REVIEW 4 major objections 3 minor 28 references
Equitable Federated Learning with NCA
T0 review · 4 major / 3 minor · reviewed 2026-08-06 · deepseek-v4-flash
Pith's one-line read A 284 KB neural cellular automaton model matches UNet segmentation in federated learning while shrinking communication and encryption overhead by orders of magnitude.
desk verdict Useful measurement study of a tiny NCA backbone in FL, but the segmentation-parity claim needs much stronger evaluation before it can be trusted. 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 mechanism is Med-NCA, a neural cellular automaton: a segmentation network built from a small learned rule that is applied iteratively over time steps rather than through a deep stack of layers. FedNCA couples two such automata, a coarse stage ($T_0=20$ steps) and a fine stage ($T_1=40$ steps), trained end-to-end via backpropagation through time. The same weights are reused at every time step, which is what keeps the parameter count, and therefore the update size, minuscule; that small size is what lets the CKKS homomorphic scheme encrypt the updates quickly and lets the server average ciphertexts directly.
What would settle it
Re-run the ultrasound experiment with a conventional split, for example 70% of patients in the federated training set across 5 clients and 30% held out, and compare Dice to the UNet baseline with confidence intervals; if FedNCA no longer matches UNet, the equality claim is refuted. A complementary check is to measure actual wall-clock time for one federated round over a simulated 1 Mbps link to confirm the claimed 500–2000× communication saving in practice.
Extended reading notes
Core claim
The paper's central discovery is that the Med-NCA backbone is not merely a lightweight alternative to UNet but is naturally suited to federated learning: because each client's model is so small, the server can aggregate encrypted updates without decryption, and the whole training loop fits on a low-cost phone. The authors demonstrate this with a two-stage NCA in which a coarse stage runs 20 update steps on a downsampled image and a fine stage runs 40 steps on the upsampled features, trained end-to-end by backpropagation through time. Under federated averaging with the CKKS homomorphic scheme, this model achieves 74% Dice on fetal abdominal ultrasound and 78% Dice on chest X-ray lung segmentation, values the authors report as equal to UNet and TransUNet, while transmitting 500–2000× fewer bytes per round and encrypting weights in under 20 milliseconds versus about 27 seconds for a UNet.
Load-bearing premise
The central claim rests on the ultrasound experiment's inverted data split: 118 of 169 patients (70%) are held out for testing while only 51 patients are split across the 5 federated clients, with no rationale given, so the reported 74% Dice may reflect an unusually small training set and an easy test set rather than a true match to baseline quality.
Editorial extensions
If this is right
- Federated training becomes possible on smartphones and tablets costing under €300, removing the need for GPU clusters in low-resource clinics.
- Per-round communication cost drops to roughly a five-hundredth to a two-thousandth of a UNet's, so clients on weak or metered connections can join frequent aggregation rounds.
- Homomorphic encryption becomes practical for federated segmentation: encrypting and decrypting updates takes under 20 milliseconds, enabling secure aggregation even against untrusted servers.
- Compression techniques such as 4-bit quantization and top-k sparsification, which the authors find degrade the Dice of UNet and TransUNet, become unnecessary for FedNCA, preserving accuracy while saving bandwidth.
Reading between the lines
- The paper does not report the byte size of the ciphertexts per round; measuring the actual encrypted payload would tell whether the 500–2000× saving survives CKKS packing overhead.
- If the NCA design generalizes, the same two-stage backbone could be applied to federated training for other dense prediction tasks, such as lesion detection or multi-organ segmentation, where communication constraints are the bottleneck.
- The authors' results on a single ultrasound split suggest a broader empirical program: benchmarking FedNCA across multiple client counts, non-i.i.d. data distributions, and modalities would test how the efficiency gains trade off against segmentation quality.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper proposes FedNCA, a federated learning system for medical image segmentation built on the Med-NCA backbone. The central claims are that FedNCA matches the segmentation quality of UNet and TransUNet (Dice 74% on fetal ultrasound, 78% on chest X-ray) while reducing communication cost by 500–2000x, enabling training on smartphones, and reducing homomorphic-encryption runtime by about 1800x compared with TransUNet. The authors argue this makes privacy-preserving federated learning practical in low- and middle-income countries with limited bandwidth and hardware.
