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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 s1 (see
Materials and methods).