VISUAL RECOGNITION OF THE STATIONARY ENVIRONMENT IN LEOPARD FROGS

Abstract

Leopard frogs (Rana pipiens) rely on vision to recognize behaviorally meaningful aspects of their environment. The optic tectum has been shown to mediate the frog's ability to recognize and respond to moving prey and looming objects. Nonetheless, atectal frogs are still able to appropriately respond to non-moving aspects of their environment. There appears to be independent visual systems operating in the frog: one system for recognizing moving objects; and another system for recognizing stationary objects. Little is known about the neural mechanisms mediating the recognition of stationary objects in frogs. Our laboratory showed that a retino-recipient area in the anterior lateral thalamus--the NB/CG zone--is involved in processing visual information concerning stationary aspects of the environment. This thesis aims to characterize the frog's responses to a range of stationary stimuli, and to elucidate the thalamic visual system that mediates those responses. I tested leopard frogs' responses to different stationary stimuli and found they respond in stereotypical ways. I discovered that leopard frogs are attracted to dark, stationary, opaque objects; and tested the extent of this attraction under different conditions. I found that frogs' preference to move toward a dark area versus a light source depends on the intensity of the light source relative to the intensity of ambient light. Unilateral lesions applied to the NB/CG zone of the anterior lateral thalamus resulted in temporary deficits in frogs' responses to stationary stimuli presented in the contralateral visual field. Deficits were observed in response to: dark objects, entrances to dark areas, light sources, and gaps between stationary barriers. However, responses to moving prey and looming stimuli were unaffected. Interestingly, these deficits tended to recover after about 6 days in most cases. Recovery time ranged from 2 - 28 days. The NB/CG zone is anatomically and functionally connected to a structure in the posterior thalamus called the "PMDT." The PMDT has no other connections in the brain. Thus, I have discovered a "satellite" of the NB/CG zone. Preliminary evidence suggests that the PMDT is another component of the visual system mediating stationary object recognition in the frog.