ORIGINAL PAPER B. Kimmerle á A.-K. Warzecha á M. Egelhaaf Object detection in the ¯y during simulated translatory ¯ight Accepted: 20 March 1997 Abstract Translatory movement of an animal in its environment induces optic ¯ow that contains informa- tion about the three-dimensional layout of the sur- roundings: as a rule, images of objects that are closer to the animal move faster across the retina than those of more distant objects. Such relative motion cues are used by ¯ies to detect objects in front of a structured back- ground. We confronted ¯ying ¯ies, tethered to a torque meter, with front-to-back motion of patterns displayed on two CRT screens, thereby simulating translatory motion of the background as experienced by an animal during straight ¯ight. The torque meter measured the instantaneous turning responses of the ¯y around its vertical body axis. During short time intervals, object motion was superimposed on background pattern mo- tion. The average turning response towards such an object depends on both object and background velocity in a characteristic way: (1) in order to elicit signi®cant responses object motion has to be faster than back- ground motion; (2) background motion within a certain range of velocities improves object detection. These properties can be interpreted as adaptations to situations as they occur in natural free ¯ight. We con®rmed that the measured responses were mediated mainly by a control system specialized for the detection of objects rather than by the compensatory optomotor system re- sponsible for course stabilization. Key words Figure-ground discrimination á Relative motion á Object detection á Insect visual system á Optic ¯ow Abbreviations OM object motion á rBM rotatory background motion á tBM translatory background motion á TFO temporal frequency of the moving pattern de®ned as ``object'' á TFB temporal frequency of the moving pattern de®ned as ``background'' Introduction In order to navigate safely in their environment animals have to gather information about the structure of their surroundings. This implies that objects that might in- terfere with the path of locomotion (either as targets or obstacles) have to be discriminated from their back- ground. Motion cues can play an important role in ob- ject detection (Helmholtz 1866). An object is seen more readily if it is moving relative to its background. Relative motion on the observer's retina can be elicited by a moving object as well as by a stationary object if the observer is in motion. During translatory self-motion the retinal image velocity of a stationary object depends on its angular position with respect to the observer's direction of motion as well as on its distance from the observer. A closer object passes by more quickly than does the background. Relative motion induced by the self-motion of the observer is, thus, also suited to pro- vide a basis for depth vision (review: Collett and Harkness 1982). Many animals ranging from insects to primates can use relative motion cues to detect objects in front of their background (Braddick 1974; Miles and Kawano 1987; Srinivasan 1993). In particular ¯ies (Kimmerle et al. 1996) as well as bees (Srinivasan et al. 1990) and hawkmoths (Pfa and Varju 1991) were shown to be able to use relative motion information that is elicited by their own locomotion to detect stationary objects elevated above the ground. J Comp Physiol A (1997) 181: 247±255 Ó Springer-Verlag 1997 B. Kimmerle (&) á A.-K. Warzecha á M. Egelhaaf Centre for Visual Sciences, Research School of Biological Sciences, Australian National University, Canberra A.C.T. 2601, Australia Present address of all authors: Lehrstuhl fuÈr Neurobiologie, FakultaÈt fuÈr Biologie, UniversitaÈt Bielefeld, Postfach 10 01 31, D-33501 Bielefeld, Germany, Fax: +49-521/1066038; e-mail: bernd.kimmerle@biologie.uni-bielefeld.de