Old and new news about single-photon sensitivity in human vision

It is sometimes said that ``our eyes can see single photons,'' when in fact the faintest flash of light that can reliably be reported by human subjects is closer to 100 photons. Nevertheless, there is a sense in which the familiar claim is true. Experiments conducted long after the seminal work of Hecht, Shlaer, and Pirenne in two distinct realms, those of human psychophysics and single-cell physiology, now admit a more precisem conclusion to be drawn about our visual apparatus. Finding a single framework that accommodates both kinds of result is a nontrivial challenge, and one that sets severe quantitative constraints on any model of dim-light visual processing. I will present one such model and compare it to a recent experiment.