Neuronal organization underlying visually elicited prey orienting in the frog--I. Effects of various unilateral lesions.

We have studied the effects on frog orienting behavior of three lesions: unilateral optic nerve section, unilateral tectal lobe ablation, and unilateral transverse hemisection of the neuraxis at a level just caudal to the optic tectum. Unilateral optic nerve section and unilateral tectal lobe ablation produce very similar deficits in visually elicited responses to prey items, an absence of responses for stimuli at locations within the monocular field of one eye. Unilateral hemisection, in contrast, results in abnormalities in visually elicited responses over a wider area, encompassing the entire ipsilateral visual hemifield. The hemisection deficit also differs in character from that following optic nerve section or tectal lesion. Within the affected hemifield, frogs do not fail to respond to stimuli but rather respond with abnormally directed movements. The movements, regardless of stimulus eccentricity on the horizontal, are always forwardly directed. While not varying with horizontal eccentricity, the movements do vary with stimulus elevation and distance. The variation with stimulus distance in the affected hemifield is somewhat different from that in the opposite hemifield. We conclude from the behavior that remains after hemisection lesions that there must exist bilateral descending tectofugal paths capable of triggering movements which vary with stimulus elevation and distance, and a crossed descending tectofugal path capable of triggering turns into one visual hemifield. That the deficit area is larger following a hemisection than following tectal lobe ablation indicates that the hemisection has affected the ability of both tectal lobes to trigger turns in one direction. A possible interpretation of this finding is that the lesion has interrupted not only the crossed descending tectofugal path from one tectal lobe but an uncrossed descending tectofugal path from the other. This hypothetical pathway as well as the others mentioned is incorporated in a model of the organization of the post-tectal circuitry involved in orienting.