REVIEW 4 major objections 6 minor 20 references
The paper claims that SQS, a signed and shrunk quadratic activation for gated linear units, preserves the bilinear weight spectrum that makes eigenvector interpretability possible while matching state-of-the-art performance.
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 →
SQS is a new GLU activation that reportedly preserves bilinear weight spectra for interpretability while achieving competitive performance on MNIST, FMNIST, and Tiny Stories.
T0 review reviewed 2026-08-05 challenge →
load-bearing objection SQS is a new, plausibly competitive activation, but the paper never defines the matrix it eigen-decomposes, so the central interpretability claim is unsupported. the 4 major comments →
Preserving Bilinear Weight Spectra with a Signed and Shrunk Quadratic Activation Function
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
Core claim
On the paper's terms: SQS is defined as σ(x) = (x/|x|)·(|x| − c)/(1 + (λ|x|)^p)^(1/p), which for p = 1 becomes (x − c·sgn(x))/(1 + λ|x|). This replaces the identity gate of a bilinear GLU with a signed, shifted, shrinking quadratic that is quasi-linear for very small and very large inputs. The discovery claim is that the bilinear layer's interpretability mechanism—each output logit computed as x^T A x and decomposable into weighted eigen-directions—survives inside this nonlinear gate, so the trained model's weights can be read as class-specific eigenfeatures. The experimental evidence is that SQS-GLU eigenvectors are highly similar to those of a bilinear MLP (cosine similarity above 0.5, nea
What carries the argument
The central object is the signed, shifted, shrunk quadratic activation SQS. With p = 1 it reduces to σ(x) = (x − c·sgn(x))/(1 + λ|x|): quasi-linear for very small and very large inputs, approximately quadratic in between while retaining sign. Inside a GLU, (Wx)⊙σ(Vx), it keeps each logit close to a sum of squared projections along eigenvectors, so an interaction matrix A extracted from the trained weights retains the spectral structure of a bilinear layer. The shrink term bounds large activations and the signed shift avoids the flat gradients of a pure quadratic; hyperparameters c and λ control the shift and shrinkage, and the paper sets λ = 0.5, c = 0.01, p = 1 for its experiments.
Load-bearing premise
The load-bearing premise is that a usable, meaningful interaction matrix can be extracted from a trained SQS-GLU by treating it like a bilinear layer, even though the SQS gate is nonlinear and input-dependent; the paper does not specify how this extraction is done.
What would settle it
Run an SQS-GLU and a bilinear MLP on the same data, extract each logit's interaction matrix, then rank each logit by how much dropping its least-important eigenvectors changes classification decisions; if SQS-GLU's top eigenvectors do not carry the class information, the weight-spectra interpretability claim is refuted. Alternatively, if SQS-to-bilinear eigenvector cosine similarity drops consistently below about 0.5 across many seeds, the claim that SQS preserves bilinear spectra fails.
If this is right
- Weight-based mechanistic interpretability becomes available in a performant GLU, without the activation-space machinery of sparse-autoencoder analyses.
- SQS-GLU reaches competitive loss and perplexity on language modeling with TinyStories, offering a drop-in transformer activation that keeps training speed comparable to ReLU, GELU, and SwiGLU.
- On vision benchmarks, SQS converges faster than ReLU-GLU and bilinear MLPs, being first to reach 80%, 85%, and 90% accuracy.
- The eigenvectors of an SQS-GLU can be used to explain individual classification decisions, inheriting the bilinear layer's decomposition into ranked eigen-directions.
Where Pith is reading between the lines
- A formal, published definition of how the interaction matrix is extracted from a trained SQS-GLU would let others verify the eigenvector-similarity result without reimplementing the paper's undocumented procedure in Section 4.1.
- Because SQS's shrink term bounds large activations, it likely acts as an implicit regularizer; one testable extension is measuring weight spectra and generalization under lower weight decay to see whether λ can substitute for part of explicit regularization.
