Take A Shortcut Back: Mitigating the Gradient Vanishing for Training Spiking Neural Networks

The Spiking Neural Network (SNN) is a biologically inspired neural network infrastructure that has recently garnered significant attention. It utilizes binary spike activations to transmit information, thereby replacing multiplications with additions and resulting in high energy efficiency. However, training an SNN directly poses a challenge due to the undefined gradient of the firing spike process. Although prior works have employed various surrogate gradient training methods that use an alternative function to replace the firing process during back-propagation, these approaches ignore an intrinsic problem: gradient vanishing. To address this issue, we propose a shortcut back-propagation method in our paper, which advocates for transmitting the gradient directly from the loss to the shallow layers. This enables us to present the gradient to the shallow layers directly, thereby significantly mitigating the gradient vanishing problem. Additionally, this method does not introduce any burden during the inference phase. To strike a balance between final accuracy and ease of training, we also propose an evolutionary training framework and implement it by inducing a balance coefficient that dynamically changes with the training epoch, which further improves the network's performance. Extensive experiments conducted over static and dynamic datasets using several popular network structures reveal that our method consistently outperforms state-of-the-art methods.

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Peer review

Reviewer Jb1s6/10 · confidence 5/52024-06-18

Summary

The paper trains SNNs using surrogate gradient learning. In order to mitigate the gradient vanishing problem, the paper proposed the Shortcut Back-propagation method and utilizes an evolutionary algorithm framework to balance the training of shallow and deep layers. The effectiveness of the proposed method is demonstrated through many experiments.

Strengths

1) The shortcut backpropagation method and the evolutionary training method are novel. 2) This paper can well handle the gradient vanishing problem. 3) The paper is well-written. 4) The paper shows the effectiveness of the proposed methods through many experiments.

Weaknesses

1) The author should add more mathematical proof to demonstrate that the mentioned residual structure in SNN is not very effective? The introduction of shortcut branches might add complexity to the network architecture, which could affect the interpretability of the model. 2) Some recent SOTA works should be compared with too. The authors can also compare with paper [1][2] which obtains really good results by MS-ResNet-18 backbone with 1 or 6 timesteps on large imageNet datasets. [1]Yao M, Zhao G, Zhang H, et al. Attention spiking neural networks[J]. IEEE transactions on pattern analysis and machine intelligence, 2023. [2] Qiu X, Zhu R J, Chou Y, et al. Gated attention coding for training high-performance and efficient spiking neural networks[C]. Proceedings of the AAAI Conference on Artificial Intelligence. 2024, 38(1): 601-610.

Questions

1) Why are the bolded values not always the best values?

Rating

6

Confidence

5

Soundness

3

Presentation

3

Contribution

3

Limitations

I find no limitation about the paper.

Reviewer 6ctb6/10 · confidence 4/52024-07-07

Summary

This paper proposes a simple method to mitigate the gradient vanishing problem in the training of SNNs. This method introduces some early classification heads (including a pooling layer and a fully connected layer) to the SNN. Because the gradients from the early classification heads pass fewer surrogate gradients, this method aids the SNN in addressing the gradient vanishing problem. The authors also suggest an evolutionary training framework that changes the loss function to gradually adjust how important early classification head outputs are during the training phase. The proposed methods are only alive in the training phase and will not affect the inference phase of SNN.

Strengths

This proposed method partially alleviates the gradient vanishing problem in the training of SNN with surrogate gradients. Furthermore, the method has demonstrated excellent performance across multiple datasets. The Short-BP method can be easily integrated into the SNN training process without introducing excessive computational overhead. Furthermore, the evolutionary training framework effectively mitigates the short-BP problem, which may make the network pay more attention to early classification heads than the final SNN output. The writing in this paper is clear and concise.

Weaknesses

1. In this paper, the author only demonstrates a change in gradient distribution in the first layer. Presenting the changes in the men and variance of the absolute gradients for each layer would provide a more direct proof of their argument. 2. The author should provide a more detailed mathematical proof to explain why the use of surrogate gradients in deep SNN would lead to gradient vanishing, as well as why direct use of residual learning will not address the problem. 3. The author has not demonstrated their method on much deeper network architectures where the gradient vanishing problem is more severe.

Questions

1. How is the network divided into multiple blocks? Are there any additional rules for the insertion position and number of early classification heads? 2. The results of using short-BP to train ResNet 18 in Table 1 and Table 2 are quite different. There may be a transcription error here.

Rating

6

Confidence

4

Soundness

3

Presentation

3

Contribution

3

Limitations

NA

Reviewer runr6/10 · confidence 5/52024-07-11

Summary

This paper proposes shortcut connections between layers to mitigate the gradient vanishing problem in SNNs. Additionally, the authors present a way to phase out the shortcut connections over training so that inference can be done without these additional connections. The experiments show that this method improves training performance in several image classification tasks.

Strengths

1.The idea is small, but interesting and effective enough. 2.The performance improvement over the existing SNN methods is noticeable. 3.The paper is well-written.

Weaknesses

1.The proposed method will increase the training time. 2.In the experimental section, some newer methods should be compared with this method. 3.Figure 2 lacks horizontal and vertical coordinates, and the readability and comprehensibility of the picture need to be improved.

Questions

1.Does the proposed method lead to an increase in the calculation of gradient backpropagation? How much is the increased training time.

Rating

6

Confidence

5

Soundness

4

Presentation

3

Contribution

4

Limitations

None.

Reviewer Jb1s2024-08-08

Thank you for your reply. I think this is a nice bit of discussion and could be added to the manuscript. In light of the additional discussion, I'd like to raise my score to a 6. This is an interesting piece of work and would be a nice addition to NeurIPS.

Program Chairsdecision2024-09-25

Decision

Accept (poster)

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