Roberto Lent, Frederico Azevedo, Carlos Andrade-Moraes, and Ana Pinto offer several important contentions. In their introduction, they wonder
whether quantitative neuroscience can be described properly as a
discipline? They ponder where the idea of a 100 billion neurons came
from and point out that "its origin in the literature is unknown" (1)
They agree that it is a supposition that sizes of the brain or areas of
the brain correlate with cognitive abilities. The authors
moreover observe that the cerebellum has greater role in cognitive and
affective functions than is typically imagined. They show why
quantifying the number of brain cells is a serious business (it will
help resolve challenges in studies of dementia) and equally why the
arbitrary number of 100 billion needs to analyzed in the context of the
functional role of glial cells. They ask why, despite considerable
controversy, it has been widely accepted that 147,000 (on
average) neurons populated the cortical columns. They wonder whether
it really is reasonable to claim humans as outliers among animals in
terms of ecephalization quotients, given that there appear to be
different species-specific scaling rules.
-----The first dogma states that the cerebral cortex is the pinnacle of brain evolution – based on the observations that its volume is greater in more ‘intelligent’ species, and that cortical surface area grows more than any other brain region, to reach the largest proportion in higher primates and humans. The second dogma claims that the human brain contains 100 billion neurons, plus 10-fold more glial cells. These round numbers have become widely adopted, although data provided by different authors have led to a broad range of 75–125 billion neurons in the whole brain. The third dogma derives from the second, and states that our brain is structurally special, an outlier as compared with other primates. Being so large and convoluted, it is a special construct of nature, unrelated to evolutionary scaling. Finally, the fourth dogma appeared as a tentative explanation for the considerable growth of the brain throughout development and evolution – being modular in structure, the brain (and particularly the cerebral cortex) grows by tangential addition of modules that are uniform in neuronal composition. In this review, we sought to examine and challenge these four dogmas, and propose other interpretations or simply their replacement with alternative views.
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