IC-810Individual sparse autoencoder features in Pythia-70m-deduped are monosemantic and have predictable causal effects on output logits, as demonstrated by an apostrophe feature whose ablation primarily suppresses the 's' token
Robert Huben, Hoagy Cunningham, Logan Riggs Smith, Aidan Ewart, Lee Sharkey
Case studies on Pythia-70m-deduped (layer 4, r=4, alpha=0.0014) show that individual dictionary features correspond to single human-understandable concepts. An apostrophe feature activates only on apostrophe tokens (and conceptually similar punctuation at lower activations), in contrast to the default residual stream dimension which is polysemantic. Ablating this feature (less-than-rank-one ablation) primarily reduces the logit for the following 's' token, consistent with its role in predicting possessives and contractions. A closing-parenthesis feature in layer 5 has a causal circuit: previous-layer features detecting dates, acronyms, and phrases cause it to activate, and its unembedding produces closing-parenthesis tokens. 12k logits are negatively affected by the apostrophe ablation (threshold 0.1).
Evidence
interventional
Key metric
12k logits negatively affected (threshold 0.1); top suppressed token is 's'
Caveat
Case studies are on a small number of hand-selected features; the authors do not claim all features are monosemantic. The ablation uses less-than-rank-one ablation, which only lowers the activation to the point where the feature is no longer active.