Attractor separation and signed cycles in asynchronous Boolean networks

TL;DR

This paper investigates how the structure of interaction graphs in asynchronous Boolean networks influences the separation of attractors, introducing new notions of separability and linking graph motifs to dynamical properties.

Contribution

It introduces new concepts of attractor separability in Boolean networks and establishes conditions on the interaction graph that guarantee attractor isolation.

Findings

Attractors can be separated by subspaces if the graph has at most one positive or negative cycle.

No path from a negative to a positive cycle allows separation by trap spaces.

Presence of a complete signed digraph on two vertices is linked to inseparable attractors.

Abstract

The structure of the graph defined by the interactions in a Boolean network can determine properties of the asymptotic dynamics. For instance, considering the asynchronous dynamics, the absence of positive cycles guarantees the existence of a unique attractor, and the absence of negative cycles ensures that all attractors are fixed points. In presence of multiple attractors, one might be interested in properties that ensure that attractors are sufficiently "isolated", that is, they can be found in separate subspaces or even trap spaces, subspaces that are closed with respect to the dynamics. Here we introduce notions of separability for attractors and identify corresponding necessary conditions on the interaction graph. In particular, we show that if the interaction graph has at most one positive cycle, or at most one negative cycle, or if no positive cycle intersects a negative cycle, then the attractors can be separated by subspaces. If the interaction graph has no path from a negative to a positive cycle, then the attractors can be separated by trap spaces. Furthermore, we study networks with interaction graphs admitting two vertices that intersect all cycles, and show that if their attractors cannot be separated by subspaces, then their interaction graph must contain a copy of the complete signed digraph on two vertices, deprived of a negative loop. We thus establish a connection between a dynamical property and a complex network motif. The topic is far from exhausted and we conclude by stating some open questions.