Optokinetic horizontal head nystagmus in the frog after regeneration of transected retinal fibres.

The recovery of optokinetic responses during regeneration of transected retinal fibres was studied. Regenerated optic fibres were demonstrated by the cobaltfilling technique. After transection of the chiasma in the midline in most of the cases regenerating retinal fibres did not cross, but terminated in ipsilateral visual centres. Aberrant fibres were found in the telencephalon, the periventricular region of the diencephalon, anterior-, posterior- and ventral tegmental commissures and in the isthmic nucleus. In one group of animals optokinetic responses did not return after regeneration of retinal fibres. Re-innervation was either symmetrical on both sides from one eye, or terminal fibres were disorganized. In the second group of animals normal optokinetic nystagmus (OKN) returned. In these cases the fibres were symmetrically distributed on both sides or, alternatively, the crossed fibres outnumbered the ipsilateral ones. The third group of animals showed reversed OKN, spontaneous turning of the head, circling or head nystagmus. Here the majority of regenerated fibres termined ipsilaterally. When the OKN returned, the retinal projection was always restored in the pretectal region. Contrary to our earlier work, we concluded that the basal optic nucleus is not essential for horizontal optokinetic head nystagmus because in a few animals, retinal fibres did not invated this nucleus, and in spite of this the OKN could be evoked. We think now that the most important structure (as an input channel) for horizontal OKN of frogs is the pretectal region.