A look at the dogma

12/01/2012 by admin.

The authors of a new paper (citation below) have doubts about some well known ‘facts’ in neuroscience. I have to admit I assumed that the numbers were backed by evidence. I may have used these numbers in posts and so I feel bound to share the doubts with readers. Here is the abstract:

Owing to methodological shortcomings and a certain conservatism that consolidates wrong assumptions in the literature, some dogmas have become established and reproduced in papers and textbooks, derived from quantitative features of the brain. 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.

They give the conventional wisdom as: intelligence is related brain size, brain size is related to cerebral cortical size. But they point out that the cerebellum keeps its ratio with the cortex, the cerebellum has more neurons that the cortex by a factor of 4 and, further, has a lot to do with intelligence. Although the neuron density is much higher in the cerebellum, there are many fewer glial cells.

The idea that humans are outliers on graphs of body size against brain size ignores the difference in various types of animals. We are not dramatically off the line for higher primates.

Cortical columns are thought to contain 147,000 neurons in each cylinder of about 1mm squared at the surface (except for a few areas) across all mammals. But this has not stood up to evidence. Surface area increases more slowly than neuron numbers and there is not a firm relationship between density and thickness.

Here are some figures and their error ranges given in the paper:

Whole Brain

1509(+- 300) grams / 171(+- 14) billion cells / 86(+- are neurons and 85(+-10) glia

Cerebral cortex grey and white matter

1233(+- 234) grams / 77(+-8) billion cells / 16(+- 2) are neurons and 61(+-7) glia

81% of brain mass 19% of brain neurons

Cerebellum

154(+- 19) grams / 85(+- 7) billion cells / 69(+- 7) are neurons and 16 (+- 2) glia

10% of brain mass 80% of brain neurons

Basal ganglia and diencephalon and brainstem

118(+- 45) grams / 8.5(+- 1.5) billion cells / 0.7(+- 0.1) are neurons and 7.7(+-1.5) glia

8% of brain mass 1% of brain neurons

Surprised?

Lent, R., Azevedo, F., Andrade-Moraes, C., & Pinto, A. (2012). How many neurons do you have? Some dogmas of quantitative neuroscience under revision European Journal of Neuroscience, 35 (1), 1-9 DOI: 10.1111/j.1460-9568.2011.07923.x

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