“The brain has hubs?”
If you read only one neuroimaging paper this week, make it this paper in PLoS Biology by Hagmann and colleagues. It’s a really remarkable combination of technical wizardry, creativity, and pretty, pretty pictures of the brain. What Hagmann et al have done is assemble rock-solid evidence that a network of brain regions located primarily in posterior midline cortex serves as the structural ‘core’ of the broader cortical connectivity map. Whereas most brain regions show sparse connectivity, typically talking to only a handful of other nearby regions , regions in the structural core are much more densely connected with one another and with other regions throughout the cortex. Hagmann et al. support this basic conclusion with five or six different analyses, each using a different network topology metric (herein lies the technical wizardry), but the bottom line is that they obtain much the same result no matter how they looked at the data.
What’s really striking about this study is that it’s arguably the best example to date (or at least, the best example that I know of–I don’t follow this literature closely) of the power that new structural MRI techniques provide to assess in vivo brain connectivity in humans. In this case, the authors used diffusion spectrum imaging, a technique that lets the researcher construct whole-brain images of white matter fiber density and then (using some sophisticated post-processing) plot the trajectories of those tracts. The authors defined a connection between regions as the presence of at least one fiber with end-points in both regions (the more terminating fibers, the stronger the connection). Given an N x N matrix (where N = 998 different brain regions in this case!) of connectivity strengths between regions, they could then apply the suite of network topology metrics to produce those pretty, pretty figures.
Lest you think this all sounds like black magic (as I suspect a reviewer or two did), Hagmann et al. provide evidence that these structure-based connectivity maps (a) are reliable across hemispheres and scanning sessions; (b) degrade gracefully in the presence of noise; (c) conform nicely to connectivity data obtained from more conventional anatomical tract tracing techniques in monkeys; and (d) are quantitatively very similar to maps obtained using functional resting-state data in the same participants. The sheer breadth of analysis in this paper is really quite striking, and you’d have to nit-pick to find faults with the methodology.
That said, there’s one critical question that these results don’t really address, and that’s what the findings mean from a functional standpoint. it’s easy to make the general argument that a small-world network structure is A Good Thing ™ for the brain to have; but the (arguably) more interesting question is why the hubs are located in these particular brain regions. The fact that a majority of the hubs (including posterior cingulate, precuneus, lateral parietal cortex, and superior temporal sulcus) are components of the brain’s “default” or task-negative network is clearly no coincidence. So what functional purpose does this pattern of connectivity serve? Why do those regions that are maximally activated at rest have the broadest pattern of connectivity with the rest of the cortex? Or is it perhaps the other way around, so that these regions develop their default status precisely because they receive inputs from multiple sources, and are ideally situated to mediate transitions between different task sets? Clearly, many questions remain to be addressed (warning: a horribly cliched ending to this post is imminent), but the Hagmann et al. paper will probably turn out to be a pretty important piece of the puzzle (see, I warned you).
Hat-tip: Neurophilosophy.