Significance. If the claims were fully supported, FedNCA would be a useful contribution to resource-constrained federated learning: the model is genuinely tiny, the paper reports measurements on real low-cost smartphones, and the code is public. The combination of a cellular-automata backbone with homomorphic encryption is a sensible direction. However, the significance of the paper rests on the 'equal segmentation quality at drastically reduced cost' claim, and that claim is currently not well supported by the experimental design. The inconsistent communication-reduction factors also make the quantitative headline unreliable. With a stronger evaluation and consistent accounting, the work could be a solid systems contribution, but in its present form the evidence is too thin for the paper's central claims.
major comments (4)
- [§4 and Fig. 3] The segmentation-parity claim is not statistically supported. The ultrasound experiment reserves 118 of 169 patients for testing and trains on 51 patients across 5 clients; the X-ray experiment uses only 50 images total, with 25 for testing and 25 for training split across 5 clients. All results are from a single run, with no standard deviations, no multiple seeds, and no significance test. With a 25-image test set, the reported Dice values of 74% and 78% carry wide confidence intervals, so the claim that FedNCA 'achieves equal segmentation quality' to UNet/TransUNet cannot be distinguished from noise. The paper should report error bars over repeated runs and, ideally, a proper patient-wise split rationale, especially given the inverted 70%-test/30%-train split.
- [§3 and §5] The communication-reduction factors are mutually inconsistent and unsubstantiated. Section 3 claims a 'nearly 500×' reduction versus U-Net, Section 5 claims '2000× lower communication overhead,' and the same section later states that compressed U-Net/TransUNet updates remain 'at least 300×' larger. The paper also states in Section 1 that Med-NCA has '5000× fewer parameters than a UNet' without deriving this number. These discrepancies matter because communication cost is the paper's headline efficiency measure. The authors should provide a single, clearly defined accounting of model sizes and per-round transmission bytes (including metadata and compression overhead) and derive each claimed ratio explicitly.
- [§4 XRay paragraph] The X-ray dataset is misidentified. The text says 'The MIMIC-III [27] dataset consists of chest XRay images,' but MIMIC-III is a clinical database, not a chest X-ray image collection; the relevant imaging resource is MIMIC-CXR. This is not a trivial citation slip, because it affects reproducibility and the exact composition of the 50-image subset. The dataset source and patient-level split should be corrected and described precisely.
- [§5, Homomorphic encryption paragraph] The 1400x/1800x homomorphic-encryption speedup is reported without the parameters that determine its validity: CKKS security level, polynomial modulus degree, scale, and whether the same plaintext encoding and ciphertext parameters are used for all three models. Without these details the comparison is hard to reproduce and the claimed speedup may partly reflect an unfavorable choice of encryption parameters for the larger models. Please specify the full HE configuration and, if possible, report ciphertext sizes and end-to-end round times.
minor comments (3)
- [Throughout] There are numerous typographical errors and spacing issues, e.g., 'FederatedLearning' in the abstract, 'fine and course NCA' in Algorithm 1, 'Intrinisic gradient compression' in reference [6], and 'Lenovo T ab P11' in Figure 4. The manuscript should be carefully proofread.
- [Fig. 3 caption] The caption states that FedNCA 'achieves the best Dice,' but the text claims only equality with baselines; these statements should be reconciled, especially since no error bars are shown.
- [Fig. 3 and Baselines paragraph] The notation 'top-25' and 'top-01' is ambiguous: does 'top-01' mean 1% or 0.1%? Define the sparsification percentage clearly and report the resulting transmission cost per round for each setting.
Circularity Check
No significant circularity: FedNCA's efficiency and quality claims are backed by direct measurements, not by construction or self-citation chains.
full rationale
The paper's central chain is architectural: a lightweight Med-NCA backbone produces small model updates, which yields lower communication costs and faster homomorphic encryption. Each link in this chain is assessed by direct measurement in Section 5 (Dice scores, transmission cost in MiB, training time on smartphones, and encryption/decryption runtime), not by a fitted parameter or by a result that is equivalent to its input. The self-citations (Med-NCA [10], NCA edge-device training [11]) are used as prior architectural building blocks and as motivation; they are not invoked as a uniqueness theorem, do not forbid alternative architectures, and do not supply the quantitative reduction numbers, which are empirically measured. No equation in the paper reduces another equation to an input by definition. The inverted 70% test split and single-run Dice comparisons are serious evaluation-validity concerns, but they are evidentiary weaknesses rather than circular reasoning. Therefore, the circularity score is 0.
