Fig. 1. The experimental tank with landmark and aperture shown (not to scale). The wooden cover, lined with aluminum screen and grounded to reduce electrical interference, was removed to show the placement of the four lamps (L). The aperture, in black, was centred 25.5 cm from the floor, and the landmarks (white circles) were centred 6 cm from the floor and extended 12.5 cm on both sides of the divider wall. The electrolocation boundaries are marked on both sides of the divider wall and the depth is marked in 5 cm increments.