REVIEW 4 major objections 4 minor 42 references
Breaking the Conventional Forward-Backward Tie in Neural Networks: Activation Functions
T0 review · 4 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read The paper claims that during backpropagation the activation function's derivative affects only gradient magnitude, not direction, so forward and backward passes can be decoupled without hurting learning.
desk verdict A well-run empirical study of surrogate gradients that overreaches: the theoretical propositions are mathematically wrong, and the paper's own experiments contradict its universal claims. 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 load-bearing identity is the Jacobian decomposition in backpropagation: ∇_{Z_{l-1}} G_l = W_l^T ⊙ φ'_l(W_l Z_{l-1}), with the untied variant D_{l-1}^{untied} = (J_l)^T D_l ⊙ g(Z_{l-1}) for strictly positive g. This separates the linear operator that carries directional information from the activation derivative that only scales each component, and it is what lets the paper claim direction dominance and then replace the derivative with arbitrary positive proxies.
What would settle it
Train a multi-layer network (for instance a small Transformer) with Heaviside forward activations and, in every layer, a backward replacement that is strictly positive but random in magnitude, then compare test performance against standard backprop under the same schedule. If the decoupled run fails to approach the backprop baseline on a nontrivial dataset, the claim that only gradient direction matters is contradicted. A cheaper check: on a trained two-layer MLP, measure the angle between the standard gradient update and the untied-gradient update over many steps; if the angle often exceeds 9
Extended reading notes
Core claim
The paper argues that the conventional tie between forward activations and backward gradients in neural network training is unnecessary. Its central proposition is that the direction of weight updates is independent of the activation function's derivative: the derivative enters the backpropagation equations only as an element-wise scalar multiplier, so it changes the magnitude of each update but not which direction each component points. That direction is set by the linear weight matrices and the activations from the preceding layer. From this, the paper derives that any strictly positive function of the pre-activations can replace the activation derivative in the backward pass without chang
Load-bearing premise
The proof assumes that replacing the activation derivative with any strictly positive function that preserves each component's sign is enough for stochastic gradient descent to keep learning, without proving that the loss actually descends or that update magnitudes cannot matter.
Editorial extensions
If this is right
- Activation functions with large flat or nondifferentiable regions, including the Heaviside step, become trainable by choosing an independent backward gradient proxy.
- Simpler backward computations (constants, rectangles) can reduce computational and memory overhead while matching or beating tied gradients, as observed in the LeNet5 experiments.
- Gradient jamming experiments show that randomizing gradient magnitude preserves learning for ReLU and linear activations, supporting the primacy of direction over magnitude.
- Common optimization techniques — momentum, clipping, scaling, noise, normalization — can be re-read as magnitude modulations that leave the gradient direction intact.
- Binary neural network training, previously justified by heuristics, receives a theoretical rationale: non-differentiable binary activations can be trained with straightforward positive backward proxies.
Reading between the lines
- If direction dominance generalizes to deep Transformers and graph networks as Proposition 2 asserts, one could design backward passes that store only signs or random projections instead of full activation derivatives, potentially cutting training memory; this is testable but untested in the paper.
- The paper's own data show logistic activations collapse under full random jamming while ReLU and linear do not, suggesting that bounded saturating forward functions rely on magnitude information more than the theory's sign-only argument would predict.
- The propositions give a formal footing for straight-through estimators; a natural next experiment is comparing constant positive proxies with stochastic ones whose mean equals the true derivative, to see whether distribution shape (not just positivity) matters for convergence speed.
- A cheap falsifying check follows from Eq. (24): if per-neuron positive scaling g(Z) can rotate the aggregate update more than 90 degrees from the true gradient on realistic batches, the claim 'sign preservation suffices' would not hold for a full update vector.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript argues that the direction of weight updates in neural networks is independent of activation-function derivatives and is predominantly determined by linear connections. It formalizes this as three propositions, then supports it with experiments on a Single Unit Classifier, an MLP, and LeNet-5 in which the backward gradient is replaced by constant, rectangular, triangular, or stochastic functions, and with BNN experiments using Heaviside activations. The paper concludes that the conventional forward-backward symmetry can be relaxed without impairing learning.
