Recognition: unknown
Class Unlearning via Depth-Aware Removal of Forget-Specific Directions
Pith reviewed 2026-05-10 11:42 UTC · model grok-4.3
The pith
By projecting out forget-specific directions layer by layer with depth-aware scaling, a closed-form method achieves class unlearning closer to full retraining than prior approaches.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
DAMP computes class prototypes in the input space of each learnable operator, extracts forget directions as residuals relative to retain-class prototypes, and applies projection-based weight updates scaled by a depth-aware rule derived from probe separability, thereby removing targeted class information from internal representations without gradient optimization and producing behavior closer to retraining from scratch.
What carries the argument
Depth-Aware Modulation by Projection (DAMP), a one-shot closed-form procedure that isolates forget directions via residuals to retain prototypes and projects them out of the network weights with layer-specific scaling.
If this is right
- Selective forgetting on forget classes improves while retain-class performance is preserved better than some prior methods.
- Residual forget-class structure detectable in deep representations is reduced.
- The technique applies across convolutional and transformer architectures without modification.
- Multi-class forgetting extends directly via low-rank subspace removal.
- Unlearning completes in one closed-form step with no optimization loop or full retraining data.
Where Pith is reading between the lines
- Forget knowledge appears localized in identifiable directional components of activation space that can be isolated using class prototypes.
- Depth-dependent scaling may prove necessary for other internal model edits beyond unlearning.
- The method implies that intervening at multiple layers is required for genuine representational change rather than output-level suppression alone.
- Similar directional removal could extend to editing other forms of embedded knowledge if suitable prototypes can be estimated.
Load-bearing premise
The forget directions extracted as residuals relative to retain-class prototypes at each layer accurately isolate only the targeted knowledge.
What would settle it
Train a linear probe on activations from deep layers after DAMP application; if the probe still classifies forget-class examples with high accuracy, the claim that forget-specific information has been removed fails.
Figures
read the original abstract
Machine unlearning aims to remove targeted knowledge from a trained model without the cost of retraining from scratch. In class unlearning, however, reducing accuracy on forget classes does not necessarily imply true forgetting: forgotten information can remain encoded in internal representations, and apparent forgetting may arise from classifier-head suppression rather than representational removal. We show that existing class-unlearning methods often exhibit weak or negative selectivity, preserve forget-class structure in deep representations, or rely heavily on final-layer bias shifts. We then introduce DAMP (Depth-Aware Modulation by Projection), a one-shot, closed-form weight-surgery method that removes forget-specific directions from a pretrained network without gradient-based optimization. At each stage, DAMP computes class prototypes in the input space of the next learnable operator, extracts forget directions as residuals relative to retain-class prototypes, and applies a projection-based update to reduce downstream sensitivity to those directions. To preserve utility, DAMP uses a parameter-free depth-aware scaling rule derived from probe separability, applying smaller edits in early layers and larger edits in deeper layers. The method naturally extends to multi-class forgetting through low-rank subspace removal. Across MNIST, CIFAR-10, CIFAR-100, and Tiny ImageNet, and across convolutional and transformer architectures, DAMP more closely resembles the retraining gold standard than some of the prior methods, improving selective forgetting while better preserving retain-class performance and reducing residual forget-class structure in deep layers.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes DAMP (Depth-Aware Modulation by Projection), a one-shot closed-form weight-surgery technique for class unlearning. It first critiques existing methods for weak selectivity, residual forget-class structure in deep layers, and reliance on final-layer bias shifts rather than true representational removal. DAMP extracts forget directions at each layer as residuals relative to retain-class prototypes in the input space to the next operator, projects them out, and applies a parameter-free depth-aware scaling rule (smaller edits early, larger edits deeper) derived from probe separability to preserve utility. The method extends to multi-class forgetting via low-rank subspaces. Experiments across MNIST, CIFAR-10/100, Tiny ImageNet, and both convolutional and transformer architectures claim that DAMP more closely matches the retraining gold standard than prior approaches in selective forgetting, retain-class preservation, and reduction of deep-layer forget structure.
