Fantastic Semantics and Where to Find Them: Investigating Which Layers of Generative LLMs Reflect Lexical Semantics

Zhu Liu, Cunliang Kong, Ying Liu, Maosong Sun

Abstract
Large language models have achieved remarkable success in general language understanding tasks. However, as a family of generative methods with the objective of next token prediction, the semantic evolution with the depth of these models are not fully explored, unlike their predecessors, such as BERT-like architectures. In this paper, we specifically investigate the bottom-up evolution of lexical semantics for a popular LLM, namely Llama2, by probing its hidden states at the end of each layer using a contextualized word identification task. Our experiments show that the representations in lower layers encode lexical semantics, while the higher layers, with weaker semantic induction, are responsible for prediction. This is in contrast to models with discriminative objectives, such as mask language modeling, where the higher layers obtain better lexical semantics. The conclusion is further supported by the monotonic increase in performance via the hidden states for the last meaningless symbols, such as punctuation, in the prompting strategy. Our codes are available at https://github.com/RyanLiut/LLM_LexSem.
Anthology ID:
2024.findings-acl.866
Volume:
Findings of the Association for Computational Linguistics: ACL 2024
Month:
August
Year:
2024
Address:
Bangkok, Thailand
Editors:
Lun-Wei Ku, Andre Martins, Vivek Srikumar
Venue:
Findings
SIG:
Publisher:
Association for Computational Linguistics
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Pages:
14551–14558
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URL:
https://preview.aclanthology.org/add_missing_videos/2024.findings-acl.866/
DOI:
10.18653/v1/2024.findings-acl.866
Bibkey:
Cite (ACL):
Zhu Liu, Cunliang Kong, Ying Liu, and Maosong Sun. 2024. Fantastic Semantics and Where to Find Them: Investigating Which Layers of Generative LLMs Reflect Lexical Semantics. In Findings of the Association for Computational Linguistics: ACL 2024, pages 14551–14558, Bangkok, Thailand. Association for Computational Linguistics.
Cite (Informal):
Fantastic Semantics and Where to Find Them: Investigating Which Layers of Generative LLMs Reflect Lexical Semantics (Liu et al., Findings 2024)
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https://preview.aclanthology.org/add_missing_videos/2024.findings-acl.866.pdf
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