Title: INTERPRETABLE DEEP NEURAL NETWORK MODELS: HYBRID OF IMAGE KERNELS AND NEURAL NET-WORKS

Abstract: We introduce interpretable components into a Deep Neural Network (DNN) to explain its decision mechanism. Instead of reasoning a decision from a given trained neural network, we design an interpretable neural network architecture before training. Weight values in the first layers of a Convolutional Neural Network (CNN) and a ResNet-50 are replaced by well-known predefined kernels such as sharpening, embossing, color filters, etc. Each filter's relative importance is measured with a variant of the saliency map and Layer-wise Relevance Propagation (LRP) proposed by (Simonyan et al., 2014; Bach et al., 2015) . We suggest that images processed by predefined kernels still contain enough information for DNNs to extract features without degrading performances on MNIST, and Ima-geNet datasets. Our model based on the ResNet-50 shows 92.1% top-5 and 74.6% top-1 accuracy on the ImageNet dataset. At the same time, our model provides three different tools to explain individual classification and overall properties of a certain class: the relative importance scores with respects to (1) each color, (2) each filter, and (3) each pixel of the image.

1. INTRODUCTION:

A Neural Network (NN) is a nonlinear and complex system with countless configurations. Tracing and analyzing these networks are extremely difficult but a necessary task to ensure the reliability of DNNs in critical areas like Pharmaceutics, Finance, and criminal justice. Furthermore, comprehensions on the DNN's decision-making processes can promote the design of an optimal NN model for a given data set.

In the previous literature, an interpretation of an NN model is mostly based on trained models. Obtaining parameters, weights and biases from trained NN models, features (subsets of inputs) are extracted based on relative importance scores. Although these methods can successfully identify important features, it is still difficult to tell why such a selection has been made. For example, Figure 1 shows 'gradient × input' scores based on (Shrikumar et al., 2016; Bach et al., 2015) . Red dots imply high scores, but they do not explain how and why these dots are important.

On the other hand, DNNs are highly flexible. Numerous different types of data can be fed into the NN: RGB, YCbCr, grayscale in different color spaces, the Discrete Cosine Transformed (DCT) (Lionel et al., 2018) of frequency domain representation, the scale-invariant feature transformed (SIFT) (Liang et al., 2016) , bag-of-feature (BoF) model (Brendel & Bethge, 2019) , and so on. DNNs still can be trained to extract features from these different forms of data, but it is not completely understood how. We believe that the flexibility of DNNs also can be used to explain classification mechanisms. If we input several different images -processed differently from a single image -into DNNs and measure how frequently or significantly each image is used, we can deduce which features or characteristics are used for classification.

In this paper, we propose interpretable DNN architectures called General Filter Neural Net (GFNN) and General Filter ResNet (GF-ResNet). We introduce human-interpretable components into a DNN: well-known image kernels such as edge detection, sharpening, embossing, etc. as shown in Table 1 . Figure 1 (a) and (b) depict some of these filters and their effects on the original image. We substitute the weights of the first layers of a CNN and a ResNet-50 with these filters. Filtered by the kernels, a set of transformed images are generated from the input images and they are fed into the subsequent neural networks. We train the neural network and measure relative importances of each kernel by a method, based on saliency map and Layer-wise Relevance Propagation (LRP) (Simonyan et al., 2014; Bach et al., 2015) , which will be described in Section 2.

2. FILTER SCORES AND FILTER SCORE IMAGES:

Our interpretable model is based on two previous feature extraction methods: the saliency map and LRP (Simonyan et al., 2014; Bach et al., 2015) . These approaches were focused on the importance scores of the input image pixels. Simonyan et al. proposed that the gradient of output layers w.r.t. pixels of a specific input reflects the relative importance of each pixel (Simonyan et al., 2014) . The magnitude of the gradients indicates the change of which pixels in an input influence most in the class scores. On the other hand, Bach et al. suggested a method for propagating the importance scores through layers called LRP (Bach et al., 2015) . LRP approach also provides the importance scores w.r.t. each pixel of an input image. The magnitude of the LRP scores indicates the contributions to the class of each pixel in the image positively or negatively. Shrikumar et al. showed that the LRP approach is equivalent to the element-wise product of the saliency map and the input (Shrikumar et al., 2016) .