- The p parameter, kept at 1 for speed, could interpolate between a smooth quadratic and a near-linear gate, potentially exposing a dataset-specific interpretability–performance trade-off curve.
- A testable extension is applying SQS to transformer MLP blocks and checking whether the eigenvectors of the combined weight matrix correspond to interpretable features such as syntactic roles in generated stories.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes Signed Quadratic Shrink (SQS), an activation function intended for use inside Gated Linear Units (GLUs). The central claim is that SQS-GLUs achieve performance competitive with standard activation functions such as SwiGLU and GELU while preserving the weight-based interpretability properties of bilinear MLPs, whose logits can be written as x^T A_a x and analyzed via eigendecomposition. Experiments on MNIST, Fashion-MNIST, and Tiny Stories compare SQS-GLU with ReLU-GLU, GELU-GLU, SwiGLU, and Bilinear GLU. The paper reports qualitative eigenvector visualizations for SQS-GLU, cosine similarities between SQS-GLU and bilinear MLP eigenvectors, and loss/accuracy/perplexity numbers. The main contribution is the SQS activation itself and the claim that it reconciles performance with interpretability.
Significance. If the central claim were established, the paper would be a meaningful contribution: designing a nonlinear activation that preserves the bilinear spectral structure would allow weight-based mechanistic interpretability in high-performing GLU architectures, addressing a known limitation of bilinear MLPs. The idea of modifying a quadratic form with a shrink factor to obtain a practical activation is interesting, and the paper provides a concrete, cheaply computable activation. However, the paper's core evidence for interpretability is not currently valid: no procedure is given to map a trained SQS-GLU to an interaction matrix, the hyperparameters underlying the interpretability figures were selected using the very same interpretability criterion, and the derivation of the activation is internally inconsistent. As such, the significance is currently speculative rather than demonstrated.
major comments (4)
- [Section 4.1] The paper's central interpretability claim is unsupported because the interaction matrix for an SQS-GLU is never defined. For a bilinear MLP, g_a(x)=(W_a^T x)(V_a^T x)=x^T A_a x with A_a symmetric, so eigendecomposition is immediate. For SQS-GLU, g_a(x)=(W_a^T x)σ(V_a^T x), where σ(v) is nonlinear and input-dependent (Algorithm 1). No fixed symmetric matrix A_a satisfies g_a(x)=x^T A_a x for all x. The text says 'we follow the same procedure' as Pearce et al., but for bilinear layers that procedure is unambiguous, whereas for SQS-GLU it is not. Without a concrete definition of how A_a is extracted from the trained weights, the eigenvectors in Figure 2 and Appendix B, and the cosine similarity comparisons, have no clear meaning. This is the load-bearing gap for the paper's main claim.
- [Section 3.1 and Algorithm 1] The activation is not derived consistently with the stated motivation. The paper begins with a quadratic activation g(x)=((|x|+c)^2-c^2)/(1+(λ|x|)^p)^{1/p}, then states that factoring out |x| and adding directional information yields σ(x)=sign(x)(|x|-c)/(1+(λ|x|)^p)^{1/p}. Factoring |x| from the numerator would produce |x|(|x|+2c), not |x|-c. Algorithm 1 implements y=(x-c*s)/(1+λ*x*s), which is the p=1 case of the latter, not of the derived quadratic. Moreover, the claim in the text that σ is 'quasi-linear for |x|<<1 and |x|>10' is false for the implemented function: for large |x|, σ(x) tends to sign(x)/λ (a constant), and at x=0 there is a discontinuity (σ(0)=-c if sign(0)=1). This undermines the theoretical motivation for why SQS should preserve bilinear structure.
- [Section 4] The hyperparameter selection is circular with respect to the interpretability claim. The paper states that λ=0.5, c=0.01 were chosen because they 'yield the most interpretable eigenvectors whilst maintaining performance,' and the same runs are then used to report the eigenvector interpretability and cosine similarity with bilinear MLPs. Since interpretability is assessed visually and by similarity to bilinear eigenvectors—criteria that were used to pick hyperparameters—the results in Figure 2 and Section 4.1 are selected on the outcome variable. A valid evaluation would require a separate validation set for hyperparameter selection and a pre-specified interpretability metric, or at least a demonstration that results are robust across a range of hyperparameters.
