I just started reading about Donald O. Hebb and his theories about cell assemblies. I am stuck on something really simple. From what I have read, if two neurons have a synaptic connection and they fire simultaneously, then that connection will be strengthened. What I can't understand is this - if neuron A has a connection with neuron B... Does neuron A fire and cause neuron B to fire, and then while neuron B is firing, neuron A is still firing and that makes it a simultaneous event? A->B Or does neuron C cause neuron A to fire while neuron D simultaneously causes neuron B to fire and that's what makes it a simultaneous event? C->A D->B I hope I am making sense. Thanks for any help.