REVIEW 3 major objections 3 minor 30 references
Toward Simple and Robust Contrastive Explanations for Image Classification by Leveraging Instance Similarity and Concept Relevance
T0 review · 3 major / 3 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read Contrastive explanations for image classifiers get shorter as concept relevance rises, and their length is stable under a 180-degree rotation but not under a 10-degree rotation or Gaussian noise.
desk verdict The main claim about relevance and explanation length is likely baked into the binning scheme; the robustness comparison is useful but contains a factual misreading of the noise results. 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 mechanism is the instance-similarity foil selection: for each explained image, Algorithm 1 chooses as the contrast the single opposite-class image whose embedding has the maximum cosine similarity, then differences the two images' concept sets. The explanation is the set of unique concepts—those appearing in only one of the two images—rendered as a natural-language sentence of the form 'classified as teapot instead of vase because it contains ... and does not contain ...'. Concept relevance comes from Concept Relevance Propagation, which isolates the neurons associated with a concept through a masked backward pass; the paper then sorts concepts by relevance and cuts them into four quartile ranges based on cumulative relevance. Explanation length, the count of concepts in the unique set, is the paper's operational measure of complexity, and it is the quantity compared across relevance ranges and across original versus augmented images.
What would settle it
Re-run the explanation algorithm on the same test images while varying the foil: use the second-most-similar image, a random image from the contrast class, or an average over several near neighbours, and recompute explanation lengths within the four relevance ranges. If the monotone increase from very strong to very low relevance disappears, reverses, or becomes non-significant under alternative foil selection, then the paper's central length result is an artifact of the single nearest-neighbor pairing rather than a general property of concept relevance.
Extended reading notes
Core claim
On its own terms, the paper discovers a monotone relation between concept relevance and explanation complexity: explanations assembled from very strong concepts are short and focused, while explanations from very low relevance concepts are long and diffuse. For each target image, the method picks the contrast-class image whose embedding is most cosine-similar, extracts per-concept relevance scores for both images with Concept Relevance Propagation, and keeps only the unique concepts that appear in one image but not the other; explanation length is the number of those concepts. Grouping concepts by cumulative relevance quartiles (top 25%, 25–50%, 50–75%, bottom 25%) yields median lengths of roughly 17, 40, 78, and 100, and the ANOVA test confirms the differences are not chance. The robustness finding is nuanced: a 180-degree rotation leaves the top three relevance ranges statistically unchanged (only the very low range shifts, $p = 0.0002$), while a 10-degree rotation shortens explanations in the strong and low ranges, and Gaussian noise significantly lengthens explanations in all four ranges. The paper interprets this as support for $H_1$ and partial support for $H_2$, and frames the noise sensitivity as the model losing focus on a small discriminative concept set.
Load-bearing premise
The load-bearing assumption is that the single opposite-class image the model judges most similar is a representative foil; if a different foil image were chosen, the set of unique concepts and every measured explanation length could change, and with them the R1 and R2 results.
Editorial extensions
If this is right
- Restricting explanations to the top relevance quartile can reduce median explanation length from about 100 concepts to about 17 while still reporting the same contrast.
- Explanation length can serve as a measurable, model-internal proxy for explanation complexity in concept-based methods.
- Robustness evaluation should include small geometric perturbations: a 10-degree rotation changes explanation length in the strong and low relevance ranges, while a 180-degree rotation leaves the top ranges unchanged.
- Gaussian noise should be treated as a serious stress test for concept-based explanation stability, since it significantly lengthens explanations in every relevance range.
Reading between the lines
- An implication the paper leaves implicit is that the entire length distribution rests on the choice of a single foil image; varying the foil selection, such as averaging over several near neighbours, would show whether the monotone trend is a property of concept relevance or an artifact of that one nearest-neighbor pairing.
- The noise result suggests explanation length could double as a detector of model uncertainty or shortcut reliance, but the paper does not test this link.
