Visual acuity in the archerfish: Behavior, anatomy, and neurophysiology

Abstract

Archerfish are known for their remarkable behavior of shooting water jets at prey hanging on vegetation above water. Motivated by the fish's capacity to knock down small prey as high as two meters above water level, we studied the role of the retina in facilitating their excellent visual acuity. First, we show behaviorally that archerfish (Toxotes jaculatrix) can detect visual structures with a minimum angle of resolution in the range of 0.075°-0.15°. Then, combining eye movement measurements with a ray tracing method, we show that the image of a target on the retina coincides with the area centralis at the ventro-temporal retina. Moving down to retinal neural circuits, we then examine the ratio by which retinal ganglion cells multiplex visual information from the photoreceptors. Measuring the anatomical densities of both cell types in the area centralis, we found photoreceptor spacing to be 5.8 μm, which supports a minimum angle of resolution as low as 0.073°. Similarly, the average spacing of the ganglion cells was 5.7 μm. Based on electrophysiological measurements we found the smallest receptive fields of ganglion cells in that area to be in the range of 8-16 μm, which translates to an angular width of 0.1°-0.2°. These findings indicate that retinal ganglion cells in the area centralis stream information to the brain at a comparable resolution with which it is sampled by the photoreceptors. Thus, the archerfish can be used as an animal model for studying how visual details are streamed to the brain by retinal output.