The Shattered Gradients Problem: If resnets are the answer, then what is the question? -
Imagine you make a neural network mapping a scalar to a scalar. After you initialise this network in the traditional way, randomly with some given variance, you could take the gradient of the input with respect to the output for all reasonable values (between about - and 3 because networks typically assume standardised inputs). As the value increases, different rectified linear units in the network will randomly switch on, drawing a random walk in the gradients; another name for which is brown noise. ![]( However, do the same thing for deep networks, and any traditional initialisation you choose, and you'll see the random walk start to look like white noise. One intuition given in the paper is that as different rectifiers in the network switch off and on the input is taking a number of different paths though the network. The number of possible paths grows exponentially with the depth of the network, so as the input varies, the gradients become increasingly chaotic. **The explanations and derivations given in the paper are much better reasoned and thorough, please read those if you are interested**. Why should we care about this? Because the authors take the recent nonlinearity [CReLU][] (output is concatenation of `relu(x)` and `relu(-x)`) and develop an initialisation that will avoid problems with gradient shattering. The initialisation is just to take your standard initialised weight matrix $\mathbf{W}$ and set the right half to be the negative of the left half ($\mathbf{W}_{\text{left}}$). As long as the input to the layer is also concatenated, the left half will be multiplied by `relu(x)` and the right by `relu(-x)`. Then: $$ \mathbf{W}.\text{CReLU}(\mathbf{x}) = \begin{cases} \mathbf{W}_{\text{left}}\mathbf{x} & \text{ if } x > 0 \\ \mathbf{W}_{\text{left}}\mathbf{x} & \text{ if } x \leq 0\end{cases} $$ Doing this allows them to train deep networks without skip connections, and they show results on CIFAR-10 with depths of up to 200 exceeding (slightly) a similar resnet. [crelu]: