The ability to determine the distance to objects is an important feature of most visual systems, but little is known about the neuronal mechanisms for distance estimation. Larval zebrafish execute different visual behaviors depending on how far they are from their prey; at medium distances, they converge their eyes and approach, but when the prey is close enough, they execute a strike and suck the prey into their mouths. In order to study distance estimation, we developed a head-fixed strike assay. We found that we could evoke strike behavior in head-fixed larvae, and quantify head elevation movements to classify the behavior as a strike. Strikes were strongly dependent on distance to prey, allowing us to use them to study distance estimation. Light intensity is rapidly attenuated as it...[ Read more ]

The ability to determine the distance to objects is an important feature of most visual systems, but little is known about the neuronal mechanisms for distance estimation. Larval zebrafish execute different visual behaviors depending on how far they are from their prey; at medium distances, they converge their eyes and approach, but when the prey is close enough, they execute a strike and suck the prey into their mouths. In order to study distance estimation, we developed a head-fixed strike assay. We found that we could evoke strike behavior in head-fixed larvae, and quantify head elevation movements to classify the behavior as a strike. Strikes were strongly dependent on distance to prey, allowing us to use them to study distance estimation. Light intensity is rapidly attenuated as it travels through water, so we hypothesized that larvae could use intensity as a distance cue. We found that increasing stimulus intensity could cause larvae to strike at prey that would normally be out of range, and decreasing the intensity could lower the strike rate for even very proximal stimuli. In order for this strategy to work over the range of natural lighting conditions, there should be some other parameter involved, and we found that the relative amount of UV light in the background also modulated the behavior, and this scaled over a range of light intensities. Finally, we presented prey in the binocular vs. monocular visual field and found that monocular prey did evoke strikes, although the binocular input produced more. These results suggest that strike behavior is optimally evoked by bright UV dots in the binocular zone with minimal UV background light, and provide a foundation to study the neuronal mechanisms of distance estimation.