Title: REVISITING DENOISING AUTO-ENCODERS

Abstract: Denoising auto-encoders (DAE)s were proposed as a simple yet powerful way to obtain representations in an unsupervised manner by learning a map that approximates the clean inputs from their corrupted versions. However, the original objective function proposed for DAEs does not guarantee that denoising happens only at the encoding stages. We argue that a better representation can be obtained if the encoder is forced to carry out most of the denoising effort. Here, we propose a simple modification to the DAE's objective function that accomplishes the above goal.

1. INTRODUCTION:

Auto-encoders (AE)s are unsupervised learning algorithms that capture structure in data by finding an representation of the inputs (encoding) from which they can be recovered at least approximately. By learning a transformation G (encoding) from the input x ∈ X to z = G(x) ∈ Z, the autoencoder tries to capture the structure of the input. To guarantee the transformation G preserves the information about x, a decoder G-1 (an approximate inverse map) is also learned such that a measure of fidelity E[D(X, X)] between the input and its reconstruction is optimized.

Learning non-linear encoding and decoding mappings has proved to be a non-trivial problem. For example, there is no consensus on what the dimensionality of the encoding space should be. On one side of the spectrum, AE architectures such as bottle-neck networks, map input data x ∈ X ⊆ R d to a lower dimensional space Z ⊆ R p , and then map it back to input space X through dimensionality expansion. The intuition behind bottleneck networks is that the lower dimensional space forces the encoder to capture meaningful relations between the variables in the input space. While this is clear when one is restricted to linear mappings, the problem becomes less well-understood if nonlinear mappings are allowed. The idea of using a low dimensional latent space has been recently employed in the context of modern generative models such as variational auto-encoders (VAE)s Kingma & Welling (2013) and generative adversarial networks (GAN)s Goodfellow et al. (2014) .

On the other hand, AE architectures can contain over-complete representations where inputs are mapped to a high dimensional encoding spaces; much higher than the dimensionality of the input space (dim(X ) < dim(Z)). Over-complete representations have proved very useful in supervised learning. However, effective learning of this high dimensional mappings relies on specialized mechanisms that attempt to avoid trivial solutions or on the large amount of constraint imposed by the supervised task. Approaches such as sparse encoding, which can be applied to over-complete scenarios, use the concept of effective dimensionality. A sparsity constraint induces an active set of variables with an expected L 0 norm smaller than the input space dimensionality. Many sparse coding procedures require solving an optimization problem at inference time since the encoding mapping is not explicit. However, it has been shown that efficient inference can be made possible by training a nonlinear feed-forward network to mimic the output of a sparse encoding algorithm Ranzato et al. (2006) and more recently with the proposal of techniques that impose very strong sparsity constraints such as winner take all auto-encoders Makhzani & Frey (2015) .

An important set of techniques that do not necessarily fall into either extreme in terms of dimensionality are based on the concept of robustness of the representation. A representation is said to be robust if is able to retain information about the input even if the input or the representation undergo perturbations. For example, contractive auto-encoders (CAE)s Rifai et al. (2011a; b) penalize the sensitivity of the encoding map G(x) to perturbations of the input x by minimizing the expected Frobenius norm of the Jacobian E[J G (X)] of G while maximizing fidelity of the reconstruction. Within this category, learning representations by local denoising, or better known as de-noising auto-encoders, provide a very general way to carry out unsupervised learning. De-noising autoencoders Vincent et al. (2008) were conceived as a very intuitive way to capture robustness in the encoding-decoding mapping by simply minimizing the error between the output reconstruction of the auto-encoder when a corrupted version of the input is feed to the map and the uncorrupted input. However, as already pointed out in Rifai et al. (2011a) , the DAE objective minimizes the reconstruction error, therefore DAEs lack of explicit robustness in the encoding phase. A workaround to the above problem can make DAEs an excellent choice for unsupervised learning. DAEs can be a very appealing alternative due to their simplicity for training and elegant interpretation. Due to their simplicity in the training criterion, DAEs have the potential to adapt to different kinds of architectures and to scale with the application. We argue that the current limitation of current DAEs can be overcome by making a very simple modification to the training objective leading to learning better representations in terms of robustness. The modified objective for DAE and is justification are the main contributions of our work.

