REVIEW 2 major objections 5 minor 33 references
endoExplain: A reproducible protocol for auditing score-localisation discordance in colonoscopy image classifiers
T0 review · 2 major / 5 minor · reviewed 2026-08-02 · deepseek-v4-flash
Pith's one-line read A high classifier score and a plausible heatmap do not by themselves show where a colonoscopy model is looking, and this paper demonstrates that with a reproducible audit protocol.
desk verdict A thorough, honest protocol paper whose central point—high scores don't imply localisation—is well supported; the mask-reference caveat is real but doesn't sink it. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the paired, score-conditioned localisation audit: for each image, a temperature-scaled positive score s_i, a CAM map H_i^m, and a binary lesion mask M_i are locked together, and the primary endpoint is a pointing-game indicator F_i^m(c) = 1(s_i >= c) * 1[argmax H_i^m not in M_i], reported conditionally on s_i >= c. This indicator, plus a separate top-20% overlap measure and a top-20% deletion logit-drop with a random-deletion control, is what carries the argument: it makes calibration, spatial agreement, perturbation response, and transfer separately inspectable on identical image-mask pairs.
What would settle it
A decisive test would be to occlude or inpaint everything outside the lesion mask and measure the target logit on the same 172 high-score images: if the model still assigns scores at or above 0.90 when only the mask contents remain, then the mask is the evidence location and the reported discordance rates are meaningful; if scores collapse when context is removed, then the mask is not the right ground truth and the 4.1-62.2% range is an artifact of the reference, not of the CAMs.
Extended reading notes
Core claim
On the paper's own terms, the central discovery is that score and heatmap are not interchangeable evidence. After fitting a scalar temperature on validation data only, 172 test images had scaled score at least 0.90; on those identical images, Grad-CAM++ failed to peak inside the annotated lesion 62.2% of the time while HiResCAM failed 5.8% and Eigen-CAM 4.1%. The discordance persisted across score thresholds, architectures, and training seeds, although the ranking of methods was not universal. Spatial alignment and deletion response disagreed within the same maps: in the full 1,000-image audit, Eigen-CAM had the highest mean overlap but a negative mean logit drop, while random deletion produ
Load-bearing premise
The protocol's load-bearing premise is that the binary lesion mask is the correct spatial reference for 'where the evidence lies'; the paper itself states in its limitations that the mask does not encode every context feature legitimately used by a classifier, so if context outside the mask carries legitimate evidence, then 'peak-outside-lesion' does not necessarily mean the model is looking at the wrong place.
Editorial extensions
If this is right
- If the central claim is right, a high temperature-scaled score threshold cannot be used to claim the model has found the lesion; 4.1-62.2% of such images have a CAM peak outside the mask depending on the map.
- If the central claim is right, heatmap displays should show the CAM method, score-calibration status, and threshold, because CAM rankings are not universal across architectures, seeds, and datasets.
- If the central claim is right, a positive deletion response is not evidence of attribution specificity, since an exact-area random-deletion control produced a larger mean logit drop than all CAMs in the high-score population.
- If the central claim is right, external deployment of a frozen model requires dataset-specific spatial auditing, since mean overlap changed across the three external mask cohorts and a centre baseline reached similar overlap.
- If the central claim is right, classifier performance claims need source-category composition, because aggregate AUPRC was dominated by dyed-lifted polyps and standard-polyp sensitivity was 0.333 on six test images.
Reading between the lines
- Editorial inference, not in the paper: a natural testable extension would be to repeat the audit on a model trained with context-erased inputs, such as inpainting the lesion, to see whether outside-mask peaks carry causal evidence; the paper's own limitation section says the mask does not encode every legitimate context feature, which is exactly the assumption this test would probe.
- Editorial inference, not in the paper: the protocol could be turned into a runtime monitor for deployed computer-aided detection, flagging frames where score exceeds a threshold but the CAM peak falls outside the polyp boundary, and logging those frames for human review.
