articles Zebrafish larvae innately begin responding to moving stimuli shortly after hatching. In their optomotor response, which is elicited by large moving stimuli presented from below or the side 1,2 , larvae swim in the direction of perceived motion. The distance they travel in a given time indicates the effectiveness of the stimulus. By observing the response of many larvae to computer-animated displays, we could determine which attributes of a moving stimulus the zebrafish visual system detects. If luminance-defined features drift smoothly or jump in space, they can produce strong sensations of motion. A number of proposed models explain how motion information can be extracted. In a simple model, a point-to-point comparison is made between the luminance pattern and a spatially displaced copy of the pattern that was seen a short time before 3 . The displacement that gives the best fit tells the brain the direction and speed of movement. A more complex strategy is to look at the Fourier motion energy in the visual scene Although there is evidence that humans can use both feature matching and motion energy to detect movement 7 , they may also sense motion when presented with stimuli in which only secondorder features such as contrast, texture or flicker are moving Here we find that the fish larvae detect moving features of visu- A moving grating elicits innate optomotor behavior in zebrafish larvae; they swim in the direction of perceived motion. We took advantage of this behavior, using computer-animated displays, to determine what attributes of motion are extracted by the fish visual system. As in humans, first-order (luminance-defined or Fourier) signals dominated motion perception in fish; edges or other features had little or no effect when presented with these signals. Humans can see complex movements that lack first-order cues, an ability that is usually ascribed to higher-level processing in the visual cortex. Here we show that second-order (non-Fourier) motion displays induced optomotor behavior in zebrafish larvae, which do not have a cortex. We suggest that second-order motion is extracted early in the lower vertebrate visual pathway. al stimuli in a way that is qualitatively similar to humans: both firstorder and second-order cues drive their behavioral response. Our demonstration of second-order motion detection in fish challenges the idea that higher-level, cortical mechanisms are necessary to explain this capacity of the visual system. RESULTS Optomotor responses to Fourier motion The assay used to measure optomotor responses is similar to the one described previously 2 (Methods). Movies showing drifting gratings evoke strong optomotor responses in almost all fish in a clutch. Fish do not respond to a moving grating with a stripe width narrower than approximately 9°, which is slightly less than the predicted resolution limit of the larval cone mosaic, 6°at this age In the following experiments, responses were normalized to the effect of a designated strong stimulus, a 100% contrast square wave subtending 100°of visual angle per cycle and moving at 1 Hz for 30 seconds Although the fish seemed to follow a motion signal in the movies, it was possible that they were tracking features such as light or dark regions or edges that were being displaced. We did an experiment to show that the optomotor response is truly a response to motion. A motion display was shown of a sine wave grating tha
