Successor Heads: Recurring, Interpretable Attention Heads In The Wild

Rhys Gould, Euan Ong, George Ogden, Arthur Conmy

Keywords: Interpretability, Large Language Models, Natural Language Processing, Science of Deep Learning, Representation Learning

TL;DR: We find "Successor Heads": a type of attention head that transforms numeric representations in language models. We find successor heads across language models of different sizes and find the abstract number representations that make them work.

Abstract: In this work we present successor heads: attention heads that increment tokens with a natural ordering, such as numbers, months, and days. For example, successor heads increment ‘Monday’ into ‘Tuesday’. We explain the successor head behavior with an approach rooted in mechanistic interpretability, the field that aims to explain how models complete tasks in human-understandable terms. Existing research in this area has found interpretable language model components in small toy models. However, results in toy models have not yet led to insights that explain the internals of frontier models and little is currently understood about the internal operations of large language models. In this paper, we analyze the behavior of successor heads in large language models (LLMs) and find that they implement abstract representations that are common to different architectures. They form in LLMs with as few as 31 million parameters, and at least as many as 12 billion parameters, such as GPT-2, Pythia, and Llama-2. We find a set of ‘mod 10’ features that underlie how successor heads increment in LLMs across different architectures and sizes. We perform vector arithmetic with these features to edit head behavior and provide insights into numeric representations within LLMs. Additionally, we study the behavior of successor heads on natural language data, identifying interpretable polysemanticity in a Pythia successor head.

Submission Number: 116