ABSTRACT
The human brain consists of functionally specialized areas, which flexibly interact and integrate forming a multitude of complex functional networks. The principles underlying this functional differentiation and integration remain unknown. Here, we demonstrate that a fundamental principle ubiquitous in nature - harmonic modes - explains the orchestration of the brain’s functional organization. Applied to the functional connectivity in resting state averaged across 812 participants, harmonic modes give rise to functional harmonics revealing the communication channels of the human brain. Remarkably, the isolines of the continuous functional harmonic patterns (gradients) overlap with the borders of cortical areas. Furthermore, each associated with a different spatial frequency, the functional harmonics provide the frequency-ordered building blocks to reconstruct any pattern of brain activity. We show that 47 brain activation patterns elicited by 7 different task categories in the Human Connectome Project task battery can be reconstructed from a very small subset of functional harmonics, uncovering a parsimonious description of the previously unknown relationship between task and resting state brain activity. Crucially, functional harmonics outperform other well-known basis functions such as those used in principle component analysis (PCA) or independent component analysis (ICA) in both, reconstructing the task activation maps as well as explaining the emergence of functionally specialized regions. Thus, our findings not only unify two competing views of the brain’s functional organization, i.e. modular vs gradiental perspective, by revealing that the functional specialization of the human cortex occurs in a gradiental manner across multiple dimensions in the functional harmonic basis, but also evidence that this basis underlies task-elicited human brain function.
Footnotes
We added comparison of functional harmonics to alternative function bases, including principal and independent components. We also updated the null model to use rotated versions of the functional harmonics surface maps.