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Thalamic structures relaying impulses originating from optic stimulation of the retina and electric stimulation of the tectum on the way to the general cortex, hyperstriatum dorsal ventricular ridge and striatum were studied in turtles Emys orbicularis. Anodal polarization of the n. rotundus temporarily, and the lesion of this nucleus irreversibly, suppressed the main negative component of responses evoked by tectal stimulation and light flashes in the dorsal ventricular ridge without affecting them in the general cortex. Polarization and lesions of the lateral thalamic region, including nucleus geniculatum lateralis produced an opposite effect. A conditioning single stimulation of the tectum or n. rotundus suppressed flash-evoked potentials in the hyperstriatum. However evoked potentials and single unit responses appeared following stimulation of the lateral thalamic region. They exhibited blocking interactions with the tectum-evoked responses. A conclusion is made that in the turtle the main pathways conducting visual information to the cortex and hyperstriatum are different: in the former case it is relayed in the lateral geniculatum region in the latter--in n. rotundus, although an overlap of the projections in dorsal ventricular ridge and striatum can be suggested.

Visual and somatic representations are established in the subcortical striatal sections of the forebrain (pallial thickening, dorsal ventricular ridge and putamen) of immobilized weakly anaesthetized turtles. According to electrophysiological characteristics they are similar to the corresponding sensory representations in the general cortex. The absence of reliable discrepancy between the latencies of potentials in the cortex and striatum evoked by light flash or by the electric stimulation of the dorsal thalamus indicates that visual projectional fibres ascending from the lateral geniculate nucleus terminate both in cortical and striatal structures. Differences in the latency distribution of single unit responses in the cortex and striatum to visual and thalamic stimulations can be due to the existence, besides the geniculo-telencephalic pathway, of a rotundo-telencephalic visual channel having direct connections with the striatum and polysynaptic ones with the general cortex. Significant differences in latency between the evoked potentials and neuronal responses in cortical and subcortical structures to electric stimulation of the skin shows that somatic projectional fibres on their way to the cortex are relayed in the striatum, thus indicating a lower corticolization degree of the somatosensory system as compared with that of the visual one.