[Morphofunctional organization of the mesencephalic visual center of the midbrain tectum of steppe and swamp turtle].

Neuronal receptive fields (RF) and dendritic arborizations (DA) in the midbrain tectum of the tortoise A. horsfieldi have been investigated and the results obtained were compared to those reported earlier for the tortoise E. orbicularis. It was shown that in A. horsfieldi, neurons with small RF (less than 10 degrees) are 3.2 times less numerous, whereas those with large RF (more than 30 degrees) are 1.6 times more numerous than in E. orbicularis. Ellipsoidal RF are predominant in A. horsfieldi, elongated ones -- in E. orbicularis. A. horsfieldi neurons more frequently react to large objects (36%), than to small ones (3%), whereas in E. orbicularis the ratio of these neurons is inverse (13 and 23% respectively). Neurons of A. horsfieldi are less effective in differentiation of the form of the object, the direction of its movement and changes in illumination. In highly specialized neurons of the species compared, the size of RF may correspond to the size of DA. The differences observed are presumably associated with differences in the ecology of the animals investigated.