This prolonged period of activity is normally concentrated electric impulses, usually around 100 Hz. It is called "coincidence" detection in that it only strengthens the synapse if there was sufficient activity in both the presynaptic and postsynaptic cells. If the postsynaptic cell does not become sufficiently depolarized then there is no coincidence detection and LTP/LTD do not occur.
The extended period of activity concentrates electrical impulses at around 100 Hz. This process is called "coincidence" detection because it only reinforces the synapse when there is sufficient activity in both presynaptic and postsynaptic cells. If the postsynaptic cell does not become sufficiently depolarized, coincidence detection fails, and LTP/LTD do not occur.