LW - Open Source Automated Interpretability for Sparse Autoencoder Features by kh4dien

The Nonlinear Library

Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Open Source Automated Interpretability for Sparse Autoencoder Features, published by kh4dien on July 31, 2024 on LessWrong. Background Sparse autoencoders recover a diversity of interpretable, monosemantic features, but present an intractable problem of scale to human labelers. We investigate different techniques for generating and scoring text explanations of SAE features. Key Findings Open source models generate and evaluate text explanations of SAE features reasonably well, albeit somewhat worse than closed models like Claude 3.5 Sonnet. Explanations found by LLMs are similar to explanations found by humans. Automatically interpreting 1.5M features of GPT-2 with the current pipeline would cost $1300 in API calls to Llama 3.1 or $8500 with Claude 3.5 Sonnet. Prior methods cost ~$200k with Claude. Code can be found at https://github.com/EleutherAI/sae-auto-interp. We built a small dashboard to explore explanations and their scores: https://cadentj.github.io/demo/ Generating Explanations Sparse autoencoders decompose activations into a sum of sparse feature directions. We leverage language models to generate explanations for activating text examples. Prior work prompts language models with token sequences that activate MLP neurons (Bills et al. 2023), by showing the model a list of tokens followed by their respective activations, separated by a tab, and listed one per line. We instead highlight max activating tokens in each example with a set of <>. Optionally, we choose a threshold of the example's max activation for which tokens are highlighted. This helps the model distinguish important information for some densely activating features. We experiment with several methods for augmenting the explanation. Full prompts are available here. Chain of thought improves general reasoning capabilities in language models. We few-shot the model with several examples of a thought process that mimics a human approach to generating explanations. We expect that verbalizing thought might capture richer relations between tokens and context. Activations distinguish which sentences are more representative of a feature. We provide the magnitude of activating tokens after each example. We compute the logit weights for each feature through the path expansion where is the model unembed and is the decoder direction for a specific feature. The top promoted tokens capture a feature's causal effects which are useful for sharpening explanations. This method is equivalent to the logit lens (nostalgebraist 2020); future work might apply variants that reveal other causal information (Belrose et al. 2023; Gandelsman et al. 2024). Scoring explanations Text explanations represent interpretable "concepts" in natural language. How do we evaluate the faithfulness of explanations to the concepts actually contained in SAE features? We view the explanation as a classifier which predicts whether a feature is present in a context. An explanation should have high recall - identifying most activating text - as well as high precision - distinguishing between activating and non-activating text. Consider a feature which activates on the word "stop" after "don't" or "won't" (Gao et al. 2024). There are two failure modes: 1. The explanation could be too broad, identifying the feature as activating on the word "stop". It would have high recall on held out text, but low precision. 2. The explanation could be too narrow, stating the feature activates on the word "stop" only after "don't". This would have high precision, but low recall. One approach to scoring explanations is "simulation scoring"(Bills et al. 2023) which uses a language model to assign an activation to each token in a text, then measures the correlation between predicted and real activations. This method is biased toward recall; given a bro...