Abstract

In order to safely move through the environment, visually-guided animals use several types of visual cues for orientation. Optic flow provides faithful information about ego-motion and can thus be used to maintain a straight course. Additionally, local motion cues or landmarks indicate potentially interesting targets or signal danger, triggering approach or avoidance, respectively. The visual system must reliably and quickly evaluate these cues and integrate this information in order to orchestrate behavior. The underlying neuronal computations for this remain largely inaccessible in higher organisms, such as in humans, but can be studied experimentally in more simple model species. The fly Drosophila, for example, relies heavily on such visual cues during its impressive flight maneuvers. Additionally, it is genetically and physiologically accessible. Therefore, it is regarded as an ideal model organism for exploring neuronal computations underlying visual processing. During my PhD-thesis, I characterized neurons presynaptic to direction selective lobula plate tangential cells by exploiting the genetic toolbox of the fruit fly in combination with in-vivo imaging. The use of genetically encoded calcium indicators and two-photon microscopy allowed me to directly investigate response properties of small columnar neurons upstream of lobula plate wide field neurons. In the highly collaborative environment of our lab my imaging experiments were complemented by several other approaches, including electrophysiological and behavioral experiments, along with modeling which resulted in the publications that comprise this cumulative dissertation. Measuring calcium signals in T4 and T5 cells in the first study, established that both populations of neurons exhibit direction selective response properties. Furthermore, T4 cells only respond to moving bright edges, whereas T5 cells encode exclusively dark edge motion. Silencing the synaptic output of T4 and T5 separately, we were able to determine that both lobula plate tangential cell responses as well as the turning behavior of walking flies were impaired only to bright or dark edges, respectively. We thus proposed that the detection of the direction of visual motion must happen either presynaptic to, or on the dendrites of T4 and T5 neurons, and that this computation takes place independently for brightness increments and decrements. The second paper published in 2014 was motivated by an anatomical study that found an asymmetric wiring between L2 and L4 cells with the dendrites of Tm2 in the distal medulla. Using two-photon calcium imaging and neuronal silencing combined with postsynaptic electrophysiological recordings, we probed the contribution of L4 and Tm2 in the OFF pathway of Drosophila motion vision. We found that while Tm2 have small, isotropic, laterally inhibited receptive fields, L4 cells respond to both, small and large field darkening. Blocking the output of both cell types resulted in a strong impairment of OFF motion vision. In contrast to the anatomical prediction, we did not observe any directional effects for either of the cells.