Multivariate visibility graphs: from time series to temporal networks An application to BOLD fMRI data Lucas Lacasa School of Mathematical Sciences Queen Mary University of London @wetuad Sebastiano Stramaglia Physics Department and INFN University of Bari @SebinoStram Speranza Sannino Dept of Electrical Engineering University of Cagliari Daniele Marinazzo Department of Data Analysis Ghent University @dan_marinazzo
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Multivariate visibility graphs:from time series to temporal networks
An application to BOLD fMRI data
Lucas LacasaSchool of Mathematical SciencesQueen Mary University of London
@wetuad
Sebastiano StramagliaPhysics Department and INFNUniversity of Bari
@SebinoStram
Speranza SanninoDept of Electrical EngineeringUniversity of Cagliari
Daniele MarinazzoDepartment of Data Analysis
Ghent University@dan_marinazzo
Visibility graphs were defined in computational geometry/computer science as the backbone graph capturing visibility paths (intervisible locations) in landscapes
• Each node represents a location• Two locations are connected by a link if they are visible
Visibility graphs were defined in computational geometry/computer science as the backbone graph capturing visibility paths (intervisible locations) in landscapes
• Each node represents a location• Two locations are connected by a link if they are visible
Captures different dynamical regimes and main features
Empirical study -- Visibility multiplex of multivariate financial series
.com bubble
Great recession
Financially stable
period
Comparison with a standard mutual information multivariate analysis
Multiplex visibility graph Multivariate series
Network structure for correlated data
Why visibility, and why applied to fMRI?
• Useful: both informative and descriptive
• Robust to processing
• Computationally easy and efficient
• Amenable to analytical insight
• Versatile, in both multivariate and univariate settings
• Novel, building a bridge between time series and networks
More motivations specific to fMRIPeaks of BOLD signal encode relevant information on coactivation and correlated activity
Tagliazucchi et al. 2012, Wu et al. 2013, Liu and Duyn 2013
Plus, visibility allows looking both at local and global activity
Examples of visibility in BOLD time series
Visibility graphs for BOLD signals
Modular temporal networks
Different modules: different temporal regimes, mainly adjacent points
TR
Dynamic functional connectivity (Hutchison et al. 2013, Leonardi et al. 2013, Hansen et al. 2015, Hindriks et al. 2016, Kudela et al. 2017)
Dynamical functional connectivity in the visibility framework: partition distance and Sorensen index
Application to a large open fMRI dataset
• Parcellation into 278 ROIs (Shen’s 2013 template)• Natural visibility graph for each ROI• Pairwise mutual information across ROIs• Average MI for each intrinsic connectivity
network, defined according to Yeo
Average MI across RSNs
Differences in the limbic system
• MANCOVAN with age, and framewise displacement as covariates• p-value 0.005, Bonferroni-Holm corrected• Shift function to assess differences across quantiles
In the same regions:• lower Regional Homogeneity (ReHo) Zang et al. (2004)• higher coefficient of variation of the BOLD signal Wu and Marinazzo (2016)• lower value of the fractional amplitude of low-frequency fluctuations (fALFF) Zou et al. (2008)