Visual circuits of the avian telencephalon: evolutionary implications.

Birds and primates are vertebrates that possess the most advanced, efficient visual systems. Although lineages leading to these two classes were separated about 300 million years ago, there are striking similarities in their underlying neural mechanisms for visual processing. This paper discusses such similarities with special emphasis on the visual circuits in the avian telencephalon. These similarities include: (1) the existence of two parallel visual pathways and their distinct telencephalic targets, (2) anatomical and functional segregation within the visual pathways, (3) laminar organization of the telencephalic targets of the pathways (e.g. striate cortex in primates), and (4) possible interactions between multiple visual areas. Additional extensive analyses are necessary to determine whether these similarities are due to inheritance from a common ancestral stock or the consequences of convergent evolution based on adaptive response to similar selective pressures. Nevertheless, such a comparison is important to identify the general and specific principles of visual processing in amniotes (reptiles, birds, and mammals). Furthermore, these principles in turn will provide a critical foundation for understanding the evolution of the brain in amniotes.