- [Table 1 and Figure 3] The performance claims are weaker than the text suggests. On MNIST and FMNIST, SQS final loss is higher than GELU and SwiGLU (e.g., MNIST 0.0834 vs 0.0680 for GELU, 0.0728 for SwiGLU), and SQS only clearly outperforms ReLU and Bilinear MLPs. On Tiny Stories, SQS's advantages are tiny (e.g., final loss 1.9020 vs 1.9021 for ReLU) and no error bars, number of seeds, or significance tests are reported for Tiny Stories, unlike MNIST/FMNIST where 5-run averages are stated. The claim that SQS is 'first to reach 80%, 85%, and 90% accuracy' is only supported by Figure 3, not by a quantitative table. For a new activation function, the performance evidence is insufficient to support the stated competitiveness.
minor comments (6)
- [Section 3.1, equation for GLU] The formula (Wx)⊙σ(Vx) = x/||x||· Wx ⊙ (|Vx|−c(Wx))/(...) appears to be a typographical error: it multiplies Wx by x/||x|| and subtracts c(Wx) in the numerator, which does not match the scalar definition of σ. It should be (Wx)⊙( sign(Vx)⊙(|Vx|-c)/(1+λ|Vx|) ) for p=1.
- [Algorithm 1] The comment 'parameter p is defined but not used in the forward pass' indicates that Algorithm 1 only implements p=1. If p is intended to be a general hyperparameter, an algorithm for p>1 should be provided, and the derivative for backpropagation should be stated. Also, using sign(0)=1 creates a discontinuity at 0; the implications for training and for the claim of quasi-linearity should be discussed.
- [Section 4.1] There is a malformed citation 'Bilinear MLP [2? ]' in the text. Several figure references are duplicated or misnumbered: 'Figures 4 5 7 7 8' in Appendix B, and Figure 9 has only the caption 'Caption' with no actual caption text.
- [Appendix A] Table 2 does not list the model dimension or embedding size for the Section 4.1 experiments, despite these being essential for reproducing the eigenvector visualizations. Table 3 lists model dim only for Section 4.2; please clarify whether Section 4.1 uses the same architecture.
- [Figure 2] Cosine similarity values are reported as averages over 5 runs but no error bars or standard deviations are shown, nor is it explained how eigenvectors are aligned across runs or across architectures. Without this, the claim that similarity is 'never below .5' is not well supported.
- [Abstract and Conclusion] The paper claims 'weight-based interpretability' as a general property, but interpretability is only evaluated on MNIST and Fashion-MNIST; no interpretability analysis is presented for the Tiny Stories language model, so the broader claim is overreaching.
Circularity Check
Interpretability claim is partly circular: λ/c are tuned to maximize eigenvector interpretability, and the interaction-matrix extraction is undefined (or bilinear by default), so 'preserving bilinear weight spectra' is either tautological or unverified.
specific steps
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fitted input called prediction
[Section 4, Experiments (first paragraph) and Section 4.1]
"For all uses of SQS, we setλ =.5, c =.01 (as we find that this yields the most interpretable eigenvectors whilst maintaining performance on par with current activation functions), andp = 1 (for computational efficiency."
The hyperparameters λ and c are explicitly chosen to maximize the interpretability of the eigenvectors. The paper then uses models trained with these selected hyperparameters to report the central claim that 'SQS-GLU facilitates the learning of interpretable features as revealed through eigenvector decomposition'. The interpretability result is therefore the selection criterion restated as a finding, not an independent prediction; it is statistically forced by the hyperparameter choice.
