Responses to visual stimuli in thalamic neurons of the turtle Emys orbicularis.

The responses to movement of shaped stimuli and diffuse illumination were studied for 300 neurons in three thalamic regions of the bog turtle Emys orbicularis L.' characteristics of their receptive fields (RF) were also studied. According to their responses to stimuli of various sizes, neurons were divided into two types. Neurons of Type I (85%) were activated by stimuli of any size and has large, small or medium RF. Weak or medium adaptive changes were found more often (63%) than strong changes (37%). Some of the neurons (33%) did not react to diffuse light; the remainder gave responses of various types. 57% of the neurons exhibited spontaneous activity, and 9% of the units were direction sensitive. Neurons of Type II (15%) responded only to presentation of stimuli of large sizes and were characterized by large RF, strong adaptation (89%) and weak reactivity to diffuse light (46%). 35% of the neurons exhibited spontaneous activity, and 20% were direction sensitive.