Our resting brain networks can be formed by multiple architectures.

There has been a lot of interest lately (see Jonah's Lehrer's nice summary) in the resting, default, or 'mind wandering' state of our brains. It recruits functional networks with rich endogenous dynamics which typically distributed over both cerebral cortices. An interdisciplinary collaboration involving Ralph Adolphs, whose experiments were carried out by Michael Tyszka, asked the question of whether these resting states, as one might suppose, require the presence of the corpus callosum, the large bundle of fibers connecting the two hemispheres. What they found is that a normal complement of resting-state networks and intact functional coupling between the hemispheres can emerge in the absence of the corpus callosum, suggesting that resting brain networks can be formed by multiple architectures. Their abstract:

Temporal correlations between different brain regions in the resting-state BOLD signal are thought to reflect intrinsic functional brain connectivity. The functional networks identified are typically bilaterally distributed across the cerebral hemispheres, show similarity to known white matter connections, and are seen even in anesthetized monkeys. Yet it remains unclear how they arise. Here we tested two distinct possibilities: (1) functional networks arise largely from structural connectivity constraints, and generally require direct interactions between functionally coupled regions mediated by white-matter tracts; and (2) functional networks emerge flexibly with the development of normal cognition and behavior and can be realized in multiple structural architectures. We conducted resting-state fMRI in eight adult humans with complete agenesis of the corpus callosum (AgCC) and normal intelligence, and compared their data to those from eight healthy matched controls. We performed three main analyses: anatomical region-of-interest-based correlations to test homotopic functional connectivity, independent component analysis (ICA) to reveal functional networks with a data-driven approach, and ICA-based interhemispheric correlation analysis. Both groups showed equivalently strong homotopic BOLD correlation. Surprisingly, almost all of the group-level independent components identified in controls were observed in AgCC and were predominantly bilaterally symmetric. The results argue that a normal complement of resting-state networks and intact functional coupling between the hemispheres can emerge in the absence of the corpus callosum, favoring the second over the first possibility listed above.