Fig. 1. Top view of an experimental tank used to train Gnathonemus petersii to discriminate visual patterns shown at a fixed distance. The fish rests in a daytime shelter whose front end is blocked by a movable transparent Plexiglas screen. Through this screen, the fish views two patterns printed on white cards that are fixed in a pattern holder on the front screen of the tank. The fish therefore views the visual patterns from a fixed vantage point. When the screen is lifted, a trained fish swims straight towards one of the two patterns, where a partition prevents it from seeing the other pattern.
