Friday, July 2, 2010
For more than a century, neuroscientists believed that the brains
of humans and other mammals differed from the brains of other
animals, such as birds (and so were presumably better). This belief
was based, in part, upon the readily evident physical structure of
the neocortex, the region of the brain responsible for complex
cognitive behaviors.
A new study, however, by researchers at the University of
California, San Diego School of Medicine finds that a comparable
region in the brains of chickens concerned with analyzing auditory
inputs is constructed similarly to that of mammals.
"And so ends, perhaps, this claim of mammalian uniqueness," said
Harvey J. Karten, MD, professor in the Department of Neurosciences
at UCSD's School of Medicine, and lead author of the study,
published this week in the Proceedings
of the National Academy of Sciences Online Early
Edition.
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, Karten 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.
"Animals like birds were viewed as lovely automata capable only of
stereotyped activity."
But this kind of thinking presented a serious problem for
neurobiologists trying to figure out the evolutionary origins of
the mammalian cortex, he said. Namely, where did all of that
complex circuitry come from and when did it first evolve?
Karten's research supplies the beginnings of an answer: From an
ancestor common to both mammals and birds that dates back at least
300 million years.
The new research has contemporary, practical import as well, said
Karten. The similarity between mammalian and avian cortices adds
support to the utility of birds as suitable animal models in
diverse brain studies.
"Studies indicate that the computational microcircuits underlying
complex behaviors are common to many vertebrates," Karten said.
"This work supports the growing recognition of the stability of
circuits during evolution and the role of the genome in producing
stable patterns. The question may now shift from the origins of the
mammalian cortex to asking about the changes that occur in the
final patterning of the cortex during development."
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University of California - San Diego
Thanks to
University of California - San Diego for this article.