Topographic Arrangement of the Rotundo-entopallial Projection in the Pigeon (Columba livia)

Abstract

The tectofugal pathway (retina – optic tectum – nucleus rotundus – entopallium) is a prominent route mediating visual discrimination in diurnal birds. Several lines of evidence have shown that at the tecto-rotundal stage this pathway is composed of multiple parallel channels. Anatomical studies show that the nucleus rotundus is composed of at least four subdivisions, according to differences in cytoarchitectonic, histochemical, and hodological properties. Each of these subdivisions is in receipt of a highly convergent, nontopographic tectal projection, originating from a distinct subset of tectorotundal neurons. Physiological studies show that neurons of different subdivisions respond specifically to different visual dimensions, such as color, luminance, twodimensional motion, and in-depth motion. At present it is less clear whether or to what extent this channel segregation is preserved at the telencephalic stage of the tectofugal pathway. The entopallium shows no obvious subdivisions or laminations. Nevertheless, tracttracing experiments show that separate portions of the entopallium receive efferent projections from different rotundal subdivisions, in a way that maintains the rostrocaudal order of these subdivisions. In the present study we investigate in detail the topography of the rotundo-entopallial projection by means of anterograde and retrograde neuronal tracers. Our results confirm the zonal topography proposed by previous studies and indicate that each zone in the entopallium receives a direct and topographically organized projection from its corresponding rotundal subdivision. These results suggest that the spatial arrangement of the different rotundal functional modules is preserved at the entopallial level.