On the Multilingual Ability of Decoder-based Pre-trained Language
  Models: Finding and Controlling Language-Specific Neurons
Takeshi Kojima Itsuki Okimura Yusuke Iwasawa Hitomi Yanaka Yutaka Matsuo
Current decoder-based pre-trained language models (PLMs) successfully
demonstrate multilingual capabilities. However, it is unclear how these models
handle multilingualism. We analyze the neuron-level internal behavior of
multilingual decoder-based PLMs, Specifically examining the existence of
neurons that fire ``uniquely for each language'' within decoder-only
multilingual PLMs. We analyze six languages: English, German, French, Spanish,
Chinese, and Japanese, and show that language-specific neurons are unique, with
a slight overlap (< 5%) between languages. These neurons are mainly distributed
in the models' first and last few layers. This trend remains consistent across
languages and models. Additionally, we tamper with less than 1% of the total
neurons in each model during inference and demonstrate that tampering with a
few language-specific neurons drastically changes the probability of target
language occurrence in text generation.