In our account of visual orientation we still discuss the first requirement, the stabilization of the animal. The last two chapters have dealt with optomotor behavior under open loop conditions. It is obvious from the properties of these responses that they serve to stabilize the trajectory of the fly against involuntary perturbations. This assertion, however, does not tell us how these course control functions operate when the fly moves freely. The simplest answer to this question would be that the mechanisms work permanently as long as the fly is confronted with visual motion (cf. Chap. 3; Optomotor Equilibrium). We will see in the second part of this book, however, that course control is more sophisticated. Even the basic function is modified according to more general requirements.
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