Fig. 1. Experimental set-up in (A) top view and (B) front view. Fish were trained to swim on a straight line through the middle of the experimental tank to reach a goal compartment where they received a food reward. Before a trial, an individual fish was kept in a starting compartment for at least 5 min. Six horizontal light sheets (thin illuminated layers of laser light) were installed in the middle of the tank. The water was seeded with neutrally buoyant particles. Cameras 1 and 2 recorded the movements of the seeding particles from above the tank. Cameras 3 and 4 recorded the fish's movements.