neopallium

Neopallium is a term used in vertebrate neuroanatomy to refer to the most recently evolved portion of the pallium, the outer layer of the forebrain. The name combines neo- meaning new with pallium. In mammals, the neopallium largely corresponds to the neocortex, a six-layered sheet involved in higher-order sensory processing, cognition, planning, and voluntary motor control. In many non-mammalian vertebrates, the term is used for homologous dorsal pallial regions, though their cytoarchitecture is not necessarily six-layered.

In birds and some reptiles, the dorsal pallium comprises structures such as the hyperpallium, nidopallium, and mesopallium, which participate in complex sensory integration and learning. These regions are functionally analogous to parts of the mammalian neopallium, but differ in organization and lamination patterns, reflecting distinct evolutionary trajectories.

Developmentally, neopallial tissue arises from the dorsal pallium during embryogenesis. In mammals, the neocortex is the principal component of the neopallium and is characterized by six cortical layers with specific patterns of connectivity linking sensory areas to association and motor regions. In other vertebrates, neopallial areas perform similar roles in sensory processing and higher-order cognition, but their structural arrangement diverges from the mammalian six-layer pattern.

Evolutionarily, the neopallium emerged in the common amniote lineage and expanded substantially in mammals, giving rise to the neocortex. In birds and reptiles, analogous dorsal pallial structures support sophisticated cognition despite the absence of a laminated neocortex.

Notes on usage: terminology varies; some authors reserve neopallium for the dorsal pallium in non-mammals, others equate it with the mammalian isocortex. The term thus denotes the evolutionarily newer pallial portion across vertebrates.