Electrophysiological characteristics of the visual system have been investigated on an ascending phylogenetic scale in submammalian vertebrates. I n cyclostomes and plagiostomes the only system of vision is retino-tectal. In lampreys spread of excitation from visual centers to descending paths of the spinal cord occurs readily, as a manifestation of diffuse, primitive brain. As a result of progressive cephalization, a new retino-thalamo-cortical system arises in amphibia and reptilia. The retino-tectal system, however, retains its leading role. Optic tectum visual potentials are of short latency, displaying stability of pattern and aptitude to reproduce stimulation rhythm, making them comparable to primary responses of the mammalian cortex. The phylogenetically more recent retino-thalaniocortical system has not attained structural or functional maturity. Due to its diffuse organization, visual potentials generated in the surface cortical layer are of long latency, displaying aptitude to fatigue and recruiting on rhythmical stimulation, sensitivity to nembutal; being unlike the primary responses of the mammalian cortex, they should rather be regarded as responses of a non-specific type. Electrical stimulation of dorsal thalamic structures in the frog, turtle and varanus also fails to evoke cortical responses of a primary type. In the course of phylogenetic development, the retino-thalamo-cortical system evidently goes through a stage (amphibia, reptiles), when representation of the function of vision is of a mainly non-specific nature.

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The study of the neuronal organization of the hedgehog associative cortical zone revealed that besides primitive characteristics this area possesses a number of progressive features. An electrophysiological approach revealed a great number of polysensory neurons. The method, based on the HRP retrograde axonal transport, enabled us to discover the connections between primitive associative LP and MD thalamic nuclei and the associative cortex. Motor conditioned reflexes connected with visual discrimination as well as reflexes to signals of different modalities were elaborated quite easily. Conditioning to a simultaneous visual-auditory complex was found impossible in hedgehogs. Bilateral ablation of associative area results in disruption of the fine components of sensorimotor integration. It is likely that a primitively organized associative system in hedgehogs participates in the formation of sensorimotor synthesis, but fails to realize complex acts of intersensory integration.