Assumptions & free parameters
free parameters (3)
- NCA time steps T0, T1 =
T0=20, T1=40
- Number of FL clients =
5
- Ultrasound train/test split =
51 train, 118 test (30%/70%)
assumptions (4)
- domain assumption Med-NCA architecture as described in [10] has the stated parameter count and behavior.
- domain assumption CKKS homomorphic encryption correctly supports averaging of encrypted model weights without accuracy loss in this setting.
- domain assumption The transmission cost includes all relevant metadata for each method, making the 300-2000x comparisons valid.
- domain assumption Training on the listed smartphones is representative of low-cost devices in LMICs.
Cite this review
Pith. "Pith review of Equitable Federated Learning with NCA." pith.science (2026). https://pith.science/paper/IE4A6O6M
@misc{pith2026250621735,
author = {Pith},
title = {Pith review of: Equitable Federated Learning with NCA},
year = {2026},
howpublished = {\url{https://pith.science/paper/IE4A6O6M}},
note = {Machine review of arXiv:2506.21735}
}
read the original abstract
Federated Learning (FL) is enabling collaborative model training across institutions without sharing sensitive patient data. This approach is particularly valuable in low- and middle-income countries (LMICs), where access to trained medical professionals is limited. However, FL adoption in LMICs faces significant barriers, including limited high-performance computing resources and unreliable internet connectivity. To address these challenges, we introduce FedNCA, a novel FL system tailored for medical image segmentation tasks. FedNCA leverages the lightweight Med-NCA architecture, enabling training on low-cost edge devices, such as widely available smartphones, while minimizing communication costs. Additionally, our encryption-ready FedNCA proves to be suitable for compromised network communication. By overcoming infrastructural and security challenges, FedNCA paves the way for inclusive, efficient, lightweight, and encryption-ready medical imaging solutions, fostering equitable healthcare advancements in resource-constrained regions.
Figures
Reference graph
Works this paper leans on
-
[27]
Mimic-iii, a freely accessible critical care database.Scientific data, 3(1):1–9, 2016
Alistair EW Johnson, Tom J Pollard, Lu Shen, Li-wei H Lehman, Mengling Feng, Mohammad Ghassemi, Benjamin Moody, Peter Szolovits, Leo Anthony Celi, and Roger G Mark. Mimic-iii, a freely accessible critical care database.Scientific data, 3(1):1–9, 2016
2016
-
[1]
Federated learning for medical image analysis: A survey.Pattern Recognition, page 110424, 2024
Hao Guan, Pew-Thian Yap, Andrea Bozoki, and Mingxia Liu. Federated learning for medical image analysis: A survey.Pattern Recognition, page 110424, 2024
work page 2024
-
[2]
Fatemeh Mosaiyebzadeh, Seyedamin Pouriyeh, Reza M Parizi, Quan Z Sheng, Meng Han, Liang Zhao, Giovanna Sannino, Caetano Mazzoni Ranieri, Jó Ueyama, and Daniel Macêdo Batista. Privacy-enhancing technologies in federated learning for the internet of healthcare things: a survey.Electronics, 12(12):2703, 2023
work page 2023
-
[3]
Federated learning for smart healthcare: A survey.ACM Computing Surveys (Csur), 55(3):1–37, 2022
DinhCNguyen,Quoc-VietPham,PubuduNPathirana,MingDing,ArunaSenevi- ratne, Zihuai Lin, Octavia Dobre, and Won-Joo Hwang. Federated learning for smart healthcare: A survey.ACM Computing Surveys (Csur), 55(3):1–37, 2022
work page 2022
-
[4]
Tadeusz Ciecierski-Holmes, Ritvij Singh, Miriam Axt, Stephan Brenner, and San- dra Barteit. Artificial intelligence for strengthening healthcare systems in low- and middle-income countries: a systematic scoping review.npj Digital Medicine, 5(1):162, 2022
work page 2022
-
[5]
Raghavendra Selvan, Bob Pepin, Christian Igel, Gabrielle Samuel, and Erik B Dam. Equity through access: A case for small-scale deep learning.arXiv preprint arXiv:2403.12562, 2024
work page Pith review arXiv 2024
-
[6]
Intrinisic Gradient Compression for Federated Learning
