REVIEW 4 major objections 5 minor 43 references
Training neural networks without backpropagation using particles
T0 review · 4 major / 5 minor · reviewed 2026-08-11 · deepseek-v4-flash
Pith's one-line read The paper reports that a multi-layer perceptron can be trained without backpropagation by giving each neuron its own particle swarm, reaching accuracies on Rice and Dry Bean datasets comparable to a backpropagation-trained MLP.
desk verdict The per-neuron PSO idea is clear and implementable, but the validation-loss gate in Algorithm 1 may be selecting on the same held-out fold whose accuracy is reported, which means the central 'similar to MLP' claim is not yet established. 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 per-neuron particle swarm. Each neuron node hosts k PSO particles, each particle being a candidate weight vector for that node; the velocity update includes the standard PSO terms plus a craziness term $c_3 r_3$ to keep particles exploring. While one node is updated, all other network weights are fixed, so the node's loss subproblem depends only on its own particle positions. The per-node best particles are combined into the network weights, and a validation-loss gate (Equation 5) decides whether the new combined weights are kept, which restricts runaway behavior. This machinery replaces the backward pass with k forward loss evaluations per node, repeated batchwise across epochs.
What would settle it
Train the method on a synthetic classification task whose solution requires two hidden neurons to change their weights simultaneously, so that no single-neuron update reduces the loss; if per-neuron PSO with validation gating cannot match a backpropagation-trained baseline on that task, the central claim fails.
Extended reading notes
Core claim
The central claim is that individual neuron nodes can be trained independently with particle swarm optimization and that the collective behavior of these separately trained neurons solves classification problems as effectively as backpropagation. The author splits the network into sub-problems: for each node, k particles carry candidate weight vectors; the loss is computed for each particle while all other node weights are frozen, and the minimum-loss particle is chosen. A validation-loss comparison after each batch decides whether the new combined weights replace the previous global best, which the author says contains the runaway problem of unconstrained per-node updates. The reported result is that the proposed method follows the same loss-reduction path as gradient descent and reaches accuracies within the spread of the MLP baseline on the Rice and Dry Bean datasets, while avoiding backpropagation entirely.
Load-bearing premise
The load-bearing assumption is that updating one neuron at a time while freezing every other neuron, and accepting the new weights only when validation loss improves, is enough to navigate the network's joint loss landscape to a good solution.
Editorial extensions
If this is right
- Training requires no backward pass, so errors do not need to be transported backward through the network; weight updates come from forward loss evaluations alone.
- Each neuron's weight update is independent, so node updates across the network can be computed in parallel without waiting for other neurons.
- The method is insensitive to the choice of activation function in the reported tests: replacing ReLU with Tanh left the loss-reduction behavior unchanged.
- Because updates are separated per node, the approach opens the door to arbitrary, non-standard connectivity between neurons without needing a global backpropagation rule.
- On the Rice and Dry Bean datasets, the reported accuracy, specificity, and F1-scores land within one standard deviation of the MLP baseline in Tables 1 and 2.
Reading between the lines
- A comparison the paper does not report is against plain coordinate-wise random search without the PSO velocity memory; without it, the contribution of the swarm dynamics versus the per-neuron decomposition alone is untested.
- The validation-loss gate repeatedly selects network states on the same validation data, which can bias the reported metrics; a clean test would hold out a third split for final evaluation.
- The method's cost is k forward passes per neuron per batch, which the paper itself flags as redundant computation; scaling to larger networks would likely require sharing or approximating per-neuron loss evaluations.
- If the approach scales, the per-neuron scalar reward resembles local learning rules in neuroscience, where each neuron aligns to a global performance score instead of receiving a backpropagated gradient.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes a derivative-free training method for multi-layer perceptrons in which each neuron is equipped with its own set of PSO particles and is updated coordinate-wise while all other weights are held fixed. A validation-loss gate in Algorithm 1 accepts or rejects whole-network updates, and the method is evaluated on four synthetic problems and two real tabular datasets (Rice and Dry Bean). The authors claim that this per-neuron PSO approach overcomes both local-minima problems of gradient descent and the search-space limitation of whole-network PSO, achieving accuracies and loss curves comparable to backpropagation-trained MLPs.
