Parameter-Efficient Audio-Visual Dynamic Facial Expression Recognition with Mamba Fusion Adapters and Frame-Level Feature Arrangement.

Dynamic Facial Expression Recognition (DFER) has recently attracted significant interest due to its vital role in enabling empathetic and human-compatible technologies. Developing models that remain robust under in-the-wild variability is a key motivation for DFER research and its practical applications. Improving models through multimodal learning, which leverages audio and video data for richer, complementary representations, is one promising direction. However, existing methods still rely heavily on modality-specific encoders and coarse-grained content-level alignment, which hinders their ability to capture fine-grained emotional semantics and dynamic cross-modal interactions. To address this, we adopt parameter-efficient fine-tuning (PEFT) to facilitate audio-visual interaction. This strategy offers key advantages: (1) freezing parameters preserves upstream pretrained knowledge, ensuring that the model focuses solely on learning modules for audio-visual interaction and modal fusion; (2) a Mamba Fusion Adapter (MFAdapter) is inserted at each encoder layer to perform causal, audio-conditioned fusion over a frame-aligned token sequence, enabling efficient multi-level cross-modal injection with linear complexity; and (3) a Frame-level Feature Arrangement (FFA) strategy is introduced as a deterministic index-based arrangement scheme that arranges audio tokens at the video-frame rate, providing a frame-indexed temporal prior that supports the causal scan in MFAdapter; FFA reduces, but does not eliminate, coarse segment-level mismatch and is not claimed as verified frame-level synchronization. Notably, our method achieves competitive performance on DFEW and MAFW while updating 2.7% (4.7 million) of the model parameters, with the best WAR of 58.70% on MAFW among all compared methods and 76.62%/65.25% (WAR/UAR) on DFEW under the official five-fold cross-validation protocols.