Human retina trades single-photon detection for high-fidelity contrast encoding

We lack a fundamental understanding of how the spike output of the retina enables human visual perception. Here we show that human vision at its ultimate sensitivity limit depends on the spike output of ON but not OFF parasol (magnocellular) ganglion cells. Surprisingly, nonlinear signal processing in the retinal ON pathway precludes perceptual detection of single photons in darkness, but enables quantal-resolution discrimination of differences in light intensity.