Most of the neural network creation functions in the toolbox, like feedforwardnet, automatically assign processing functions to the network input and outputs. These are called process functions and can be overridden after the network.
For this case, the feedforwardnet has removeconstantrows and mapminmax as the processing functions. The mapminmax does normalize the input in the range [-1 1] for your data.
For more details refer the following link for input and output process functions.
