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Abstract. A prominent model of visual motion detection is the so-called correlation or Reichardt detector. Whereas this model can account for many properties of motion vision, from humans to insects (review, Borst and Egelhaaf 1989), it has been commonly assumed that this scheme of motion detection is not well suited to the measurement of image velocity. This is because the commonly used version of the model, which incorporates two unidirectional motion detectors with opposite preferred directions, produces a response which varies not only with the velocity of the image, but also with its spatial structure and contrast. On the other hand, information on image velocity can be crucial in various contexts, and a number of recent behavioural experiments suggest that insects do extract velocity for navigational purposes (review, Srinivasan et al. 1996). Here we show that other versions of the correlation model, which consists of a single unidirectional motion detector or incorporates two oppositely directed detectors with unequal sensitivities, produce responses which vary with image speed and display tuning curves that are substantially independent of the spatial structure of the image. This surprising feature suggests simple strategies of reducing ambiguities in the estimation of speed by using components of neural hardware that are already known to exist in the visual system. IntroductionThe simplest way in which a visual system can determine how fast and in what direction an object travels would be to determine how long the object needs to cover the distance between two given points. Indeed,`featuretracking' mechanisms have been proposed along these lines to explain motion vision (Braddick 1980;Ullman 1983) and are sometimes used in machine vision (Murray and Buxton 1990). The disadvantage of such schemes, however, is that they need to identify the object, or features within it, before carrying out the tracking. In another class of models, this problem is avoided by using information on local spatiotemporal changes of intensity to measure velocity. In one subclass of these`intensity-based' models, the so-called gradient models, image speed is determined by computing the ratio of the local temporal and spatial gradients of intensity (Fennema and Thompson 1975;Limb and Murphy 1975). Gradient models have the property that they measure the speed of an image independently of its spatial structure. Modi®cations of this scheme have been proposed for measuring image velocity in two dimensions and for overcoming problems associated with low and sparsely distributed contrasts (Hildreth and Koch 1987;Johnston et al. 1992;Srinivasan 1990).Another subclass of the intensity-based models is the so-called Reichardt or correlation detector, which extracts a motion signal from the spatiotemporal correlations that are present in the moving image (Reichardt 1957). In this model, which has been very successful in describing motion sensitivity in animal vision from insects to primates (review, Borst and Egelhaaf 1989), the signal fr...

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Summary. Tethered flying Drosophila melanogasterchange the posture of their caudal body appendages in response to visual stimuli. In the present paper the relevance of lateral abdomen deflections for flight control is analysed. During abdomen deflections the line of action of the gravitational force is shifted with the fly's centre of mass. The line of action of aerodynamic drag forces is displaced accordingly, because friction is increased on the side of the body to which the abdomen is deflected. These two passive forces, together with the average flight forces generated actively by the wings, induce a yaw moment. In still air, the axis of this torque is tilted about 30 ~ backwards relative to the vertical body axis. It will be called "yaw axis of the flight mechanics". Two sets of observations support the notion of a combined yaw motor output. (a) The elementary motion detectors mediating the lateral abdomen deflection and the dynamics of the response resemble that of the optomotor response measured as yaw torque or as variation of wing beat amplitudes. (b)The asymmetric directional selectivity of the motion detecting system mediating the abdomen deflection corresponds to the orientation of the yaw axis of the flight mechanics. To explain the asymmetry, a nonlinear transfer characteristic is assumed in the motion detecting system.