Human thought depends on the behavior of billions of neurons, nerve cells that signal each other.
With the exception of neural synchrony models, the artificial neural networks discussed so far ignore the firing pattern of neurons. They look only at the average rate of firing, represented by a real number, which is the activation of the artificial neuron. Then there are around 100 different per-second firing rates it could have, firing anywhere from 0 to 100 time each second. This is a rate code. 100 different possibilities, which is enormously more than a rate code can handle.