[Corticalization of 2 regions of the visual system in the evolution of vertebrates]

Data on the evolution of the visual system in the phylogenetic scale of vertebrates are reported. Visual projections in the telencephalon of cyclostomes (lampreys) and retinothalamo-telencephalic channel in elasmobranches (skates) are shown. In amphibians (frogs) and reptiles (turtles) the existence of two visual subsystems (retino-thalamo-telencephalic and retino-tecto-thalamo-telencephalic) is determined, both being overlapped partially at the level of n. geniculatus lateralis and totally in the telencephalon. In turtles earlier visual and tectal impulses towards the telencephalon proved to be relayed in n. geniculatus lateralis and later ones--in n. rotundus. In mammals (rats) visual tecto-cortical connections are shown to have one synaptic relay in thalamic structures since cortical potentials evoked by stimulation of the colliculus superior in rats posses a rather short latency. Significant latency shortening of tectal visual responses in monkeys, as compared with the latency of the respective potentials in turtles (chronic identical experiments), supports the idea of the progressive development of the tectal division of the visual system in the vertebrate phylogony. It is concluded that both divisions of the visual system--retino-thalamic and retino-tectal underwent corticalization at earlier evolutionary stages.