2. DENOISING AUTO-ENCODERS:

Let X be the random variable representing the input, and X be a corrupted version obtained by an an stochastic operator q C ( X|X). The goal of the DAE is to learn a deterministic encoder-decoder composition

f (•) = G-1 (G(•)) such that, for Y = f ( X) the quantity E[D(Y, X)] is minimized.

The corruption process leads to learning non-trivial transformations of the input in particular for the over-complete cases where identity mapping can be learned if there is no appropriate capacity control mechanism in place. Let G and G-1 be parametrized by θ = {θ enc , θ dec }. Parameter learning of the DAE can be formulated as the following optimization problem:

EQUATION

where Xi ∼ q C ( X|X = x i ). As we already mentioned, a solution to (1) does not impose explicit restrictions to the encoder. In this approach, only the dimensionality and the range of the encoder can be controlled by modifying its architecture. Therefore, there exist the possibility that the encoder could be learning a feature map, for which some dimensions still carry the effects of perturbing the original input. In many cases, the actual denoising could be taking place in the decoding phase. Below, we present a simple way to overcome the above limitation based on a modification to the DAE objective (1).

2.1. MODIFIED DENOISING OBJECTIVE:

In order to explicitly enforce robustness in the encoding phase, we can measure the amount of distortion in the encoder by comparing the resulting feature values for the original uncorrupted input against the feature values obtained from encoding its corrupted counterpart. Likewise D in (1), the difference between can be measured by a function D enc (•, •), for instance cross-entropy can be used for encoder that map input values to [0, 1]. This leads to the following modified optimization problem:

EQUATION

where λ is a tradeoff parameter. The original DAE objective can be simple obtained by setting λ to infinity.

2.2. ANALYSIS OF THE MODIFIED DAE:

The modified objective (2) appears as an intuitive way to explicitly enforce robustness in the encoding phase. Unlike the plain objective where Y = f (X), the stochastic operator q C in conjunction with the encoder function G(.) lead to a random map from an instance x of X to the random variable Z|X = x. In this case, the minimization of the reconstruction error leads to maximization of a lower bound on the mutual information between Z and X1 . However, the information maximization perspective does not provide insights about how the noise acts as capacity control on the encoder. Since the only constraint on the entropy H(Z) is the encoder architecture, mutual information I(X; Z) = H(X) -H(X|Z) = H(Z) -H(Z|X) does not tell us much about the conditional entropy H(Z|X), which is directly related to the encoder. Bear in mind that the corruption process is sill under consideration, thus the map from X to Z is stochastic. The conditional entropy H(Z|X) can be upper bounded by H(Z|G(X))2 :

EQUATION

which corresponds to the population version of the second term of the objective (2).

2.3. ARCHITECTURAL CONSIDERATIONS AND THE ENCODING LOSS:

The above formulation assumes that the encoding loss term matches the conditional log likelihood fun-ction. Thus, the encoding loss should also match the encoding architecture. For instance, if we use sigmoidal units, we can assume a multivariate Bernoulli distribution of the code and use cross-entropy as the encoding loss. Architectures inducing a continuous code space may require further considerations for the choice of the loss or architectural constraints. For instance, a linear encoder with the euclidean distance as the encoding loss may lead to a trivial minimization of the modified objective. By reducing the norm of the encoder weights, the distance between the codes of the clean and corrupted samples can be made arbitrarily small, only being compensated by scaling of the decoder weights. In this case, a simple way to circumvent this issue is by using tied weights in the decode. In linear case, the role of tied weights is easy to understand. However, a similar interpretation may not extend to non-linear cases. To avoid trivial shrinkage of the encoding space, and without resorting to tied weights, we can employ a normalized distance function such as squared euclidean distance divided by the total variance of the encoded data or by adding a batch normalization layer to the top layer of encoder.