- The teapot-versus-vase setup could be extended to other visually similar class pairs; if the quartile length ordering replicates, the method would be a general recipe for simple contrastive explanations rather than a dataset-specific observation.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a concept-based contrastive explanation method for image classification that pairs a target instance with the most cosine-similar instance from a contrast class, extracts concept relevance scores via Concept Relevance Propagation (CRP), and reports as an explanation the concepts unique to one of the two compared images. The method is evaluated along two axes: (R1) whether explanation length varies across relevance ranges defined as cumulative-relevance quantiles, and (R2) whether explanation length remains stable under Gaussian noise, 10-degree rotation, and 180-degree rotation. The authors report that higher-relevance concepts yield shorter explanations (H1 supported) and that robustness depends strongly on the augmentation type (H2 partially supported). The central positive claim is, however, undermined by the way relevance ranges are defined, and the discussion of the Gaussian noise result directly contradicts the tabulated and plotted results.
Significance. If the R1 finding were valid, the paper would make a useful empirical contribution to concept-based explainability, showing that relevance strength is a lever on explanation conciseness. The implementation is a concrete instantiation of earlier conceptual work (Finzel et al.), and the choice of a semantically close contrast pair is a reasonable design. However, the main statistical claim is potentially a definitional artifact of cumulative-relevance binning rather than an empirical property of the explanation method. The noise-robustness finding is also misdescribed in the Discussion, and the foil-selection strategy is not stress-tested. The paper's significance therefore depends on a re-analysis that separates bin-size effects from true relevance effects.
major comments (3)
- [Section 4] The primary result H1 is likely an artifact of the binning scheme. Relevance ranges are defined as cumulative-relevance quantiles: 'very strong' is the top 25% of total relevance, 'strong' is the next 25%, and so on. In any skewed relevance distribution, the top 25% of total relevance is concentrated in few concepts while the bottom 25% spans many concepts. Because explanation length is the number of unique concepts produced by Algorithm 1 (lines 16-18), the reported medians of approximately 17, 40, 78, and 100 closely mirror the concept counts per bin rather than any contrastive property of the explanation. The ANOVA F-value of 354.41 only shows that the four constructed groups differ in mean length; it cannot distinguish a relevance effect from a mechanical bin-size effect. The manuscript never reports the number of concepts in each relevance range or the shape of the relevance distribution. To support H1, the authors should either use fixed-size concept bins (e.g., the top 10 concepts vs. the next 10 concepts) or normalize explanation length by the number of concepts in each bin, and show that the monotonic trend survives.
- [Section 5] The Discussion misstates the Gaussian noise result. Section 4 states that 'across all relevance levels, explanation lengths are consistently and significantly lower for noisy images compared to the original ones,' and Figure 6 shows downward shifts in the distributions. Section 5, however, claims that 'Explanation lengths increased significantly across all relevance ranges when noise was added' and interprets this as the model losing focus on a small set of discriminative concepts. Table 1 confirms the Section 4 description with positive t-values (e.g., t=25.7762 for very strong relevance), indicating shorter explanations under noise. This internal contradiction directly affects the interpretation of H2 and the robustness conclusions; the Discussion must be corrected to describe the actual direction of the effect.
- [Algorithm 1] The explanation length and all downstream results depend on the selection of exactly one contrastive image, namely the embedding-nearest neighbor from the opposite class. The algorithm does not vary this selection, compare against random foils, or test sensitivity to the similarity criterion. Because uniqueness is defined against this single foil, the sets of unique concepts and hence all measured lengths in R1 and R2 could change markedly if a different foil image were used. The authors should at least report a sensitivity analysis (e.g., lengths obtained with the second- and third-nearest neighbors, or with a random foil distribution) to establish that the reported trends are not specific to one arbitrarily chosen pairing.
minor comments (3)
- [Table 1] The p-values are reported as '0.0000'; this should read '<0.0001' or similar to avoid implying an exact zero probability.
- [Section 2] There is a spacing typo in the phrase 'whereP is the observed fact' twice; 'whereP' should be 'where P'.
- [Section 4] The statement 'The maximum possible explanation length is 512' is not derived or justified; the maximum depends on the number of extractable concepts in the chosen CRP layer and how uniqueness is counted, so it should be either formally derived or stated as an upper bound with explanation.
Circularity Check
R1's 'higher relevance -> shorter explanations' is largely a definitional consequence of cumulative-relevance binning; H2 robustness results remain empirical.
