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A guideline-grounded global evaluation shows that most gradient-based explanation methods transferred from computer vision systematically follow ECG signal amplitude rather than clinically diagnostic regions, and that sign-adjusted variants

Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →

T0 review · deepseek-v4-flash

2026-07-31 23:13 UTC pith:QNPYXIDZ

load-bearing objection A transparent, reusable benchmark that convincingly demonstrates amplitude bias in gradient-based ECG attributions; the abstract just overstates the faithfulness claim. the 2 major comments →

arxiv 2607.24035 v1 pith:QNPYXIDZ submitted 2026-07-27 cs.CY cs.LG

Beyond Local Inspection: Global, Guideline-Grounded Evaluation of Post-hoc XAI Methods for ECG Classification

classification cs.CY cs.LG
keywords explainable AIECG classificationpost-hoc attributionclinical plausibilityamplitude biasguideline-based evaluationnormalized coveragetime-series XAI
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper tries to establish that popular gradient-based post-hoc explanation methods, when applied to ECG classifiers without domain adaptation, are systematically driven by signal amplitude rather than by clinically diagnostic patterns, and that this failure is invisible in single-sample heatmaps. To show this, it aggregates explanations across thousands of heartbeats and compares their coverage of cardiologist-defined diagnostic regions (PR interval, ST segment, QRS morphology) on four conditions spanning low- and high-amplitude criteria. The evidence: input-multiplied methods such as Integrated Gradients and LRP-ε reach Spearman correlations with absolute signal amplitude up to 0.69 and assign as little as 4.6% of relevance to the ischemia-defining ST segment; nine of the thirteen methods score below chance on at least one condition. Sign-adjusted variants, which multiply gradients by only the sign of the input, remain above chance on all four pathologies, with LRP-ε/SIGN reaching the highest worst-case normalized coverage (41.5%). A sympathetic reader cares because choosing an explanation method for clinical AI is currently guided by convention and visual plausibility, and the paper offers a quantitative, guideline-grounded way to detect systematic attribution failures before deployment.

Core claim

The paper claims that ECG explanation quality should be judged globally, against clinical guidelines, and that on this test most gradient-based attribution methods fail. Comparing 13 methods on the same four trained CNNs and the same true-positive predictions, it shows that input-multiplied and baseline-subtracted methods (Gradient×Input, Integrated Gradients, DeepSHAP, GradSHAP, LRP-ε, SmoothGrad×Input) concentrate relevance on the high-amplitude R-peak and T-wave regardless of pathology, while sign-adjusted versions keep relevance on the low-amplitude PR interval and ST segment that define AV block and ischemia. Nine of thirteen methods fall below a random baseline on at least one conditio

What carries the argument

The framework's central mechanism is a five-step global aggregation pipeline that turns local attribution maps into population-level relevance histograms over R-peak-aligned 500-timestep heartbeats, then scores them against guideline-derived binary reference masks (PR interval for AV block, ST segment for ischemia, QRS complex for both bundle branch blocks). Two metrics carry the argument: normalized coverage (NCov), a skill score expressing how much of the positive relevance falls in the masked region relative to a random baseline, and its minimum across pathologies (NCovmin), which ranks methods by their weakest condition rather than their average. A Spearman correlation between absolute r

Load-bearing premise

The central load-bearing premise is that the cardiologist-derived reference masks are the correct target and that the predictively valid CNNs, applied only to true-positive cases, act as a pattern proxy — meaning coverage scores plausibility, not faithfulness, and the paper explicitly states it did not test the method-model linkage directly.