Even though previous approaches were developed independently, they are closely related to each other. Especially, when the activation function is a piece-wise linear function, saliency map and LRP approach are rooted in the same equation. In this section, we will (1) discuss the mathematical relation between the saliency map and the importance scores in LRP, and (2) show that their methods can be generalized to any layers to monitor important nodes.

2.1. PIECE-WISE LINEAR NEURAL NETWORK:

In this section, we will define a trained neural network model mathematically. The basic assumption of this work is that the activation functions for NN models are Rectified Linear Units (ReLU) functions. ReLU (x) is linear if x ≥ 0, but is equal to 0 if x < 0. In other words, ReLU (x) is a piece-wise linear function. We will show the mathematical relations between the piece-wise linear function and the saliency map and importance scores in LRP.

2.1.1. FRAMEWORK:

For a given neural network model M with (n l + 1) layers, w l i,j denotes the weight from the i th node in the l th layer to the j th node in the (l + 1) st layer, and L l i and b l i denote the i th node (or its value) and the i th bias in the l th layer respectively. Especially, let L 0 and L n l be the input layer and the output layer with dimensions of n in and n out respectively. All indices such as l, i, j, n in , n out , and n l are integers.

If rectified linear units function, ReLU(x), is the activation function for an NN model M , then the value of a node L l+1 j is

L l+1 j = ReLU i (w l i,j • L l i ) + b l+1 j (1)

by the definition of artificial neurons. Observe that even though the weights w l i,j are a constant for a trained model M , L l i is a variable, dependent on an input I. We also denoted M I as the state when an input I is fed into a trained model M . The difference between M and M I is that while M is a structure, M I is a snapshot of M with the nodes of L l j loaded with their values from the input I.

2.1.2. OUTPUT LAYERS WITH RESPECT TO INPUT LAYER:

For an NN model M I , the first layer is the input layer, i.e., L 0 i = I i for i = 0, . . . , n in . Similarly, the output layer is the last layer, i.e., L n l j = O j , where O j denotes the j th node in the output layer where 0 ≤ j ≤ n out . Jung et al. showed that for any NN models M I ,

O j = i (α i,j • I i ) + c j (2)

where α i,j , and c j are constants within a polytope-shape partition defined by the ReLU functions of each node in each layer (Jung et al., 2019) . Equation 2shows that the output layer of an NN is a piece-wise linear function of the input layer. Specifically, each parameter to ReLU function defines a half-space in the input space I such that {I i : i (α i,j • I i ) + c j ≥ 0} and their intersections define a polytope n, the input space. Because α i,j and c j are constants within each polytope, the output layer of an NN is a piece-wise linear function of the input layer.

The coefficients, α i,j , are Simonyan et al.'s gradients w.r.t. the input pixels. Similarly, multiplication of coefficients and pixels of an input, α i,j • I i , are the same as the scores calculated by LRP or the gradient × input.

2.1.3. FILTER SCORES:

We generalize importance scores such that they are defined for any layers of an NN. We will define the importance scores of the first layer in a CNN as filter scores since they imply how important each filter is.

Proposition 1. For any NN models M I and any l,

O j = i (α i,j • L l i ) + c j (3)

where α i,j and c j are some constants.

It is not difficult to prove the proposition since we can substitute L l i for I i in Eqaution 2 without loss of generality. Using Proposition 1, we can define the scores similar to the gradient and the gradient × input for any layer. We define filter scores as the sum of each kernel's importance scores, i.e. α i,c • L 1 i , where c is the largest nodes of the output layer. For example, if a DNN classifies an image as 0, then the filter score for filter number 37, e.g. an embossing kernel in our model, is given by

i∈K (α i,0 • L 1 i )

where K is a group of indices belong to the 37 th kernel. The filter score image is a layer-wise equivalent version of the gradient × input.

3. AN ARCHITECTURE OF GENERAL FILTER NEURAL NETWORK:

The main idea behind the interpretable model is to design a neural network along with humanunderstandable components. If the effects of these components are well-studied for images, we can interpret a DNN accordingly. As for an image classification task, image kernels such as edge filters, etc. are well-understood and they are good candidates for the human-understandable components. In addition, the dimensions of image kernels are perfectly suitable for replacing the first layer of the convolutional neural networks.