-
self definitional
[Section 4 (R1 evaluation), relevance-range definition and Fig. 3]
"To compute the ranges, all concepts are first sorted in descending order of relevance for a given prediction, where very strong relevance is the top 25% of total relevance, strong are concepts that lie between 25% and 50%, low are concepts that account for the 50% to 75% and very low is the remaining 25% of the cumulative relevance. ... For very strong relevance, the median explanation length is around 17. ... For very low relevance, we can see that the median value of explanation length is around 100."
The relevance ranges are cumulative-relevance quantiles over concepts sorted in descending relevance. For any non-increasing relevance vector, the number of concepts needed to accumulate each successive quarter of total relevance is non-decreasing, so the 'very strong' bin contains the fewest concepts and the 'very low' bin the most by construction. The R1 outcome, explanation length, is the number of concepts in the explanation for that range (Fig. 3). Thus the reported monotone medians (about 17, 40, 78, 100) mostly mirror the mechanically increasing bin sizes.
full rationale
The circularity is confined to R1/H1. The relevance ranges are not independent treatment groups: they are constructed by taking the top 25% of cumulative relevance, then the next 25%, and so on, so the top range contains few concepts and the bottom range contains many whenever the relevance scores are non-increasing. Explanation length is measured by the number of concepts in the explanation for that range, so the monotonic trend in median lengths is largely a built-in property of the binning scheme. The paper never reports the number of concepts per relevance range or a fixed-size-bin control, and the ANOVA cannot distinguish the mechanical bin-size effect from any genuine relevance effect. This is not a self-citation-chain problem: the definitional reduction is fully visible in Section 4's own definitions, independent of the authors' earlier papers [9,10]. R2/H2, by contrast, is an empirical comparison of original versus augmented explanations using paired t-tests; those results do not reduce by construction and remain self-contained evidence. The foil-selection issue in Algorithm 1 is a representativeness limitation, not circularity, because choosing the nearest neighbor does not by itself force the measured length trend. Overall, one central prediction (H1) is largely definitional while H2 retains independent empirical content, yielding partial circularity.
Assumptions & free parameters
free parameters (3)
- Relevance range boundaries =
25%, 50%, 75% of cumulative relevance
- CRP layer index =
40
- Augmentation parameters =
10 and 180 degrees; Gaussian noise sigma not specified
assumptions (5)
- domain assumption Shorter explanations are preferable for human understanding.
- domain assumption CRP concept relevance scores faithfully reflect the model's decision process and are human-understandable.
- domain assumption The single nearest-neighbor embedding is a representative contrastive foil.
- ad hoc to paper Cumulative relevance quantiles are a valid way to group concepts into relevance levels.
- standard math ANOVA and paired t-test assumptions (normality, independence) hold for explanation lengths.
Cite this review
Pith. "Pith review of Toward Simple and Robust Contrastive Explanations for Image Classification by Leveraging Instance Similarity and Concept Relevance." pith.science (2026). https://pith.science/paper/D5OXZ4RP
@misc{pith2026250623975,
author = {Pith},
title = {Pith review of: Toward Simple and Robust Contrastive Explanations for Image Classification by Leveraging Instance Similarity and Concept Relevance},
year = {2026},
howpublished = {\url{https://pith.science/paper/D5OXZ4RP}},
note = {Machine review of arXiv:2506.23975}
}
read the original abstract
Understanding why a classification model prefers one class over another for an input instance is the challenge of contrastive explanation. This work implements concept-based contrastive explanations for image classification by leveraging the similarity of instance embeddings and relevance of human-understandable concepts used by a fine-tuned deep learning model. Our approach extracts concepts with their relevance score, computes contrasts for similar instances, and evaluates the resulting contrastive explanations based on explanation complexity. Robustness is tested for different image augmentations. Two research questions are addressed: (1) whether explanation complexity varies across different relevance ranges, and (2) whether explanation complexity remains consistent under image augmentations such as rotation and noise. The results confirm that for our experiments higher concept relevance leads to shorter, less complex explanations, while lower relevance results in longer, more diffuse explanations. Additionally, explanations show varying degrees of robustness. The discussion of these findings offers insights into the potential of building more interpretable and robust AI systems.
Figures
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Reference graph
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