Collision-sensitive neurons in the optic tectum of the bullfrog (Rana catesbeiana)

Behavioral analysis showed that the frog displays collision avoidance behavior when the visual angle of a looming object reaches a constant value. In the present study, we identified collision-sensitive neurons in the frog tectum and examined their response characteristics. Only when the focus of expansion of image of an approaching object was located at the center of their receptive field did the collision-sensitive neurons get activated. And the peak firing rate occurred when the approaching object had reached a visual angle (θ) of 14.8 ± 3.3° (n = 16). We suggested that the size of retinal image, but not time-to-collision, is critical cue for the frog avoidance behavior.