Prey orienting in frogs: Accounting for variations in output with stimulus distance

Abstract

We have studied the responses of leopard frogs,Rana pipiens, to live mealworms presented at different distances on the mid-sagittal plane. The response of normal frogs to stimuli at nearer distances consists of a direct snap whose amplitude increases with stimulus distance. For greater distances, the response consists of a forward hop whose amplitude also varies with stimulus distance. Over an intermediate range of distances, responses may be either snaps or hops. Whichever response occurs is of appropriate amplitude. The distance at which frogs switch from predominantly snapping responses to predominantly hopping responses increases with body size. Like normal frogs, unilaterally blinded frogs respond to stimuli at nearer distances with snaps whose amplitude varies with stimulus distance, switch from snapping to hopping over an intermediate range of distances, and respond to stimuli at greater distances with hops whose amplitude also increases with stimulus distance. In many cases, unilateral blinding did however result in a decrease in the distance at which the frogs switched from snapping to hopping. Such changes were not accompanied by the changes in snap or hop amplitude which would be expected if unilateral blinding resulted in generalized changes in distance judgement. Normal variations in snap amplitude and switches from snapping to hopping were also observed in frogs subjected to unilateral eye removal prior to the metamorphic eye migration which creates the adult binocular visual field. These results imply that neither distance discrimination nor any of the kinds of variation in motor output which occur with increasing stimulus distance necessarily depend on binocular cues. The behaviors studied also appear to be largely independent of normal binocular experience. More generally, our results suggest that the movement triggered by a stimulus at a particular location is not determined entirely by the retinal and superficial tectal region activated but rather reflects a combination of a retinal local sign signal with other kinds of information. The latter probably include signals related to stimulus distance and body posture, and may include signals related to body size as well.