Significance. If valid, the claimed result would provide a formal justification for straight-through estimators and would license training non-differentiable activation functions, which is practically valuable. The paper has strengths: a clear research question, a reproducible experimental protocol with 20 runs per setting, several datasets and architectures, and honest limitations regarding more complex architectures. However, the theoretical core is not sound: the proof of Proposition 1 mischaracterizes element-wise multiplication as scalar scaling, and Proposition 3 does not establish a descent or convergence property. The experiments are not sufficient to rescue the general claim, since several configurations show clear performance collapses. I therefore cannot recommend publication as a theoretical contribution; an empirical-only claim would require substantial reframing.
major comments (4)
- [Section 3, Eq. (22), Proposition 1] Equation (22) writes ∇_{Z_{l-1}} G_l = W_l^T ⊙ φ'_l(W_l Z_{l-1}) and then treats φ'_l as an 'element-wise scalar multiplier'. If φ'_l has distinct values across neurons, each row/column of W_l^T is scaled by a different factor, so the vector direction is not preserved. For example, with W_l^T = [[1,0],[0,1]] and D_l = (1,1), φ' = (1,0) gives a different direction than φ' = (0,1). Thus Proposition 1 is invalid as stated, and the BNN justification in Section 5 relies on this statement.
- [Section 3, Proposition 3, Eq. (24)] Strict positivity of g only preserves the sign of each component of (J_l)^T D_l at one layer. It does not preserve the sign of the full gradient with respect to earlier weights after composing layers, and it does not imply that the update is a descent direction. Concrete two-layer linear counterexample: W2^T = [[1,-1],[0.5,1]], x = (1,1), dL/da2 = (1,1), φ2' = (1,0.001), surrogate g2 = (0.001,1), φ1' = g1 = (1,1). Then δ1 = (0.999,0.501), δ̃1 = (-0.999,1.0005), and δ1·δ̃1 ≈ -0.497 < 0. The surrogate update for W1 moves opposite to the true gradient. Hence Eq. (24) does not establish the feasibility claim.
- [Section 4.8, Full-Jamming] Full-Jamming replaces the backward gradient by a uniform random value in [0,1]. Since all sampled values are nonnegative, negative feedback gradients have their signs flipped. The text claims that this 'ensures that only the sign (directionality) of the gradient is preserved', which is contradicted by the definition and Figure 8(a). This mischaracterization weakens the interpretation that direction, not magnitude, drives learning.
- [Section 4, Tables 3-5] The empirical support is mixed. In Table 3, the untied Log/1 configuration is consistently 2-5 points below tied across most settings. In Table 4, Log/1 has standard deviations as high as 0.289 and near-chance means at low learning rates. In Table 5, ReLU/1 collapses to about 0.11 accuracy for LR=0.05 and 0.1 at all batch sizes. These results do not support the unqualified conclusion that decoupling 'does not impair learning'; they indicate that some surrogate choices severely hurt training. The claim should be restricted to specific surrogates and regimes.
minor comments (4)
- [Section 4.3 / Figure 6] The text says '(a) constant, (b) rectangular, (c) triangular (d)', but Figure 6 has only (a)-(c) and the caption labels (b) Rectangular and (c) Triangular. Please align the numbering.
- [Section 2, Eq. (3)] Typo: 'proximated' should be 'approximated'.
- [Figure 8 caption] The caption refers to 'Experiment 6', but the section is Experiment 5 (Section 4.8).
- [Section 3, Eq. (22)] The notation W_l^T ⊙ φ'_l(...) is dimensionally ambiguous; use diag(φ'_l(...))W_l or state the Hadamard convention explicitly.
