Fig. 1. Experimental set-up and generation of visual stimuli. (A) The set-up for delivering visual stimuli is located inside a sealed Faraday cage. Four monitors are located at 20 cm to the sides, in front and above the animal (the upper monitor is not shown). Stimulus signals generated by PC1 were directed to any one of the four monitors through a selector switch located outside the Faraday cage. A second computer (PC2) was used in combination with PC1 to record the response of the crab. (B) The looming stimulus was a simulated projection of an approaching object from one of the monitor screens. The simulation corresponded to a black square object of 5 cm that approached from a distance of 70 cm at a constant velocity of 20 cm s^–1 (see Materials and methods).