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.

2 Comments:

Blogger Michael W. Cole said...

It seems that you are a bit too quick to dismiss the concept of "brain area". I agree that it is problematic, but I don't agree with your conclusion that the idea is ready no longer useful.

For instance, you seem to dismiss the fact that brain areas have well-defined functional properties (they respond the same to similar situations), which should be enough reason to support the utility of the "brain area" concept.

One might argue that the concept's trouble comes from the simplistic psychological concepts we tend to attach to each brain area. There is no reason the brain should be tidy enough to fit well withing our little labeling system.

I agree that assuming these areas are discrete and non-overlapping is a problem. We should maybe consider the possibility of gradients of function. Or we could consider Fuster's concept of 'cognits' (http://www.citeulike.org/user/kristina/article/693908) (widely distributed nets of bound representational features), though current methods seem inadequate for fully embracing this idea.

(My blog:
http://www.neurevolution.net)

9:02 PM  
Blogger neubrain said...

I am not dismissing the concept of 'brain area', but merely pointing out its inadequecies. Where is the evidence that "brain areas" have well-defined functional properties, besides for areas involved in early states of sensori-motor processing? Even if these "brain areas" respond the same to similar situations does not imply you can infer or otherwise attribute function to these areas. It only means you can correlate activity in certain regions of the brain with certain situations. And that doesn't tell you much of anything, particularly in light of our collective ignorance regarding the different mechanisms in which neurons encode information (firing rate is only one of several ways).

The possibility of gradients of function is getting ahead of ourselves since we can't assign function for most regions of the brain. More likely is that we define and enumerate neurochemical gradients, and then try to relate these to function or at least to information processing.

In the Fuster reference you made, his model, which postulates that memory and knowledge are represented by distributed, interactive, and overlapping networks of neurons in association cortex, is by no means new. For example, see Mesulam's "From sensation to cognition" published in Brain in 1998. These network models still rely on the assumption of discrete "brain areas", which is the very assumption that my post calls into question and which is determined to be an inadequate concept.

6:25 PM  

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