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: Transcoders enable fine-grained interpretable circuit analysis for language models, published by Jacob Dunefsky on April 30, 2024 on The AI Alignment Forum.
Summary
We present a method for performing circuit analysis on language models using "transcoders," an occasionally-discussed variant of SAEs that provide an interpretable approximation to MLP sublayers' computations. Transcoders are exciting because they allow us not only to interpret the output of MLP sublayers but also to decompose the MLPs themselves into interpretable computations. In contrast, SAEs only allow us to interpret the output of MLP sublayers and not how they were computed.
We demonstrate that transcoders achieve similar performance to SAEs (when measured via fidelity/sparsity metrics) and that the features learned by transcoders are interpretable.
One of the strong points of transcoders is that they decompose the function of an MLP layer into sparse, independently-varying, and meaningful units (like neurons were originally intended to be before superposition was discovered). This significantly simplifies circuit analysis, and so for the first time, we present a method for using transcoders in circuit analysis in this way.
We performed a set of case studies on GPT2-small that demonstrate that transcoders can be used to decompose circuits into monosemantic, interpretable units of computation.
We provide code for training/running/evaluating transcoders and performing circuit analysis with transcoders, and code for the aforementioned case studies carried out using these tools. We also provide a suite of 12 trained transcoders, one for each layer of GPT2-small. All of the code can be found at
https://github.com/jacobdunefsky/transcoder_circuits, and the transcoders can be found at
https://huggingface.co/pchlenski/gpt2-transcoders.
Work performed as a part of Neel Nanda's MATS 5.0 (Winter 2024) stream and MATS 5.1 extension. Jacob Dunefsky is currently receiving funding from the Long-Term Future Fund for this work.
Background and motivation
Mechanistic interpretability is fundamentally concerned with reverse-engineering models' computations into human-understandable parts. Much early mechanistic interpretability work (e.g.
indirect object identification) has dealt with decomposing model computations into circuits involving small numbers of model components like attention heads or MLP sublayers.
But these component-level circuits operate at too coarse a granularity: due to the relatively small number of components in a model, each individual component will inevitably be important to all sorts of computations, oftentimes playing different roles. In other words, components are polysemantic.
Therefore, if we want a more faithful and more detailed understanding of the model, we should aim to find fine-grained circuits that decompose the model's computation onto the level of individual feature vectors.
As a hypothetical example of the utility that feature-level circuits might provide in the very near-term: if we have a feature vector that seems to induce gender bias in the model, then understanding which circuits this feature vector partakes in (including which earlier-layer features cause it to activate and which later-layer features it activates) would better allow us to understand the side-effects of debiasing methods.
More ambitiously, we hope that similar reasoning might apply to a feature that would seem to mediate deception in a future unaligned AI: a fuller understanding of feature-level circuits could help us understand whether this deception feature actually is responsible for the entirety of deception in a model, or help us understand the extent to which alignment methods remove the harmful behavior.
Some of the earliest work on SAEs aimed to use them to find such feature-level circuits (e.g.
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