SINCE Wertheimer's classic paper1, research in motion perception has been concerned with the study of visual illusions such as φ-motion. Various phenomena of this type are easy to elicit by successive changes of the light flux in spatially distinct photoreceptors, and easy to explain by the specific properties of the motion detectors, although there are reports to the contrary2. The present account deals with a less easily comprehensible illusion which is elicited by simultaneous changes of the light flux in differently illuminated receptors3-5. The phenomenon has previously been ascribed to the prolonged latency of the 'light-on' responses at lower levels of illumination which converts simultaneous stimuli into successive signals6, but this does not explain the illusion satisfactorily3. MacKay and co-workers were the first to attribute the apparent motion to the adaptive properties of the input channels of the motion detectors4. We show that the illusion can be induced in the fruitfly, Drosophila melanogaster, as well as in man. The course control response to motion provides a quantitative assay of the illusion in the fly. The results suggest that the illusion originates in the distortion of the visual signals before motion detection.
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