Thalamic neuronal reactions of the tortoise, Emys orbicularis, to visual stimuli

Reactions of 300 neurons and characteristics of their receptive fields (RF) in response to moving stimuli and diffuse illumination have been investigated in three hypothalamic regions of the tortoise Emys orbicularis. With respect to their reactions to different stimuli, the neurons investigated were divided into two groups. Neurons of the 1st type (85%) were activated by the stimuli of any size exhibiting large, small and mean RF. Adaptive changes were usually weak and mean (63%); strong changes were less frequent (37%). Some of the neurons (33) did not react to diffuse illumination, whereas other ones produced reactions of different types: 57% of the neurons exhibited a spontaneous activity, 9%--directional sensitivity. Neurons of the 2nd type (15%) reacted only to large stimuli and were characterized by large RF, strong adaptation (89%), low reactivity to diffuse illumination (46%); 35% of the neurons revealed the spontaneous activity, 20%-directional sensitivity.