Significance. If the empirical claims hold, the paper offers a simple, forward-only, derivative-free training scheme with per-neuron parallelism that is easy to implement and that the authors have released as open-source code. Those are genuine strengths. However, the evidence is limited to two small tabular datasets and synthetic examples, and the central comparability claim is weakened by the evaluation-protocol ambiguity discussed below, by missing baseline error bars, and by an unproven premise about decomposing the joint loss into per-neuron subproblems. The contribution is incremental relative to the large literature on evolutionary and derivative-free neural-network training, but the per-neuron decomposition is a distinct and potentially useful variation.
major comments (4)
- [Algorithm 1, Steps 6 and 14; Sections 6.5 and 6.6] The final network is selected based on validation loss (Steps 6 and 14), but the paper never states that a separate test split, not used for this selection, is reserved inside each cross-validation fold. Sections 6.5 and 6.6 only say '4-fold cross-validation' and '13-fold cross-validation.' If the validation set used in Step 14 is the same held-out fold whose accuracy is reported in Tables 1 and 2, the reported accuracies are optimistically biased because the network has effectively been selected on the test set. Please clarify the exact data split (e.g., train/validation/test within each fold) and, if the validation gate used the test fold, re-run the experiments with nested splits before the comparability claim can be assessed.
- [Sections 4.1 and 5; Eq. (5)] The method's core premise is that fixing all other weights turns each neuron into an independent subproblem, but the joint loss is not separable into per-neuron losses. Updating each neuron by its own best particle does not necessarily decrease the joint loss; the paper itself notes in Section 4 that 'the nodes together perform runaway operations' and relies on the validation-loss gate in Algorithm 1 for stability. No convergence or quality guarantee is provided for this coordinate-wise PSO procedure. Please either provide a formal statement of what the per-neuron updates optimize (e.g., a descent or monotone-improvement condition on the joint loss) or substantially temper the Abstract and Introduction claims that the method 'overcome[s] the problem of gradient descent and the limitation of the PSO algorithm.'
- [Tables 1 and 2] The comparability claim rests on the last three rows of Tables 1 and 2, but standard deviations are reported only for the proposed PSO rows, not for the MLP or other baselines. Overlapping or comparable error bars are necessary to support 'performance is similar to MLP.' Please report means and standard deviations (or confidence intervals) for all methods and specify the MLP architecture, optimizer, learning rate, epochs, and any regularization used for the baselines.
- [Section 6 (all subsections)] The free PSO parameters—inertia weight w, acceleration constants c1 and c2, craziness constant c3, number of particles per neuron, batch size, weight initialization range, and stopping criterion—are not reported anywhere in Section 6 or in Algorithm 1. Without these values, the experiments are not reproducible from the text alone; the GitHub link is helpful but does not replace a hyperparameter table. Please add a table listing the values used for each dataset and synthetic experiment.
minor comments (5)
- [Figure 3] The flowchart text is garbled and largely unreadable in the PDF; please replace it with a clear vector figure with legible font.
- [Section 6.2] The MLP and the proposed method use different output-layer configurations (one output node with MSE loss vs. two output nodes with BCE loss), so the loss curves in Figures 4–7 are not directly comparable. Please explain how the comparison is intended despite the differing losses.
- [Sections 6.1–6.4 and 7] The phrase 'follows the gradient descent' is misleading because the proposed method is derivative-free; it appears to mean only that the loss curve decreases similarly. Please rephrase to avoid implying gradient-based behavior.
- [Algorithm 1, Step 7 and Eq. (5)] Step 7 says loss values are set to zero after each epoch, but Eq. (5) accumulates batch losses into f_j_ik. Please clarify whether the accumulated loss is reset per epoch and how the personal-best comparison in Steps 9–11 interacts with this reset.
