HFS and LFS LTD do not occlude each other. (A) Pooled data (mean ± SEM, n = 5) of normalized fEPSPs plotted as a function of time. Two HFS (arrows) delivered to the mossy fibers (20 min apart), using a stimulation intensity that induced the maximal response, induced LTD that reached saturation. Forty minutes later, the stimulation intensity was increased to evoke a fEPSP similar in amplitude to the preconditioning one. A subsequent LFS (filled bar) produced a further LTD. (B) Pooled data (mean ± SEM, n = 5) of normalized fEPSPs plotted as a function of time. Two LFS (filled bars) delivered to the mossy fibers, using a stimulation intensity that induced the maximal response (20 min apart), induced LTD that reached saturation. As in A, the stimulation intensity was increased to evoke a fEPSP similar in amplitude to the preconditioning one. A HFS train delivered to the same pathway produced a further depression of the fEPSP.