2.4. RELATION TO OTHER APPROACHES:

In a similar spirit to the proposed modified denoising auto-encoder objective, contractive autoencoders achieve robustness in the encoder by explicitly computing a regularization term based on the L 2 of the Jacobian of the encoder. In the regime of small Gaussian perturbations the modified objective can be approximated by the Jacobian of the encoding transformation. Nevertheless, the setting of the modified denoising auto-encoder is more general in terms of distances and forms of input corruption, which can lead to different properties to the ones obtained by manipulating the Jacobian of the transformation. Another interpretation of the learning algorithm is as a Siamese network of the encoders, where the goal is to map the clean and corrupted input to the "same" code.

3. EXPERIMENTS:

This section describe some of the experiments we have carried out with the modified DAE. We qualitatively illustrate how in over-complete representations, the DAE benefits from adding the penalty to the encoding space.

3.1. SYNTHETIC DATA:

Gaussian distributed data: The first example corresponds to a set of data point drawn from a bivariate Gaussian distribution with zero mean and covariance matrix

EQUATION

) -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 (a) Linear g(x) = x -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 (b) ReLU g(x) = max{0, x} -4 -3 -2 -1 0 1 2 3 4 -4 -3 -2 -1 0 1 2 3 4 (c) logsig g(x) = 1/(1+exp(-x))

Figure 1 : Outputs for different activation functions of the modifed DAE with an over-complete representation when the inputs are Gaussian distributed.

We use a linear encoding, that is g(x) = x, that is also over-complete since the encoder project the 2-dimensional data points to 10 different directions. The conventional auto encoder would over fit being able to achieve zero reconstruction error, but it won't be able to implicitly retain what is thought to be the structure in the data. We also compare this output to the outputs of two nonlinear auto-encoders, one uses the logsig units g(x) = 1/(1 + exp(-x)), and the other a rectified linear units (ReLU) g(x) = max{0, x}. Figure 1 shows the outputs of the three auto-encoders on the we described how the modified objective can be understood as minimizing an upper bound on the conditional entropy of the code given the inputs assuming the corruption process to be part of the encoding. By simply minimizing the distance between the encoding of the uncorrupted inputs and the encoding of their corrupted counterparts, we show that robustness of the auto encoder can be guaranteed at the encoding phase. Experiments using over-complete bases showed that the modified DAE was able to learn useful encoding mapping (representation) and can also learn a regularized input-output in an implicit manner.

-8 -6 -4 -2 0 2 4 6 8 -4 -3 -2 -1 0 1 2 3 4 (a) Linear g(x) = x -8 -6 -4 -2 0 2 4 6 8 -4 -3 -2 -1 0 1 2 3 4 (b) ReLU g(x) = max{0, x} -8 -6 -4 -2 0 2 4 6 8 -4 -3 -2 -1 0 1 2 3 4 (c) logsig g(x) = 1/(1 + exp(-x)) -8 -6 -4 -2 0 2 4 6 8 -4 -3 -2 -1 0 1 2 3 4 (d) SatLU g(x) = max{0, x}-max{0, x-1}

:

Note that the information maximization view applies to conventional auto-encoder, as well.The main difference being the stochastic map

2 This follows from the data processing inequality.

Figures and Tables:

Figure 2: Outputs for different activation functions of the modified DAE with an over-complete representation when the inputs are a mixture of Gaussian distributions.Gaussian distributed data. It can be seen that the outputs approximately align with what corresponds roughly to the first principal component of the data. Notice, that no bottleneck neither shrinkage of parameters was explicitly defined. The parameters of our cost function are λ = 1 for the distortion trade-off, and σ = 0.5 for the noise level. Mixture of Gaussians: The second example, employs a mixture of three Gaussian distributions to show the output of modified DAE in a nonlinear scenario, where an over-complete representation

Peer review (iclr2017):
The paper proposes to add an additional term to the denoising-autoencoder objective. The new term is well motivated, it introduces an asymmetry between the encoder and decoder, forcing the encoder to represent a compressed, denoised version of the input. The authors propose to avoid the trivial solution introduced by the new term by using tied weights or normalized Euclidean distance error (the trivial solution occurs by scaling the magnitude of the code down in the encoder, and back up in the decoder). The proposed auto-encoder scheme is very similar to a host of other auto-encoders that have been out in the literature for some time. The authors evaluate the proposed scheme on toy-data distributions in 2D as well as MNIST. Although the work is well motivated, it certainly seems like an empirically unproven and incremental improvement to an old idea.