Wednesday, January 24, 2007

What is a Brain Area?

What is a "brain area"? More recently, I have become aware of the inadequateness of the concept of "brain area", or at any rate, to call into question the basis for such a concept. This basis is three-fold, as noted by Felleman/van Essen: cortical areas (or in general, brain areas) are defined by 1) connectivity, 2) functional maps, and 3) chemical or architectonic signatures. However, for the most part, parcellations of the primate (and non-primate) brain have been based on studies using Nissl- or myelin-stained material that are over a century old, and investigators have come up with widely different parcellation schemes for the brain, which in my opinion, is a prominent warning sign that the notion of "cortical area" is ill-defined. Further anatomical studies of the brain have confirmed this point to me. And so, while I recognize the utility to conventionally naming different brain areas on the basis of Nissl-stained material or otherwise, I do not believe we currently possess an adequate conceptual understanding of what really constitutes a "brain area". In early sensori-motor areas, this concept seems applicable since we are talking about mappings from sensory receptor sheets onto the cortex, which get mapped onto well-defined areas of the brain, but other areas of the brain are not like this, and there is no reason a priori to expect that these association and limbic parts of the brain should be nicely parcellated into anything like discrete non-overlapping brain areas.

Part of the problem involves considering useful alteratives to this notion of discrete non-overlapping brain areas which is prevalent in the neuroscience community, and which heavily biases interpretions of experiments. It is largely a conceptual problem, but I am confident that a revolution in our notion of "brain area" will be forthcoming in the near future. Such an overhaul in this precious concept is requisite to a better understanding of the brain.

What I find amusing is that neuroscience textbooks never address this conceptual issue, though it is widely recognized by many prominent neuroscientists as a central problem. This has the peculiar effect that students of neuroscience often learn about their subject, thinking that all of the fundamental conceptual issues have been worked out and that the field of neuroscience rests on a firm foundation. This is not the case, and I would not be surprised if this shaky foundation crumbles, and that many of the "mysteries" of the brain's organization and function, when viewed in a new light and a new foundation, do not seem that mysterious after all, but rather obey a very precise and well-defined logic and reason.

The observation that the concept of "brain area" is ill-defined means, in part, that current attempts to analyze whole-brain connectivity using graph theory are based on incorrect data and incorrect assumptions, since we may legitimately question whether the nodes in the graph have any real meaning. So claims like "the brain is a small-world network" purported by some are empty, and are merely the consequence of following the recent fad in "network science", where anyone and everyone attempts to show that their favorite system is a so-called small-world network. How unoriginal and blase! If only these people could think for themselves instead of parroting the latest fad. The worst part of it is when these people actually publish such nonsense since it misleads other people (usually laymen, but also some neuroscientists) who don't know any better.