Contrary to what was once believed about differences existing between human and bird brains, a new study has discovered that there are similarities.
Generally speaking, the brains of mammals have long been presumed to be more highly evolved and developed than the brains of other animals, in part based upon the distinctive structure of the mammalian forebrain and neocortex - a part of the brain's outer layer where complex cognitive functions are centered.
Specifically, the mammalian neocortex features layers of cells (lamination) connected by radially arrayed columns of other cells, forming functional modules characterized by neuronal types and specific connections.
Early studies of homologous regions in nonmammalian brains had found no similar arrangement, leading to the presumption that neocortical cells and circuits in mammals were singular in nature.
For 40 years, Harvey J. Karten, from the University of California, San Diego School of Medicine, lead author of the study, and colleagues have worked to upend this thinking.
In the latest research, they used modern, sophisticated imaging technologies, including a highly sensitive tracer, to map a region of the chicken brain (part of the telencephalon) that is similar to the mammalian auditory cortex. Both regions handle listening duties.
They discovered that the avian cortical region was also composed of laminated layers of cells linked by narrow, radial columns of different types of cells with extensive interconnections that form microcircuits that are virtually identical to those found in the mammalian cortex.
The findings indicate that laminar and columnar properties of the neocortex are not unique to mammals, and may in fact have evolved from cells and circuits in much more ancient vertebrates.
"The belief that cortical microcircuitry was a unique property of mammalian brains was largely based on the lack of apparent lamination in other species, and the widespread notion that non-mammalian vertebrates were not capable of performing complex cognitive and analytic processing of sensory information like that associated with the neocortex of mammals," said Karten.
The study has been published this week in the Proceedings of the National Academy of Sciences Online Early Edition.