During locomotion, the visual system is subjected to continuous changes of the retinal images termed ‘optic flow’. Turns to one side are accompanied by motion of the whole retinal image in the opposite direction. Optic flow elicited during rotation around a body axis is independent of the distance of the objects in the visual surround. In contrast, during translation, the retinal image of an object moves faster than the retinal image of its background. Thus, the retinal images of object and background move relative to each other. Visual systems have developed strategies to use the information provided by optic flow for different orientation tasks. Many animals compensate global rotational image motion around a body axis by eye, head and/or body movements in the opposite direction. These optokinetic or optomotor responses serve to stabilize the whole retinal image or at least part of it (Miles and Wallman, 1993) and have been interpreted in insects as a mechanism to compensate for unintended deviations from a straight path of locomotion. Compensatory optomotor responses have been studied in great detail in the fly (e.g. Fermi and Reichardt, 1963; Götz, 1964, 1975; Wolf and Heisenberg, 1990). Relative motion in the retinal image, in contrast, signals the presence of an object in the visual field. Relative motion cues can therefore be used to discriminate an object from its background. Detection and fixation of objects solely defined by relative motion have been investigated, for example, in humans (e.g. van Doorn and Koenderink, 1982, 1983; Regan and Beverly, 1984), in monkeys (e.g. Miles and Kawano, 1987), in bees (e.g. Srinivasan et al., 1990; Kern et al., 1997) and in flies (Virsik and Reichardt, 1976; Reichardt et al., 1983; Egelhaaf, 1985a; Kimmerle et al., 1996, 1997). In flies, flight control and visual orientation can be investigated on different levels ranging from free-flight behaviour to the neuronal and subcellular levels (for reviews, see Egelhaaf and Borst, 1993; Egelhaaf and Warzecha, 1999). Both optomotor turning behaviour and fixation responses can be investigated under controlled stimulus conditions using tethered flies in a flight simulator. In previous studies using the flight simulator, flies were shown to fixate an object in the frontal part of their visual field even if the object could be discriminated from the background only by means of relative motion (Virsik and Reichardt, 1976; Reichardt et al., 1983; Egelhaaf, 1985a). These experiments were performed in a cylindrical arena in which both the object and the background could be rotated only around the fly’s vertical body axis. Hence, flight situations in an environment were simulated in which no translational optic flow was present, corresponding to a situation in which object and background were at an infinite distance from the fly. In a realistic stationary three-dimensional environment, rotation of the animal around one of its body axes alone does not provide any relative motion cues. Therefore, the turning responses of tethered flies to objects defined by relative motion were subsequently investigated during simulated translational flight (Kimmerle et al., 1997). However, in these 1723 The Journal of Experimental Biology 203, 1723–1732 (2000) Printed in Great Britain © The Company of Biologists Limited 2000 JEB2524 The ability of flies to detect and fixate objects moving relative to their background was investigated in a flight simulator during translational tethered flight. The fly experienced optic flow that depended on its own actions and reactions in a similar way as in free-flight (closed-loop) conditions. Fixation of an object required turning responses towards it. The simulated distances between the fly, object and background were varied systematically by changing the velocities with which the object and the background pattern moved from the frontal to the back part of the fly’s visual field. Fixation responses were only elicited when the object was simulated to be closer than the background. The fly’s fixation performance was better with close than with more distant objects. Since, under many stimulus conditions, fixation responses were either elicited or entirely failed to be elicited, it is concluded that object fixation behaviour is gated in the visuo-motor pathway. Key words: flight, object fixation, vision, optic flow, closed loop, blowfly, Lucilia sp. Summary Introduction OBJECT FIXATION BY THE BLOWFLY DURING TETHERED FLIGHT IN A SIMULATED THREE-DIMENSIONAL ENVIRONMENT BERND KIMMERLE*, JUDITH EICKERMANN AND MARTIN EGELHAAF Lehrstuhl für Neurobiologie, Fakultät für Biologie, Universität Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany *Present address: Institut für Neurobiologie, Freie Universität Berlin, Königin-Luise-Straße 28–30, D-14195 Berlin, Germany (e-mail: kimmerle@zedat.fu-berlin.de) Accepted 21 March; published on WWW 10 May 2000