Many animals are able to judge the absolute size of visual objects, a skill that becomes especially useful for tasks such as selecting prey. The ability to judge absolute size, however, requires that the visual system be able to perform a critical correction to account for distance between the eye and the viewed object: dramatic differences in the size of the actual retinal image occur as the distance between viewer and object changes. A situation in which this corrective requirement is especially challenging arises for animals that need to judge the size of aerial prey from an underwater vantage point. Here, as the study of Schuster and colleagues shows, the complex optical situation arising from distance and refraction requires precise understanding of the relationship between an object's apparent size and the fish's relative position to the object.
In the experiments reported by the researchers, archer fish viewed a set of eight disks at various heights above the water surface from various horizontal distances and selected one of the disks as a shooting target based only on its absolute size. Although naïve fish often selected disks that would be too large to be swallowed, all could eventually learn to judge absolute size with great precision; in doing so, they perfectly accounted for the complex optical situation posed by their underwater viewpoint. In a series of experiments, the researchers showed that the fish do not learn this by remembering which combinations of spatial configurations and the corresponding images were rewarding in the past. Rather, the fish extracted the underlying law that connects spatial configuration and apparent size. This remarkable cognitive ability allows the fish to readily judge a target's objective size from underwater views they have never encountered before.
Stefan Schuster, Samuel Rossel, Annette Schmidtmann, Ilonka Jäger, and Julia Poralla: "Archer Fish Learn to Compensate for Complex Optical Distortions to Determine the Absolute Size of Their Aerial Prey"
Published in Current Biology, Volume 14, Number 17, September 7, 2004, pages 1565-1568.