When to Align, When to Predict: A Phase Diagram for Multimodal Learning

2026-06-09

AI Summary

This work addresses the lack of a systematic understanding of when cross-modal alignment (CA) and cross-modal prediction (CP) are effective in multimodal learning—a gap that often leads to suboptimal performance or even degradation relative to unimodal baselines. The authors propose a unified linear analytical framework under a structured signal–noise model with correlated interference, revealing complementary failure mechanisms of CA and CP. They introduce the first multimodal “phase diagram,” which delineates four distinct regimes: both methods succeed, only alignment works, only prediction works, or neither is effective. Leveraging separation ratio analysis, a unidirectional whitening mechanism, and a few-shot label-guided localization algorithm, this phase diagram enables practical guidance for method selection on real-world data. Experiments across synthetic, stereo vision, image–text, and astrophysical datasets validate its efficacy in identifying harmful multimodal configurations, offering a diagnostic tool for practitioners prior to model deployment.

Abstract

Cross-modal alignment (CA) and cross-modal prediction (CP) are the dominant paradigms for multimodal representation learning, yet there is no systematic understanding of when each succeeds, when each fails, and when cross-modal training helps at all -- a gap that leaves practitioners, especially in scientific domains like biomedicine or astrophysics, with heterogeneous instruments and multiple levels of organization and measurement, unable to diagnose why standard methods underperform the best single modality. We develop a unified linear framework that addresses both questions. Under a spiked signal-plus-noise model with structured cross-modal nuisance correlation, we derive separation ratios for both objectives that expose complementary failure modes: alignment whitens each modality and fails when nuisance is strongly correlated across views; prediction encodes whatever is cross-predictable through a one-sided whitening, with recovery governed by source-modality quality. The resulting phase diagram partitions multimodal problems into four regimes: Both, CA only, CP only, and Neither. We present a data-driven procedure to locate real-world datasets in this diagram using a small labeled subsample, identifying the preferred objective and prediction direction before any cross-modal training. Experiments on synthetic data, stereo-vision benchmarks, image-caption pairs, and real astrophysical data validate the predictions in the nonlinear regime, including the Neither regime where cross-modal training is actively harmful. Our framework lets practitioners diagnose their multimodal problem and choose the right objective before committing to training. Code to reproduce the results is available at https://github.com/IlayMalinyak/mm_align_vs_pred.

Problem

• multimodal learning
• cross-modal alignment
• cross-modal prediction
• representation learning
• phase diagram

Innovation

• multimodal learning
• cross-modal alignment
• cross-modal prediction
• phase diagram
• signal-plus-noise model