The spatial distribution of light intensity received by the eyes changes continually when an animal moves around in its environment. These retinal activity patterns contain a wealth of information on theVisual orientation greatly relies on the evaluation of the global visual motion patterns received by the eyes when an animal moves around. These motion patterns depend in a characteristic way on the trajectory described by the moving animal as well as on the particular three-dimensional structure of the visual environment z'2. Consider, for instance, two simple commonplace situations. In the first, a moving animal unintentionally deviates from its course. This results in a displacement of the entire visual scene, which contains a strong rotational component. When this rotational component is extracted from the retinal motion pattern, it can be used to control the compensatory optomotor responses of the animal. In this way, the course may be stabilized against internal and external disturbances. A different situation is encountered when the animal passes nearby objects located in front of a more distant background. The retinal images of these objects and their background then move relative to each other leading to discontinuities in the motion field. This relative motion may indicate the existence of nearby stationary or moving objects. This information can be used to discriminate objects from their background and might serve as the basic cue in various visual orientation tasks, such as fixation of stationary objects or pursuit of moving targets. These types of global retinal motion patterns do not only occur when the animal moves around bodily. Similar motion patterns may also arise during head and eye movements. This review concentrates on recent studies on the visual system of the fly, which has proved, during the past few decades, to be a suitable model system for the elucidation of the neuronal computations underlying various behavioural motion-dependent tasks 3'4. We analyse the basic mechanisms by which the nervous system of the fly processes coherent largefield motion, and relative motion between objects and background, and how these motion patterns are exploited in mediating optomotor course stabilization and object-induced orientation. Whether related mechanisms play a role in evaluating global retinal motion patterns in other species has yet to be established, although it is not unreasonable to expect that this will be the case. This has already been shown for the mechanisms underlying other motion information processing tasks. The basic mechanism of local movement detection for instance (see below), which was initially discovered in the insect visual system, was later also found in vertebrates, including iTlan ~8.

We studied the effects of various patterns as contextual stimuli on human orientation discrimination, and on responses of neurons in V1 of alert monkeys. When a target line is presented along with various contextual stimuli (masks), human orientation discrimination is impaired. For most V1 neurons, responses elicited by a line in the receptive field (RF) center are suppressed by these contextual patterns. Orientation discrimination thresholds of human observers are elevated slightly when the target line is surrounded by orthogonal lines. For randomly oriented lines, thresholds are elevated further and even more so for lines parallel to the target. Correspondingly, responses of most V1 neurons to a line are suppressed. Although contextual lines inhibit the amplitude of orientation tuning functions of most V1 neurons, they do not systematically alter the tuning width. Elevation of human orientation discrimination thresholds decreases with increasing curvature of masking lines, so does the inhibition of V1 neuronal responses. A mask made of straight lines yields the strongest interference with human orientation discrimination and produces the strongest suppression of neuronal responses. Elevation of human orientation discrimination thresholds is highest when a mask covers only the immediate vicinity of the target line. Increasing the masking area results in less interference. On the contrary, suppression of neuronal responses in V1 increases with increasing mask size. Our data imply that contextual interference observed in human orientation discrimination is in part directly related to contextual inhibition of neuronal activity in V1. However, the finding that interference with orientation discrimination is weaker for larger masks suggests a figure-ground segregation process that is not located in V1.

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Drosophila mutants transformed with a chimaeric gene that expresses the ocellar visual pigment in the major class of photoreceptor cells of the retina were used to investigate the properties of this minor pigment. The photoreceptor cells in which this opsin was misexpressed showed new spectral characteristics and physiology.