What would settle it

Train an ischemia classifier that is deliberately prevented from using the ST segment (e.g., by masking that region during training) yet still reaches high accuracy through other features, and run LRP-ε/SIGN through the pipeline. If it still shows high normalized coverage of the ST mask, then the coverage score reflects the method's bias toward the mask rather than clinically meaningful model support, and the ranking among methods would not be evidence of clinical plausibility.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

  • Global, guideline-grounded evaluation can expose systematic attribution failures that local heatmap inspection cannot, so studies relying on visual plausibility alone are insufficient evidence for clinical trust.
  • Amplitude-multiplied and baseline-subtracted methods (Integrated Gradients, DeepSHAP, LRP-ε, SmoothGrad×Input) should not be assumed reliable for low-amplitude ECG criteria; their high average coverage is driven by bundle branch block cases where diagnostic regions coincide with high-amplitude waveforms.
  • Methods should be selected by worst-case normalized coverage across conditions rather than mean coverage, because a method is deployed once and then meets whichever pathology appears.
  • The SIGN adjustment restores above-chance attribution on all four pathologies for all three SIGN variants, with LRP-ε/SIGN the best; this positions sign-adjusted gradients as a low-cost default for time-series XAI.
  • The framework is method- and model-agnostic, so the same mask-and-coverage pipeline can audit different models and architectures, or be extended to other physiological time series with guideline-defined regions.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The amplitude-bias mechanism likely generalizes beyond ECG: any physiological time series whose diagnostic features sit in low-amplitude segments (e.g., EEG event-related potentials or PPG dicrotic notch timing) would be vulnerable to the same input-multiplication bias, and sign-adjusted gradients are a natural first test.
  • Because the paper uses the trained model as a fixed pattern proxy and does not test faithfulness, a high-coverage method could still be exploiting a shortcut shared with the mask; combining this framework with model-randomization or feature-deletion tests would separate method bias from model behavior.
  • A clinician-facing implication left implicit: presenting global guideline-coverage scores alongside local heatmaps would give cardiologists a decision-relevant summary of whether an explanation method tracks the region they actually read; structured clinician studies could test whether coverage scores change trust or diagnostic agreement.
  • The mask-based metric is blind to morphological shape within the QRS complex, as the paper notes; extending it to concept-level attribution (e.g., concept relevance propagation) would let coverage register whether relevance follows the notched or slurred morphology that distinguishes the bundle branch blocks.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Referee Report

2 major / 4 minor

Summary. The paper proposes a global, guideline-grounded evaluation framework for post-hoc XAI methods in ECG classification. Using four binary CNN classifiers trained on PTB-XL, the authors apply 13 gradient-based attribution methods to true-positive test predictions, extract and align beats, and aggregate relevance across beats and leads. They compare these aggregated attributions against guideline-derived reference masks (PR interval for AV block, ST segment for ischemia, QRS complex for RBBB/LBBB) using coverage (Eq. 4), normalized coverage relative to a random baseline (Eq. 5), and worst-case normalized coverage over pathologies (Eq. 6). They also report Spearman correlations between absolute relevance and absolute input amplitude. The main results are that input-multiplied or baseline-dependent methods (Integrated Gradients, LRP-ε, DeepSHAP, SmoothGrad×Input) strongly correlate with signal amplitude (mean SCC up to 0.69) and have negative normalized coverage on low-amplitude diagnostic regions (e.g., LRP-ε: 4.6% coverage on the ischemia ST segment), while SIGN-adjusted variants maintain positive worst-case coverage across all four pathologies. The paper concludes that methods transferred from computer vision systematically overlook diagnostically decisive low-amplitude regions and recommends SIGN-based adaptations.

Significance. The framework is a genuinely useful contribution to time-series XAI evaluation: it is open-source, reusable, uses expert-defined reference masks, aggregates over many heartbeats to reveal systematic patterns, and reports worst-case reliability rather than only averages. The bootstrap confidence intervals and mask-boundary sensitivity analysis strengthen the internal validity of the comparisons. If the central assumption holds—that the trained CNNs rely on the guideline-defined regions when making true-positive predictions—the finding that many popular attribution methods are amplitude-dominated in ECG would be clinically important and would motivate domain-adapted XAI. The authors are transparent that they measure clinical plausibility (guideline agreement), not faithfulness, and they acknowledge this in the Methods and Limitations. The significance is therefore conditional on the pattern-proxy assumption, which is plausible but not directly tested in this manuscript.

