Circumvention of extraretinal photoresponses in assessing recovery of vision following optic nerve crush in goldfish.

Earlier experiments indicated that a conditioned light stimulus that was used to investigate the recovery of vision following optic nerve crush could evoke an extraretinal photoresponse. The present experiments sought to identify a visual stimulus that does not evoke a response following removal of both eyes for use in experiments on optic nerve regeneration. A stimulus consisting of a slight up-down movement of a small ring of light, that was kept stationary between conditioning trials, was classically conditioned to shock in eyed fish and the conditioned response consisted of a suppression of branchial ventilation movements. Following bilateral enucleation or a sham operation the fish received additional sessions of conditioning trials over a period of 3 weeks. Postoperative responding to the moving light stimulus was blocked in the enucleated but not the sham-operated fish. When the ring of light was turned on and off as a conditioned stimulus, responding was extensively but not completely eliminated following enucleation. The investigation confirms that extraretinal photostimulation can be classically conditioned to shock in at least some goldfish, and it shows that such conditioning can be circumvented by using a small moving light as the conditioned stimulus.