The misnomer of “functional connectivity”
Lately I’ve actually been growing frustrated with the term “functional connectivity.” The term is used to describe an imaging strategy known as “functional connectivity MRI,” or “fcMRI.” In short, “functional connectivity MRI” is a technique in which the intensity of blood fluctuations in small regions of interest in the brain are charted over time. Then, the synchronicity of blood fluctuations for regions of interest is calculated in order to determine the degree to which those regions of the brain operate in concert with one another.
Now don’t get me wrong: I think that fcMRI is a brilliant technique. I use it in my research, and it yields indispensable information in the collective effort to understand the human brain.
That being said, I think the term “functional connectivity” is a misnomer, and I think a lot of people are assuming that “functional connectivity” tells us something about physical wiring diagrams in the brain. It does not. If anything, “functional connectivity” tells us about the software of the brain, rather than the organ’s hardware.
By analogy, let’s say that a team of sociologists is looking at the student population of a large high school, and proposes to create a diagram of “social connectivity” for the student body, in an effort to graph the cliques, clubs, and sports teams at the school. And let’s say that their technique for “social connectivity” is to look at the number of times that any two students are talking at the same time throughout the day, regardless of their location within the school. Certainly this endeavor to chart the ebb and flow of talkativeness within the student body would be interesting, and could be used to diagram lots of dynamical trends within the high school. And it is more than likely that the intensity of friendship between two students would contribute to a correlation (or an anti-correlation!) of synchronicity in their talking. But to suppose that the map of social correlation is synonymous with, or a surrogate for a diagram of “social connectivity” would be an error.
In terms of neural imaging, “functional connectivity” measures are measures of activational correlation. Period. As soon as people start assuming that the physical mapping of fiber tracts is implicit in the functional connectivity map, they are going off track. (The term “intrinsic connectivity networks” is starting to appear in the literature to describe the brain’s functional networks. I think that this only confirms and furthers the confusion.)
Now one may ask, “Isn’t this just an issue of semantics? Sure, the term ‘functional connectivity’ may be misleading. But doesn’t the confusion just create a conceptual error rather than a practical one?”
To this I would first reply that there is no such thing as a purely conceptual error. Particularly in science, all errors of concept eventually become errors of practice.
For neuroscience in particular, it is an absolute must for us to ultimately understand the underlying mechanisms that cause the functional networks to emerge from the physical wiring of the brain so that we can eventually do good translational work with the information. Gustav Deco’s lab is doing the best work that I have seen thus far in trying to mechanistically relate the physical wiring of the brain to the functional networks that emerge from the system. (I’m attaching the PDF of a 2010 SfN poster from Deco’s lab to this post. Warning: it’s dense.) As it is right now, the fcMRI data is starting to be used for markers in pathology and classification, which is great for developing new diagnostic criteria. But as far as future translational work to correct pathology and ultimately intervene at the level of brain systems–it’s going to require better understanding of the layers of regulation between the anatomical connectivity and the resultant functional networks.
A quick solution? Let’s stop using the word “connectivity” altogether to describe functional phenomena. Save the word for the anatomical stuff. Let’s instead use the more accurate and descriptive word “correlation”–the acronyms could even stay the same: fcMRI would just become “functional correlation MRI,” and the term to describe the functional networks could simply become “intrinsic correlation networks” rather than “intrinsic connectivity networks.” This might sound like nitpicking. But if it is, let’s remember that we’re constructing a scientific understanding of human nature–not a mythological one. And as is far more true of science than it is of myth, the devil is in the details.