- [Section 3.1] The statement that 'the original PSO paper had removed the craziness term' is a historical claim without a citation; please either cite the source or rephrase to describe only the version used here.
Circularity Check
Central training claim is empirically self-contained; only minor self-cited craziness-term heuristic appears, and it is not load-bearing.
full rationale
The paper's central claim is that per-neuron PSO weight updates can train MLPs to accuracies comparable to backpropagation on Rice and Dry Bean datasets. This is an external empirical benchmark, not a derivation from a fitted quantity. No equation in the paper defines the target result as an input: Eq. (4) is a per-node min-selection over particles, Eq. (5) is a batch-wise loss accumulation rule, and Algorithm 1 is a coordinate-wise search controlled by a validation-loss gate. The only author self-citation of note is the craziness term from [24,25] used in Eq. (3); it is a heuristic component of the search, not a theorem from which the central claim is deduced, so it does not make the training result circular. The validation-loss gate in Algorithm 1 could bias results if the validation data coincided with the scored test fold, but the manuscript does not state that they coincide, and k-fold evaluation is described; this is an experimental-design risk, not an exhibited circular reduction. The paper is self-contained against external datasets, so no significant circularity is found.
Assumptions & free parameters
free parameters (7)
- PSO inertia weight w =
not reported
- PSO acceleration constants c1, c2 =
not reported
- Craziness constant c3 =
not reported
- Number of particles k per neuron =
not reported
- Number of epochs =
20 for synthetic datasets, 100 for real datasets
- Batch size =
not reported
- Weight initialization range =
not reported
assumptions (3)
- domain assumption Optimizing each neuron independently, with other weights fixed, leads to a good solution of the joint loss.
- domain assumption PSO converges to an acceptable local optimum for each neuron within the chosen number of epochs.
- domain assumption Repeated use of validation loss for accepting or rejecting weight updates does not materially bias the reported test performance.
Cite this review
Pith. "Pith review of Training neural networks without backpropagation using particles." pith.science (2026). https://pith.science/paper/PBKZCNZ4
@misc{pith2026241205667,
author = {Pith},
title = {Pith review of: Training neural networks without backpropagation using particles},
year = {2026},
howpublished = {\url{https://pith.science/paper/PBKZCNZ4}},
note = {Machine review of arXiv:2412.05667}
}
read the original abstract
Neural networks are a group of neurons stacked together in multiple layers to mimic the biological neurons in a human brain. Neural networks have been trained using the backpropagation algorithm based on gradient descent strategy for several decades. Several variants have been developed to improve the backpropagation algorithm. The loss function for the neural network is optimized through backpropagation, but several local minima exist in the manifold of the constructed neural network. We obtain several solutions matching the minima. The gradient descent strategy cannot avoid the problem of local minima and gets stuck in the minima due to the initialization. Particle swarm optimization (PSO) was proposed to select the best local minima among the search space of the loss function. The search space is limited to the instantiated particles in the PSO algorithm, and sometimes it cannot select the best solution. In the proposed approach, we overcome the problem of gradient descent and the limitation of the PSO algorithm by training individual neurons separately, capable of collectively solving the problem as a group of neurons forming a network. Our code and data are available at https://github.com/dipkmr/train-nn-wobp/
