Each data point represents the slope value of fEPSPs recorded at stratum radiatum. Stimulating pulse 1 (S1) was given at stratum radiatum, S2 was given at stratum oriens, for which the corresponding reversed fEPSP values were recorded with the same recording pipette placed in stratum radiatum (compare Fig. 1A). S1+2 was third stimulation mode, where S1 and S2 were given simultaneously. The stimulus protocol S1, S2, S1+2 was continuously repeated throughout the experiment. Different concentrations of CNQX were delivered via bath perfusion and field potentials from stratum radiatum and stratum oriens were found to be reduced in parallel (panel A, representative data from a total of 7 experiments), whereas administration via MFP selectively blocked the fEPSP in one layer without affecting another (panel B, representative data from a total of 5 experiments). The results are consistent with the effect/noneffect of CNQX on the competitive fEPSP induced by simultaneous S1 and S2 (see text for further explanation). Insets show traces for fEPSPs obtained by S1 or S2 stimulation.