major comments (2)
  1. [Abstract and Conclusion] The abstract and conclusion use causal language that goes beyond the measured construct. The abstract states that methods "overlook diagnostically decisive low-amplitude regions" and concludes a "systematic failure" of CV-transferred methods. However, the Methods section ("Clinical Plausibility vs. Faithfulness") explicitly states: "we did not test that linkage directly" and that coverage measures clinical plausibility, not faithfulness. A method can have low ST-segment coverage either because it fails to expose the model's reasoning or because the model itself does not rely on the ST segment. The same-model argument (LRP-SIGN 63.8% vs LRP-ε 4.6% on ISCH) only proves that at least one of the two is not perfectly faithful; it does not identify which. The abstract and conclusion should be rephrased to say that these methods show "low agreement with guideline-defined diagnostic regions" or
  2. [Methods — Clinical Plausibility vs. Faithfulness; Discussion] The pattern-proxy assumption is load-bearing but untested. The paper relies on three indirect justifications: (i) the models are predictively valid (AUROC 0.93–0.99), (ii) the analysis is confined to true positives, and (iii) prior lead-level evidence on the same architecture/pathologies. These do not establish that the CNNs use the guideline-defined temporal regions. The statement that "one model cannot account for both" (LRP-ε/SIGN vs LRP-ε) is insufficient: two unfaithful methods can disagree arbitrarily on a fixed model. If the CNNs actually rely on R-peak/T-wave amplitude or other shortcuts, then LRP-ε could be more faithful than SIGN despite lower coverage. The paper should either add a targeted test (e.g., perturbing/ablating the guideline region and measuring the model's output change) or explicitly frame the results as "guideline agreement," not as evidence about the model's dec
minor comments (4)
  1. [Eq. (3)] The definition of Spearman's correlation is unnecessarily convoluted. A standard formulation using ranks would be clearer and shorter.
  2. [Methods — Pattern Analysis] The threshold of 0.05 for discarding positive relevance values is a free parameter. The paper does not report sensitivity to this threshold. Since the results depend on it, a small sensitivity analysis (e.g., thresholds 0.01, 0.1) would be reassuring.
  3. [Coverage metric / Limitations] The coverage metric pools relevance across all 12 leads and applies the same temporal mask to every lead. As the authors note, relevance placed in the correct temporal region but in a clinically uninformative lead is still credited. This is acknowledged, but the abstract's phrase "diagnostically decisive regions" could be more precise: the masks are temporal, not lead-specific, and the paper evaluates "temporal agreement pooled over leads."
  4. [Figures 1–2] The figures are dense and difficult to read in the manuscript layout. The 16-panel histograms in Figure 2 would benefit from larger panels or a zoomed-in view for at least one representative lead.

Circularity Check

0 steps flagged

No derivation reduces to its inputs; disclosed self-citations and an explicitly untested pattern-proxy assumption are the only caveats, not circular reductions.

full rationale

The paper's central results are empirical: four CNNs trained on PTB-XL, 13 attribution methods, and coverage/normalized-coverage/worst-case metrics (Eqs. 4-6) computed from relevance maps and guideline masks. No parameter is fitted to the target outcome and no method's score is an identity with its definition; the high SCC of input-multiplied methods is acknowledged as partly true by construction and the paper rests its conclusion on the coverage divergence instead. The SIGN recommendation is based on the same open pipeline as all methods and the self-origin is disclosed in Competing Interests. The main interpretive step is the pattern-proxy assumption, for which the paper states 'we did not test that linkage directly' and cites the authors' own prior work for lead-level agreement; this is an explicit validity limitation and a mild self-citation dependency, but it does not make the numerical comparison reduce to the guideline masks or to the authors' prior conclusions. The framework is self-contained against external data and clinical guidelines, so no circular step is identified. Score 2 reflects the minor self-citation/pattern-proxy caveat rather than any derivation-level circularity.