- Editorial inference, not in the paper: applying the same paired audit to transformer-based attention maps would reveal whether attention-as-explanation inherits the same method-dependence, since attention is not gradient-weighted the way CAMs are.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper introduces endoExplain, a reproducible protocol for auditing score–localisation discordance in colonoscopy image classifiers. Using content-hash separation of HyperKvasir development and masked images, the authors train three architectures with three seeds each, fit temperature scaling on validation data only, and evaluate five CAM methods plus random and centre baselines on identical image–mask pairs. The main empirical result is that among 172 reserved standard-polyp images with temperature-scaled score ≥0.90, peak-outside-lesion rates range from 4.1% (Eigen-CAM) to 62.2% (Grad-CAM++), while a random-deletion control also produces positive logit drops. The paper reports robustness to thresholds, architecture/seed, lesion size and centrality, and frozen external transfer to three mask cohorts, and it includes a source-category audit showing that 149/155 test positives are dyed-lifted polyps. The authors conclude that classifier score, CAM localisation, and perturbation response are separate constructs, and caution against treating a high score or visually plausible CAM as evidence of lesion localisation or model reasoning.
Significance. If the results hold, the paper makes a valuable methodological contribution: it provides a concrete, code-linked template for separating calibration, spatial alignment, perturbation response, and transfer in explainability audits. The inclusion of content-defined data roles, validation-only model selection and temperature fitting, paired per-image CAM comparison, spatial baselines, random-deletion controls, multiple seeds, and a source-category audit is unusually rigorous for this literature. The finding that method choice changes spatial interpretation on identical inputs — e.g., Grad-CAM failure 50.6% versus HiResCAM 5.8% on the same 172 images — is a strong, falsifiable demonstration that CAMs are not interchangeable. The paper's explicit recognition of its limitations, including mask incompleteness and the dyed-lifted-polyp confound, further supports the credibility of the protocol. The exact discordance rates, however, are conditional on the binary lesion mask as the spatial reference, and this assumption needs additional robustness analysis before the strongest wording is fully supported.
major comments (2)
- [§5.4, Eq. (2), Table 4] The primary endpoint treats the HyperKvasir binary lesion mask as the complete spatial reference, yet the paper itself states in Section 5.4 that the mask does not encode every context feature legitimately used by a classifier. As a result, the headline 4.1–62.2% peak-outside-lesion range is conditional on mask validity. Please add a mask-dilation sensitivity analysis (e.g., 0/5/10/20 pixels) and report whether the peak-outside rates and method ordering change. If the rates are materially affected, the conclusion that 'a high score is not evidence of lesion localisation' should be tempered to reflect that the reference itself is incomplete.
- [§4.1, Table 2, §4.2] The high-score audited pool is distribution-shifted: the model was trained almost entirely on dyed-lifted polyps (702/721 train positives), while the 1,000 spatial-audit images are standard polyps. The test set contains only 6 standard polyps, and the model's sensitivity on them is 0.333. Thus the 172 images with score ≥0.90 are a highly selected subset of standard polyp images, potentially enriched for cases where the model relies on procedural context rather than lesion appearance. Please quantify the sensitivity of the discordance rates to this compositional shift — for example, by reporting score distributions for standard vs. dyed-lifted positives or by repeating the primary analysis on strongly diluted masks — and adjust the generalization claims accordingly.
minor comments (5)
- [§3.3] The tie-breaking rule for constant maps and boundary ties is reported, but the multiplicative permutation used for tied top-area pixels is not specified. Please provide the exact seed and permutation construction in the reproducibility appendix.
- [§4.1, Table 3] Temperature scaling improves ECE and NLL but slightly worsens Brier score. This is correctly reported; it would be helpful to add a sentence explaining that a single scalar calibration can trade off different proper scores, so ECE alone should not be read as evidence of overall improvement.
- [§4.5, Figure 5] For the ETIS cohort, only 16 images are eligible at the 0.90 threshold. The text says the subset is too small for stable rankings, but the figure still shows the full bar set. Please add a visual or textual marker that the ETIS estimates have very wide intervals or omit the conditional bars for ETIS.
- [§5.4] The limitations paragraph notes that patient identifiers were unavailable and image bootstrap intervals may be too narrow. This is appropriately candid, but the paper would be strengthened by also stating explicitly how many images might share a procedure or sequence source, if this is known from the dataset metadata.
- [§2, Related work] Reference [7] (HiResCAM) is a preprint; please add a citation to the peer-reviewed version if one exists, or state that the preprint was the definitive source used.
