Intratectal targeting by optic fibers in goldfish under impulse blockade

Regenerating optic fibers in goldfish were tested for their capacity to grow from an inappropriate region of the tectum to their appropriate part of the tectum when silenced by periodic intraocular injections of tetrodotoxin. For this, fibers normally innervating the lateral posterior quadrant of one tectum were surgically redirected into the medial anterior end of the opposite host tectum. This starting position corresponds to the path normally taken by fibers to innervate the medial tectum. For comparison, fibers normally innervating the medial posterior quadrant of the opposite tectum were surgically redirected into this same position in the host tectum. To prevent cueing with resident optic fibers, host fibers were eliminated by removing the eye supplying the host tectum. The innervation by these deflected fibers was determined autoradiographically using quantitative microdensitometry. Electrically silent fibers were found to navigate toward their appropriate region just as well as fibers with impulse activity. Thus, the capacity of fibers to read positional tectal cues and to bypass foreign tectal regions was not detectably regulated by impulse activity.