Topography of primary visual centres in the brain of the chick, Gallus gallus.

In a previous paper (Ehrlich and Mark, '83b) the primary visual centres of the chick were described. In this paper patterns of retinotopy are examined by means of silver degeneration and autoradiographic techniques following discrete laser lesions of the retina. Well-defined and complete retinotopic maps are found in each of the following visual centres: tectum, lateral anterior thalamus, lateroventral geniculate nucleus, superficial synencephalic nucleus, ectomammillary nucleus, and tectal grey. In the dorsolateral anterior thalamus, pars lateralis, and external nucleus there is some evidence of a retinotopic innervation, yet not as well defined as those nuclei mentioned previously. Retinotopic maps were not observed in other retinorecipient regions. These include the ventrolateral thalamus, dorsolateral anterior thalamus, magnocellular part and rostrolateral part, pretectal optic area, and diffuse pretectal nucleus. Within the lateroventral geniculate, lateral anterior thalamus, superficial synencephalic nuclei, and the tectal grey, the ventral to dorsal retinal axis is mapped along the rostrocaudal axis, the reverse of the orientation seen in the tectum, which may have implications for explanations of how retinotopic maps are formed. The poor retinotopy n dorsolateral anterior thalamus, lateral part, is discussed with respect to the view that it may be the avian homologue of the mammalian lateral geniculate nucleus.