Identification of conserved anatomo-functional organization of the pallium supporting cognition in vertebrates – PALL-E-NODY

The majority of human cognitive skills are thought to depend on structure and on the computing capabilities of the cerebral cortex, which is found only in mammals. For this reason, the evolution of intelligence is often only considered from studies on monkeys and apes, which are phylogenetically close to humans and possess enlarged cortical areas. However, many behavioral studies suggest that birds such as corvids and parrots demonstrate cognitive capacities rivaling those of primates, such as theory of mind, tool use and its manufacture. Accumulative knowledge in comparative neuroanatomy revealed that the basic telencephalic circuitry is very similar between mammals and birds, although the avian telencephalon is organized in nuclei, instead of layers. This suggests that the crucial condition for generating similar functions is not the general morphology or cytoarchitecture, but the connectivity to achieve similar neuronal network.
We hypothesize that, although the cognitive functions involving insight behaviors found in corvids and primates may be a consequence of convergent evolution, there exist basic characteristics of functional organization of the vertebrate brains allowing for the emergence of complex cognitive functions. This project examines the molecular and anatomical traits (e.g. gene expression, connectivity patterns and neurotransmission) underlying the pallial characteristics shared in three vertebrate classes, mammals, birds, and teleost fishes.
The debate about homology of the pallium areas in different vertebrate classes has been long lasting due to its large morphological variation. Nonetheless, similar sensory-motor circuitries can be found in mammals, birds, and teleosts. Avian and teleostean pallia receive unimodal ascending sensory inputs from the diencephalon, and it in turn sends long efferent projections to subtelencephalic brain areas, as also do the mammalian cortices. In addition, recent publications and our preliminary data suggest that the sensory nuclei contributing to this circuitry both in mammals and birds express same sets of molecular markers (e.g. the Rorß and Eag2 genes in the thalamorecipient neurons), suggesting that potential genetic programs specifying similar connectivity may be shared in mammals and birds. To gain insight into pallial evolution in these species, as well as to decipher the contribution of these genes to the common connectivity, the first part of the project (Task 1 and Task 2) examines the functional significance of the genes expressed in the sensory pallial areas.
On top of the sensory-motor integration, existence of association areas with intratelencephalic connectivity would provide neuroanatomical bases for generating more flexible and adaptable behaviors. In mammals, the prefrontal cortex (PFC) is one of the main association cortices, and plays a major role in executive functions. Birds have a functional equivalent of the PFC, where dopamine neurotransmission is mandatory for the executive functions, as it is the case in the mammalian PFC. Since context-dependent flexibility of choice behavior or logical thinking can be observed in some teleost fish, it is likely that teleosts pallium possess some equivalent of executive brain area in the pallium. The second half of the project aims to examine the executive brain functions in relation to dopamine system in birds and fish (Task 3, Task 4, and Task 5).
The last Task 6 of the project will address the question of homology of pallial areas in different vertebrate groups, by verifying the expression of ventral pallial markers, the nature of the ventral pallium remaining disputed. Our project will provide fundamental new data for our understanding of the pallial functions and its evolutionary history. These approaches are also making non-mammalian species interesting models in cognitive neurosciences, for dissecting out the principal neural mechanisms underlying higher order cognitive functions.