The visual system of the trout Salmo irideus Gibb. A degeneration and radioautographic study.

The organization of the primary visual system of the trout (Salmo irideus Gibb.) was studied by Fink-Heimer methods and radioautography. In addition to normal fiber, Nissl and Golgi staining techniques, we carried out total ablation of the retina on the right side on 16 animals which survived post-operatively at 13 degrees C for 9 to 31 days. The brains were cut at 35 micrometer and then stained by the Fink-Heimer techniques. Furthermore, we injected 20 microliter (2 microCi) of 14C proline into the right eye of 8 trouts which were then fixed between 24 and 72 hours after the injection, and then prepared for radioautography. The primary visual system projections are entirely contralateral. The optic tract is divided into three bundles which pass into tectal layers 2, 4 and 6. In the trout, a very obvious deep optic layer is present which extends throughout the length of the medial part of the tectum. In addition to the accessory optic tract and to a projection at the level of the preoptic area, the trout possesses a well-developed thalamic visual system composed of a ribboned geniculate nucleus and two medial thalamic visual centres fed by the retinothalamic pathways as well as a small retinal target located in the lateral thalamus. The pretectum receives only a few small sparse retinal projections. An interpretation of the trout visual system as the whole is discussed.