LittleFly Project
Optic flow processing in Drosophila
Using the blow fly as an experimental animal, many insights into the neural processing of optic flow have been gained in the past. However, two things have escaped our analysis:
- 1. Due to the small size of the columnar elements presynaptic to the lobula plate tangential cells, the cellular implementation of the Reichardt detector still is not known.
- 2. The precise function of the various cells at the level of the lobula plate or the descending neurons for visual course control is elusive, since single cell ablation in blowflies requires a surgery which is prohibitive for later use of the animals in behavioral tests.
Here, the large repertoire of genetic intervention in Drosophila melanogaster holds great promise.
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Figure 2: Functional neuroanatomy of VS-cells in Drosophila. Click for more information.
We use optical imaging and electrophysiology to obtain a direct readout of neural activity. In particular, we perform whole cell patch clamp recordings from the large tangential neurons of the lobula plate while genetically silencing specific subpopulations of columnar neurons (Figure 1).
During such recordings, precisely defined moving visual patterns are presented to the fly’s facet eyes. Recently, such measurements revealed that lamina neurons L1 and L2 provide the major input signals to the fly motion vision system. The two pathways, L1 and L2, split the visual input into an ON-channel (L1), providing information about dark-to-bright transitions, and an OFF-channel (L2), transmitting information about bright-to-dark transitions. These findings subsequently led to the discovery that visual motion is detected in parallel by two system, one within each pathway. Another key to such studies is the anatomical reconstruction of the underlying neural circuitry. This is done (Figure 2) by using mostly genetic techniques and histochemistry to functionally characterize individual neurons and their circuitry.
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Figure 3: Yellow-Cameleon 3.6 in transgenic animals in vivo. Click for more information.
Neurons that are too small for electrical recordings have to be reached by other means. This is achieved by using genetically encoded indicators of neural activity, in particular for intracellular calcium (GECIs) and Two-Photon-Laser-Scanning-Microscopy (2PLSM). However, changes in calcium in visual interneurons appear to be tiny and GECI fluorescence and its change are notoriously small. Thus, optimal sensors that match the specific requirements of the investigated neuron have to be chosen. For this purpose presynaptic boutons of the larval Drosophila NMJ (Figure 3) have been established as a unique test system for GECIs in transgenic animals in vivo. In this system quantitative cross-correlation of fluorescence changes of GECIs to the underlying neural activity and changes in intracellular calcium has recently been achieved by injection of OGB-1. This approach and close collaboration with the Research Group of Oliver Griesbeck in our department allows the identification of new GECIs that are suitable to discern biologically meaningful signals from noise. Additional development of new hardware allows i.e. the elimination of contaminating photons that emerge from the visual stimulus. As an outcome of this work we now optically record fluorescence changes from visual interneurons that express a novel GECI.
