Progress of topographic regulation of the visual projection in the halved optic tectum of adult goldfish.

Abstract

The patterns of re-established visual projections on to the rostral half-tectum were studied following excision of the caudal tectum at various intervals after section of either the contralateral optic nerve or the ipsilateral optic tract in adult goldfish. The pattern of a newly restored retinotectal projection depended on the duration of the post-operative period given to the halved tectum before it was re-innervated by regenerating optic fibers from the retina. When the duration was such that regenerating optic fibers invaded the denervated rostral half-tectum at about 40 days or longer after excision of the caudal tectum, the remaining half-tectum was able to accommodate incoming optic fibers not only from the appropriate temporal hemiretina but also from the foreign nasal hemiretina in an orderly compressed topographic pattern. If the surgical operations were timed so that the halved tectum received regenerating optic fibers earlier than 33 days after excision of the caudal tectum, the halved tectum initially accommodated only those optic fibers originating from the temporal half of the retina at this early stage. This normal (uncompressed) pattern of the newly regenerated visual projection eventually changed into an orderly compressed pattern at a later period. Post-operative dark-deprivation of the operated fish has no significant effect on the temporal transition. The temporal transition from an initally normal pattern into an orderly compressed pattern may reflect the time course of progressive and systematic changes involved in topographic regulation of the halved tectum into a whole.