Functional Information Decomposition: A First-Principles Approach to Analyzing Functional Relationships

Information theory, originating from Shannon's work on communication systems, has become a fundamental tool across neuroscience, genetics, physics, and machine learning. However, the application of information theory is often limited to the simplest case: mutual information between two variables. A central challenge in extending information theory to multivariate systems is decomposition: understanding how the information that multiple variables collectively provide about a target can be broken down into the distinct contributions that are assignable to individual variables or their interactions. To restate the problem clearly, what is sought after is a decomposition of the mutual information between a set of inputs (or parts) and an output (or whole). In this work, we introduce Functional Information Decomposition (FID) a new approach to information decomposition that differs from prior methods by operating on complete functional relationships rather than statistical correlations, enabling precise quantification of independent and synergistic contributions.