Motor planning modulates sensory-motor control of collision avoidance behavior in the bullfrog, Rana catesbeiana

Abstract

In this study, we examined the collision avoidance behavior of the frog, Rana catesbeiana to an approaching object in the upper visual field. The angular velocity of the frog's escape turn showed a significant positive correlation with the turn angle (r(2) = 0.5741, P<0.05). A similar mechanism of velocity control has been known in head movements of the owl and in human saccades. By analogy, this suggests that the frog planned its escape velocity in advance of executing the turn, to make the duration of the escape behavior relatively constant. For escape turns less than 60°, the positive correlation was very strong (r(2) = 0.7097, P<0.05). Thus, the frog controlled the angular velocity of small escape turns very accurately and completed the behavior within a constant time. On the other hand, for escape turns greater than 60°, the same correlation was not significant (r(2) = 0.065, P>0.05). Thus, the frog was not able to control the velocity of the large escape turns accurately and did not complete the behavior within a constant time. In the latter case, there was a small but significant positive correlation between the threshold angular size and the angular velocity (r(2) = 0.1459, P<0.05). This suggests that the threshold is controlled to compensate for the insufficient escape velocity achieved during large turn angles, and could explain a significant negative correlation between the turn angle and the threshold angular size (r(2) = 0.1145, P<0.05). Thus, it is likely that the threshold angular size is also controlled by the turn angle and is modulated by motor planning.

Keywords: Collision avoidance behavior; Frog; Motor planning.

Fig. 1.. Stimulus positions and escape directions.

Vertical and horizontal eccentricities of the stimulus (ξ and Φ) are represented with concentric circles and radial lines, respectively. The frog is located at the center of the concentric circles facing 0° (upward on diagram). The stimulus position relative to the animal is represented by a large dot. The escape direction is represented by a bar extending from the dot. The bars seem to orient radially from the center of the concentric circles.

Fig. 2.. Scatter plot showing a significant positive correlation (P<0.05) between the escape direction α and the angular velocity ω of the escape behavior (r² = 0.5741).

Data points obtained from right turns are shown as they would appear after reflection around the mid sagittal plane.

Fig. 3.. Scatter plot showing a correlation between the escape direction α and behavioral duration τ of the escape behavior (r² = 0.118).

Data points obtained from right turns are shown as they would appear after reflection around the mid sagittal plane. The slope of the regression line is 0.0026 s/deg, suggesting almost constant behavioral duration.

Fig. 4.. Scatter plot showing a significant negative correlation (P<0.05) between the escape direction α and the angular threshold θth of escape behavior (r² = 0.1145).

Data points obtained from right turns are shown as they would appear after reflection around the mid sagittal plane.

Fig. 5.. Comparison between mean (± s.d.) angular thresholds (θth) triggering forward (0°≤α≤90°, 270°≤α≤360°) versus backward (90°≤α≤270°) escape jumps.

The former is significantly larger than the latter (P<0.05).

Fig. 6.

(A) Scatter plot showing no significant correlation (P>0.05) between the angular velocity ω and the angular threshold θth of escape turns less than 60° on either side of the midline. (B) Scatter plot showing a correlation between ω and θth of escape turns of more than 60° from the midline. There is a significant positive correlation between them (P<0.05).

Fig. 7.. The stimulus simulated a black square approaching at a constant speed v with size l through a path of p.

The threshold of the stimulus size on the screen placed h above the experimental stage was calculated by the equation, r  =  hl/(p − vt), where t denotes the delay between the onset of stimulus and the time when the frog start to move.