A possible neurophysiological basis for depth perception in frogs: existence of a horopter surface.

In frogs, multi-unit receptive fields (MURF) of rostral binocular tectal points (BTP) show a crossed disparity when mapped at a distance equal to the perimeter radius (i.e., 33 cm). The shape of the spatial surface where MURF of all BTP are in-register is investigated in two planes: (a) in the longitudinal plane, the locus of superimposition is a circumference passing through both eyes; (b) in the vertical plane, it corresponds to a straight line tilted towards the animal's head. This surface can be defined as the frog's horopter surface since it represents the spatial locus where objects can simultaneously stimulate corresponding retinal areas. Behavioural and electrophysiological correlations are discussed.