Laminar and columnar auditory cortex in avian brain.

The mammalian neocortex mediates complex cognitive behaviors, such as sensory perception, decision making, and language. The evolutionary history of the cortex, and the cells and circuitry underlying similar capabilities in nonmammals, are poorly understood, however. Two distinct features of the mammalian neocortex are lamination and radially arrayed columns that form functional modules, characterized by defined neuronal types and unique intrinsic connections. The seeming inability to identify these characteristic features in nonmammalian forebrains with earlier methods has often led to the assumption of uniqueness of neocortical cells and circuits in mammals. Using contemporary methods, we demonstrate the existence of comparable columnar functional modules in laminated auditory telencephalon of an avian species (Gallus gallus). A highly sensitive tracer was placed into individual layers of the telencephalon within the cortical region that is similar to mammalian auditory cortex. Distribution of anterograde and retrograde transportable markers revealed extensive interconnections across layers and between neurons within narrow radial columns perpendicular to the laminae. This columnar organization was further confirmed by visualization of radially oriented axonal collaterals of individual intracellularly filled neurons. Common cell types in birds and mammals that provide the cellular substrate of columnar functional modules were identified. These findings indicate that laminar and columnar properties of the neocortex are not unique to mammals and may have evolved from cells and circuits found in more ancient vertebrates. Specific functional pathways in the brain can be analyzed in regard to their common phylogenetic origins, which introduces a previously underutilized level of analysis to components involved in higher cognitive functions.