Effects of post-operative visual environments on reorganization of retinotectal projection in goldfish.

1. Possible influence of different visual environments on the reorganization of retinotectal projection was studied with neurophysiological mapping methods following excision of the caudal half of the optic tectum in adult goldfish. 2. Post-operative light-deprivation showed no significant effects: in the absence of visual input, the visual projection from the whole retina because compressed on to the remaining rostral half-tectum in correct retinotopic order within 4 months, regardless of whether the contralateral optic nerve was left intact, or severed and then allowed to regenerate. 3. When the operated goldfish were continually exposed to visual stimuli without any dark period (post-operative dark-deprivation), two different results were observed: if the optic nerve was sectioned, in addition to excision of the caudal tectum, an orderly field compression was observed within 70 days in the re-established retinotectal projection; on the other hand, if the optic nerve was left intact, the dark-deprived fish retained the original connexions between the remaining rostral half-tectum and the temporal hemiretina without showing any sign of field compression for up to 253 days. 4. When the dark-deprived fish was then transferred into darkness, the suppressive effect disappeared: a compression of the retinotectal projection was induced within 2 or 3 weeks after the transfer. 5. Histological preparations of the fish brains showed consistent morphologic changes in the laminar structure of the remaining half-tectum. The stratum opticum and the stratum fibrosum et griseum superficiale merged together to form a new layer which contained an intricate network of thick fibre bundles.