Depolarization of the soma of the presynaptic cell results in a significant increase in amplitude of the EPSP evoked by spikes generated by that neuron!
Figure 18. A. The presynaptic cell was caused to generate a spike approximately once per second. The membrane potential of the cell was then depolarized by 14 mV and the EPSPs evoked by this cell in the postsynaptic neuron were examined. B. Surprisingly, the amplitude of the EPSP depended upon the membrane potential of the presynaptic neuron, even though the same number and rate of action potentials were generated! This means that the height of the EPSP encodes the membrane potential of the presynaptic neuron's dendrite/soma - a fundamentally different result than expected by the rate/timing coding hypothesis of information transfer via action potentials.