Activation functions(激发函数) As mentioned previously, the activation function acts as a squashing function(压缩函数), such that the output of a neuron in a neural network is between certain values(usually o and 1, or-1 and 1). In general, there are three types of activation functions, denoted byΦ( As mentioned previously, the activation function acts as a squashing function（压缩函数）, such that the output of a neuron in a neural network is between certain values (usually 0 and 1, or -1 and 1). In general, there are three types of activation functions, denoted by Φ(.)