Evidence for presynaptic changes in LTP. When a single fiber is stimulated, a change in success rate without change in potency establishes that the alteration is presynaptically mediated. (A) A plot of paired-pulse potency (mean amplitude excluding failures) ratio during baseline and following the induction of LTP for 8 neurons. (B) A plot of paired-pulse ratio (mean amplitude including failures) during baseline and following the induction of LTP for the same neurons. Note that despite large PPF during baseline and, for some neurons, following the induction of LTP the paired-pulse potency ratio is approximately 1 both during baseline and following the induction of LTP. (C) A plot of success rate change vs amplitude change for LTP of these neurons. (D) The corresponding plot for potency. Note that for all neurons the increase in EPSC amplitude can be explained by the success rate change and that potency is unaltered. From .