Some determinants of optic terminal localization and retinotopic polarity within fibre populations in the tectum of goldfish.

1. The reorganization of the retinotectal projection which results after partial ablation of tectal tissue was examined in goldfish using electrophysiological methods. 2. Regardless of the size of a unilateral ablation of caudal tectum, an orderly and virtually complete, 'compressed', visual projection re-formed on the remaining tectum after crushing the optic nerve and allowing it to regenerate. 3. If the optic nerve was left intact after ablations of caudal tectum, compressed projections were only found when the ablations were small. Large caudal ablations involving half or more of the dorsal tectum resulted in the cut fibres transposing onto the remaining tectum and forming an overlaid, 'duplicate', projection on the remaining intact projection. 4. In approximately one third of cases the duplicate projection lay in a reversed polarity along the rostrocaudal axis of the tectum. In the remaining cases the polarity of the duplicate projection was normal. 5. Transposed projections of reversed rostrocaudal polarity could be consistently obtained by ablating temporal retina and caudal tectum, leaving an intact strip of fibres terminals along the caudal edge of the tectal remnant. 6. Compression and duplication occurred in the same way if fish were maintained in constant light. 7. After ablations of lateral tectum, leaving the optic nerve intact, compression and some disorderly duplications were found. 8. Reversed projections could be induced across the mediolateral axis of dorsal tectum by denervating the medial tectum and ablating a strip of lateral tectum. 9. Projections of normal polarity were found after the optic nerve was allowed to regenerate into tecta which had previously supported reversed polarity projections.