On the Understanding of the Visual Centers of the Midbrain

Abstract

The axons of retinal ganglion cells reach several midbrain centers, in addition to the diencephalic centers discussed in Chapter 6; and there has long been great interest in the relationship between the retinomesencephalic and the retinodiencephalic pathways. These two pathways are formed by a major division of the optic tract, a division that raises several fundamental issues: Are the two pathways formed by branches of the same axons (and hence by the same ganglion cells) or are different groups of axons (and hence of ganglion cells) involved? If different, in what ways? My own reading of the literature [see also Giolli and Towns’ (1980) review of axonal branching in the visual system] suggests no single answer to these questions. In many species, for example the rat, ganglion cells of all major functional groups project to both mid- and forebrain centers by means of branching axons; while in other species, such as the monkey, there is a substantial degree of grouping, whereby different functional groups of ganglion cells project separately to mid- or forebrain. In short, there is considerable variation between species in the relative prominence of branching and grouping of ganglion cells in forming this major division of the visual pathway.