Circularity Check
No significant circularity: the score-localisation audit uses pre-declared, validation-only fitted calibration and a hash-disjoint held-out spatial pool; no target outcome is fitted and then predicted.
full rationale
The paper's derivation chain is self-contained and non-circular. Training, checkpoint selection (validation AUPRC), and temperature fitting (validation NLL only) are completed before the spatial audit, and the 1,000 masked images were hash-separated before training. The primary outcome, Eq. (2), is a descriptive conditional failure indicator: F_im(c)=1(s_i>=c)1[argmax H_im notin M_i]. The reported 4.1%-62.2% peak-outside-lesion rates are proportions within the 172 images meeting the predeclared threshold; they are not used to fit any parameter, and the temperature parameter is not fitted to the CAM or mask outcomes. The paper explicitly reports that temperature scaling changed test ECE from 0.0167 to 0.0115 and also worsened Brier score, showing the result is not a selected fit. The acknowledged limitation in Section 5.4 that 'The lesion mask is treated as a spatial reference but does not encode every context feature legitimately used by a classifier' concerns the validity of the mask as spatial ground truth, not circularity: no conclusion is derived by assuming the conclusion. The source-category confounding (149/155 test positives dyed-lifted polyps) is likewise an interpretability and generalisability limitation that the paper itself surfaces. No load-bearing self-citation is present; the HiResCAM faithfulness property is cited from external prior work and explicitly hedged as not making a CAM a lesion mask. The random-deletion and centre baselines provide independent controls. Therefore no circular step can be exhibited, and the appropriate score is 0.
Assumptions & free parameters
free parameters (2)
- Temperature scaling T =
0.829
- Primary checkpoint selection =
EfficientNet-B0 seed 2089
assumptions (4)
- domain assumption Content separation by SHA-256 byte identity and exact 256-bit average-hash fingerprint guarantees no training/evaluation leakage into the spatial audit.
- domain assumption The binary lesion mask is an adequate reference for what the classifier should localise; disagreement with the mask is interpreted as score-localisation discordance.
- domain assumption Pointing-game peak, top-20% IoU, and deletion logit-drop are meaningful and separable properties of a CAM.
- ad hoc to paper Combining polyps and dyed-lifted-polyps into a binary polyp_family target yields a meaningful audit construct.
Cite this review
Pith. "Pith review of endoExplain: A reproducible protocol for auditing score-localisation discordance in colonoscopy image classifiers." pith.science (2026). https://pith.science/paper/ZZXKUOYE
@misc{pith2026260719372,
author = {Pith},
title = {Pith review of: endoExplain: A reproducible protocol for auditing score-localisation discordance in colonoscopy image classifiers},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZZXKUOYE}},
note = {Machine review of arXiv:2607.19372}
}
read the original abstract
Background and objective: A high classifier score and a plausible class-activation map (CAM) are often presented together, although neither establishes that the other is reliable. We introduce endoExplain as a reproducible protocol for auditing score-localisation discordance rather than as a new detector or explanation algorithm. Methods: Content hashing separated HyperKvasir development images from 1,000 masked images before training. EfficientNet-B0, ResNet-34 and ConvNeXt-Tiny were trained with three seeds each. Scores were temperature scaled using validation data only. Grad-CAM, Grad-CAM++, XGrad-CAM, HiResCAM and Eigen-CAM were evaluated on identical image-mask pairs, alongside random and centre baselines. Outcomes combined peak localisation, overlap, a top-20% deletion response, score-threshold sensitivity and adjustment for lesion size and centrality. The selected checkpoint was transferred without retraining to three external mask cohorts. Results: Temperature scaling reduced test expected calibration error from 0.0167 to 0.0115. Among 172 reserved images with scaled score at least 0.90, peak-outside-lesion rates ranged from 4.1% to 62.2% across CAMs. Method dependence remained evident across architectures and seeds, although method rankings were not universal. Spatial alignment and deletion response were not interchangeable. A random-deletion control also produced positive logit drops, limiting specificity claims based on deletion alone. External positive-mask results were dataset dependent. A source-category audit also exposed that 149/155 test positives were dyed-lifted polyps, materially bounding classifier claims. Conclusions: endoExplain makes calibration, spatial agreement, perturbation response and transfer separately inspectable. The results caution against using a score or visually persuasive CAM as evidence of lesion localisation or model reasoning.
Figures
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Reference graph
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Reviewed August 2, 2026 · model on record in the stance chip above.
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