When an animal moves through space, multiple visual features are important for orientation. First, the animal creates a self-induced optic flow pattern, which it can use for course stabilization. Second, certain objects might be approaching indicating an obstacle or an enemy one should avoid. Third, smaller objects might move around and form potentially interesting targets, which are worth following. It is the ultimate goal in systems neuroscience to understand how the activity of neural circuits creates meaningful patterns to eventually control behavior. We aim to understand how the brain of Drosophila processes the visual stimuli in these different scenarios to initiate appropriate behavioral responses.
We study the behavioral responses to these kinds of stimuli in an arena of high-speed monitors surrounding the fly where it can interact with a virtual environment like in a computer game. The fly is glued to a hook and placed in the center of the arena so that it cannot move and therefore we know precisely what the fly sees at each time point. But it can still move its wings or legs. We use two complementary systems to measure behavioral output: To study visual responses in flight, we track the movements of the two wings and calculate the difference between wing beat amplitudes which provides us with the information into which direction the fly is steering. To study walking behavior we place an air-suspended Styrofoam ball under the fly on which the fixated fly can still walk. We use a system of computer-mouse sensors tracking the rotation of the ball to measure turning and walking speeds. These systems allow us to perform so called open-loop experiments, where the visual stimulus is independent of what the fly is doing. Yet, it also allows us to directly couple behavioral responses to the visual stimulus, so that closed-loop experiments are possible.
We interfere with the neuronal activity in varies ways: Using the Gal4/UAS system we target a subset of neurons and express certain effector proteins. That is, for example, shibirets or Tetanustoxin (TNT), both inhibiting synaptic transmission. It is also possible to express Channelrhodopsins (ChR), which permits a temporally precise activation of neuronal activity. Blocking elements in the visual system of the fly gives us good indication about which neuronal circuits are necessary for certain behaviors. Activation permits us to identify whether the elements can initiate behavior on their own.
When flies are presented with a rotating pattern they respond with a turning response in the direction of pattern motion. That behavior is called the optomotor response and used for course stabilization (Heisenberg & Wolf, 1984). Using the experimental setup with walking flies we have found that medulla neurons T4 and T5 are necessary elements for that behavior (Schnell et al., 2012; Bahl et al., 2013): Flies with T4 and T5 output blocked did not perform an optomotor response anymore. They could not see the movement of the surrounding pattern and hence were unable to stabilize their course. However, these flies were not completely blind: In a different scenario flies were permitted to fixate and track a black bar. Normal flies do this reliably and quickly bring the bar to the front and keep it there. To our surprise, this behavior was still intact in flies with T4 and T5 neurons blocked. This shows, that tracking behavior is controlled by a different neuronal pathway than the one used in motion vision.
The large tangential cells in the Lobula Plate (LPTCs) are postsynaptic to T4 and T5 neurons and collect information from many visual columns. From electrophysiological recordings it is known that LPTCs respond robustly and in a direction selective manner to visual motion stimulation. These neurons were therefore considered as prime candidates for controlling the optomotor response for many years, however a direct link could not be shown. We used blind, flying flies and expressed ChR in LPTCs selective for horizontal motion (HS cells) (Haikala et al., 2013). By shining blue light on one side of the brain only the neurons in one hemisphere were activated. Although the flies could not see, that activation initiated a turning to the side of activation, which finally proved that LPTC activity is sufficient to control motion behavior.
In the future we will use further behavioral experiments to get a more precise understanding how object fixation works and aim to identify the underlying neuronal elements.
Bahl, A., Ammer, G., Schilling, T. & Borst, A. Object tracking in motion-blind flies. Nat. Neurosci. 16, 730–738 (2013).
Haikala, V., Joesch, M., Borst, A. & Mauss, A. S. Optogenetic Control of Fly Optomotor Responses. J. Neurosci. 33, 13927–13934 (2013).
Heisenberg, M. & Wolf, R. Vision in Drosophila. (Springer, Berlin, Heidelberg, New York, Tokyo, 1984).
Schnell, B., Raghu, S. V., Nern, A. & Borst, A. Columnar cells necessary for motion responses of wide-field visual interneurons in Drosophila. J. Comp. Physiol. A 198, 389–395 (2012).