Circularity Check
No significant circularity: the theoretical propositions are independent of the experiments, and no fitted parameter or self-citation is presented as a prediction.
full rationale
The paper's derivation chain is not circular. The mathematical claims (Propositions 1–3) are derived from the backpropagation update equations (10)–(11) and geometric arguments, not from fitted constants, benchmark tuning, or the experimental results. The experiments use arbitrary untied gradient surrogates (constant, rectangular, triangular, random) and report accuracy; they do not fit a surrogate to training data and then present the resulting accuracy as a prediction. There are no self-citations in the reference list and no load-bearing reliance on the authors' prior work. The discussion of Straight-Through Estimator, BinaryConnect, and XNOR-Net is explicitly comparative and does not import an unverified uniqueness theorem or ansatz. The main weakness is mathematical invalidity rather than circularity: Proposition 3 equates 'direction' with per-component sign and assumes that positive element-wise scaling cannot impair learning, which is not sufficient in deep networks—different positive per-neuron scales change the vector direction across layers, and composed surrogates can produce updates opposing the true gradient. That is a correctness gap, not a reduction of the conclusion to the input by construction. Under the hard rules, circularity requires a specific exhibited reduction (e.g., fitted parameter renamed as prediction, or a conclusion identical to a definition). No such reduction is present here, so the appropriate score is 0.
Assumptions & free parameters
free parameters (4)
- Constant surrogate gradient value =
1
- Rectangular window support =
not fully specified; Figure 8c uses |x| <= 5
- Noise normalization constant =
not reported numerically
- Triangular surrogate shape =
not specified as a formula
assumptions (5)
- standard math Backpropagation equations (10)-(11) give the exact gradient-based training dynamics.
- domain assumption For every architecture the layer Jacobian factorizes as J_l^T ⊙ phi'_l(Z_{l-1}) (Eq. 23).
- ad hoc to paper Element-wise multiplication by a strictly positive vector preserves gradient direction.
- ad hoc to paper Learning depends on direction and sign, not magnitude, of weight updates.
- domain assumption Test accuracy on balanced benchmark splits is enough to validate training effectiveness.
Cite this review
Pith. "Pith review of Breaking the Conventional Forward-Backward Tie in Neural Networks: Activation Functions." pith.science (2026). https://pith.science/paper/FIFYMSRR
@misc{pith2026250907236,
author = {Pith},
title = {Pith review of: Breaking the Conventional Forward-Backward Tie in Neural Networks: Activation Functions},
year = {2026},
howpublished = {\url{https://pith.science/paper/FIFYMSRR}},
note = {Machine review of arXiv:2509.07236}
}
read the original abstract
Gradient-based neural network training traditionally enforces symmetry between forward and backward propagation, requiring activation functions to be differentiable (or sub-differentiable) and strictly monotonic in certain regions to prevent flat gradient areas. This symmetry, linking forward activations closely to backward gradients, significantly restricts the selection of activation functions, particularly excluding those with substantial flat or non-differentiable regions. In this paper, we challenge this assumption through mathematical analysis, demonstrating that precise gradient magnitudes derived from activation functions are largely redundant, provided the gradient direction is preserved. Empirical experiments conducted on foundational architectures - such as Multi-Layer Perceptrons (MLPs), Convolutional Neural Networks (CNNs), and Binary Neural Networks (BNNs) - confirm that relaxing forward-backward symmetry and substituting traditional gradients with simpler or stochastic alternatives does not impair learning and may even enhance training stability and efficiency. We explicitly demonstrate that neural networks with flat or non-differentiable activation functions, such as the Heaviside step function, can be effectively trained, thereby expanding design flexibility and computational efficiency. Further empirical validation with more complex architectures remains a valuable direction for future research.
Figures
Figures from the paper (5 more)
Reference graph
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Reviewed August 4, 2026 · model on record in the stance chip above.
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