Laminar and columnar auditory cortex in avian brain

Abstract

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.

Fig. 1.

The location and laminar organization of the Field L/CM complex in chicks. (A) A schematic drawing illustrates the organization of the mammalian telencephalon. (B and C) Schematic drawings illustrate the organization of the avian telencephalon and major components of the auditory telencephalon at the rostral (B) and caudal (C) levels. (D and E) Laminar arrangement of the Field L/CM complex is readily recognized in sections stained with either Nissl (D) or immunocytochemistry for parvalbumin (E). The location of the photomicrographs is comparable to the box in C. (F) CM externus and internus contain a high density of parvalbumin-immunoreactive axons and somata, respectively. (G) L1 externus contains a higher density of parvalbumin-immunoreactive neurons than L1 internus. Dorsal is up and medial is right in A–E. Photos in F and G were rotated ≈60 degrees counterclockwise. CM-ext, CM externus; CM-int, CM internus; L1-ext, L1 externus; L1-int, L1 internus. [Scale bars: 500 μm in E (applies to D and E); 200 μm in G (applies to F and G).]

Fig. 2.

Columnar organization of the Field L/CM complex in chicks. (A) Extended focus view of confocal microscopic Z-series of the labeling following an injection of BDA conjugated with rhodamine into CM. Note labeled axons and neurons were restricted within a column crossing all layers. White arrows point out an axonal collateral coursing obliquely away from the column. (B–D) Photomicrograph and camera lucida reconstruction of labeled neurons and fibers following an injection of BDA into Field L2a (B) and CM (C and D). Compared with the case in C, the injection site in D was much smaller and involved fewer neurons. Photomicrograph was taken from a Giemsa-counterstained section. Shaded areas indicate the center of an injection site. Dashed lines outline the location of LaM. Arrowheads in D point out the descending axons. See Fig. S2 for more reconstructions of injections. (E and F) Camera lucida reconstructions of intracellularly filled neurons in L2a (E) and CM (F). Dendrites and axonal arborization of each neuron are illustrated separately for clarity. Note the long radially oriented axonal collaterals in both cases. [Scale bars: 200 μm in A; 200 μm in D (applies to B–D); 200 μm in F (applies to E and F).]

Fig. 3.

Granule cells in Field L2a. (A) Labeled neurons in L2a following an injection of BDA into this layer. Small granule cells are the major cell type of labeled neurons in L2a. The white star indicates a large nongranule stellate cell. (B–D) Samples of granule cells in L2a labeled following injections into L1/L2 (B), CM (C), and L1 (D). Note the small cell bodies and fine and short dendrites. Nomarski-DIC photomicrographs were taken from Giemsa-counterstained sections. (Scale bar: 20 μm.)

Fig. 4.

Comparable laminar and columnar organization of the mammalian A1 (Left) and the avian auditory pallium (Right). Blue, red, and green lines and arrows indicate their thalamic inputs, intrinsic connections, and descending projections, respectively. Orange lines and arrows indicate recurrent projections from the deep layers to the more superficial layers. Black lines and arrows indicate reentrant projections from the other side of the brain. Innervation that is comparable to the projection from the mammalian layer III on the contralateral A1 (dotted black line) has not been identified in birds. Gray regions indicate columnar functional modules. Note that this schematic drawing illustrates only the major components of this intricate network.