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Title: On the Selection of Initialization and Activation Function for Deep Neural Networks

Paper content:
The weight initialization and the activation function of deep neural networks have a crucial impact on the performance of the training procedure.
An inappropriate selection can lead to the loss of information of the input during forward propagation and the exponential vanishing/exploding of gradients during back-propagation.
Understanding the theoretical properties of untrained random networks is key to identifying which deep networks may be trained successfully as recently demonstrated by Schoenholz et al. (2017) who showed that for deep feedforward neural networks only a specific choice of hyperparameters known as the `edge of chaos' can lead to good performance.
We complete this analysis by providing quantitative results showing that, for a class of ReLU-like activation functions, the information propagates indeed deeper for an initialization at the edge of chaos.
By further extending this analysis, we identify a class of activation functions that improve the information propagation over ReLU-like functions.
This class includes the Swish activation, $\phi_{swish}(x) = x \cdot \text{sigmoid}(x)$, used in Hendrycks & Gimpel (2016),
Elfwing et al. (2017) and Ramachandran et al. (2017).
This provides a theoretical grounding for the excellent empirical performance of $\phi_{swish}$ observed in these contributions.
We complement those previous results by illustrating the benefit of using a random initialization on the edge of chaos in this context.
Deep neural networks have become extremely popular as they achieve state-of-the-art performance on a variety of important applications including language processing and computer vision; see, e.g., BID8 .
The success of these models has motivated the use of increasingly deep networks and stimulated a large body of work to understand their theoretical properties.
It is impossible to provide here a comprehensive summary of the large number of contributions within this field.
To cite a few results relevant to our contributions, BID11 have shown that neural networks have exponential expressive power with respect to the depth while BID14 obtained similar results using a topological measure of expressiveness.We follow here the approach of BID14 and BID16 by investigating the behaviour of random networks in the infinite-width and finite-variance i.i.d. weights context where they can be approximated by a Gaussian process as established by BID10 and BID9 .In
this paper, our contribution is two-fold. Firstly
, we provide an analysis complementing the results of BID14 and BID16 and show that initializing a network with a specific choice of hyperparameters known as the 'edge of chaos' is linked to a deeper propagation of the information through the network. In particular
, we establish that for a class of ReLU-like activation functions, the exponential depth scale introduced in BID16 is replaced by a polynomial depth scale. This implies
that the information can propagate deeper when the network is initialized on the edge of chaos. Secondly, we
outline the limitations of ReLU-like activation functions by showing that, even on the edge of chaos, the limiting Gaussian Process admits a degenerate kernel as the number of layers goes to infinity. Our main result
(4) gives sufficient conditions for activation functions to allow a good 'information flow' through the network (Proposition 4) (in addition to being non-polynomial and not suffering from the exploding/vanishing gradient problem). These conditions
are satisfied by the Swish activation φ swish (x) = x · sigmoid(x) used in BID4 , BID2 and BID15 . In recent work,
BID15 used automated search techniques to identify new activation functions and found experimentally that functions of the form φ(x) = x · sigmoid(βx) appear to perform indeed better than many alternative functions, including ReLU. Our paper provides
a theoretical grounding for these results. We also complement
previous empirical results by illustrating the benefits of an initialization on the edge of chaos in this context. All proofs are given
in the Supplementary Material.
We have complemented here the analysis of BID16 which shows that initializing networks on the EOC provides a better propagation of information across layers.
In the ReLU case, such an initialization corresponds to the popular approach proposed in BID3 .
However, even on the EOC, the correlations still converge to 1 at a polynomial rate for ReLU networks.
We have obtained a set of sufficient conditions for activation functions which further improve information propagation when the parameters (σ b , σ w ) are on the EOC.
The Tanh activation satisfied those conditions but, more interestingly, other functions which do not suffer from the vanishing/exploding gradient problems also verify them.
This includes the Swish function used in BID4 , BID2 and promoted in BID15 but also ELU Clevert et al. (2016) .Our
results have also interesting implications for Bayesian neural networks which have received renewed attention lately; see, e.g., Hernandez-Lobato & Adams FORMULA4 and BID9 . They
show that if one assigns i.i.d. Gaussian prior distributions to the weights and biases, the resulting prior distribution will be concentrated on close to constant functions even on the EOC for ReLU-like activation functions. To obtain
much richer priors, our results indicate that we need to select not only parameters (σ b , σ w ) on the EOC but also an activation function satisfying Proposition 4.

assistant: text: How to effectively choose Initialization and Activation function for deep neural networks