Figures
Figures from the paper (5 more)
Reference graph
Works this paper leans on
-
[1]
Gradient Followi ng Without Back-Propagation in Layered Networks
Andrew G. Barto and Michael I. Jordan. “Gradient Followi ng Without Back-Propagation in Layered Networks”. In: International Conference on Neural Networks . 1987, pp. 629–636
work page 1987
-
[2]
Gradients without Backprop agation
Atılım Güne¸ s Baydin et al. “Gradients without Backprop agation”. In: (2022). arXiv: 2202.08587 [cs.LG]
arXiv 2022
-
[3]
C. M. Bishop. Neural networks for pattern recognition . Oxford: Oxford university press, 1995
work page 1995
-
[4]
C. M. Bishop. Pattern recognition and machine learning. Berlin: Springer, 2006
work page 2006
-
[5]
How to Train Your Wide Neural Network Without Backprop: An Input-Weight Alignment Perspective
Akhilan Boopathy and Ila Fiete. “How to Train Y our Wide Ne ural Network Without Back- prop: An Input-Weight Alignment Perspective”. In: (2021). arXiv: 2106.08453 [cs.LG]
work page Pith review arXiv 2021
-
[6]
Supervised Learning in Ne ural Networks without Feed- back Networks
Robert D. Brandt and Feng Lin. “Supervised Learning in Ne ural Networks without Feed- back Networks”. In: IEEE International Symposium on Intelligent Control . 1996. DOI : 10.1109/ISIC.1996.556182
-
[7]
Classification of Rice V ari eties Using Artificial Intelligence Methods
Ilkay Cinar and Murat Koklu. “Classification of Rice V ari eties Using Artificial Intelligence Methods”. In: International Journal of Intelligent Systems and Applicat ions in Engineering 7.3 (2019), pp. 188–194. DOI : 10.18201/ijisae.2019355381
-
[8]
Adaptive Subg radient Methods for Online Learning and Stochastic Optimization
John Duchi, Elad Hazan, and Y oram Singer. “Adaptive Subg radient Methods for Online Learning and Stochastic Optimization”. In: Journal of Machine Learning Research 12 (2011), pp. 2121–2159
work page 2011
Show all 43 references
-
[9]
R. O. Duda, P . E. Hart, and D. G. Stork. Pattern classification . Wiley, 2001
2001
-
[10]
Neuroevolution in Deep Neural Networks: Current Trends and Future Challenges
Edgar Galván and Peter Mooney. “Neuroevolution in Deep Neural Networks: Current Trends and Future Challenges”. In: IEEE Transactions on Artificial Intelligence 2.6 (2021), pp. 476–
2021
-
[11]
Exponential natural evolut ion strategies
Tobias Glasmachers et al. “Exponential natural evolut ion strategies”. In: Proceedings of the 12th Annual Conference on Genetic and Evolutionary Comp utation. GECCO ’10. Association for Computing Machinery, 2010, pp. 393–400. ISBN : 9781450300728. DOI : 10.1145/1830483.1830557...
2010
-
[12]
An introduction to neural networks
Kevin Gurney. An introduction to neural networks . London: UCL Press, 1997
1997
-
[13]
Never loo k back - A modified EnKF method and its application to the training of neural networks witho ut back propagation
Eldad Haber, Felix Lucka, and Lars Ruthotto. “Never loo k back - A modified EnKF method and its application to the training of neural networks witho ut back propagation”. In: (2018). arXiv: 1805.08034 [math.NA]
2018 arXiv
-
[14]
Non-Linear Back-propagation: Doing Back-Propagation without Deriva- tives of the Activation Function
John Hertz et al. “Non-Linear Back-propagation: Doing Back-Propagation without Deriva- tives of the Activation Function”. In: IEEE Transactions on Neural Networks 8.6 (1997), pp. 1321–1327
1997
-
[15]
The Forward-Forward Algorithm: Som e Preliminary Investigations
Geoffrey Hinton. “The Forward-Forward Algorithm: Som e Preliminary Investigations”. In: (2022). arXiv: 2212.13345 [cs.LG]. 13
2022 arXiv
-
[16]
Decoupled Neural Interfaces usin g Synthetic Gradients
