Mammalian rods respond to single photons with a
hyperpolarization of about 1 mV which is accompanied by
continuous noise. Since the mammalian rod bipolar cell
collects signals from 20–100 rods, the noise from
the converging rods would overwhelm the single-photon signal
from one rod at scotopic intensities (starlight) if the
bipolar cell summed signals linearly (Baylor et al., 1984).
However, it is known that at scotopic intensities the retina
preserves single-photon responses (Barlow et al., 1971;
Mastronarde, 1983). To explore noise summation in the rod
bipolar pathway, we simulated an array of rods synaptically
connected to a rod bipolar cell using a compartmental model.
The performance of the circuit was evaluated with a discriminator
measuring errors in photon detection as false positives
and false negatives, which were compared to physiologically
and psychophysically measured error rates. When only one
rod was connected to the rod bipolar, a Poisson rate of
80 vesicles/s was necessary for reliable transmission of
the single-photon signal. When 25 rods converged through
a linear synapse the noise caused an unacceptably high
false positive rate, even when either dark continuous noise
or synaptic noise where completely removed. We propose
that a threshold nonlinearity is provided by the mGluR6
receptor in the rod bipolar dendrite (Shiells & Falk,
1994) to yield a synapse with a noise removing mechanism.
With the threshold nonlinearity the synapse removed most
of the noise. These results suggest that a threshold provided
by the mGluR6 receptor in the rod bipolar cell is necessary
for proper functioning of the retina at scotopic intensities
and that the metabotropic domains in the rod bipolar are
distinct. Such a nonlinear threshold could also reduce
synaptic noise for cortical circuits in which sparse signals
converge.