree, and a "GIS habitat association model" was created. The model was tested in 2005, and the overall model accuracy was 77.5%. The study extended the known cliff chipmunk range in Idaho west to Rock Creek, Twin Falls County, east to Weston Canyon, Franklin County, and north to the Cotterel Mountains, Cassia County. Monitoring current known locations and searching for new locations to further refine 11 understanding of the species' distribution and to determine the actual population status of cliff chipmunks in Idaho are recommended.

The widely acknowledged performance gap between processors and memory has been the subject of much research. In the Explicitly Parallel Instruction Computing (EPIC) paradigm, the combination of in-order issue and the presence of a large number of parallel function units has further worsen the problem. Prefetching, by hardware, software or a combination of both, has been one of the primary mechanisms to alleviate this problem. In this talk, we will discuss two prefetching mechanisms, one hardware and other software, suitable for implementation in EPIC processors. Both methods rely on the off-line learning of Markovian predictors. In the hardware mechanism, the predictors are loaded into a table that is used by a prefetch engine. We have shown that the method is particularly effective for prefetching into the L2 cache. Our software mechanism which we called predicated prefetch leverages on informing loads. This is used in conjunction with data remapping and offline learning of Markovian predictors. This distinguishes our approach from early software prefetching techniques that only involves static program analysis. Our experiments show that this framework, together with the algorithms used in it, can effectively remove, in the best instance, 30% of the stall cycles due to cache misses. The results also show that the framework performs better than pure hardware stride predictors and has lower bandwidth and instruction overheads than that of pure software approaches. / Singapore-MIT Alliance (SMA)