SRT-NLA Activation Verbalizer — gemma-4-31B-it (L47)

Read the hidden states of a frozen gemma-4-31B-it in natural language — whether the state came from text or from an image.

These checkpoints are the generative component of the gemma-4 SRT verbalization stack. Inject a layer-47 activation vector as a soft prefix and the frozen backbone decodes candidate text for it. Layer 47 is the backbone's cross-modal alignment peak: image soft-tokens and word tokens share one interpretant space there, so the same machinery reads visual states and text states alike.

What the stack does (zero training on any of it)

• Image to caption. An image's mean L47 state retrieves on-topic full-sentence captions from an open pool of 10,000 COCO captions: 5/5 CIFAR natural images at rank 1 (horse → "an old black and white photo with a person riding a horse", 0.680; dog 0.651; ship 0.631). Live in the SRT-Sunstone demo.
• Reports the texture that is present. Given a random-dot autostereogram whose figure exists only in binocular disparity, the stack retrieves "An abstract mosaic of tiny colored squares", a faithful sentence-level report of what a flat encoder actually receives (paper §11.6.2–.3).
• Reads text states at ceiling. Centered replay 0.994 inside a fully calibrated anchor frame (NN retrieval 0.695, random floor 0.494), so every reported number is interpretable.
• Generates verbalization candidates. Sampled best-of-K from these checkpoints climbs monotonically with K (0.51 → 0.59 centered fve at K=32); the target vector is available at inference, so oracle scoring of candidates is free.

Companion artifacts (L47 retrieval indexes for captions and text, anchors, eval results, source pools): RiverRider/srt-nla-gemma4-artifacts. Program: github.com/space-bacon/SRT (paper_nla.md §11.6–§11.7).

Which decode to use

This mirrors the program-wide finding: on every backbone studied, the deployable decode is retrieval or best-of-K, never argmax.

Files

Measurement notes (reported openly; they shape the decode table)

• Argmax decoding mode-collapses on this chat-tuned host even though the injected vector demonstrably carries the content — it halves the gold text's cross-entropy (8.70 → 4.13 nats/token). The information is in the distribution; sampling surfaces it, argmax does not.
• The best-of-K slope (+0.017/doubling) matches gpt-oss-20b and is half of base-model Qwen2.5-7B's, supporting a base-vs-instruction-tuned verbalization hypothesis (testable on the gemma-4 base checkpoint).
• In-context drafts do not help decoding: an activation-space neighbour's text adds ~0.03 nats of predictive value for the gold text. Activation similarity is not token-space predictive utility.

Reproduction protocol

• Targets must be encoded corpus text (scripts/sample_targets.py --corpus); chat-tuned gemma-4 degenerates under bare-BOS self-sampling.
• The backbone is BOS-sensitive: prefix-free re-encodes must prepend BOS (centered replay 0.615 without, 0.9986 with).
• Load with ActivationVerbalizer (srt/nla/), num_prefix_tokens=16, extraction_layer=47, backbone via srt.nla.load_frozen_backbone (Gemma4ForConditionalGeneration; AutoModelForCausalLM silently loads random weights for this architecture).