Luke Melas-Kyriazi and Franklyn Wang. Intrinisic gradient compression for fed- erated learning. arXiv preprint arXiv:2112.02656, 2021
work page Pith review arXiv 2021
-
[7]
FedFMS: Exploring Federated Foun- dation Models for Medical Image Segmentation
Yuxi Liu, Guibo Luo, and Yuesheng Zhu. FedFMS: Exploring Federated Foun- dation Models for Medical Image Segmentation . Inproceedings of Medical Image Computing and Computer Assisted Intervention – MICCAI 2024, volume LNCS 15008. Springer Nature Switzerland, October 2024
work page 2024
Show all 28 references
-
[8]
Fedcv: a federated learning framework for diverse computer vision tasks.arXiv preprint arXiv:2111.11066, 2021
Chaoyang He, Alay Dilipbhai Shah, Zhenheng Tang, Di Fan1Adarshan Naiynar Sivashunmugam, Keerti Bhogaraju, Mita Shimpi, Li Shen, Xiaowen Chu, Mahdi Soltanolkotabi, and Salman Avestimehr. Fedcv: a federated learning framework for diverse computer vision tasks.arXiv preprint arXi...
2021 arXiv
-
[9]
No one left behind: Inclusive federated learning over heterogeneous devices
Ruixuan Liu, Fangzhao Wu, Chuhan Wu, Yanlin Wang, Lingjuan Lyu, Hong Chen, and Xing Xie. No one left behind: Inclusive federated learning over heterogeneous devices. InProceedings of the 28th ACM SIGKDD Conference on Knowledge Dis- covery and Data Mining, pages 3398–3406, 2022
2022
-
[10]
Med-nca: Robust and lightweight segmentation with neural cellular automata
John Kalkhof, Camila González, and Anirban Mukhopadhyay. Med-nca: Robust and lightweight segmentation with neural cellular automata. InInternational Con- ference on Information Processing in Medical Imaging, pages 705–716. Springer, 2023. 10 N. Lemke et al
2023
-
[11]
Unsupervised training ofneuralcellularautomataonedgedevices
John Kalkhof, Amin Ranem, and Anirban Mukhopadhyay. Unsupervised training ofneuralcellularautomataonedgedevices. In International Conference on Medical Image Computing and Computer-Assisted Intervention, pages 498–507. Springer, 2024
2024
-
[12]
The evolution of mobile phone surveys in low- and middle-income countries: A study of coverage structure.International Journal of Public Opinion Research, 35(4):edad031, 10 2023
Mahmoud Elkasabi and Azam Khan. The evolution of mobile phone surveys in low- and middle-income countries: A study of coverage structure.International Journal of Public Opinion Research, 35(4):edad031, 10 2023
2023
-
[13]
Building trusted federated learning: Key technologies and challenges.Journal of Sensor and Actuator Net- works, 12(1), 2023
Depeng Chen, Xiao Jiang, Hong Zhong, and Jie Cui. Building trusted federated learning: Key technologies and challenges.Journal of Sensor and Actuator Net- works, 12(1), 2023
2023
-
[14]
Preservingprivacyandsecurityinfederatedlearning
TrucNguyenandMyTThai. Preservingprivacyandsecurityinfederatedlearning. IEEE/ACM Transactions on Networking, 32(1):833–843, 2023
2023
-
[15]
Fedml-he: An efficient homomorphic- encryption-based privacy-preserving federated learning system
Weizhao Jin, Yuhang Yao, Shanshan Han, Jiajun Gu, Carlee Joe-Wong, Srivatsan Ravi, Salman Avestimehr, and Chaoyang He. Fedml-he: An efficient homomorphic- encryption-based privacy-preserving federated learning system. arXiv preprint arXiv:2303.10837, 2023
2023 arXiv
-
[16]
Transunet: Transformers make strong encoders for medical image segmentation.arXiv preprint arXiv:2102.04306, 2021
JienengChen,YongyiLu,QihangYu,XiangdeLuo,EhsanAdeli,YanWang,LeLu, Alan L Yuille, and Yuyin Zhou. Transunet: Transformers make strong encoders for medical image segmentation.arXiv preprint arXiv:2102.04306, 2021
2021 arXiv
-
[17]
U-net: Convolutional networks for biomedical image segmentation
Olaf Ronneberger, Philipp Fischer, and Thomas Brox. U-net: Convolutional networks for biomedical image segmentation. In Medical image computing and computer-assisted intervention–MICCAI 2015: 18th international conference, Mu- nich, Germany, October 5-9, 2015, proceedings, par...
