  The rod photoreceptors in the retina are known to be sensitive to single
photons, but it has long been debated whether these single-photon signals
propagate through the rest of the visual system and lead to perception.
Recently, single-photon sources developed in the field of quantum optics have
enabled direct tests of single-photon vision that were not possible with
classical light sources. Using a heralded source based on spontaneous
parametric downconversion to generate single photons which were sent to an
observer at either an early or late time, Tinsley and Molodtsov et al. (2016)
had observers judge when the photon was seen. Based on the above-chance
accuracy in both a subset of high-confidence trials and in all post-selected
trials, they claimed to show that humans can see single photons. However, we
argue that this work suffers from three major issues: self-contradicting
results, inappropriate statistical analyses, and a critical lack of statistical
power. As a result, we cannot conclude that humans can see single photons based
on the data of this study. We present a careful examination of the statistical
analyses and the internal consistency of the data, which indicated that none of
the key evidence holds.
