Distance determined by the angular declination below the horizon

A biological system is often more efficient when it takes advantage of the regularities in its environment(1,2). Like other terrestrial creatures, our spatial sense relies on the regularities associated with the ground surfac e(2-6). A simple, but important, ecological fact is that the field of view of the ground surface extends upwards from near (feet) to infinity (horizon )(2). It forms the basis of a trigonometric relationship wherein the furthe r an object on the ground is, the higher in the field of view it looks, wit h an object at infinity being seen at the horizon. Here, we provide support for the hypothesis that the visual system uses the angular declination bel ow the horizon for distance judgement. Using a visually directed action tas k(7-10), we found that when the angular declination was increased by binocu larly viewing through base-up prisms, the observer underestimated distance. After adapting to the same prisms, however, the observer overestimated dis tance on prism removal. Most significantly, we show that the distance overe stimation as an after-effect of prism adaptation was due to a lowered perce ived eye level, which reduced the object's angular declination below the ho rizon.