Transformer在中间层用非对齐方向计算概念,这反而提升了模型稳定性。
Off-Axis, On Purpose: Where a Transformer Computes Concepts and Why it Does So

- 中间层将概念存于与输出轴正交的子空间,避免词汇干扰
- 强制每层对齐输出轴会降低有效维度,但不影响主流评测指标
- 通过特定旋转可稳定实现该几何结构,且不依赖具体旋转方式
Transformer 的最终答案位于一个特定轴上:即其解码权重所读取的方向。中间状态大多不在该轴上,这种偏离通常被视为解释障碍。我们发现这是功能性的。一个12层模型分两阶段运行:第一阶段中,每个子层都将信息写入一个近似正交于输出轴的子空间,注意力值与输出轴夹角在75至96度之间,各深度保持一致。将注意力值映射到输出轴造成的损害是等量随机旋转的64至84倍,且损伤仅来自跨标记混合——该子空间保护了概念组合不受词汇影响。在该子空间之下,坐标系本身随深度刚性旋转。第二阶段答案才最终沿轴到达,且通过累加而非旋转实现。若强制每层对齐输出轴(如早期退出训练),则在困惑度、LAMBADA 和 BLiMP 上表现与基线相当,但将概念阶段的有效维度从约25降至14,而这些基准未检测到变化。几何结构可被强加,但不能直接请求。通过损失函数指定正交投影会导致六组种子崩溃,因模型更倾向于丢弃维度以满足条件。但在阶段边界插入一个固定旋转,可稳定达到基线性能。一种稀疏旋转能被周围权重吸收,九组种子中有七组成功,优于普通训练的五组。旋转的具体形式无关紧要:二十三次实验使用十三种不同旋转均达成相同质量;两个不同种子的基线也保持概念框架正交,同时输出方向一致。这种自由可被利用:预先随机选定的基底,在训练前指定后,仍被整个概念阶段采用,性能不变。
原文摘要 · Abstract (English)
A transformer's answer lives on one axis: the direction its unembedding reads. Its intermediate states largely do not, and that off-axis position is usually treated as an obstacle to interpretation. We show it is functional. A 12-layer model computes in two phases. Through the first, every sublayer writes into a subspace held near-orthogonal to the read-out, attention 75 to 96 degrees off it at every depth. Moving attention's values onto the read-out is 64 to 84 times more damaging than a matched random rotation, and the damage is entirely in cross-token mixing: the subspace insulates composition from the vocabulary. Beneath it the frame itself turns rigidly with depth. In the second phase the answer arrives on-axis, late, and by addition rather than by turning accumulated content onto the read-out. Pressing every layer onto the read-out instead, as training for early exit does, matches the baseline on perplexity, LAMBADA and BLiMP while cutting the concept-phase workspace from about twenty-five effective dimensions to fourteen, a change none of those benchmarks register. The geometry can also be imposed, though not by asking for it. Prescribing it through the loss is a lottery: six of eight seeds collapse, because a model told to null its read-out projection obeys most cheaply by discarding dimensions. Inserting one fixed rotation at the phase boundary lands it instead, at baseline quality. A sparse rotation the surrounding weights can absorb converges on all nine seeds, against five of nine for ordinary training. Which rotation is immaterial: twenty-five runs across thirteen distinct ones reach the same quality, and two baselines from different seeds hold their concepts in near-orthogonal frames while agreeing on their read-outs. That freedom is usable: a basis drawn at random and prescribed before training is adopted across the concept phase, with quality unchanged.
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