Bird brains: They are more like ours than we thought

  In Chapter 13 of Principles of Neurobiology, we learned about the evolution of the nervous system. Towards the end of the chapter, we focused on the expansion and origins of the mammalian neocortex (13.20-13.22 & Box 13-5), the 6-layered laminar structure that covers most of the surface of the human brain and endows us with our intellectual capacities.

While a 6-layered cortex exists only in mammalian brains, mammals are by no means the only “intelligent” animals. In particular, birds prove to be exceedingly smart and can solve tasks that will challenge children. The cognitive abilities of birds are due to their large pallium, the part of the bird brain that is considered homologous to the mammalian neocortex. However, in contrast to the neocortex, the bird pallium is not arranged in layers. Instead, it contains many different nuclei, somewhat like the subcortical nuclei of the mammalian brain.

Mammalian neocortex and bird pallium share similar fiber architectures, as revealed by 3D-PLI and dye tracing. Adapted from Stacho et al. 2020.

This strikingly different organization of the two structures has caused an intense debate on which features of pallial organization — if any — are conserved between mammals and birds. New data from Stacho and colleagues (1) now revealed a fiber architecture in the pallia of pigeons and owls similar to the organization of the mammalian neocortex.

In the mammalian cortex, radial fibers that run perpendicular to the cortical surface connect the individual layers into functionally specialized processing modules. These modules are then connected by tangential fibers, running parallel to the cortical surface and crossing the radial fibers at right angles. Therefore, cortical fiber architecture is dominated by a crisscrossing of orthogonal fibers (see e.g. Figure 14-29).

To understand the fiber organization of the bird pallium, the authors probed the pallial sections with a method called three-dimensional polarized light imaging (3D-PLI). In this method, a brain section is repeatedly imaged, while polarized light is directed at it from different angles. This series of images then reveals the orientation of myelinated fibers in the sample. Despite the lack of layers, Stacho et al. found that functionally related pallial nuclei were connected by radial fibers, essentially forming specialized modules. These modules were, in turn, connected by tangential fibers. Radial and tangential fibers crossed at right angles, creating an architecture that looks very much like the one we knew about in the mammalian cortex! To back up their findings and to map the bird brain circuits at higher resolution, the authors then also traced brain connectivity using traditional anterograde and retrograde tracers (Section 14.18; 14.19), both injected into the brain or applied to ex vivo brain slices.

Stacho et al.'s findings show that mammalian and bird pallia are organized in similar ways, despite their very different structures at first glance. This insight now triggers a whole lot of new questions. First and foremost, from an evolutionary perspective: Do birds and mammals share a common circuit architecture as a result of the conservation of an architecture that was already present in the last common ancestor of the amniotes? Or are we looking at a case of convergent evolution, in which the crisscrossing of perpendicular fibers arose independently? We will need to investigate the fiber architecture in more species across the amniote tree to find out!

Reference

1.   M. Stacho, C. Herold, N. Rook, H. Wagner, M. Axer, K. Amunts, O. Güntürkün, A cortex-like canonical circuit in the avian forebrain. Science 369(6511):eabc5534 Link