Where instruction tuning landed in this pair — a propagation scan (N80 21/33, band 16–20)

#412
by tetracta - opened

We ran a before/after internal scan of meta-llama/Llama-3.1-8Bmeta-llama/Llama-3.1-8B-Instruct with a fixed probe protocol (identical prompts for both models, controlled perturbations injected at seven depths, 168 matched probes per model) and thought the result was worth putting here rather than only on our own page.

Where the difference sits. 80% of the base→instruct difference mass falls in 21 of 33 stations (a station is the output of one block; 32 layers → 33 stations). The densest five-station band is 16–20, carrying 1.54× what a uniform spread would put there and 1.36× what a flat null would. The first observable difference is station 4 — that is our probe grid's detection floor (the earliest probe is injected at layer 2), not something the tuning earned.

This one surprised us. That band is mid-network. In the same run Qwen2.5, Falcon3, OLMo-2 and Mistral all concentrated in their last five stations, so a single pair would have told us "instruction tuning is a last-layers story" — and it is not, at least not for this family.

Knowledge side. Factual recall barely moved: 0 of the 20 probe facts broke and 2 were repaired. What moved is behaviour on entities the model does not know — fake-name echo avoided went 16 → 19 of 20 (McNemar 3 improved / 0 regressed, exact two-sided p = 0.250), and a three-class judge reads refusals 0→8, echo 4→1, fabricated answers 16→11. Trajectory AUROC 0.040 → 0.983.

Data. Report: https://tetracta-model-xray-sample-reports.static.hf.space/reports/karsilastirma-llama-3-1-8b-llama-3-1-8b-instruct.html
Signed provenance/deletion attestation (its report_sha256 pins that exact file): https://www.tetracta.ai/llm_tomografi/attest/c2c7f3600f5140c895a19c1ec566175c
Machine-readable per-station profiles for this and 21 other interventions across six families: https://huggingface.co/datasets/tetracta/model-xray-gallery

If this does not match how the instruction tuning was actually done, we would genuinely like to know. The instrument is young, family differences like this are exactly what we cannot yet explain, and a correction from the people who trained the model is worth more to us than another scan.

Correction — 6 September 2026

We withdraw the Model X-Ray location, spread, concentration and derived-severity conclusions in the opening post. Any legacy knowledge-separation, portrait-visualization, lesion-response or simulated-quantization conclusion linked from it is also withdrawn and must not be used as current evidence.

We are publishing no replacement figures. Validation remains pending. Correction record: https://www.tetracta.ai/model-xray/correction/

— Tetracta

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