Fig. 1.

Overview of the experimental setup. A: top view of the experimental tank. The tank can house an individual in each of the four corner compartments, and the central arena was used as a testing area. Each compartment is watertight from the neighboring compartments to prevent the EOD of fish in their compartment from acting as a spatial cue for the fish learning the mealworm location. All analyses were performed in the active zone (a) positioned at least 15 cm away from the wall (b). Remotely controlled gate (g) lets an animal enter the central arena from the corner compartment. Red arrows indicate following objects. q, Glass tank (1.8 m × 1.8 m × 0.3 m); b, circular fence (heat-formed acrylic sheet); c, test fish; d, home base; e, one of eight graphite electrodes to measure the EOD; f, live mealworm restrained on a suction cup; g, remotely controlled gate; h, one of four landmarks; i, compartment separator. B: side view of the experimental chamber shows the aquarium tank (a) surrounded by the experimental chamber (p). The chamber wall (p) blocked external sources of light, sound and RF noise. The walls (p) were filled with fiberglass batts for soundproofing and surrounded by a Faraday cage. The chamber rested on multiple layers of vibration-absorbing materials (j). The aquarium was supported by aluminum frames on the edges (k) and was uniformly heated from below by a floor heater (l). Eight infrared illuminators (m) uniformly lit the central arena by reflecting off the ceiling (n). The ceiling panel hid an infrared-sensitive camera (o) to prevent its reflection on the water surface.