|6 classically recognized layers of the cortex|
Due to "strong adherence to tradition", they suggest in their paper's introduction, some views about the nervous system have been "extensively reproduced in papers and textbooks, becoming undisputed dogmas of neuroscience." (1) Waving at Thomas Kuhn's classic The Structure of Scientific Revolutions, the authors conclude that: "Dogmas are fundamental to science. They serve the purpose of being challenged, and eventually replaced by other transient truths." (7). In a brilliant dissection in between these remarks, the authors seek out and problematize the empirical basis of four dogmas: 1) The idea that the cerebral cortex is the highest achievement of brain evolution; 2) the claim that the human brain has 100 billion neurons and 10 times more glial cells; 3) the dogma that the human brain is exceptionally complex as compared with those of other primates, and finally 4) the claim that brains grow in evolution and development by the addition of uniform modules. Putting it simply: this is a hell of a good paper.
The authors 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 (2). The authors moreover observe that the cerebellum has greater role in cognitive and affective functions than is typically imagined (3). 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 (5). They ask why, despite considerable controversy, it has been widely accepted that 147,000 (on average) neurons populated the cortical columns (7). 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. They write:
We conclude that that the human brain is not exceptional in the absolute composition of neurons and glial cells, the main operators of its computational functions. It is, rather, a result of the linear scaling rule characteristic of primates. We are not special in nature, but only big-brained primates. Having big brains and being primates, we have acquired a gigantic number of computational units that have made us capable of superior cognitive performance.(6)Good stuff - although I would qualify the word "superior." And it raises several points related to the possibility of neurohistory (other thoughts here and here). One is a question of responsibility. Sweeping histories, grand narratives, and big stories have long been the stock-in-trade of popular histories. (H. G. Well's Outline of History, a wonderful book by the way, is but one example.) But such histories have a way of hardening ideas and dogmas that are actually controversial or in need of refinement and elaboration even as the question of scientific facticity (as the comments about dementia above make clear) has considerable importance.
Secondly doesn't the existence of these dogmas, as well as the observation we don't know the origins of the claim that we have 100 billion neurons, only elevate further the fact that in order to even begin a neurohistory project we would need a clearer, deeper, and refined history of neuroscience and neurology? The dogmas that Lent et al. describe point towards other unsettled questions. They ask, for example, "to what extent is it true that the number of neurons in the brain declines with aging?" "Do males" really have a "higher number of neurons than females?" To this one, a colleague of mine, adds, and just when "did we begin to believe that we use 10% of our brains?" And as Anne Harrington pointed out a long time in her brilliant book Medicine, Mind, and the Double Brain even the notion that the hemispheres act differently has a long history. Isn't it time for historians and philosophers to be mindful about these facts?