Compression and expansion without impulse activity in the retinotectal projection of goldfish

The capacity of regenerating optic fibers to undergo retinotopic compression and expansion in the absence of impulse activity was tested by eliminating activity with periodic intraocular injections of tetrodotoxin (TTX) during regeneration. To test for compression, the posterior half of tectum was removed and the optic nerve crushed. For expansion, the temporal half of retina was ablated and the nerve also crushed. The projection was then subsequently examined with electrophysiological mapping and autoradiographic tracing. Like electrically active fibers, silent fibers formed a retinotopically ordered projection that was compressed onto the anterior half tectum. Similarly, TTX-treated fibers from a nasal half retina formed a retinotopic projection that was expanded across the entire tectum. Except for some enlargement of receptive fields produced by the TTX, the topography was equivalent to that formed by active fibers. Thus, fibers can apparently maintain relative positions irrespective of absolute tectal position without the benefit of activity-dependent ordering. This implies the existence of an activity-independent mechanism for relative positioning that may operate across larger distances than the activity-dependent ordering responsible for fine topography and ocular dominance columns.