Periodic Reporting for period 3 - MicroCyFly (Microcircuitry of the Drosophila visual system) Reporting period: 2020-07-01 to 2021-12-31 Summary of the context and overall objectives of the project To turn sensory inputs of the outside world into appropriate actions, our brains as well as the brain of all animals must perform specific computations. These computations can be simple or complex, but often involve intricate circuit architecture. It is our major goal to understand how neural computations are implemented in the neuronal networks for the brain. We chose to study a basic, paradigmatic computation, the extraction of direction-selective signals, which is a hallmark of motion detection. While recent years have mapped out core circuits of visual motion detection in the model organism Drosophila, and highlighted many parallels to vertebrate vision, we now aim to understand circuit computations at a new level of detail. Within the “MicroCyFly” project, we develop novel genetic tools to dissect visual circuitry at the level of individual neuronal connections, or synapses, rather at the cell type level. To understand how microcircuits in the brain are generally organized to perform certain tasks is key to target many neurological and psychiatric diseases. Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far To this end, we have carefully characterized known neurons of motion detection circuits. We found physiological properties that are essential to implement theoretical models of motion detection, using linear summation mechanisms. This work also reconciles previous discrepancies in the field. In parallel, we have used a forward genetic screen to also identify novel neurons of motion-detection circuitry. This led to the identification of novel inhibitory cell types, the roles of which we have extensively characterized using physiology, especially in vivo calcium imaging experiments of neural activity, as well as behavioral analysis. Most central to the “MicroCyFly” project, we have made significant progress towards developing novel genetic tools for the synapse-specific analysis of circuit function. In vitro validation of individual components of this tool called STAB (synapse-targeted activity block) has been successful, so what we will now move to in vivo validation and testing. Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far) Our major goal towards the end of the project is to conclude the analysis of the above-mentioned circuit motives, and thus add the physiological basis and extended circuit models to motion-detection. Furthermore, circuitry is currently being dissected at the cell type level. Genetic tools for the synapse-specific analysis of circuit function would truly expand the range of possibilities in neuroscience research. We therefore aim to successfully establish the STAB tool in vivo to then apply it to the other objectives to get to a more detailed, synapse-specific analysis of visual circuitry, and to make it available to the community.