A Theory of Conditional Collapse under Low-Rank Weight-Space Ablations: I. The Single-Block Theory and Synthetic Validation
This study investigates the conditions under which activation patching and weight-space ablation yield consistent assessments of the causal influence of model components. By constructing an idealized additive residual stream model and integrating low-rank weight ablation, first-order interaction expansions, and synthetic task validation, the work establishes the first theoretical criterion for their consistency. The analysis reveals fundamental differences in how the two methods affect readout mechanisms, derives precise error expressions and interaction formulas, and empirically validates three core predictions on synthetic tasks: ablation configurations exhibit a strong negative correlation with model accuracy (Spearman ρ = −0.83), and polarity reversal phenomena are reproducible across distinct architectures.