Language Arithmetics: Towards Systematic Language Neuron Identification and Manipulation

TL;DR

This paper investigates language-specific neurons in large multilingual models, identifying their properties and demonstrating how systematic neuron manipulation can control language behavior and improve multilingual task performance.

Contribution

It introduces the LAPE method for identifying language neurons and demonstrates systematic neuron activation manipulation to steer model behavior across multiple languages and tasks.

Findings

Neurons cluster in deeper layers and are more specialized for non-Latin scripts.

Shared neurons reflect linguistic proximity among related languages.

Neuron manipulation improves multilingual task performance and language control.

Abstract

Large language models (LLMs) exhibit strong multilingual abilities, yet the neural mechanisms behind language-specific processing remain unclear. We analyze language-specific neurons in Llama-3.1-8B, Mistral-Nemo-12B, and Aya-Expanse-8B & 32B across 21 typologically diverse languages, identifying neurons that control language behavior. Using the Language Activation Probability Entropy (LAPE) method, we show that these neurons cluster in deeper layers, with non-Latin scripts showing greater specialization. Related languages share overlapping neurons, reflecting internal representations of linguistic proximity. Through language arithmetics, i.e. systematic activation addition and multiplication, we steer models to deactivate unwanted languages and activate desired ones, outperforming simpler replacement approaches. These interventions effectively guide behavior across five multilingual tasks: language forcing, translation, QA, comprehension, and NLI. Manipulation is more successful for high-resource languages, while typological similarity improves effectiveness. We also demonstrate that cross-lingual neuron steering enhances downstream performance and reveal internal "fallback" mechanisms for language selection when neurons are progressively deactivated. Our code is made publicly available at https://github.com/d-gurgurov/Language-Neurons-Manipulation.