Brain Structure and Function are Coupled in a Region- and Behaviour-Specific Way
Post by Elisa Guma
What's the science?
Brain activity is naturally shaped by the anatomical structure underlying it. Whole-brain magnetic resonance imaging techniques have allowed us to identify how the brain is connected both structurally, based on white-matter pathways, as well as functionally, based on the correlated fluctuations of brain activity in different regions over time. A large body of work has focused on understanding the way in which these networks are organized in the context of evolution, development, and disease, but the degree to which brain structure limits brain function is hard to quantify. This week in Nature Communications, Preti and Van De Ville propose a method to quantify this relationship by creating an index to define the structure-function relationship and explore its spatial patterning and behavioral relationship.
How did they do it?
The authors used diffusion-weighted magnetic resonance imaging (a measure of brain structure, sensitive to white matter tracts that connect different brain regions) and resting-state functional magnetic resonance imaging data (brain activity at rest) from 56 healthy volunteers from the Human Connectome Project, a publicly available resource. They aimed to define the way in which brain structure and function “couple”, or rather, the dependency of the functional signal on the structural signal. In order to do so, they created a “structural-decoupling index”, which allowed them to quantify the degree to which these signals were coupled (i.e. function depends heavily on structure) or decoupled (i.e. function is less dependent on structure). This was done by first building a structural-connectome of the brain, which allows the brain to be represented as a set of interconnected nodes. The building blocks of this connectome (structural harmonics) were then extracted using matrix decomposition by eigendecomposition of the graph Laplacian. The resting-state activity data was then projected onto the structural-connectome harmonics and the spatial pattern of activation at every time point was represented as a weighted linear combination of structural patterns. Based on the spatial frequency related to each harmonic, the functional signal was then split into two portions: one more coupled with the structure (related to low frequency harmonics), the other more decoupled (related to high frequencies). The amount of function/structure decoupling vs. coupling was quantified per brain region with the structural-decoupling index, to understand whether different brain structures have different degrees of coupling/decoupling. Next, they ranked regions based on the structural-decoupling index to explore the relationship of these regions to different behaviors, using a literature-based meta-analytic, public resource (Neurosynth). Finally, in order to validate these findings, the authors also generated two null models which they compared their model to.
What did they find?
They found that activity in the primary sensory regions, such as the visual, auditory, somatosensory, and motor cortex was more strongly coupled with brain structure. Conversely, the functional activity of higher-order regions such as the parietal lobe, which is part of the executive control network, the temporal lobe, including the amygdala and language areas, and orbitofrontal lobes were more decoupled from brain structure. The authors were also able to relate regions to behaviors based on the structural-decoupling index and found that regions with a low index, or rather, regions in which structure and function were highly coupled, were related to lower-order functions, such as sensory or motor functions. Regions with higher decoupling, in which function was less dependent on structure, were related to more complex functions such as memory, reward, or emotion.
What's the impact?
This study identified a novel method with which to quantify the relationship between functional brain activity and underlying brain structure. Further, the authors show that this coupling varies across structures related to different cognitive domains. This method can now be applied more broadly to understand inter- and intraindividual variability in structural and functional coupling and how this coupling might be altered psychiatric disorders.
Preti et al. Decoupling of brain function from structure reveals regional behavioral specialization in humans. Nature Communications (2019). Access the original scientific publication here.