Visual Flicker Induces Orientation Behaviour in the Fly Musca

Flies navigate under visual control1. When a fly perceives an object within its visual surroundings, it may turn towards the object. This orientation be­ haviour plays an important role in the fixation of objects and image stabilization, tracking and pos­ sibly even more complex phenomena such as pattern perception (see for example2_6) . Although this kind of behaviour normally occurs in a “closedloop” situation, it can be explained by “open-loop” experiments under steady state conditions. This has been shown in a quantitative behavioural analysis of object fixation by tethered flying flies3. As re­ ported by Reichardt3, stabilized retinal images of stationary objects fail to elicit a turning reaction whereas moving objects do. These observations are incorporated in a phenomenological theory de­ scribing the basic logical structure of the flight orientation by means of a stochastic equation of motion7: The angular velocity of the surrounding panorama (around the fly’s vertical axis) provides the visual feedback. However, from this one cannot conclude that the fly’s visual system computes in­ stantaneous angular velocities. When looking for the visual stimuli which may induce orientation, one has to bear in mind the variety of events that may be evoked by movement. It is known from earlier experiments8-12 that the direction-sensitive perception of motion in the steady state is described by the correlation of signals coming from different receptor channels. In the case of stationary (openloop) orientation behaviour, however, the results to be reported here suggest a superposition of signals from different receptor channels.