Figure 8.

The contribution of the FP to high-pass filtering by the photoreceptor-LMC synapse. A, comparison between the frequency response of a photoreceptor soma recorded in the retina (black) and the field potential (gray) recorded from the extracellular space of a lamina cartridge. Gain, expressed as the millivolt response per unit of stimulus contrast, is plotted versus frequency on logarithmic scales. B, The frequency response predicted for the transmembrane potential of a photoreceptor axon terminal in the lamina cartridge (black) is compared with the frequency response for synaptic transfer from photoreceptor soma to LMC (gray trace) (taken from a previous study by Juusola et al., 1995a). To estimate the frequency response of the transmembrane potential in the photoreceptor axon, we took the data in A and subtracted the FP response from the photoreceptor response. This procedure suggests that subtraction of the field potential from the presynaptic axon response contributes substantially to the high-pass characteristic of the synapse at frequencies <50 Hz.