Fig. 1. Placement of electrodes and evoked field potentials in a representative rat. (A ) Location of stimulating electrode (arrow) targeted at the medial perforant path (MPP) in the angular bundle. (B ) Location of the shallow (L2) at near the hippocampal fissure and deep (L1) recording electrodes in the hilus of the dentate gyrus. (C ) Average evoked potential (n = 8) recorded from the electrode L2 (top ) and L1 (bottom ) after 100 μA stimulation of the MPP. The slope of the line “ab” measured the increasing slope of the population excitatory postsynaptic potential (EPSP) before the onset of the population spike. The population spike amplitude (P1) was measured as the difference between the positive and negative peaks of the transient event. Shock artifacts indicted by dots. (D ) Average evoked potential at L2 and L1 after 20 μA stimulation of the MPP, which evoked a small population spike.