The activation function determines the mapping between input and a hidden layer. It defines the functional form for how a neuron gets activated. For example, a linear activation function could be defined as: f(x) = x, in which case the value for the neuron would be the raw input, x. A linear activation function is shown in the top panel of Figure 4.2. Linear activation functions are rarely used because in practice deep learning models would find it difficult to learn non-linear functional forms using linear activation functions. In previous chapters, we used the hyperbolic tangent as an activation function, namely f(x) = tanh(x). Hyperbolic tangent can work well in some cases, but a potential limitation is that at either low or high values, it saturates, as shown in the middle panel of the figure 4.2.
Perhaps the most popular activation function currently...
