A key function of the nervous system is to sample data from the external world, generate internal signals, and transform them into meaningful information that can be used to trigger behaviour. In order to gain insight into the underlying mechanism for signal transformation, the visual system has been extensively studied: partly owing to the stimulus being reliably presentable, and the anatomy being well described. The Drosophila visual system is one such system, with the added advantage of genetic tractability. In this thesis, I studied the filtering property of visual neurons at two levels, biophysical and circuit levels. The first study looks at signal transformation at the biophysical level, at the input of the visual system, in photoreceptors. Voltage-gated potassium channels counteract the depolarization caused by opening of light sensitive channels, and the heterogeneous properties of their kinetics can fine-tune the photoreceptor’s frequency response to fulfill the animal’s ecological requirements. Shaker (Kv1) and Shab (Kv2) have been identified as fast and slow inactivating components of the photoreceptor’s outward currents, however a current with intermediate kinetics (IKf) has not been molecularly identified, but had been postulated to be Shal (Kv4). I focused on characterizing this current using whole-cell patch clamp in wild type and mutants, and using antibodies for Shal. My results from whole-cell patch clamp indicated that IKf in adult R1-6 cells are not Shal, from their voltage dependence and insensitivity to a Kv4 blocker. This calls for alternative molecular basis for IKf, which is likely to be a slow inactivating component of Shaker, or a combination of its many splice variants. The second study looks at signal transformation at the circuit level, at the output end, in the third optic neuropil, lobula. Visual projection neurons project from the lobula to the central brain, and have been proposed to carry behaviourally relevant visual features to higher brain regions. It was recently shown that optogenetic activation of individual visual projection neuron types could induce distinct behaviours such as takeoff and backward walking, linking these visual neurons to specific behavioural programs downstream. Using in vivo two-photon calcium imaging, I recorded visually evoked calcium responses from three of these cell types. Cell types that showed induced takeoff and backward walking preferentially responded to dark looming stimuli or fragmented expanding local features, suggesting their role in behaviours triggered by object approach. To explore how this visual information is transformed in the downstream circuit, we identified several candidate neurons that receive input from this cell type by anatomical overlap, and then validated their connections using optogenetic activation and calcium imaging. One downstream cell-type that projects bilaterally had very similar response properties to its upstream partner, whereas another cell-type that projects ipsilaterally seemed to filter out some information from its upstream partner. This is one of the first studies that functionally characterizes lobula visual projection neurons and their downstream partners in Drosophila, and their response properties agree with the general idea that visual information becomes increasingly selective as it is sent to higher brain regions.

On assiste à un essor spectaculaire des méthodes optiques pour suivre l’activité de populations neuronales in vivo. Ceci a permis de mettre en évidence des motifs remarquables d’organisation fonctionnelle à l’échelle mésoscopique impliqués dans de nombreuses fonctions cérébrales physiopathologiques. Cette thèse vise à mettre en place les outils permettant de disséquer les circuits sous-tendant ces motifs remarquables selon une approche expérimentale basée uniquement sur la microscopie optique. Plus particulièrement, ces outils ont été optimisés pour décrire la région CA1 de l’hippocampe adulte et le « barrel cortex » au cours du développement. En effet, deux motifs remarquables ont récemment été mis en évidence dans ces structures, les assemblées neuronales de CA1 adulte impliquées d’une part dans des processus de mémorisation et les neurones Hubs du cortex en développement et d’autre part participant au développement postnatal des circuits neuronaux. Dans ce contexte, nous avons développé un nouveau paradigme expérimental combinant imagerie calcique biphotonique in vivo, photostimulation par illumination holographique et analyse mathématique. Nous avons optimisé le choix et la co-expression de la sonde calcique et de l’opsine dans nos conditions expérimentales, et calibré leur utilisation dans les neurones de différentes structures cérébrales. De plus, nous avons conçu et assemblé un nouveau microscope à deux voies d’excitation, une pour l’imagerie calcique et l’autre pour la photostimulation holographique in vivo. Cette nouvelle approche expérimentale est en cours de validation sur les neurones Hubs à forte connectivité du « barrel cortex » en développement. / Over The last five years we have observed a huge improvement of optical methods to monitor the activity of neuronal populations in vivo. With these new approaches, remarkable patterns of functional organization at the mesoscopic scale that are involved in many pathophysiological brain functions were highlighted. This thesis aims to develop tools allowing us to dissect the circuits underlying these remarkable patterns according to an experimental approach based on all optical microscopy. These tools have been optimized to describe the functional organization of CA1 neurons in the adult hippocampus as well as in the barrel cortex during development. Two remarkable patterns have recently been identified in these structures, first, adult CA1 neural assemblies involved in memory processes and second, Hub cortical neurons that shape neuronal circuit during development. We have developed a new experimental paradigm combining in vivo two photon calcium imaging, holography photostimulation and mathematical analysis. We optimized the choice and co-expression of calcium probe (GCaMP6s) and opsin (Chronos and ChR2H134R) in our experimental conditions and calibrated their use in neurons of different brain structures. In addition, we designed and assembled a new two-path excitation microscope, one for calcium imaging and the other for in vivo holography photostimulation. This new experimental approach is being validated on Hub neurons with high connectivity in the developing barrel cortex.