In other words, our model is a hybrid of image kernels and CNNs as shown in Figure 2 (a). In a regular CNN, the weights of the first layer are obtained by a training. We replace them with 40 different filters as shown in Table 1 : 32 different types of edge detecting filters, 2 second-order derivative filters, 3 sharpening filters, embossing, and 2 blurring filters. Especially, we used directional Robert, Prewitt, Sobel, and Frei-Chen filters for edge detection. As for the architecture, two convolutional and max-pooling layers are added in the subsequent layers. 3 × 3 × 40 × 64 and 3 × 3 × 62 × 128 filters are used for each convolutional layer respectively with an appropriate padding. We use 2×2 max-pooling after the convolutional layers. The first and the second dense layers have dimensions of 2048 and 625 respectively. Such image filters are not directly applicable to color images since each pixel has 3 dimensions. Furthermore, the dimension of layers should match the subsequent layer of ResNet-50 and CNNs. Hence, color images are divided into 3 single layers and the same pre-defined kernels are convoluted over each layer separately. We concatenate all resulting layers into a single block and 1 × 1 × 96 × D filters are applied to reduce the depth of the blocks as shown in Figure 2 (b) , where D depends on subsequent layers.

ResNet-50 contains 50 layers including 5 different types of building blocks (He et al., 2016) . We substitute our interpretable components with D = 64 for the first kernel (7×7×3×64) of ResNet-50 called GF-ResNet. Similarly, our interpretable components are followed by two convolutional layers and max-pooling layers in the case of the CNNs for color images. 5×5×64×64 and 3×3×64×128 filters are used for each convolutional layer respectively with an appropriate padding. We use 2×2 max-pooling after the convolutional layers. 

3.1. EFFECTS OF THE FILTERS:

In the previous literature regarding interpretable DNNs, the performance has to be sacrificed over interpretability (Brendel & Bethge, 2019; Liang et al., 2016; Xiao et al., 2015) . We hypothesized that the features extracted in their models are only a limited number of features and it might have lowered the accuracies. To verify the hypothesis, we changed the number of edge filters for GFNN from 32 to 4 and trained it with MNIST dataset as shown in Figure 3 (a) . As the number of filters decreases, the features extracted from each filter are also reduced as well. As expected, the accuracy declines as the number of filters decreases. There is a relatively large drop particularly from 8 to 4. It is because there are 8 different directions for edge filters and those with the same direction can compensate each other. When there are only 4 filters, the compensation is not possible. Suggested interpretable models achieved higher or equivalent accuracies even though they have fewer parameters as shown in Figure 3 (b) . A regular CNN and the interpretable model are trained 50 times with different initial weights. The highest, lowest and average accuracies of the interpretable model are higher than those of the regular CNN. Therefore, the human-interpretable components do not necessarily deteriorate the performance of NNs but can enhance it. The first convolutional kernels of a regular CNN are initialized with the same values as the predefined kernels. However, setting the initial values is not good enough to improve CNNs, rather it worsens the accuracy. Random filters are also applied instead of predefined kernels to verify the role of the image processing kernels. Interestingly, the normalized random filters actually produced a better accuracy than a regular CNN does. Surprisingly, in Figure 3 (d), the random filters perform better than normal ResNet Figure 4 : The average filter scores of each filter for the entire MNIST data set. Filter numbers represent kernel types as described in Table 1 . and GF-ResNet with ImageNet dataset. This might be due to the regularization as we observed in the batch normalization and the dropout.

A similar tendency is found in GF-ResNet with ImageNet data as shown in Figure 3 (c) and (d) . As the number of filters decreases, the accuracies decrease. The difference is that the accuracy loss in GF-ResNet is not as severe as the loss in MNIST even with 4 filters. The accuracy drops significantly when there are only the same directional filters. The normalized random filters also sightly improve the accuracy as shown in MNIST. Therefore, as we hypothesized, there is a loss of information after the images are filtered if the pre-defined kernels are chosen badly. In other words, our pre-defined filters extract enough features from the images for the complex classification such as ImageNet data.

