Steering Block / Layer Selection
The point
Where and how many sites you intervene on changes both the effective strength of the intervention and whether a localization claim is testable.
- Too few (single site on a deep stack) → downstream norm/attention denoise it → looks like "no effect".
- Too many (whole stack) → off-distribution collapse → the effect is collapse-driven and no longer localized.
Choosing where to intervene
Pick the site(s) by a screening method, by heuristic, or both.
By method (pre-screen the layers):
- Gradient-based — rank layers by gradient / attribution score w.r.t. the target.
- Activation-based — rank by an activation signal (probe accuracy, diff-mean magnitude, separability).
By heuristic:
- Mid-to-late layers usually work best — start there.
- If a single layer shows no effect, intervene on 3–5 layers instead.
- To sweep the whole stack, use spaced intervals (e.g. every 2–3 layers) rather than every layer.
What to intervene on (transformer)
Target the attention, the MLP, or the residual stream — choose by what the claim is about.
Note: circuit discovery generally studies attention heads + MLP.
Two rules to keep it honest
- Match the claim. If the claim is about a region (e.g. "early layers 0–7"), intervene on a window covering that region and use matched windows elsewhere as null controls — a single site can't adjudicate a regional claim. Report the localization you actually observe; don't rewrite the claim to fit the data.
- Never copy a raw index across models of different depth. "Layer 16 of 32" → scale by relative depth (0.5 → layer 24 of 48), then check neighbors.
Composition
Lock the site set here first (the coefficient plateau is site-dependent), then sweep α via ../steering-coefficient-tuning/. Re-sweep the coefficient whenever the site set changes.