K/V-Cache Interventions Dissociate Representation Alignment from Persona Expression in Decoder-Only Language Models
K/V-cache interventions transplant target-conditioned key/value trajectories into source-persona generation, enabling structured persona control in decoder-only language models Strong representation-level alignment (V-gap 0.84-0.94) does not guarantee behavioral expression; only mid-layer replacement (layers 9-20) achieves both target-marker expression and preserved lexical diversity Position perturbations (lag and shuffle) uniformly suppress target-persona expression, revealing a common behavio
Analysis
TL;DR
- K/V-cache interventions transplant target-conditioned key/value trajectories into source-persona generation, enabling structured persona control in decoder-only language models
- Strong representation-level alignment (V-gap 0.84-0.94) does not guarantee behavioral expression; only mid-layer replacement (layers 9-20) achieves both target-marker expression and preserved lexical diversity
- Position perturbations (lag and shuffle) uniformly suppress target-persona expression, revealing a common behavioral failure mode rather than a strict dissociation
- Representation similarity metrics alone are insufficient predictors of downstream persona expression; the K/V cache is a controllable but structurally constrained intervention surface
- The intervention is characterized as trajectory-level transplantation rather than isolated persona-representation injection, with controls confirming representational shifts are not solely explained by imported token history
Why It Matters
This research directly addresses a critical challenge in AI safety and controllability: how to precisely manipulate model behavior without degrading output quality. For practitioners building persona-driven or role-playing systems, understanding that representation alignment and behavioral expression can dissociate means that standard interpretability metrics may be misleading when evaluating intervention strategies. The findings also have implications for alignment research, as they demonstrate that intermediate layers (particularly mid-layers) play a disproportionately important role in persona expression.
Technical Details
- Intervention method: K/V-cache transplantation where target-conditioned key/value trajectories are inserted into source-persona generation across 13 configurations on Llama-3.1-8B
- Layer-band analysis: Early, mid (layers 9-20), and late layer replacements all achieved strong V-space alignment (V-gap: 0.91, 0.89, 0.84 respectively), but only mid-layer replacement combined substantial target-marker expression with preserved lexical diversity (TTR 0.77 vs. 0.65 for full replacement)
- Position perturbation experiments: Both lag and shuffle perturbations uniformly suppressed target-persona expression, indicating structural constraints in the intervention surface
- Control experiment: Same-token-sequence decoding under source vs. target conditioning reproduced the sign and layer localization of the L28 representational shift, confirming the shift is not explained solely by imported token history
- Key metrics: V-gap (representation alignment), TTR (type-token ratio for lexical diversity), L28 representational shift analysis
Industry Insight
- Practitioners should not rely solely on representation-level similarity metrics when designing persona control or alignment interventions; behavioral outcomes may diverge significantly from representational alignment scores
- Mid-layer K/V cache interventions appear to be the optimal sweet spot for persona expression, suggesting that interpretability efforts targeting these layers could yield the highest return for behavior manipulation
- The structural constraints revealed by position perturbations indicate that trajectory coherence matters as much as layer selection, pointing toward the need for temporally-aware intervention strategies rather than static cache replacement approaches
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