Behavioral paradigm. (a) Experimental setup. The water had a depth of 10 cm, and the infrared camera was placed just above the tank. In experiments 1 and 2, the stimuli were projected to the tank through multiple reflections of the mirrors. In experiment 3, the stimuli were projected directly to the tank. (b) Experimental stimuli. Upright BM and non-BM (or inverted BM) stimuli were represented as the animations of six white dots. (c) Experimental procedure. Upright BM and non-BM (or inverted BM) stimuli were either projected onto two opposite sides of the tank separately in a within-subject design (experiments 1 and 2) or onto one side of the tank in a between-subject design (experiment 3). The habituation period took at least 5 minutes until the fish swam in the tank in a normal pattern and speed. The experiment period included a 1-minute baseline and 4-minute test.