Input and output characteristics of collision avoidance behavior in the bullfrog were examined using computer graphics to model a looming stimulus. The means of time-to-collision of avoidance behavior in response to looming visual stimuli approaching at a velocity of either 2 or 4 m/s were significantly different (t(141) = 7.93, p < 0.05). On the other hand, mean threshold sizes of visual stimuli triggering avoidance behavior were not significantly different in either case (t(201) = 0.54, p > 0.05). Furthermore, we showed that the mean threshold sizes changed in a predictable manner depending on the distance between the displayed stimulus and the animal. These results strongly suggest that the frog displays collision avoidance behavior when the visual angle of a looming object reaches a constant value (about 20°). The mean maximum velocities of the avoidance behavior in response to the two visual stimuli were not significantly different (t(198) = 1.44, p > 0.05). However, we found that the frog could control the velocity depending on the location of an approaching object in its dorsal visual field. Finally, we demonstrated that habituation significantly affected the mean probability of avoidance behavior occurrence (ANOVA, at 2 m/s, F(2,15) = 14.25; at 4 m/s, F(2,15) = 13.35, p < 0.05), but not those of time-to-collision, threshold size and maximum velocity (ANOVA, at 2 m/s, F(2,13) = 0.07, F(2,14) = 0.46 and F(2,14) = 0.70, respectively; at 4 m/s, F(2,15) = 0.50, F(2,14) = 0.68 and F(2,14) = 0.41, respectively, p > 0.05). Thus, frog collision avoidance behavior seems to have an all or none – like property.

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