Gap-junctional coupling of mammalian rod photoreceptors and its effect on visual detection.

The presence of gap junctions between rods in mammalian retina suggests a role for rod-rod coupling in human vision. Rod coupling is known to reduce response variability, but because junctional conductances are not known, the downstream effects on visual performance are uncertain. Here we assessed rod coupling in guinea pig retina by measuring: (1) the variability in responses to dim flashes, (2) Neurobiotin tracer coupling, and (3) junctional conductances. Results were consolidated into an electrical network model and a model of human psychophysical detection. Guinea pig rods form tracer pools of 1 to ∼20 rods, with junctional conductances averaging ∼350 pS. We calculate that coupling will reduce human dark-adapted sensitivity ∼10% by impairing the noise filtering of the synapse between rods and rod bipolar cells. However, coupling also mitigates synaptic saturation and is thus calculated to improve sensitivity when stimuli are spatially restricted or are superimposed over background illumination.