Axiom & Free-Parameter Ledger

3 free parameters · 5 axioms · 0 invented entities

The central evaluation depends on expert-defined temporal masks and the assumption that a predictively valid model's true-positive explanations should align with guideline regions. These are domain assumptions the authors disclose; the main hand-chosen numeric inputs are the mask boundaries and the 0.05 relevance threshold, tested only partially (mask shifts) or not at all (threshold).

free parameters (3)
  • Reference mask temporal boundaries = Clinical segments in standardized 500-step beat (exact boundaries not given in paper)
    Central to the coverage metric; hand-set temporal segments defined by two cardiologists and verified against median beats; sensitivity analysis around ±20 steps does not eliminate dependence on the central choice.
  • Positive-relevance filtering threshold = 0.05
    Normalized positive relevance below 0.05 is discarded; affects all coverage and histogram results; no sensitivity analysis for this threshold is reported.
  • Number of extracted beats per recording = 2
    Fixed sampling-density choice for the aggregation pipeline; reasonable, but a free protocol choice.
axioms (5)
  • domain assumption Guideline-defined diagnostic regions are an appropriate ground truth for explanation quality.
    Reference Patterns: the published guidelines (29-31) are treated as the gold standard for what a trustworthy explanation should emphasize.
  • domain assumption Predictively valid CNNs on true-positive predictions are a pattern proxy; method rankings on a fixed model are meaningful.
    Methods, Clinical Plausibility vs. Faithfulness: all methods share the same fixed model per pathology, and coverage differences are attributed to the methods.
  • domain assumption Only positive relevance supports the predicted class; negative relevance carries no guideline support.
    Pattern Analysis: 'Restricting the analysis to positive relevance allowed us to isolate features supporting the predicted class'; this may under-credit methods that encode evidence in negative relevance.
  • domain assumption Temporal-only masks applied across all leads sufficiently represent diagnostic criteria.
    Limitations: masks encode only the temporal component of the criteria and apply across all leads; lead focus is only visible in histograms, not in the coverage metric.
  • standard math Spearman rank correlation is the appropriate measure of amplitude dependence.
    Equation (3); a standard non-parametric measure of monotone dependence, appropriate for the stated goal but not a causal measure.

pith-pipeline@v1.3.0-alltime-deepseek · 21141 in / 17033 out tokens · 151568 ms · 2026-07-31T23:13:01.205669+00:00 · methodology

0 comments
read the original abstract

Explainable AI (XAI) is used to assess whether artificial intelligence models rely on meaningful patterns, yet explanations that appear plausible for individual predictions may systematically misrepresent model behavior. This is particularly problematic in medicine, where models may rely on irrelevant signal characteristics rather than disease-specific patterns without being recognizable. We address this challenge using electrocardiogram (ECG) data, for which clinical guidelines provide explicit knowledge about diagnostically relevant signal regions. We introduce a global, guideline-grounded framework that aggregates explanations across heartbeats to evaluate them against clinically defined regions of interest. Using four binary classifiers trained on PTB-XL, we assess 13 gradient-based methods across two categories of patterns: low-amplitude segments and high-amplitude QRS morphology. Our results reveal a systematic failure of methods transferred from computer vision. Their explanations often follow signal amplitude rather than clinical relevance, with mean Spearman correlations up to 0.69, leading them to overlook diagnostically decisive low-amplitude regions. For ischemia, LRP-$\epsilon$ assigns only 4.6% of relevance to the ST segment, compared with 63.8% for LRP-SIGN. Nine of 13 methods fall below chance for at least one condition, indicating inconsistent reliability across patterns. These findings show that global, domain-grounded evaluation can uncover systematic explanation failures not obvious from sample-level heatmaps.

Figures

Figures reproduced from arXiv: 2607.24035 by Birgit A{\ss}mus, Jennifer Hannig, Michael Guckert, Nils Gumpfer, Samuel Sossalla.

Figure 1
Figure 1. Figure 1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Guideline-informed reference masks and the global attribution patterns evaluated against them. Panel a marks the regions that diagnostic guidelines29–31 designate for each pathology, namely PR-interval prolongation for AV-block, ST-segment deviation for ischemia, and QRS morphology in V1-V2 and V5-V6 for the bundle branch blocks. The masks encode only the temporal extent of these criteria and apply across … view at source ↗

discussion (0)

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