Towards Understanding Modality Interaction in Multimodal Language Models via Partial Information Decomposition
Pith reviewed 2026-06-28 17:38 UTC · model grok-4.3
The pith
Partial Information Decomposition separates unique, redundant, and synergistic contributions from vision and language in multimodal models.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Applying PID to model outputs shows that reasoning-oriented tasks exhibit high synergy between sensory and linguistic inputs whereas expert and knowledge-oriented tasks show stronger language-unique reliance; these profiles generalize across model families and predict sensitivity to modality-level interventions, while Sensory PID applied to omni-modal models identifies a visual dominance bottleneck even on audio-visual tasks.
What carries the argument
Partial Information Decomposition (PID) applied at the decision level to quantify unique, redundant, and synergistic information contributions from each input modality.
If this is right
- Reasoning and grounding tasks tend to exhibit high synergy between modalities.
- Expert and knowledge-oriented tasks show stronger reliance on language-unique information.
- Modality-use profiles generalize across model families and predict sensitivity to modality interventions.
- PID-guided reweighting yields initial gains on multimodal reasoning and grounding tasks.
- Sensory PID reveals visual information dominance as a bottleneck in tri-modal fusion.
Where Pith is reading between the lines
- This could support task classification by information requirements that applies across architectures.
- Reweighting informed by PID profiles might reduce the cost of modality-specific fine-tuning.
- Extending the method to additional modalities could expose systematic underuse of certain inputs in current systems.
Load-bearing premise
The PID quantities computed from model outputs on existing benchmarks faithfully reflect the underlying causal contributions of each modality rather than artifacts of the chosen tasks or output format.
What would settle it
Computing PID profiles on the same models but with a new benchmark that uses different output formats or task structures and finding that the reported modality-use patterns and intervention predictions no longer hold.
Figures
read the original abstract
Understanding modality interaction in multimodal large language models (MLLMs) is central to reliable deployment. We introduce Partial Information Decomposition (PID) as a decision-level framework that separates unique, redundant, and synergistic contributions of sensory and linguistic inputs, beyond representation alignment and outcome-based evaluation. Across vision--language benchmarks, PID reveals recurring modality-use profiles: reasoning and grounding-oriented tasks tend to exhibit high synergy, whereas expert and knowledge-oriented tasks show stronger language-unique reliance. These profiles generalize across model families and predict sensitivity to modality-level interventions. We further extend PID to tri-modal systems with Sensory PID, treating language as a control variable to decompose video--audio information gain. Applied to omni-modal models, Sensory PID reveals a sensory synergy bottleneck dominated by visual information even on audio--visual fusion tasks. Finally, PID-guided reweighting provides initial evidence for improving multimodal reasoning and grounding performance.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces Partial Information Decomposition (PID) as a decision-level framework to separate unique, redundant, and synergistic contributions of sensory and linguistic inputs in multimodal LLMs. It reports recurring modality-use profiles across vision-language benchmarks (high synergy for reasoning/grounding tasks, language-unique reliance for expert/knowledge tasks), shows these profiles generalize across model families and predict sensitivity to modality interventions, extends the approach to tri-modal Sensory PID (with language as control), identifies a visual-dominated sensory synergy bottleneck, and provides initial evidence that PID-guided reweighting can improve multimodal reasoning and grounding performance.
Significance. If the PID quantities are shown to capture intrinsic causal modality contributions rather than benchmark artifacts, the framework would supply a new information-theoretic tool for analyzing and intervening on modality interactions that goes beyond representation alignment or outcome metrics. The reported cross-family generalization, predictive validity for interventions, and reweighting results would constitute a substantive contribution to understanding multimodal model behavior.
major comments (2)
- [Evaluation and Results] The central claim that PID reveals intrinsic modality-use profiles (rather than artifacts of task design or output format) is load-bearing for all downstream results on generalization and intervention sensitivity. The manuscript must include an explicit invariance test: recompute PID on semantically equivalent but format-altered prompts (e.g., rephrased questions or changed answer formats) and demonstrate stability of the unique/redundant/synergistic quantities; without this, the recurring profiles could be downstream of benchmark construction.
- [Abstract and Evaluation] Profiles are derived from the same benchmarks subsequently used to validate generalization across model families and to predict intervention sensitivity. This creates a circularity risk: the task groupings that define the profiles may already encode the modality requirements that later appear as predictive patterns. An independent hold-out set of tasks or an a priori task taxonomy (not derived from the PID outputs) is needed to break the dependence.
minor comments (2)
- [Abstract] The abstract states results but supplies no equations for the PID decomposition, no dataset or model details, and no description of controls, making it impossible for a reader to assess whether the reported profiles are supported by the computation.
- [Tri-modal Extension] Notation for the tri-modal Sensory PID extension (language as control variable) is introduced without an explicit equation or diagram showing how the three-way decomposition is computed from model outputs.
Simulated Author's Rebuttal
We thank the referee for the constructive feedback. The two major comments identify important robustness checks that will strengthen the claims regarding intrinsic modality-use profiles. We address each below and will incorporate the requested analyses in the revised manuscript.
read point-by-point responses
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Referee: [Evaluation and Results] The central claim that PID reveals intrinsic modality-use profiles (rather than artifacts of task design or output format) is load-bearing for all downstream results on generalization and intervention sensitivity. The manuscript must include an explicit invariance test: recompute PID on semantically equivalent but format-altered prompts (e.g., rephrased questions or changed answer formats) and demonstrate stability of the unique/redundant/synergistic quantities; without this, the recurring profiles could be downstream of benchmark construction.
Authors: We agree that demonstrating invariance to prompt and output format variations is essential to support the claim that PID quantities reflect intrinsic modality interactions rather than benchmark artifacts. In the revised manuscript we will add a dedicated invariance experiment: for a representative subset of tasks we will recompute PID using semantically equivalent but rephrased questions and altered answer formats, reporting the resulting changes (or lack thereof) in unique, redundant, and synergistic values. This will directly test stability and address the concern that observed profiles may be downstream of specific task construction. revision: yes
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Referee: [Abstract and Evaluation] Profiles are derived from the same benchmarks subsequently used to validate generalization across model families and to predict intervention sensitivity. This creates a circularity risk: the task groupings that define the profiles may already encode the modality requirements that later appear as predictive patterns. An independent hold-out set of tasks or an a priori task taxonomy (not derived from the PID outputs) is needed to break the dependence.
Authors: We acknowledge the circularity risk. To break the dependence, the revised manuscript will introduce an a priori task taxonomy drawn from established multimodal evaluation literature (distinguishing reasoning/grounding tasks from knowledge/expert tasks) that is defined independently of our PID computations. We will then evaluate generalization and intervention sensitivity on a hold-out set of tasks excluded from the initial profile derivation, confirming that the reported patterns persist. These additions will be included in the updated version. revision: yes
Circularity Check
No significant circularity in derivation chain
full rationale
The paper introduces PID as an external information-theoretic tool and applies it post-hoc to model outputs on benchmarks to identify modality profiles. No equations or steps in the provided text reduce a claimed prediction or uniqueness result to a fitted parameter or self-citation by construction. The generalization and intervention-sensitivity observations are presented as empirical findings rather than tautological outputs of the input data definitions. The derivation remains self-contained against external benchmarks and does not invoke load-bearing self-citations or ansatzes that collapse to the target claims.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption PID decomposition of model outputs on benchmarks separates unique, redundant, and synergistic modality contributions in a causally meaningful way.
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