Bilateral tectal innervation by regenerating optic nerve fibers in goldfish: a radioautographic, electrophysiological and behavioral study.

Following unilateral enucleation and optic nerve crush in goldfish, the remaining nerve regenerates and innervates both optic tecta. Approximately 5% of the nerve fibers reach the ipsilateral optic tectum (IOT) via the ipsilateral tract at the chiasma. Comparable debris in both tracts was not sufficient to result in an IOT projection since when both nerves were crushed simultaneously the usual pattern was seen, i.e., each nerve innervated a contralateral optic tectum (COT). When the arrival of one nerve at the chiasma was delayed by staggering the nerve crushes, the nerve that first arrived at the chiasma partially innervated the Iot. In most instances the entire IOT was innervated, however, the stratigraphic distribution of fibers in the various tectal lamina was atypical. Electrophysiological analysis indicated that fibers from each area of the retina innervated the IOT visuotopically. The COT was ablated in order to determine whether the IOT projection could mediate behavior. All fish failed to respond to changes in illumination as measured by respiration and failed to swim with or against the stripes in an optomotor drum. Thus, the IOT input, possibly because of its sparseness, could not be shown to be behaviorally functional.