Two common and distinct forms of variation in human functional brain networks

Abstract

The cortex has a characteristic layout with specialized functional areas forming distributed large-scale networks. However, substantial work shows striking variation in this organization across people, which relates to differences in behavior. While a dominant assumption is that cortical 'variants' represent boundary shifts in the borders between regions, here we show that variants can also occur at a distance from their typical position, forming ectopic intrusions. Both forms of variants are common across individuals, but the forms differ in their location, network linkages, and activations during tasks. Sub-groups of individuals share similar variant properties, but sub-grouping on the two variant forms appears independent. This work argues that individual differences in brain organization commonly occur in two dissociable forms, border shifts and ectopic intrusions, that must be separately accounted for in the analysis of cortical systems across people. This work expands our knowledge of cortical variation in humans and helps reconceptualize the discussion of how cortical systems variability arises and links to individual differences in behavior.

Controversies and progress on standardization of large-scale brain network nomenclature

“…A second, more recently recognized form of inter-individual variation is variation in the spatial position and extent of network nodes (Figure 5C) (78,80,85,86,90,(95)(96)(97)(98)(99)101,(125)(126)(127)(128)(129)(130)(131). Such variations can take the form of areal expansions, contractions, or displacements that lead to variation in the exact positions of network borders across individuals (125,126). More extreme individual spatial variations can relocate a node outside of the initialized parcel boundary, and not spatially overlap with other sample participants (98,123).…”

“…Single cortical areas representing network nodes can be split into multiple discontinuous regions in some individuals, while still clearly exhibiting the same properties of the unified area (35). Individual-specific brain networks also exhibit ectopic intrusions, in which a punctate region within a brain network has strong, idiosyncratic connectivity with a different network (85,86,123,125). All individuals exhibit some form of topographical variation in their brain networks (125).…”

“…Individual-specific brain networks also exhibit ectopic intrusions, in which a punctate region within a brain network has strong, idiosyncratic connectivity with a different network (85,86,123,125). All individuals exhibit some form of topographical variation in their brain networks (125). However, it is unclear if these violations of regional spatial contiguity reflect a differential sampling of regions on different hierarchical scales.…”

Hemispheric Asymmetries of Individual Differences in Functional Connectivity

“…This procedure takes into account the homogeneity of functional connectivity within a contiguous region, as well as the proportion of the variants' territory that was dominated by a single network. Homogeneity was assessed using principal component analysis of a variant's vertices' seed maps, then calculating the variance explained by the first principal component of the variant (Dworetsky, Seitzman, Adeyemo, Smith, et al, 2021;Gordon et al, 2016). Network dominance was assessed via a vertex-wise template matching technique that assigned each vertex to the canonical network that was most similar to the vertex's seed map based on Dice coefficient (see (Dworetsky, Seitzman, Adeyemo, Smith, et al, 2021)).…”

“…Homogeneity was assessed using principal component analysis of a variant's vertices' seed maps, then calculating the variance explained by the first principal component of the variant (Dworetsky, Seitzman, Adeyemo, Smith, et al, 2021;Gordon et al, 2016). Network dominance was assessed via a vertex-wise template matching technique that assigned each vertex to the canonical network that was most similar to the vertex's seed map based on Dice coefficient (see (Dworetsky, Seitzman, Adeyemo, Smith, et al, 2021)). Variants that did not meet the threshold of 66.7% homogeneity and 75% network dominance in the individual network map were split along the network boundaries of a vertex-wise network map, producing the final analyzed network variants.…”