Modality-Specific Low-Rank Factors

• Modality-specific low-rank factors are tailored parameterizations that enforce individual low-rank constraints per data modality, enhancing model efficiency and interpretability.
• They are applied in neural network adaptation, multimodal fusion, and tensor completion to minimize overfitting while preserving key structural features.
• Careful rank selection per modality reduces parameter cost and improves performance by matching intrinsic data dimensionalities in varied domains.

Modality-specific low-rank factors are structured parameterizations, penalties, or decompositions that explicitly model—and typically constrain—the intrinsic rank of model parameters or latent variables separately and independently for each data modality or modality-combination in a multi-modal or multi-view system. This paradigm emerges in numerous contexts: neural network adaptation (LoRA), matrix/tensor decomposition for data integration, low-rank multimodal fusion, low-rank regularization in regression, multi-mode tensor factorization, and domain-specific generative modeling (e.g. time-frequency models). Properly chosen modality-specific ranks often yield superior parameter efficiency, interpretability, and performance compared to global low-rank constraints, especially when intrinsic dimensionalities differ among modalities.

1. Core Mathematical Formulations

The central methodology is to represent a parameter update, weight tensor, regression block, or latent factor matrix for each modality $m$ (or tuple of modalities) as a low-rank object with modality-specific rank and/or factorization:

• Matrix-structured (e.g. LoRA, block regression):

$$W_m' = W_m^0 + B_m A_m,\qquad B_m \in \mathbb{R}^{d_\text{out} \times r_m},\; A_m \in \mathbb{R}^{r_m \times d_\text{in}}$$

Each modality gets its own adapter $(A_m, B_m)$ and rank $r_m$ (Gupta et al., 16 May 2024).

• Block regression (multi-omics):

$$B^{(m)} = U^{(m)} V^{(m)^\top},\qquad U^{(m)} \in \mathbb{R}^{p_m \times r_m},\; V^{(m)} \in \mathbb{R}^{q \times r_m}$$

Estimated via blockwise nuclear norm penalties for each $B^{(m)}$ (Mai et al., 2019).

• Multimodal fusion (tensor-based):
  • CP- or Tucker-decomposition, with separate modality factors:

  $$W \approx \sum_{i=1}^{r} W_1^{(i)} \otimes W_2^{(i)} \otimes \cdots \otimes W_M^{(i)}$$

  Each $W_m^{(i)}$ is specific to modality $m$ (Liu et al., 2018, Sahay et al., 2020). - Mode-specific low-rank factors in Tucker or other decompositions (e.g., TensLoRA):

  $$T \approx \mathcal{G} \times_1 U^{(1)} \times_2 U^{(2)} \cdots$$

  $U^{(i)}$ may be tuned per modality, task, or axis (Marmoret et al., 22 Sep 2025).

• Multi-mode tensor completion:

$\mathcal{Y} \approx \sum_{n=1}^N w_n (\mathcal{G}_n \times_n U^{(n)}),\qquad U^{(n)} = \text{mode-%%%%7%%%% factor},~ r_n \ll I_n$

Each mode/factor is regularized by its own nuclear/log penalty (Zeng, 2020).

• Generative signal models:

$\alpha_{f n} \sim \mathcal{N}_c(0, [W H]_{f n}),\qquad \text{with %%%%8%%%% low-rank across time-frequency}$

The dictionary $D$ and factorization $W H$ are chosen for the particular modality (Févotte et al., 2018).

Modality-specific low-rankness is typically enforced by either explicit parameterization (as above) or by penalizing a local nuclear norm or log-norm term for each block/mode in an objective.

2. Algorithmic Paradigms for Modality-Specific Low-Rank Adaptation

Fine-tuning via Modality-Specific Low-Rank Adapters

In low-rank adaptation of foundation/time-series models, each target modality (e.g., MeanBP, HR) receives a dedicated set of LoRA adapters $(A^m, B^m)$. Only these parameters are tuned; the rest of the model remains frozen, minimizing overfitting and parameter cost. Empirical ablation shows that small rank values $r_m$ (typically 2–8) suffice for >95% of full fine-tuning performance, especially for tasks with limited modality-specific data (Gupta et al., 16 May 2024).

Composite Nuclear Norm and Block-Wise Optimization

For multi-view regression (e.g., drug sensitivity from multi-omics data): $$\min_{\{B^{(m)}\}} \frac{1}{2n} \|Y - \sum_{m=1}^K X^{(m)} B^{(m)}\|_F^2 + \lambda \sum_{m=1}^K w_m \|B^{(m)}\|_*$$ Blockwise proximal gradient (singular value thresholding) is used: each $B^{(m)}$ is updated independently, so each modality can adapt its effective rank $r_m$ to its signal (Mai et al., 2019).

Multimodal Fusion via Modality-Specific Tensors

Low-rank fusion replaces full-order tensor weights by factorizations where each component is a modality-specific matrix/tensor. Efficient computation combines per-modality projections (e.g., $z_m = U_m^T x_m$), followed by elementwise multiplication and summation, reducing parameter and compute complexity from $O(\prod d_m)$ to $O(r \sum d_m)$ (Liu et al., 2018, Sahay et al., 2020).

Mode-Specific Tensor Factorization

In Tucker or CP-based tensor adaptations (e.g., TensLoRA), rank parameters are selected per tensor mode, allowing compression or expansion along specific axes such as layer, projection, or feature, tailored to the redundancy or diversity of each axis (modality, task, or group) (Marmoret et al., 22 Sep 2025).