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other
[Section 4.1, Figure 2 caption; Appendix B.0.1]
"we follow the same procedure elucidated by Pearce et al. [2]. More specifically, we analyze a shallow Feed-Forward network consisting of an embedding projection, an SQS-GLU, and an output projection ... Figure 2 displays the top eigenvectors for five classes in both datasets. ... Following the methodology of Pearce et al. [2], we compute the eigen-spectrum of the interaction matrices extracted from the SQS-GLU layer."
For bilinear MLPs, the procedure defines an interaction matrix A_a = sym(W_a V_a^T) because the logit is exactly x^T A_a x. For SQS-GLU the logit is (W_a^T x) σ(V_a^T x) with nonlinear σ, so no exact interaction matrix exists. The paper never defines how the interaction matrix is extracted from the SQS-GLU layer. If the same bilinear formula is used, then the 'SQS-GLU eigenvectors' are by construction the bilinear eigenvectors of the weight outer product, making the claimed preservation of bilinear weight spectra tautological; if a different formula is used, the claim is unverifiable. In either case the central claim does not rest on a demonstrated property of the SQS function.
full rationale
The performance comparison (Section 4.2, Table 1) is self-contained against external baselines and is not circular. However, the paper's central novel claim—that SQS enables weight-based interpretability by preserving bilinear weight spectra—is substantially circular or underdefined. First, the hyperparameters λ=.5, c=.01 are selected specifically to yield 'the most interpretable eigenvectors'; the same runs are then presented as evidence that SQS-GLU learns interpretable eigenfeatures. Thus the interpretability result is an artifact of the selection process, not an independent empirical finding. Second, the paper never defines the interaction matrix whose eigenvectors are plotted for SQS-GLU. The bilinear MLP has a unique, well-defined interaction matrix A_a = sym(W_a V_a^T) because its logit is a quadratic form; SQS-GLU's logit contains a nonlinear gate σ, so no such matrix exists. If the authors reuse the bilinear extraction, then the eigenvectors are by construction those of a bilinear approximation, making the 'preservation' claim true by definition rather than by the network's behavior; if they use a different extraction, the procedure is omitted. The references to Pearce et al. and Sharkey are external, not self-citations, and do not themselves create circularity. The performance results and speed measurements remain independent, but the interpretability claim—the paper's raison d'être—reduces to a fitted selection and an undefined extraction, warranting a score of 6.
Axiom & Free-Parameter Ledger
free parameters (3)
- λ =
0.5
- c =
0.01
- p =
1
axioms (3)
- domain assumption Bilinear interaction matrices' eigendecomposition yields interpretable features.
- ad hoc to paper An interaction matrix can be assigned to an SQS-GLU.
- ad hoc to paper SQS is quasi-linear for small and large |x|.
invented entities (1)
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Signed Quadratic Shrink (SQS) activation function
no independent evidence
Cite this review
Pith. "Pith review of Preserving Bilinear Weight Spectra with a Signed and Shrunk Quadratic Activation Function." pith.science (2026). https://pith.science/paper/57ANZWSS
@misc{pith2026250901874,
author = {Pith},
title = {Pith review of: Preserving Bilinear Weight Spectra with a Signed and Shrunk Quadratic Activation Function},
year = {2026},
howpublished = {\url{https://pith.science/paper/57ANZWSS}},
note = {Machine review of arXiv:2509.01874}
}
read the original abstract
Understanding the inner workings of machine learning models is critical for ensuring their reliability and robustness. Whilst many techniques in mechanistic interpretability focus on activation driven analyses, being able to derive meaningful features directly from the weights of a neural network would provide greater guarantees and more computational efficiency. Existing techniques for analyzing model features through weights suffer from drawbacks such as reduced performance and data inefficiency. In this paper, we introduce Signed Quadratic Shrink (SQS), an activation function designed to allow Gated Linear Units (GLUs) to learn interpretable features without these drawbacks. Our experimental results show that SQS achieves performance competitive with state-of-the-art activation functions whilst enabling weight-based interpretability
Figures
Reference graph
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This paper was first reviewed by deepseek-v4-flash on August 5, 2026.
discussion (0)
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