Hippocampal Pallium and Map-Like Memories through Vertebrate Evolution

Abstract

The hippocampus in humans and other mammals is essential for episodic and relational memories. Comparative evidence indicates that a hippocampal pallium homologue is present in birds, reptiles, amphibians, ray-finned fishes, cartilaginous fishes and agnathans. Some of their characteristics, such as the topological position and the pattern of connectivity, appear remarkably well conserved. We review here substantial data showing that in all the vertebrate groups studied up to date, from fish to mammals, the hippocampus plays a fundamental role in spatial memory. In these vertebrates groups, the hippocampal pallium homologue is involved in the use of map-like, relational representations of the objective space that provide stable allocentric frames of reference, thus allowing flexible navigation. These similarities suggest a common evolutionary ancestry and indicate that the functional properties of the hippocampus appear early in the vertebrate phylogenesis and are retained through the independent evolution of the vertebrate lineages.