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摘要: 近年来, 深度学习在图像分类、目标检测及场景识别等任务上取得了突破性进展, 这些任务多以卷积神经网络为基础搭建识别模型, 训练后的模型拥有优异的自动特征提取和预测性能, 能够为用户提供“输入–输出”形式的端到端解决方案. 然而, 由于分布式的特征编码和越来越复杂的模型结构, 人们始终无法准确理解卷积神经网络模型内部知识表示, 以及促使其做出特定决策的潜在原因. 另一方面, 卷积神经网络模型在一些高风险领域的应用, 也要求对其决策原因进行充分了解, 方能获取用户信任. 因此, 卷积神经网络的可解释性问题逐渐受到关注. 研究人员针对性地提出了一系列用于理解和解释卷积神经网络的方法, 包括事后解释方法和构建自解释的模型等, 这些方法各有侧重和优势, 从多方面对卷积神经网络进行特征分析和决策解释. 表征可视化是其中一种重要的卷积神经网络可解释性方法, 能够对卷积神经网络所学特征及输入–输出之间的相关关系以视觉的方式呈现, 从而快速获取对卷积神经网络内部特征和决策的理解, 具有过程简单和效果直观的特点. 对近年来卷积神经网络表征可视化领域的相关文献进行了综合性回顾, 按照以下几个方面组织内容: 表征可视化研究的提起、相关概念及内容、可视化方法、可视化的效果评估及可视化的应用, 重点关注了表征可视化方法的分类及算法的具体过程. 最后是总结和对该领域仍存在的难点及未来研究趋势进行了展望.Abstract: In recent years, deep learning has made breakthrough progress on image classification, object detection, and scene recognition tasks. These tasks mostly build recognition models based on the convolutional neural network (CNN). The trained models have excellent automatic feature extraction and prediction performance, which is able to provide users with “input-output” end-to-end solutions. However, due to the distributed feature coding and the increasingly complex model structure, users cannot yet accurately understand the internal knowledge representation of the model as well as the potential reasons for a specific decision. On the other hand, the application of the CNN models in some high-risk areas also requires a full understanding of the reason for their decisions, so as to get user's trust. Therefore, the interpreting ability of CNN has gradually attracted attention. Researchers have proposed a serious of methods for understanding and interpreting CNN, including post-hoc interpretation methods and building self-explainable models. These methods have their respective focuses and advantages, performing feature analysis and decision interpretation of CNN from various aspects. As one of the important CNN interpreting ability methods, representation visualization can visually present the features learned by CNN and the correlation between the input and output. In this way, a straightforward understanding of CNN internal features and decision-making can be obtained in a simple and intuitive way. This paper gives a comprehensive review of the related literatures on CNN representation visualization research in recent years, and organizes the content according to the following aspects: the introduction of representation visualization research, related concepts and contents, visualization methods, visualization effect evaluation, and the application of visualization. The classification of the representation visualization methods and the specific algorithms are our focus. Finally, the difficulties and future trends in the field are prospected, and the full text is summarized.
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表 1 梯度方法及其变种的特点比较
Table 1 Comparison of the characteristics of the gradient method and its variants
方法 显著图生成依据 特点 VBP 普通梯度 过程简单, 但存在梯度噪声问题 GBP 每一层使用 ReLU 过程简单, 但存在梯度噪声问题 积分梯度 梯度图的平均 过程复杂, 需多次迭代, 耗时 平滑梯度 梯度图的平均 过程复杂, 需多次迭代, 耗时 整流梯度 阈值过滤后的梯度 过程较复杂, 阈值的选取需要经验 表 2 类激活映射方法的比较
Table 2 Comparision of the class activation mapping methods
方法 通道权重 优点 缺点 CAM Softmax 层权重 类别区分性 依赖 GAP 层 Grad-CAM 各通道的梯度平均值 类别区分性, 结构通用 梯度不稳定 Grad-CAM++ 各通道的梯度平均值, 高阶梯度 类别区分性, 结构通用 梯度不稳定, 高阶梯度计算复杂 Score-CAM 对各通道的预测值 类别区分性, 结构通用, 权重稳定 权重计算过程复杂, 重复迭代耗时 表 3 可视化方法的特点比较
Table 3 Comparison of characteristics of visualization methods
方法分类 方法名称 发表年份 细粒度/
区域级类别相关 在线/
离线模型明晰/
模型不可知可视化视角 局部解释/
全局解释扰动 简单扰动[13, 42–43] 2014、2018 区域级 否 离线 模型不可知 输出类 局部 有意义的扰动[44] 2017 区域级 否 离线 模型明晰的 输出类 局部 生成式扰动[45–46] 2019 区域级 是 离线 模型明晰 输出类 局部 反向传播 梯度类反向传播 VBP[22–23] 2010、2013 细粒度 否 离线 模型明晰 输出类 局部 GBP[50] 2014 细粒度 否 离线 模型明晰 输出类 局部 Smooth gradient[52] 2017 细粒度 否 离线 模型明晰 输出类 局部 Integrated gradient[53] 2017 细粒度 否 离线 模型明晰 输出类 局部 Rectified gradient[54] 2019 细粒度 否 离线 模型明晰 输出类 局部 规则类反向传播 Deconvolution[13] 2013 细粒度 否 离线 模型明晰的 神经元/层 局部 LRP[58] 2015 细粒度 否 离线 模型明晰 输出类 局部 DTD[61] 2017 细粒度 否 离线 模型明晰 输出类 局部 CLRP[59]、SGLRP[60] 2018、2019 细粒度 是 离线 模型明晰 输出类 局部 类激活映射 CAM[62] 2015 区域级 是 在线 模型明晰 输出类 局部 Grad-CAM[63–64] 2016、2017 区域级 是 离线 模型明晰 输出类 局部 Grad-CAM++[65] 2018 区域级 是 离线 模型明晰 输出类 局部 Score-CAM[66] 2019 区域级 是 离线 模型明晰 输出类 局部 激活最大化 AM[81] 2009 细粒度 是 离线 模型明晰 神经元/输出类 全局 DGN-AM[82] 2016 细粒度 是 离线 模型明晰的 神经元/输出类 全局 注意为掩码 通道注意力[18] 2017 区域级 否 在线 模型明晰的 层 局部 空间–通道注意力[72] 2018 区域级 否 在线 模型明晰 层 局部 类别注意力 — 区域级 是 在线 模型明晰 层 — 其他方法 LIME[78] 2016 区域级 是 离线 模型不可知 输出类 局部 SHAP[79] 2017 细粒度 是 离线 模型不可知 输出类 局部 表 4 CNN表征可视化相关的综述文献统计
Table 4 Review literature statistics related to CNN representation visualization
文献 发表年份 侧重内容 [103] 2016 几种典型的特征可视化方法 (如扰动、反向传播、
激活最大化等), 以及相互之间的关系分析[104] 2017 特征可视化的必要性, 基于反向传播的可视化方法 [105] 2017 模型可视化, 不限于 CNN 可解释性领域 [19] 2018 基于反向传播的可视化方法
(AM、VBP、DTD 和 LRP 等)[106] 2018 自解释的 CNN [20] 2018 可解释性的概念, 相关文献分类 [107] 2018 人工智能的可解释性 [102] 2019 机器学习的可解释性方法与评估 [108] 2020 机器学习的可解释性 [109] 2020 深度学习的可解释性 [110] 2020 人工智能的可解释性 [111] 2020 人工智能的可解释性 -
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