The brain is composed of several anatomically clearly separated structures. This parcellation is often extended into the isocortex, based on anatomical, physiological, or functional differences. Here, we derive a parcellation scheme based purely on the spatial structure of long-range synaptic connections within the cortex. To that end, we analyzed a publicly available dataset of average mouse brain connectivity, and split the isocortex into disjunct regions. Instead of clustering connectivity based on modularity, our scheme is inspired by methods that split sensory cortices into subregions where gradients of neuronal response properties, such as the location of the receptive field, reverse. We calculated comparable gradients from voxelized brain connectivity data and automatically detected reversals in them. This approach better respects the known presence of functional gradients within brain regions than clustering-based approaches. Placing borders at the reversals resulted in a parcellation into 41 subregions that differs significantly from an established scheme in nonrandom ways, but is comparable in terms of the modularity of connectivity between regions. It reveals unexpected trends of connectivity, such as a tripartite split of somatomotor regions along an anterior to posterior gradient. The method can be readily adapted to other organisms and data sources, such as human functional connectivity.

We generalized a technique to find borders between brain regions based on functional data for use with voxelized connectivity data instead. Instead of maximizing a connectivity measurement, it draws borders where qualitative trends of connectivity reverse. The method does not adjust the borders between regions in established brain hierarchies, but instead creates a completely new hierarchy and associated parcellation. When we applied the technique to mouse isocortex connectivity, the results differed significantly from established parcellations, especially around primary sensory areas, yet they matched them in terms of modularity of connectivity. We conclude that it reveals and formalizes previously unappreciated trends of intracortical organization.

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Competing Interests: The authors have declared that no competing interests exist.

Handling Editor: Olaf Sporns

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