3.2. INTERPRETATION OF THE FILTERS:

GFNN and GF-ResNet are the NN models incorporated with the human interpretable components. To interpret each classification, we need to calculate the gradient × L2, where the interpretable images are produced, for each input image. Since there are 40 and 96 filters for GFNN and GF-ResNet respectively, 40 and 96 different filter score images are created for a given input image. For example, Figure 1 (c) shows 4 different filter score images out of 40 images. To analyze the contribution of each filter, we added every pixel values for each filter score image, i.e., filter scores.

In case of GF-ResNet, we can extract more information w.r.t. colors and pixels. Instead of adding the filter score images w.r.t. the filter, we can add them w.r.t. pixels. The figures on the right side of Figure 1 show which pixels in the image are important with the different methods; saliency map, the gradient × input and the gradient × L2. GF-ResNet can highlight the region of the object better than other methods. By summing the filter score image w.r.t. the colors, i.e. color filter scores, we can understand which color is important. For example, in Figure 6 

4. EXPERIMENTS ON MNIST DATASET AND IMAGENET DATASETS:

In this section we trained the proposed model with the Modified National Institute of Standards and Technology database (MNIST) and the Imagenet Large Scale Visual Recognition Challenge 2012 classification dataset (Krizhevsky et al., 2019) and performed analyses on the results. Our implementation for ResNet and GF-ResNet follows the practice in (He et al., 2016; Krizhevsky et al., 2019) except the per-pixel mean subtraction and PCA color augmentation: a 224×224 random-crop, its horizontal flip, and batch normalization. We train models with stochastic gradient descent with a mini-batch of 100. The initial learning rate is 0.1 and it is divided by 10 for every 15 epochs until 90 epochs. A weight decay and a momentum are 0.0001 and 0.9 respectively. GFNN and GF-ResNet are successfully trained without degrading the performance as shown in Figure 3 . West and east filters are the most important for the number 1s, which are written in the vertical direction. Furthermore, people frequently write number 1s slightly tilted in the southwest direction. Thus, northwest filter scores are relatively high compared to other filters.

Interesting characteristics are found in the number 0s. Because number 0s are round shapes, result images of all directional filters should include some parts of 0s as shown in Figure 1 (b) . However, the DNN mostly uses the west and east filters for the classification of number 0s. Figure 5 shows that the DNN looks for the vertical parts of 0s.

Contrarily, as for the number 5s, south filters are important. That is, horizontal parts of images are essential to classify the number 5s. The filter score image section of the number 5 in Figure 5 , top horizontal straight lines of the number 5 have the most contribution to the classification.

Edge detecting filters usually have top 3 scores except for number 8s. Sharpening filters have the second-highest scores for the number 8. It is because the sharpening filters produce two distinct round shapes as shown in the right-side three images of Figure 5 . In other words, two round shapes are distinct features for the number 8.

We can classify numbers into three categories: vertical, horizontal, and other numbers. Vertical components are important for 0s, 1s, 4s, and 6s. For the numbers 2s, 3s and 5s horizontal parts are essential. Other numbers use roughly all directional filters except for south filters.

Furthermore, embossing filters are not used at all for any numbers. On the contrary, blurring filters are frequently used. Embossing filters are another kind of directional edge filters, especially for black and white images. Thus, their effects on images are duplicated with other edge filters.

4.2. INTERPRETATION OF GF-RESNET:

The interpretable components of GF-ResNet provide 32 filter score images. There are three tools to interpret the individual and overall classification with GF-ResNet based on the filter score images; the color and edge filter scores, and the summation of filter score images w.r.t. the input pixels.

Figure 6 (a) shows the average filter scores for the entire 'green mamba' class. As expected, the green filter score is the most important since the mamba is green. The edge filter scores show that 6 (a) also suggests that the NN generally uses the blue color to locate the green mamba in the image by eliminating the background since the bright colors have high magnitudes in all color channels. For example, the first row on the right side of Figure 1 shows a green mamba in the blue background. The filter score image with respects to the input pixels indicates that the NN spots the green mamba successfully by subtracting blue colors from the red channel.

The edge filter scores indicate important edges in the image. If the target object is horizontally located, south and north filters become important in the image. Furthermore, if there are several edges with various objects in the image, then the edges of the target objects are chosen as the most important contributors. For example, when a tree branch and a green mamba are in the same image, the NN separates them by utilizing different color edge filters.

