Torus Graphs for Multivariate Phase Coupling Analysis

TL;DR

This paper introduces torus graphs, a new class of graphical models for analyzing multivariate phase coupling in circular data, with applications in neuroscience, demonstrating improved identification of phase interactions over traditional methods.

Contribution

The paper develops torus graphs based on exponential family models to better analyze multivariate phase interactions on a torus, advancing beyond existing methods.

Findings

Torus graphs accurately identify conditional associations in phase data.

Standard phase locking value fails to capture multivariate interactions.

Torus graphs produce intuitive results in brain phase data analysis.

Abstract

Angular measurements are often modeled as circular random variables, where there are natural circular analogues of moments, including correlation. Because a product of circles is a torus, a d-dimensional vector of circular random variables lies on a d-dimensional torus. For such vectors we present here a class of graphical models, which we call torus graphs, based on the full exponential family with pairwise interactions. The topological distinction between a torus and Euclidean space has several important consequences. Our development was motivated by the problem of identifying phase coupling among oscillatory signals recorded from multiple electrodes in the brain: oscillatory phases across electrodes might tend to advance or recede together, indicating coordination across brain areas. The data analyzed here consisted of 24 phase angles measured repeatedly across 840 experimental trials (replications) during a memory task, where the electrodes were in 4 distinct brain regions, all known to be active while memories are being stored or retrieved. In realistic numerical simulations, we found that a standard pairwise assessment, known as phase locking value, is unable to describe multivariate phase interactions, but that torus graphs can accurately identify conditional associations. Torus graphs generalize several more restrictive approaches that have appeared in various scientific literatures, and produced intuitive results in the data we analyzed. Torus graphs thus unify multivariate analysis of circular data and present fertile territory for future research.