Resting state inter and intra hemispheric human brain functional connectivity

Resting-state functional connectivity between neuroanatomical regions has attracted significant attention in recent years. In the process of obtaining the resting-state functional connectivity map of the human brain from blood-oxygen-level-dependent fMRI signals, it is common to average the signals from left and right hemispheres. This averaging can introduce unappreciated complexities and unintended consequences not related to the research question of interest. In this paper, we mathematically demonstrate that measures of functional connectivity obtained by averaging homologous regions from the both hemispheres become undesirably dependent on four inter-hemispheric connectivity measures. We explore this finding in real-world fMRI data from 25 healthy young participants. We show that inter-hemispheric averaging has a mixed effect on the results and may introduce correlation artifacts to the connectivity map. Furthermore, we show mathematically and demonstrate with Monte Carlo simulations of null data that inter-hemispheric averaging will not alter human brain connectivity map at rest only and if only there are no inter-hemispheric correlations.