Fig. 1. (A) Photograph of the experimental set-up using the half-field stimulus drum. Two of these semicircular constructions are placed around a circular observation tank, covering 360° of the animal's horizontal and 55° of its vertical visual fields. The video monitor displays a field of view of 3 mm and in the photograph shows the eyes and head of a sandlance. (B) Illustration of one semicircular construction. (C) Diagram to illustrate the position of the fish relative to the two semicircular parts of the drum. Each half-circle is powered by a separate power supply, allowing independent variations in stimulus speed and direction.