Fig. 1: Testing the role of optic flow in distance estimation: training and testing set-up.

a Training—the fish was trained to swim 0.8 m to an overhead infrared proximity sensor (IR Detector) which when the fish passed beneath it, detected a voltage change and via the Arduino computer caused the aquarium lights to switch on, signalling to the fish to return to the start area for a food reward. Training was conducted with the tunnel walls and floor patterned with alternating 0.02-m-wide black and white vertical stripes (set-up 1). b Testing—during testing trials, the infrared proximity sensor controlling the overhead lights was removed. The fish was therefore unable to cause the lights to turn on during testing trials. This was to test whether the fish had learned the correct distance, or to swim to the infrared detector landmark to encounter the light stimulus. The fish was tested with four different background combinations: (1) 0.02 m vertical stripes; (2) 0.01 m vertical stripes; (3) 0.02 m wide checkerboard pattern; (4) horizontal stripes of width 0.02 m.