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Title: Information Plane Analysis of Deep Neural Networks via Matrix--Based Renyi's Entropy and Tensor Kernels

Paper content:
Analyzing deep neural networks (DNNs) via information plane (IP) theory has gained tremendous attention recently as a tool to gain insight into, among others, their generalization ability.
However, it is by no means obvious how to estimate mutual information (MI) between each hidden layer and the input/desired output, to construct the IP.
For instance, hidden layers with many neurons require MI estimators with robustness towards the high dimensionality associated with such layers.
MI estimators should also be able to naturally handle convolutional layers, while at the same time being computationally tractable to scale to large networks.
None of the existing IP methods to date have been able to study truly deep Convolutional Neural Networks (CNNs), such as the e.g.\ VGG-16.
In this paper, we propose an IP analysis using the new matrix--based R\'enyi's entropy coupled with tensor kernels over convolutional layers, leveraging the power of kernel methods to represent properties of the probability distribution independently of the dimensionality of the data.
The obtained results shed new light on the previous literature concerning small-scale DNNs, however using a completely new approach.
Importantly, the new framework enables us to provide the first comprehensive IP analysis of contemporary large-scale DNNs and CNNs, investigating the different training phases and providing new insights into the training dynamics of large-scale neural networks.
Although Deep Neural Networks (DNNs) are at the core of most state-of-the art systems in computer vision, the theoretical understanding of such networks is still not at a satisfactory level (Shwartz-Ziv & Tishby, 2017) .
In order to provide insight into the inner workings of DNNs, the prospect of utilizing the Mutual Information (MI), a measure of dependency between two random variables, has recently garnered a significant amount of attention (Cheng et al., 2018; Noshad et al., 2019; Saxe et al., 2018; Shwartz-Ziv & Tishby, 2017; Yu et al., 2018; .
Given the input variable X and the desired output Y for a supervised learning task, a DNN is viewed as a transformation of X into a representation that is favorable for obtaining a good prediction of Y .
By treating the output of each hidden layer as a random variable T , one can model the MI I(X; T ) between X and T .
Likewise, the MI I(T ; Y ) between T and Y can be modeled.
The quantities I(X; T ) and I(T ; Y ) span what is referred to as the Information Plane (IP).
Several works have demonstrated that one may unveil interesting properties of the training dynamics by analyzing DNNs in the form of the IP Goldfeld et al., 2019; Noshad et al., 2019; Chelombiev et al., 2019) .
Figure 1 , produced using our proposed estimator, illustrates one such insight that is similar to the observations of Shwartz-Ziv & Tishby (2017) , where training can be separated into two distinct phases, the fitting phase and the compression phase.
This claim has been highly debated as subsequent research has linked the compression phase to saturation of neurons (Saxe et al., 2018) or clustering of the hidden representations (Goldfeld et al., 2019) .
Contributions We propose a novel approach for estimating MI, wherein a kernel tensor-based estimator of Rényi's entropy allows us to provide the first analysis of large-scale DNNs as commonly found in state-of-the-art methods.
We further highlight that the multivariate matrix-based approach, proposed by , can be viewed as a special case of our approach.
However, our proposed method alleviates numerical instabilities associated with the multivariate matrixbased approach, which enables estimation of entropy for high-dimensional multivariate data.
Further, using the proposed estimator, we investigate the claim of Cheng et al. (2018) that the entropy H(X) ≈ I(T ; X) and H(Y ) ≈ I(T ; Y ) in high dimensions (in which case MI-based analysis would be meaningless) and illustrate that this does not hold for our estimator.
Finally, our results indicate that the compression phase is apparent mostly for the training data, particularly for more challenging datasets.
By utilizing a technique such as early-stopping, a common technique to avoid overfitting, training tends to stop before the compression phase occurs (see Figure 1 ).
This may indicate that the compression phase is linked to the overfitting phenomena.
Figure 1 : IP obtained using our proposed estimator for a small DNN averaged over 5 training runs.
The solid black line illustrates the fitting phase while the dotted black line illustrates the compression phase.
The iterations at which early stopping would be performed assuming a given patience parameter are highlighted.
Here, patience denotes the number of iterations that need to pass without progress on a validation set before training is stopped to avoid overfitting.
It can be observed that for low patience values, training will stop before the compression phase.
For the benefit of the reader, the bottom right corner displays a magnified version of the first four layers.
In this work, we propose a novel framework for analyzing DNNs from a MI perspective using a tensor-based estimate of the Rényi's α-order entropy.
Our experiments illustrate that the proposed approach scales to large DNNs, which allows us to provide insights into the training dynamics.
We observe that the compression phase in neural network training tends to be more prominent when MI is estimated on the training set and that commonly used early-stopping criteria tend to stop training before or at the onset of the compression phase.
This could imply that the compression phase is linked to overfitting.
Furthermore, we showed that, for our tensor-based approach, the claim that H(X) ≈ I(T ; X) and H(Y ) ≈ I(T ; Y ) does not hold.
We believe that our proposed approach can provide new insight and facilitate a more theoretical understanding of DNNs.

assistant: text: The authors propose a tensor-kernel based estimator for mutual information estimation between high-dimensional layers in a neural network.