[Paper Review] Role of inter-hemispheric connections in functional brain networks

Johann H. Martínez, Javier M. Buldú+3 more|arXiv (Cornell University)|Oct 31, 2017

Functional Brain Connectivity Studies37 references3 citations

TL;DR

This study investigates how inter-hemispheric functional connections shape centrality distribution in human brain networks using EEG-derived functional networks. By modeling the left and right hemispheres as interconnected sub-networks, it demonstrates that centrality balance depends critically on the strategic placement of inter-hemispheric links—specifically, peripheral-peripheral (PP) or central-central (CC) connections—which determine whether one hemisphere dominates or both remain functionally balanced.

ABSTRACT

Today the human brain can be modeled as a graph where nodes represent different regions and links stand for statistical interactions between their activities as recorded by different neuroimaging techniques. Empirical studies have lead to the hypothesis that brain functions rely on the coordination of a scattered mosaic of functionally specialized brain regions (modules or sub-networks), forming a web-like structure of coordinated assemblies (a network of networks). The study of brain dynamics would therefore benefit from an inspection of how functional sub-networks interact between them. In this paper, we model the brain as an interconnected system composed of two specific sub-networks, the left (L) and right (R) hemispheres, which compete with each other for centrality, a topological measure of importance in a networked system. Specifically, we consideredfunctional brain networks derived from high-density electroencephalographic (EEG) recordings and investigated how node centrality is shaped by interhemispheric connections. Our results show that the distribution of centrality strongly depends on the number of functional connections between hemispheres and the way these connections are distributed. Additionally, we investigated the consequences of node failure on hemispherical centrality, and showed how the abundance of inter-hemispheric links favors the functional balance of centrality distribution between the hemispheres.

Motivation & Objective

• To understand how inter-hemispheric connections influence the distribution of node centrality in functional brain networks.
• To investigate the role of inter-hemispheric connectivity in maintaining functional balance between the left and right brain hemispheres.
• To evaluate network robustness against node failure by analyzing the impact of inter-hemispheric link configurations on centrality loss.
• To identify optimal connection strategies (PP vs. CC) that maximize or minimize hemispheric centrality, depending on network configuration.

Proposed method

• Modeled the human brain as a network-of-networks with left (L) and right (R) hemispheres as distinct sub-networks connected via inter-hemispheric links.
• Used eigenvector centrality to quantify node importance, with hemispheric centrality computed as the sum of individual node centralities.
• Proposed a competition parameter $\Omega_L$ to assess how close the actual centrality distribution is to optimal configurations (maximal or minimal $C^L$).
• Performed iterative rewiring of inter-hemispheric links by repositioning weights in descending order of strength to identify configurations maximizing/minimizing $C^L$.
• Defined local impact $l^{L}_{imp}(i)$ as the percentage drop in left-hemisphere centrality after node $i$ removal, and local contribution $lc^L(i)$ as the node’s share in total hemispheric centrality.
• Evaluated robustness across three stages of inter-hemispheric connectivity (complete, relative, slight), analyzing centrality loss under node failure.

Experimental results

Research questions

• RQ1 How does the distribution of inter-hemispheric connections affect the centrality balance between the left and right brain hemispheres?
• RQ2 What connection strategies (PP vs. CC) optimize or minimize hemispheric centrality, and how do they influence functional dominance?
• RQ3 How does the abundance of inter-hemispheric links affect the robustness of centrality distribution under node failure?
• RQ4 To what extent does the network’s functional balance depend on the strategic placement of connector nodes between hemispheres?

Key findings

• The distribution of centrality between hemispheres strongly depends on the number and distribution of inter-hemispheric connections.
• Peripheral-peripheral (PP) connections favor the hemisphere with higher $\lambda_1$ (dominant sub-network), increasing its centrality accumulation.
• Central-central (CC) connections benefit the less dominant hemisphere by allowing it to achieve higher centrality than in other configurations.
• When $\Omega_L \approx 0$, neither hemisphere is in an optimal configuration, indicating functional balance between hemispheric centrality.
• A higher number of inter-hemispheric links enhances functional balance by reducing the dominance of one hemisphere over the other.
• Node failure has a greater impact on centrality when hubs are removed, and the local contribution $lc^L(i)$ quantifies each node’s share in hemispheric centrality.