2015
-
[18]
Growing neural cellular automata.Distill, 5(2):e23, 2020
Alexander Mordvintsev, Ettore Randazzo, Eyvind Niklasson, and Michael Levin. Growing neural cellular automata.Distill, 5(2):e23, 2020
2020
-
[19]
Topology-aware federated learning in edge computing: A comprehensive survey
Jiajun Wu, Fan Dong, Henry Leung, Zhuangdi Zhu, Jiayu Zhou, and Steve Drew. Topology-aware federated learning in edge computing: A comprehensive survey. ACM Computing Surveys, 56(10):1–41, 2024
2024
-
[20]
Communication- efficient federated learning over capacity-limited wireless networks.IEEE Trans- actions on Cognitive Communications and Networking, 2024
Jaewon Yun, Yongjeong Oh, Yo-Seb Jeon, and H Vincent Poor. Communication- efficient federated learning over capacity-limited wireless networks.IEEE Trans- actions on Cognitive Communications and Networking, 2024
2024
-
[21]
Federated dropout—a simple approach for enabling federated learning on resource constrained devices.IEEE wireless communications letters, 11(5):923–927, 2022
Dingzhu Wen, Ki-Jun Jeon, and Kaibin Huang. Federated dropout—a simple approach for enabling federated learning on resource constrained devices.IEEE wireless communications letters, 11(5):923–927, 2022
2022
-
[22]
Fedsz: Leveraging error-bounded lossy compression for federated learning communications
Grant Wilkins, Sheng Di, Jon C Calhoun, Zilinghan Li, Kibaek Kim, Robert Un- derwood, Richard Mortier, and Franck Cappello. Fedsz: Leveraging error-bounded lossy compression for federated learning communications. In2024 IEEE 44th Inter- national Conference on Distributed Compu...
2024
-
[23]
In- verting gradients-how easy is it to break privacy in federated learning?Advances in neural information processing systems, 33:16937–16947, 2020
Jonas Geiping, Hartmut Bauermeister, Hannah Dröge, and Michael Moeller. In- verting gradients-how easy is it to break privacy in federated learning?Advances in neural information processing systems, 33:16937–16947, 2020
2020
-
[24]
Source inference attacks in federated learning
Hongsheng Hu, Zoran Salcic, Lichao Sun, Gillian Dobbie, and Xuyun Zhang. Source inference attacks in federated learning. In2021 IEEE International Con- ference on Data Mining (ICDM), pages 1102–1107. IEEE, 2021
2021
-
[25]
Homomorphic encryption for arithmetic of approximate numbers
Jung Hee Cheon, Andrey Kim, Miran Kim, and Yongsoo Song. Homomorphic encryption for arithmetic of approximate numbers. In Advances in Cryptology– ASIACRYPT 2017: 23rd International Conference on the Theory and Applications of Cryptology and Information Security, Hong Kong, Chi...
2017
-
[26]
Fetal ab- dominal structures segmentation dataset using ultrasonic images.Mendeley Data, 2023
Roberto; Santos Luís Otávio; Soares Muylaert Barroso Felipe; Zimmermann Loureiro Chaves Thiago Da Correggio, Karine Souza; Noya Galluzzo. Fetal ab- dominal structures segmentation dataset using ultrasonic images.Mendeley Data, 2023
2023
-
[28]
Adaptive gradient sparsification for efficient federated learning: An online learning approach
Pengchao Han, Shiqiang Wang, and Kin K Leung. Adaptive gradient sparsification for efficient federated learning: An online learning approach. In2020 IEEE 40th international conference on distributed computing systems (ICDCS), pages 300–
Reviewed August 6, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.