Fig. 1. Experimental set-up and shadow velocities. (A) Top view of the experimental tank. Water containing food odor flows into a tunnel on the right side and exits on the left. Animals enter the tunnel from the start compartment and approach the food odor release point. A pair of bath electrodes is attached to the tunnel walls, 8 cm from the tunnel entrance. The bath is grounded with a ground wire. Shadows always move from right to left over the tank. (B) Side view of the set-up. Animals inside the tank are filmed with a camera positioned above the tank. The camera is connected to a TV monitor. Bath electrodes are connected to an amplifier and an oscilloscope. Signals are recorded on a computer. The shadow is produced by swinging a plastic rectangle through a light beam directed onto the tank. The tank wall facing the light and shadow apparatus is covered. (C) Average shadow velocities and accelerations derived from five repeated measurements with silicone photodiodes placed between the right tunnel wall and the bath electrodes. Velocities and accelerations for all three shadows (slow, medium, fast) are highly consistent as evidenced by small standard deviations.