Re-investigation of the role of the accessory optic system and pretectum in the horizontal optokinetic head nystagmus of the frog. Lesion experiments.

The effects of various lesions in the accessory optic system and pretectal area were studied on the horizontal optokinetic head nystagmus (OKN) in the frog Rana esculenta. For histological control the cobalt-filling technique and haematoxylineosin staining were used. Isolated complete transection of the basal optic root caused only a slight reduction in the frequency of the OKN. Transection of the marginal optic tract on one side reduced the frequency as if one eye had been removed. Electrolytic destruction of the basal optic nucleus on one side did not influence the optokinetic responses significantly. In monocular animals, however, an obvious reduction occurred in the frequency of the OKN when the basal optic nucleus corresponding to the seeing eye had been destroyed. If the lesion involved the brain tissue surrounding the basal optic nucleus, nystagmus could not be evoked. Complete destruction of the lateral pretectal neuropil, which receives retinal afferents, strongly reduced the frequency or abolished the OKN. We conclude that the pretectal region is the most important structure at the input side in the initiation of this visuomotor response. The basal optic nucleus together with the neighbouring tegmental area may also play some role in the regulation of the OKN.