| Abstract: | Extracellular studies have been made on the background activity and reactions to visual stimuli in neurons of nucleus rotundus and nucleus suprapeduncularis of the thalamus in the tortoise E. orbicularis. Reactions of on-off type to diffuse light flashes were recorded predominantly in both of the nuclei, whereas specific reactions to movement stimuli were found in the neurons of n. rotundus. The receptive area of the neurons in both of the nuclei involves all the visual field of one or both of the eyes. The structure of the receptive area of the neurons of n. suprapeduncularis is relatively homogeneous, whereas the receptive area of the neurons in n. rotundus exhibits a zone with an increased excitability under the horizontal meridian. After a damage of tectum opticum, the heterogeneity of the receptive areas and the specific responses to movement in cells of n. rotundus disappeared, while thresholds of the reactions increased. In the neurons of n. suprapeduncularis homogeneous structure of the receptive area remained unaffected, the thresholds of the reactions being increased as well. |