Decoupled and Combination-Aware Extensions

Complex multi-modal scenarios (e.g., missing/incomplete multimodality) motivate adapter architectures that feature both modality-specific and "modality-combination" specific low-rank factors, with additional "shared" adapters for cross-modality generalization (Zhao et al., 15 Jul 2025, Zhao et al., 9 Nov 2025). Dynamic weighting schemes adjust training schedule based on representation separability.

3. Trade-offs, Selection, and Parameterization of Ranks

Selection of modality-specific rank $r_m$ is central. Empirical findings include:

• Performance Plateau: Performance (e.g., MAPE, accuracy) typically improves rapidly with small increases in $r_m$ (e.g., $r=1\to2$), then plateaus; higher values yield negligible additional gain at substantial parameter cost (Gupta et al., 16 May 2024, Liu et al., 2018).
• Guideline: Choose the smallest $r_m$ where accuracy "levels off" (the "elbow" of the curve). For small/medium models, $r=2$ often suffices; for larger or more complex modalities, increase only if significant gains are observed with larger $r$.
• Rank heterogeneity: Blockwise or modewise rank adaptation is often superior to applying a global (shared) rank, especially when intrinsic dimensionalities differ by modality (Mai et al., 2019, Zeng, 2020).
• Compression and expressiveness: In tensor-based LoRA (TensLoRA), mode-specific rank scheduling allows targeting representation capacity to axes with higher redundancy, e.g., using smaller $r_\text{layer}, r_\text{projection}$ for ViT and higher $r_\text{feature}$ (Marmoret et al., 22 Sep 2025).

Paper/Approach	Rank Selection Paradigm	Empirical Rank Range	Comments
LoRA (time-series)	Sweep, select at plateau	$r$=2–8	>95% perf at <2% params; separate $A^m, B^m$ per modality
Multimodal Fusion (LMF)	Cross-validated, sweep	$r$=2–8	Choose by validation MAE; unstable for $r>8$
Composite nuclear norm	Blockwise, inspect spectrum	data-driven	$r_m$ estimated per block; refine by re-fitting
Tensor LoRA (TensLoRA)	Modewise tuning	various	$r_1,\ldots,r_N$ per mode; trade off parameter budget
MCULoRA	Private + shared per combination	task/combination	Scheduling reflects learning difficulty

4. Interpretability and Structural Insights

Low-rank factors confer interpretability and diagnostic power:

• Latent features: The columns of modality-specific left factors (e.g., $U^{(m)}$ in regression, $W_m^{(i)}$ in fusion) reveal groups of features (e.g., co-expressed genes, temporal patterns) particularly influential for individual modalities (Mai et al., 2019).
• Variance explained: The modal or block-specific rank (or norm of singular values) reflects the effective latent dimensionality of the modality; sharp drops in the singular spectrum signify the cutoff for intrinsic structure (Zeng, 2020).
• Sparsity and orthogonality: Additional $\ell_1$ and orthogonality constraints (as in solrCMF) yield sparse, disjoint latent factors, cleanly partitioned into globally shared, partially shared, and individual structures (Held et al., 16 May 2024).

5. Applications in Domains and Models

Modality-specific low-rank factors pervade diverse application areas:

• Time series foundation models: LoRA with per-modality adapters for ICU vital-sign forecasting boosts adaptation efficiency with minimal overfit on limited data (Gupta et al., 16 May 2024).
• Omics and integrative genomics: Composite low-rank block regression improves drug sensitivity prediction compared to global low-rank or elementwise-sparse models (Mai et al., 2019).
• Multimodal sentiment/emotion analysis: Tensor low-rank fusion with modality-specific factors reduces model size and computation by up to 10× while retaining performance (Liu et al., 2018, Sahay et al., 2020).
• Multi-mode tensor completion: Mode-specific low-rankness enables robust imputation in video, MRI, and hyperspectral imagery, outperforming global-rank tensor methods (Zeng, 2020).
• Neural backbone adaptation: TensLoRA and MoRA generalize LoRA to support mode/modal-specific compression and cross-modal low-rank sharing, yielding parameter-efficient adaptation for both vision and language (Marmoret et al., 22 Sep 2025, Zhao et al., 9 Nov 2025).
• Incomplete/missing modalities: Aggregating private and shared low-rank adapters, with dynamic training adjustment, achieves new state-of-the-art robustness to missing data (Zhao et al., 15 Jul 2025).

6. Limitations, Extensions, and Open Issues

While modality-specific low-rank factors deliver superior expressiveness-to-parameter trade-offs and interpretability, limitations and active research areas include:

• Rank selection automation: Most studies rely on grid-search or elbow heuristics; formal model-selection or minimal-norm approaches (e.g., via nuclear norm minimization) are not routinely employed in practice.
• Scaling to non-matrix forms: Extension to higher-order tensors (e.g., via CP or Tucker) is application-dependent; computational efficiency and identifiability of such factorizations remain open challenges (Marmoret et al., 22 Sep 2025).
• Interaction with sparsity and sharedness: Joint enforcement of orthogonality, sparsity, and global/partial/individual structure (as in solrCMF) can complicate optimization and interpretation, requiring advanced ADMM or block-coordinate descent with manifold constraints (Held et al., 16 May 2024).
• Lack of theory for deep architectures: Most empirical guidance comes from regression or shallow fusion; theoretical guarantees for deep neural contexts are limited.
• Task- and modality-dependence: Intrinsic modality ranks may vary with downstream task, requiring retraining if objectives or data distributions shift significantly. No universal selection rule has been endorsed.

A plausible implication is that robust practical workflows will continue to integrate heuristic rank sweeps, domain-informed prior knowledge, and inspection of singular-spectrum profiles to optimally calibrate modality-specific low-rank factors for each novel application.