Fig. 1.

Spontaneous miniature excitatory postsynaptic currents (mEPSCs) are indistinguishable from mEPSCs evoked by depolarization of a voltage-clamped cone. A: spontaneously occurring mEPSCs recorded from a horizontal cell in bright light. B: mEPSCs evoked in the same horizontal cell by depolarizing a presynaptic cone to −30 mV in the presence of a bright background light. C: there was no significant difference between the cumulative amplitude histograms of spontaneously occurring mEPSCs (—) and mEPSCs recorded during the depolarizing test step (- - -). Cumulative amplitude histograms in this figure were averaged from 6 cone/horizontal cell pairs. D: quantal amplitudes determined from the steep initial slope of the rise in the relationship between variance and mean amplitude of horizontal cell EPSCs evoked by pairs of test steps to −40, −30, or −20 mV applied to cones (8.2 ± 2.2 responses at each potential, 30-s interstimulus interval, n = 6 cone/horizontal cell pairs).