# Understanding back-propogation

Understanding the back-propagation algorithm for training neural networks can sometimes be challenging, because often there is a lot of confusing terminology which varies between sources. Also it is commonly described just in terms of the mathematics. Here I present a diagrammatic explanation of back-propagation for the visually inclined. I also summarize the non-linear stages that are commonly used, and provide some philosophical insight.

The forward pass though a neural net consists of alternating stages of linear multiplication by a weight matrix and non-linear activation functions which transform the output of each linear unit independently. We can write the transformation in vector form as ${\bf z}={\bf Wx}$, and ${\bf y}=g({\bf z})$ where ${\bf x}$ is the input, ${\bf z}$ is the output of the linear stage, ${\bf y}$ is the output of the non-linear stage, and $g({\bf z})$ is the activation function which acts on each element of ${\bf z}$ independently. For subsequent stages, the input ${\bf x}$ is the output ${\bf y}$ of the previous stage.

# Energy pooling in neural networks for digit recognition Having trained a two layer neural network to recognize handwritten digits with reasonable accuracy, as described in my previous blog post, I wanted to see what would happen if neurons were forced to pool the outputs of pairs of rectified units according to a fixed weight schedule.

I created a network which is almost a three layer network where the output of pairs of the first layer rectified units are combined additively before being passed to the second fully connected layer. This means that the first layer has a 28×28 input and a 50 unit output (hidden layer) with rectified linear units, and then pairs of these units are averaged to reduce the neuron count to 25, and then the second fully connected layer reduces this down to 10. Finally the softmax classifier is applied.