The pixel-wise interpretation of GF-ResNet's filter score images performs better than the saliency map and the gradient×input as shown in Figure 1 . Our model successfully selects the regions of the target object in the image even when other methods fail. We think that the interpretable components of the GF-ResNet force the NN to select filter-created features, i.e. different edges in the image so that the interpretability and the accuracy are improved.

5. CONCLUSION:

In this paper, we successfully implemented predefined and commonly used image kernels into CNNs and ResNet-50, named as GFNN and GF-ResNet. Our models were trained with MNIST, and Im-ageNet datasets and produced impressive performances for large-scale object recognition tasks (up to 92.1% top-5 and 74.6% top-1 on ImageNet). Filter scores and filter score images defined in 2.1.3 are useful metrics for quantifying relative importances of each predefined kernels. We showed how DNNs utilize image kernels to classify digits and images according to their shapes and colors with various datasets.

A APPENDIX:

A.1 LIST OF IMAGE PROCESSING KERNELS. 0 1 A.1.2 EMBOSSING 1 0 0 0 0 0 0 0 -1 A.1.3 BLURRING 1/9 1/9 1/9 1/9 1/9 1/9 1/9 1/9 1/9 1/16 1/8 1/16 1/8 1/4 1/8 1/16 1/8 1/16 A.1.4 EDGE: EAST

A.1.1 SHARPENING:

1 1 -1 1 -2 -1 1 1 -1 5 -3 -3 5 0 -3 5 -3 -3 1 0 -1 1 0 -1 1 0 -1 1 0 -1 2 0 -2 1 0 -1 A.1.5 EDGE: NORTHEAST 1 -1 -1 1 -2 -1 1 1 1 -3 -3 -3 5 0 -3 5 5 -3 0 -1 -1 1 0 -1 1 1 0 0 -1 -2 1 0 -1 2 1 0 A.1.6 EDGE: NORTH -1 -1 -1 1 -2 1 1 1 1 -3 -3 -3 -3 0 -3 5 5 5 -1 -1 -1 0 0 0 1 1 1 -1 -2 -1 -1 0 1 0 1 2 A.1.7 EDGE: NORTHWEST -1 -1 1 1 -2 1 1 1 1 -3 -3 -3 -3 0 5 -3 5 5 1 -1 0 -1 0 1 0 1 1 -2 -1 0 -1 0 1 0 1 2 A.1.8 EDGE: WEST -1 1 1 -1 -2 1 -1 1 1 -3 -3 5 -3 0 5 -3 -3 5 -1 0 1 -1 0 1 -1 0 1 -1 0 1 -2 0 2 -1 0 1

A.1.9 EDGE: SOUTHWEST

1 1 1 -1 -2 1 -1 -1 1 -3 5 5 -3 0 5 -3 -3 -3 0 1 -1 -1 0 1 -1 -1 0 0 1 2 -1 0 1 -2 -1 0 A.1.10 EDGE: SOUTH 1 1 1 1 -2 1 -1 -1 -1 5 5 5 -3 0 -3 -3 -3 -3 1 1 1 0 0 0 -1 -1 -1 1 2 1 0 0 0 -1 -2 -1 A.1.11 EDGE: SOUTHEAST 1 1 1 1 -2 -1 1 -1 -1 5 5 -3 5 0 -3 -3 -3 -3 1 1 0 1 0 -1 0 -1 -1 2 1 0 1 0 -1 0 -1 -2

A.1.12 EDGE: SECOND ORDER DERIVATIVE OPERATEORS 0 1 A.1.13 FIRST ORDER DERIVATIVE 0 0 -1 0 1 0 0 0 0 -1 0 0 0 1 0 0 0 0

1 0 -1 1 0 -1 1 0 -1 -1 -1 -1 0 0 0 1 1 1 1 0 -1 2 0 -2 1 0 -1 -1 -2 -1 0 0 0 1 2 1   1 0 -1 √ 2 0 - √ 2 1 0 -1     -1 - √ 2 -1 0 0 0 1 √ 2 1  

:



Figures and Tables:

Figure 1: Left. The interpretable neural network model divides input images into several pieces. Each piece is fed into subsequent networks. (a) Schematic images of the 7 th , 15 th , 23 rd and 31 st filters of GFNN. (b) Each filter is convoluted onto the original images. (c) The filter scores images as defined in Section 2.1.3, i.e., the gradient × L2 layer. Right. A relative importance can be traced by several interpreting methods.; Saliency map, gradient × input , and the sum of filter score images with respect to the input pixels.

