Figure 4. Decreasing stimulus intensity to produce smaller excitatory postsynaptic potential responses did not alter facilitation produced by paired pulse stimuli separated by 120 ms. Superimposed excitatory postsynaptic potential recordings show comparable facilitation observed at four different stimulus intensities (top). Graphs (bottom) show the experimental time course for excitatory postsynaptic potential amplitudes and facilitation, expressed as an amplitude ratio (EPSP2/EPSP1). Facilitation did not change at six different levels of stimulation, although excitatory postsynaptic potential amplitudes were reduced to approximately 10% of control at the lowest stimulus intensity. An overshoot in response amplitude was observed when stimulus intensity was returned to control levels (at approximately 70 min).