AbsTopK: Rethinking Sparse Autoencoders For Bidirectional Features
Sparse autoencoders (SAEs) are widely used for LLM interpretability, but existing variants often impose non-negativity that prevents single features from representing bidirectional concepts. We derive SAE variants from unrolled proximal gradient updates, identify this structural limitation, and propose AbsTopK SAE with magnitude-based hard thresholding. Across four LLMs and seven probing/steering tasks, AbsTopK improves reconstruction and interpretability while enabling single features to encode contrasting concepts.
Zhu X, Khalili M M, Zhu Z. AbsTopK: Rethinking Sparse Autoencoders For Bidirectional Features. ICLR 2026.