CENTRIFUGAL PROJECTIONS UPON THE RETINA - AN ANTEROGRADE TRACING STUDY IN THE PIGEON (COLUMBA-LIVIA)

Previous work has shown that the avian retina receives two types of ce ntrifugal fibers from the brain. These types can be distinguished base d on the size and the morphology of their terminal endings and have be en termed convergent and divergent. The centrifugal fibers arise from the isthmooptic nucleus (ION) and the surrounding ectopic cell region (ECR). We used injections of anterograde tracers either to the ION/ECR or to the ECR only to determine the morphology, depth of termination, and regional distribution of the centrifugal fibers arising from each . We found that the ECR gives rise only to the divergent type of the c entrifugal fiber, whereas the ION gives rise mainly to the convergent type but may also send some fibers of the divergent type. Most of the fibers project contralaterally, although a few from the ECR project ip silaterally. The terminals of either type are not uniformly distribute d throughout the retina; instead, they are found mainly in the inferio r, midtemporal, to nasal portion of the retina and appear to avoid the fovea and most of the red field. By comparison, the ION receives a ma jor projection from portions of the tectum that receive input from the fovea and the red field in a type of neural loop. The neural loop doe s not project to the same point (homotopic), but projects from the red field to the inferior retina (heterotopic), as was recently proposed by Holden (1990; Vis. Neurosci. 4:493-497). The distribution of centri fugal axons corresponds to displaced ganglion cells that selectively i nnervate the nuclei of the accessory optic system (AOS), including the nucleus of the basal optic root (dorsal, ventral, and lateral) and th e nucleus lentiformis mesencephali, pars magnocellularis. We suggest t hat the centrifugal axons act by increasing the gain on the AOS, there by enhancing retinal stabilization of gaze with improved accuracy of p ecking of small objects. (C) 1995 Wiley-Liss, Inc.