John Hopfield in the early 1980's investigated a particular kind of neural network which is now commonly referred to as the Hopfield network or Hopfield model. In the Hopfield network, there are no special input or output neurons (see McCulloch-Pitts), but all are both input and output, and all are connected to all others in both directions (with equal weights in the two directions). Input is applied simultaneously to all neurons which then output to each other and the process continues until a stable state is reached, which represents the network output.