Figure 2: (a) The architecture of GFNN for grayscale images. The difference between a regular CNN and our model is the substitution of the kernels in the first layer by predefined ones. (b) Our interpretable components for color images. Predefined kernels are applied to each color components and resulting layers are concatenated. 1×1×96×D filters, not predefined, are convoluted to reduce the dimension.

Figure 3: (a) Accuracy comparison when the number of predefined kernel filters decreases with MNIST dataset. Accuracy drops as 32 edge filters in Table 1 decrease to 4 filters. (b) Accuracy comparison between different conditions with MNIST. (c) Accuracy comparison when the number of predefined kernel filters decreases with ImageNet dataset. (d) Accuracy comparison between different conditions with ImageNet.

(b), color filter scores are used to show how GF-ResNet can eliminate the background.

Figure 5: Typical MNIST images of 0, 5 and 8. Rows show the most significant filters for the given numbers, and their filter score images.

Figure 6: (a) The interpretation of an entire 'green mamba' class. The color filter scores and edge filter scores are shown. (b) An example of the 'steel arch bridge' class. The original image and its color filter scores are on the left side. The red and blue channels of the original image are shown in the center. The magnitude of each color channel is shown with colors. The sky background is not considered for classification as indicated in the color filter score.

List of filter types and their corresponding filter numbers. The first order derivatives filters are used only when it is indicated. Full list of filters can be found in Appendix

Table content: <table><tr><td>Kernel Types</td><td colspan="2">Filter Numbers Kernel Types</td><td>Filter Numbers</td></tr><tr><td>Edge Detecting East</td><td>0-3</td><td colspan="2">Second Order Derivative 32, 33</td></tr><tr><td colspan="2">Northeast 4-7</td><td>Sharpening</td><td>34-36</td></tr><tr><td>North</td><td>8-11</td><td>Embossing</td><td>37</td></tr><tr><td colspan="2">Northwest 12-15</td><td>Blurring</td><td>38, 39</td></tr><tr><td>West</td><td>16-19</td><td>First Order Derivative*</td><td>40-47</td></tr><tr><td colspan="2">Southwest 20-23</td><td/><td/></tr><tr><td>South</td><td>24-27</td><td/><td/></tr><tr><td colspan="2">Southeast 28-31</td><td/><td/></tr></table>

Peer review (iclr2020):
=== A. Summary === The paper proposes to hand-design (as opposed to learning) the 3x3 kernels in the very first layer of an image classifier CNN to be the well-known kernels that perform sharpening, embossing, blurring, and edge detecting, etc. After that, the network is trained to perform image classification (i.e. these kernels are kept frozen and only other layers* parameters are updated during training) on MNIST or ImageNet. The authors argue that the hand-designed kernels are well-understood and showed qualitative examples of how they can be used to interpret what filters are often used to classify which class of images. Additionally, the paper shows that the integration of the hand-designed filters almost kept the network accuracy unchanged. === B. Decision === Reject. I vote for Reject because the work here, i.e. replacing the 1st layer 3x3 filters with the hand-designed ones, does not provide any new or surprising insights into what we have already known about the first layer kernels of CNNs or CNNs in general. It has been since 2012 that Krizhevsky et al. showed the 1st layer filters learned during the training already learned Gabor-like filters and kernels that detector colors. That is, they are already quite interpretable from human perspectives. Can*t one reproduce this paper with the learned filters and draw similar insights? The choices of the hand-designed kernels in the paper are arbitrary i.e. not supported by any reason. === C. Suggestions for improvement === Interpreting CNNs is an important quest and designing interpretability into CNNs is a viable approach! However, I suggest the authors seriously think about the motivation of the work i.e. focus on identifying what questions are not well-understood in CNNs and how your methods answer those questions.