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arXiv 2602.15438cs.LGcs.AIstat.ML
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Logit距离界限表征相似性

Logit Distance Bounds Representational Similarity

IT University of Copenhagen, Denmark(丹麦IT大学);University of Trento, Italy(意大利特伦托大学);University of Copenhagen, Denmark(丹麦哥本哈根大学)

Beatrix M. G. Nielsen, Emanuele Marconato, Luigi Gresele, Andrea Dittadi, Simon Buchholz

AI总结:

研究判别模型在分布接近时内部表征是否线性相似,提出基于logit差异的距离度量,证明其能保证线性相似性,并用于蒸馏实验提升学生模型表征相似性。

摘要:

对于包括自回归语言模型在内的广泛判别模型族,可辨识性结果表明,如果两个模型诱导相同的条件分布,则它们的内部表示在可逆线性变换下相等。我们问当分布接近而非相等时,是否类似结论近似成立。基于Nielsen等人(2025)关于KL散度接近不一定意味着高线性表征相似性的观察,我们研究了一种基于logit差异的分布距离,并表明该距离的接近确实能产生线性相似性保证。具体地,我们基于模型的可辨识性类定义了一个表征不相似性度量,并证明它受logit距离限制。我们进一步表明,当模型概率远离零时,KL散度是logit距离的上界;然而,得到的界在实践中无法提供有意义的控制。因此,基于KL的蒸馏可以匹配教师的预测,但未能保留线性表征属性,例如人类可解释概念的线性探针可恢复性。在合成和图像数据集上的蒸馏实验中,logit距离蒸馏产生的学生模型具有更高的线性表征相似性,并能更好地保留教师的线性可恢复概念。

英文摘要:

For a broad family of discriminative models that includes autoregressive language models, identifiability results imply that if two models induce the same conditional distributions, then their internal representations are equal up to an invertible linear transformation. We ask whether an analogous conclusion holds approximately when the distributions are close instead of equal. Building on the observation of Nielsen et al. (2025) that closeness in KL divergence need not imply high linear representational similarity, we study a distributional distance based on logit differences and show that closeness in this distance does yield linear similarity guarantees. Specifically, we define a representational dissimilarity measure based on the models' identifiability class and prove that it is bounded by the logit distance. We further show that, when model probabilities are bounded away from zero, KL divergence upper-bounds logit distance; yet the resulting bound fails to provide nontrivial control in practice. As a consequence, KL-based distillation can match a teacher's predictions while failing to preserve linear representational properties, such as linear-probe recoverability of human-interpretable concepts. In distillation experiments on synthetic and image datasets, logit-distance distillation yields students with higher linear representational similarity and better preservation of the teacher's linearly recoverable concepts.

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