arXiv Computation and Language By Momose Oyama, Yusuke Takase, Hidetoshi Shimodaira

Language Model Maps for Prompt-Response Distributions via Log-Likelihood Vectors

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The paper introduces a method that represents language models as log‑likelihood vectors over prompt‑response pairs, enabling the construction of model maps that compare conditional distributions. Squared Euclidean distances in this vector space approximate KL divergence, and experiments show that these maps reveal global structure related to model attributes and task performance. The approach also captures systematic shifts from prompt changes, supports additive compositionality for predicting downstream scores, and offers PMI vectors to mitigate unconditional distribution effects, thereby aiding analysis and prediction of input‑dependent behavior.

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arXiv AI
Sep 1

Mechanism Shift During Post-training from Autoregressive to Masked Diffusion Language Models

The study investigates how post‑training of large autoregressive language models (ARMs) into masked diffusion models (MDMs) affects their internal computation. Across two 7B ARM‑MDM families and four diagnostic tasks, the authors find that MDMs retain much of the ARM’s high‑attribution pathways on prefix‑dominant tasks, but reorganize computation toward earlier layers on globally constrained tasks. Component‑level probes reveal that ARMs depend on sharply specialized components, whereas MDMs show weaker specialization and more diffuse output‑space alignment.

By Injin Kong, Hyoungjoon Lee, Yohan Jo