The Angular Threshold for Frog Collision Avoidance Behavior Changes Depending on Not Only the Stimulus Location But Also the Behavioral Strategy

Abstract

In our previous study, we showed that the frog, Rana catesbeiana, produces collision avoidance behavior when the retinal size of an approaching object reaches about 20^o. In this study, we examined whether the threshold value is unchangeable under any conditions. The avoidance behavior of the frog to looming stimulus moving toward the animal on a collision course was examined by using computer graphics and frame by frame analysis. The distance between an animal and a display on which the looming stimulus was presented was changed from 20 cm to be 40cm. The mean angular thresholds are compared among three different areas on the screen of a display. Area B at a distance of 20 cm and area C at 40 cm subtended the same visual angle on the center of each display. Area A at a distance of 20 cm corresponds to outside the visual angle. Statistical analysis showed that the mean angular threshold in areas A and B were not significantly different (p>0.05). The mean angular thresholds in areas B and C were not significantly different, either (p>0.05). However, the analysis showed that the mean angular threshold in area C was significantly smaller than that in area A (p<0.05). These results suggest that the difference in angular threshold depending on the location of a display is caused by both visual field specificity and integration of size and depth information of an approaching object. We also examined the relationship between the angular threshold of visual stimulus and the direction of evoked escape jumps which is crucial for successful survival. The analysis showed that the angular threshold for a backward jump is significantly smaller than that for a forward jump. These results show that the angular threshold size can be changed depending on both stimulus condition and the behavioral strategy or prospective behavior performed by the animal.