Proceedings: Interactions between optic fibres in their regeneration to specific sites in the goldfish tectum.

Interactions between optic fibres in their regeneration to specific sites in the goldfish tectum BY J. E. COOK* and T. J. HORDER. University Laboratory of Physiology, Parks Road, Oxford OX1 3PT To explain the termination of regenerating optic fibres at particular retinotopically ordered positions in the optic tectum Sperry (1943) proposed that fibres, 'chemospecifically' differentiated according to retinal origin, selected terminal sites on the basis of matching distinctive tectal cues. However, this cannot explain, without further qualifications, the finding in goldfish (Gaze & Sharma, 1970) that the entire optic fibre array will re-establish terminals retinotopically within the rostral halftectum after ablation of the caudal half. During the course of regeneration in a series of goldfish (5.4-6.0 cm nose to tail-fin base, maintained at 280C) in which the right optic nerve had been cut together with removal of the caudal half of the left tectum, we mapped electrophysiologically (Jacobson & Gaze, 1964: tungsten electrodes) the arrangement of localized visual responses, believed to originate from optic fibre terminal arborizations, on the rostral halftectum which retains its original gross anatomical features. The earliest regenerated responses were due to fibres representing the naso-dorsal field located within the rostral half-tectum just as they are in a complete retinotectal projection, but these gradually moved forward on