Significance. If the central claims hold, DAMP offers an efficient, optimization-free alternative to retraining or gradient-based unlearning that targets internal representations rather than classifier heads. Its closed-form nature, extension to multi-class cases, and explicit depth-dependent modulation are strengths that could make verifiable class forgetting more practical in computer vision pipelines. The comparative resemblance to retraining on multiple benchmarks and architectures would be a notable advance if supported by rigorous ablations.
major comments (2)
- [Method (DAMP extraction and projection steps)] Method description (DAMP procedure): forget directions are defined as residuals relative to retain-class prototypes (class means) at each layer. This first-order statistic may fail to isolate targeted knowledge when class-conditional distributions exhibit substantial intra-class variance or multi-modality, and sequential projections alter the input distribution to later layers, so directions computed on the original model may misalign after earlier edits. This assumption is load-bearing for the claim of true representational removal closer to retraining.
- [Depth-aware scaling rule (abstract and method)] Depth-aware scaling rule: presented as parameter-free, yet derived from probe separability measurements on the data. This introduces a data-dependent step that reduces the independence of the final performance claims and risks new failure modes on retain classes if separability does not track actual utility loss.
minor comments (2)
- [Abstract] Abstract: the phrase 'more closely resembles the retraining gold standard than some of the prior methods' is vague; explicit naming of the strongest baselines and quantitative deltas would improve clarity.
- [Method] The manuscript would benefit from an explicit statement of the precise mathematical form of the projection update and the separability-based scaling formula (including any constants or thresholds) to allow direct reproduction.
Simulated Author's Rebuttal
Thank you for the detailed and constructive review. We address each major comment point by point below, indicating where revisions will be made to improve clarity and rigor.
read point-by-point responses
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Referee: Method description (DAMP procedure): forget directions are defined as residuals relative to retain-class prototypes (class means) at each layer. This first-order statistic may fail to isolate targeted knowledge when class-conditional distributions exhibit substantial intra-class variance or multi-modality, and sequential projections alter the input distribution to later layers, so directions computed on the original model may misalign after earlier edits. This assumption is load-bearing for the claim of true representational removal closer to retraining.
Authors: We thank the referee for identifying these methodological assumptions. The use of class prototypes enables the closed-form extraction of forget-specific directions without optimization, and our experiments show consistent reduction of forget-class structure in deep layers across datasets with varying degrees of intra-class variation. Nevertheless, we agree that first-order statistics may be insufficient for highly multi-modal distributions and that sequential application of projections could introduce misalignment. In the revised manuscript we will (i) add an explicit limitations paragraph discussing these cases and (ii) include a new ablation that recomputes directions after each layer edit versus the current one-shot computation, quantifying any difference in forgetting and utility metrics. These additions will better support the claim of representational removal. revision: yes
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Referee: Depth-aware scaling rule: presented as parameter-free, yet derived from probe separability measurements on the data. This introduces a data-dependent step that reduces the independence of the final performance claims and risks new failure modes on retain classes if separability does not track actual utility loss.
Authors: We agree that the terminology 'parameter-free' requires clarification. The scaling factors are automatically derived from separability probes rather than being manually tuned, but they do rely on data measurements. In the revision we will update the abstract and method section to describe the rule as 'hyperparameter-free yet data-informed.' We will also add analysis showing the observed correlation between probe separability and retain-class utility across the reported benchmarks, together with a brief discussion of potential edge cases where this correlation might weaken. These changes will make the data dependence transparent without altering the method's practical advantages. revision: yes
Circularity Check
No circularity in derivation chain
full rationale
The paper introduces DAMP as a closed-form, one-shot projection method for class unlearning. Its steps—prototype computation, residual direction extraction, and depth-aware scaling from probe separability—are presented as algorithmic choices rather than a mathematical derivation or prediction that reduces to the inputs by construction. Performance claims are empirical comparisons to retraining on held-out benchmarks (MNIST, CIFAR, etc.), with no self-citation load-bearing the core argument, no fitted parameter renamed as a prediction, and no ansatz or uniqueness theorem invoked circularly. The scaling rule, while data-informed, is an explicit design choice for utility preservation and does not force the reported outcomes tautologically.
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Forget-specific directions can be isolated as residuals between forget-class and retain-class prototypes computed at the input to each learnable operator.
- domain assumption Probe separability at each depth provides a reliable signal for scaling the magnitude of the edit without harming retain-class performance.
Forward citations
Cited by 1 Pith paper
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Classification-Head Bias in Class-Level Machine Unlearning: Diagnosis, Mitigation, and Evaluation
Class-level unlearning shortcuts via bias suppression in the classification head; new bias-aware training mechanisms and bias-specific metrics are introduced to diagnose and reduce this dependence.
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