Fig 1. Overhead view of a circular test aquarium showing the four compartments with the same volume and area and the white, opaque chamber used in the motivation test.

A) Each compartment was covered on the inside with adhesive paper of a different color (red, yellow, green or blue). The central compartment was formed by the junction of the background colors of the other compartments. The aquarium diameter was 40 cm; the water column height was 15 cm. The outer walls and the internal partitions were opaque and of the same color as the rest of the compartment. The test aquaria were randomly distributed within the testing area of the lab and the position of the colored compartments was changed for each fish to minimize any possible environmental bias in our results. B) Transparent locks attached to the bottom of the chamber prevented the complete opening of the transparent doors. Thus, fish could touch and move the doors, but could not completely open them and leave the central chamber. Chamber size = 10 x 10 x 22 cm.