Max Jaderberg et al. “Decoupled Neural Interfaces usin g Synthetic Gradients”. In: (2016). arXiv: 1608.05343 [cs.LG]
2016 arXiv
-
[17]
One forward is enough for neural ne twork training via likelihood ratio method
Jinyang Jiang et al. “One forward is enough for neural ne twork training via likelihood ratio method”. In: (2023). arXiv: 2305.08960 [cs.LG]
2023 arXiv
-
[18]
Particle swarm optimization
James Kennedy and Russell C. Eberhart. “Particle swarm optimization”. In: Proceedings of the IEEE International Conference on Neural Networks . 1995, pp. 1942–1948
1995
-
[19]
Eberhart, and Shi Y
James Kennedy, Russell C. Eberhart, and Shi Y. Swarm intelligence. San Francisco: Morgan Kaufmann Publishers, 2001
2001
-
[20]
Adam: a Method for S tochastic Optimization
Diederik P . Kingma and Jimmy Lei Ba. “Adam: a Method for S tochastic Optimization”. In: International Conference on Learning Representations . 2015, pp. 1–13
2015
-
[21]
Multiclass Classifica tion of Dry Beans Using Computer Vision and Machine Learning Techniques
Murat Koklu and Ilker Ali Ozkan. “Multiclass Classifica tion of Dry Beans Using Computer Vision and Machine Learning Techniques”. In: Computers and Electronics in Agriculture 174.105507 (2020). DOI : 10.1016/j.compag.2020.105507
2020
-
[22]
Back-Propagation Without Weight Trans- port
John F. Kolen and Jordan B. Pollack. “Back-Propagation Without Weight Trans- port”. In: IEEE International Conference on Neural Networks (ICNN’94 ). 1994. DOI : 10.1109/ICNN.1994.374486
1994
-
[23]
Ensemble Kalman inversion: a derivative-free technique for machine learning tasks
Nikola B Kovachki and Andrew M Stuart. “Ensemble Kalman inversion: a derivative-free technique for machine learning tasks”. In: Inverse Problems 35.095005 (2019)
2019
-
[24]
Binary classifica tion posed as a quadratically con- strained quadratic programming and solved using particle s warm optimization
Deepak Kumar and A. G. Ramakrishnan. “Binary classifica tion posed as a quadratically con- strained quadratic programming and solved using particle s warm optimization”. In: S¯ adhan¯ a 41.3 (2016), pp. 289–298. DOI : 10.1007/s12046-016-0466-y
2016 doi
-
[25]
Quadratically co nstrained quadratic pro- gramming for classification using particle swarms and appli cations
Deepak Kumar and A. G. Ramakrishnan. “Quadratically co nstrained quadratic pro- gramming for classification using particle swarms and appli cations”. In: (2014). arXiv: 1407.6315 [cs.AI]
2014 arXiv
-
[26]
Backpropagation and the br ain
Timothy P . Lillicrap et al. “Backpropagation and the br ain”. In: Nature Reviews Neuroscience 21 (2020), pp. 335–346. DOI : 10.1038/s41583-020-0277-3
2020 doi
-
[27]
Random feedback weights su pport learning in deep neural net- works
Timothy P . Lillicrap et al. “Random feedback weights su pport learning in deep neural net- works”. In: (2014). arXiv: 1411.0247 [q-bio.NC]
2014 arXiv
-
[28]
Random synaptic feedback w eights support error back- propagation for deep learning
Timothy P . Lillicrap et al. “Random synaptic feedback w eights support error back- propagation for deep learning”. In: Nature Communications 7.13276 (2016). DOI : 10.1038/ncomms13276
2016 doi
-
[29]
Th e HSIC Bottleneck: Deep Learning without Back-Propagation
Wan-Duo Kurt Ma, J.P . Lewis, and W . Bastiaan Kleijn. “Th e HSIC Bottleneck: Deep Learning without Back-Propagation”. In: AAAI Conference on Artificial Intelligence . 2020, pp. 5085–5092
2020
-
[30]
Simple Evolutio nary Optimization Can Rival Stochastic Gradient Descent in Neural Networks
Gregory Morse and Kenneth O. Stanley. “Simple Evolutio nary Optimization Can Rival Stochastic Gradient Descent in Neural Networks”. In: Proceedings of the Genetic and Evolu- tionary Computation Conference 2016 . GECCO ’16. Association for Computing Machin- ery, 2016, pp. 477–4...
