Figure 1.

Summary of the applied stimulation protocols. During all experiments, the perforant pathway was stimulated with 20 consecutive stimulation trains. The first stimulation train (indicated by red boxes) was applied 2 min after starting fMRI. Each stimulation train lasts for 8 s and was followed by 52 s rest (indicated by the gray boxes). During the first set of experiments, continuous 1, 5, 10, or 20 Hz pulses were applied during each stimulation train. During these stimulation frequencies, each applied pulse elicited one population spike; consequently, under these conditions, the input (i.e. applied pulses) and output activity (number of population spikes) vary in a similar way (Experiment 1). In a second set of experiments, high frequency pulses (5, 10, or 20) were superimposed on a 1 Hz stimulation protocol. Because only one population spike was generated to the first pulse of the burst during these conditions, whereas all subsequent pulses only generated fEPSPs, the number of population spikes was similar. Consequently, only the incoming activity varied, whereas the output activity remained similar (Experiment 2). During the third set of experiments, the perforant pathway was additionally stimulated with 5 Hz-4 pulses and 10 Hz-2 pulses; thus, there were four different experimental conditions in which 20 pulses were applied but different numbers of population spikes were generated (Experiment 3).