Evolution of brains, cognition, and consciousness.

Most current hypotheses about the neural basis for consciousness, including higher-level consciousness, are based on mammalian neural features, particularly focusing on thalamocortical circuitry. It is postulated here that since higher-level consciousness is correlated with higher-level cognitive abilities in humans and other mammals, this correlation also holds for nonmammalian taxa; that the level of cognitive ability may be proportional to the level of consciousness; and that the neural basis for both is either the same or highly overlapping. Recent studies, ingeniously designed to be species-sensitive, have revealed a host of highly cognitive abilities in birds, including manifestations of working memory (delayed match-to-sample, episodic memory, transitive inference, and multistability) and category formation, language and numerical comprehension, tool manufacture and cultural transmission of tool design, theory of mind, and Piagetian object permanence to a high level. Comparisons of the neural circuitry reveal extensive similarity in thalamic nuclei and ascending systems to the pallium. However, the avian pallium lacks the laminar architecture and therefore cortical columns of mammals, and avian pallial projection neurons are multipolar rather than specifically pyramidal as in mammals. Neural features common to both mammals and birds, which thus may be crucially involved in the generation of both higher-level cognitive abilities and higher-level consciousness, include large, multipolar, glutamatergic neurons with extensive and densely spiny dendrites, with extensive interconnectedness as well as inter-regional connections, GABAergic inputs from local interneurons, and connections with thalamic nuclei that are regulated by the thalamic reticular nucleus as well as basal ganglia loops.