2016
-
[31]
Random Gradient-Free Min imization of Convex Func- tions
Y . Nesterov and V . Spokoiny. “Random Gradient-Free Min imization of Convex Func- tions”. In: F oundations of Computational Mathematics 17 (2017), pp. 527–566. DOI : 10.1007/s10208-015-9296-2
2017 doi
-
[32]
Derivativ e-free optimization: a review of algo- rithms and comparison of software implementations
Luis Miguel Rios and Nikolaos V . Sahinidis. “Derivativ e-free optimization: a review of algo- rithms and comparison of software implementations”. In: Journal of Global Optimization 56 (2013), pp. 1247–1293. DOI : 10.1007/s10898-012-9951-y
2013 doi
-
[33]
Learning representations by back-propagating errors
David E. Rumelhart, Geoffrey E. Hinton, and Ronald J. Wi lliams. “Learning representations by back-propagating errors”. In: Nature 323 (1986), pp. 533–536
1986
-
[34]
Evolution Strategies as a Scalable Alternative to Reinforcement Learning
Tim Salimans et al. Evolution Strategies as a Scalable Alternative to Reinforcement Learning
-
[35]
Schneider et al
Andreas C. Schneider et al. What should a neuron aim for? Designing local objec- tive functions based on information theory . 2024. arXiv: 2412.02482 [cs.IT]. URL : https://arxiv.org/abs/2412.02482
2024
-
[36]
Deep Neuroevolution: Gene tic Algorithms Are a Competitive Al- ternative for Training Deep Neural Networks for Reinforcem ent Learning
Felipe Petroski Such et al. “Deep Neuroevolution: Gene tic Algorithms Are a Competitive Al- ternative for Training Deep Neural Networks for Reinforcem ent Learning”. In: (2017). arXiv: 1712.06567 [cs.NE]. URL : http://arxiv.org/abs/1712.06567. 14
2017 arXiv
-
[37]
Efficient natural evolution strategies
Yi Sun et al. “Efficient natural evolution strategies”. In: Proceedings of the 11th Annual Con- ference on Genetic and Evolutionary Computation . GECCO ’09. Association for Computing Machinery, 2009, 539–546. ISBN : 9781605583259. DOI : 10.1145/1569901.1569976. URL : https://do...
2009
-
[38]
An Evolut ionary Algorithm of Lin- ear complexity: Application to Training of Deep Neural Netw orks
S. Ivvan V aldez and Alfonso Rojas Domínguez. “An Evolut ionary Algorithm of Lin- ear complexity: Application to Training of Deep Neural Netw orks”. In: (2019). arXiv: 1907.05951 [cs.NE]. URL : http://arxiv.org/abs/1907.05951
2019 arXiv
-
[39]
Natural evolution strategies
Daan Wierstra et al. “Natural evolution strategies”. I n: J. Mach. Learn. Res. 15.1 (2014), pp. 949–980. ISSN : 1532-4435
2014
-
[40]
Neural Network Learni ng without Backpropagation
Bogdan M. Wilamowski and Hao Y u. “Neural Network Learni ng without Backpropagation”. In: IEEE Transactions on Neural Networks 21.11 (2010), pp. 1793–1803
2010
-
[41]
A Gradient-Guided Evolutionar y Approach to Training Deep Neural Networks
Shangshang Y ang et al. “A Gradient-Guided Evolutionar y Approach to Training Deep Neural Networks”. In: IEEE Transactions on Neural Networks and Learning Systems 33.9 (2022), pp. 4861–4875. DOI : 10.1109/TNNLS.2021.3061630. 15
2022
-
[493]
DOI : 10.1109/TAI.2021.3067574
2021
-
[2017]
URL : https://arxiv.org/abs/1703.03864
arXiv: 1703.03864 [stat.ML]. URL : https://arxiv.org/abs/1703.03864
Reviewed August 11, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.