Vision in goldfish following bilateral tectal ablation

Visual sensitivity of optic tectum-ablated goldfish was investigated using a classical conditioning technique. Intact fish were screened to obtain individuals which showed suppression of breathing movements in response to the visual conditioned stimulus (CS) in the presence or absence of adapting illumination. Following bilateral optic tectum ablation, responding was blocked in light-adapted but not dark-adapted fish. Response threshold testing revealed no significant postoperative changes in visual sensitivity. Small remnants of tectal tissue containing cellular elements of the periventricular gray zone and optic axon terminals were detected in some ablates but there was no evident relationship to response threshold. Optic nerve crush blocked responding in ablates and recovery occurred within 2–3 weeks postaxotomy confirming that the response was mediated by retinal as opposed to extraretinal photostimulation. The experiments support the findings by others that tectum ablation results in decreased visual sensitivity and that conditioned visual responding can be obtained. However, we find no support for the suggestion that visual sensitivity in the ablates depends on functional recovery of regenerating optic axons which innervate non-tectal visual nuclei. Instead, the results indicate that the normal retinal projections to the non-tectal nuclei can mediate visual responding and, in addition, that postoperative conditioning experience facilitates recovery of response in ablates which initially appear to be blind to the CS.