Investigating noun-noun compound relation representations in autoregressive large language models

AbstractThis paper uses autoregressive large language models to explore at which points in a given input sentence the semantic information is decodable. Using representational similarity analysis and probing, the results show that autoregressive models are capable of extracting the semantic relation information from a dataset of noun-noun compounds. When considering the effect of processing the head and modifier nouns in context, the extracted representations show greater correlation after processing both constituent nouns in the same sentence. The linguistic properties of the head nouns may influence the ability of LLMs to extract relation information when the head and modifier words are processed separately. Probing suggests that Phi-1 and LLaMA-3.2 are exposed to relation information during training, as they are able to predict the relation vectors for compounds from separate word representations to a similar degree as using compositional compound representations. However, the difference in processing condition for GPT-2 and DeepSeek-R1 indicates that these models are actively processing the contextual semantic relation information of the compound.