Researchers exploring the boundaries of human vision have made the rather remarkable discovery that our eyes essentially have no lower limit when it comes to light detection, and are able to pick up single particles of light radiation, known as photons. Though such faint sources of illumination are not strong enough to generate an actual visual image, and are therefore more “felt” than “seen”, the fact that we are able to register individual photons at all is not only of huge scientific significance, it’s also totally badass.

The retina – which is the part of the human eye that detects light – contains two types of photoreceptor cells, known as rods and cones. Of these, rods are particular useful at seeing in low light, as they are sensitive to extremely weak stimuli, although they are only able to detect white light, and therefore rely on the cones to help them generate color vision.

Previous studies dating back several decades have shown that these rods are powerful enough to pick up light signals containing as few as five photons, though researchers have been left in the dark over whether they can go any lower than this. Testing this out has proven somewhat challenging, firstly because creating a device capable of firing single photons into people’s eyes isn’t easy, and secondly because the neural signals generated from these stimuli are likely to be so faint that they can barely be measured.

^{The retina contains two types of photoreceptor cells, of which rods are responsible for detecting extremely low light signals. They send an impulse along the optical nerve to the brain. Designua/Shutterstock}

However, writing in the journal Nature Communications, a team of researchers describes how they designed an experiment during which three volunteers stared into a dark chamber, and periodically heard auditory notes. Some of these notes were accompanied by a single photon, which was fired directly into their eye from a quantum optical light source.

After each tone, participants indicated whether or not they thought they had “seen” a photon, while providing a rating of how confident they were of their answer. Given that photons are so small, however, the chance of these particles missing their target or being deflected before they can stimulate the rod cells to generate an optical impulse is quite high, with the researchers calculating that only about 6 percent of all photons fired actually resulted in a signal reaching the brain. As a result, they expected participants to miss most of the photons, guessing wrong on a high number of trials.

To account for this, they made sure to repeat the experiment on a mass scale, eventually conducting 30,767 trials. When looking at the results, the study authors found that participants correctly identified the presence of a photon with a frequency far greater than the averaged possibility would have been had they simply been guessing at random.

Significantly, results showed that those who said they were highly confident of having detected a photon were particularly accurate, suggesting that when a photon hits the retina, people really do know about it.

Interestingly, the researchers found that when people saw a photon, their probably of correctly detecting another one was considerably increased for the next five seconds. From this finding, the authors suggest that when rods pick up a photon, they become temporarily “primed” to continue detecting light at similar levels of intensity. In other words, the “gain” – or light sensitivity – of the visual system is temporarily enhanced.

Though the researchers say that the brain circuitry underlying this capability is not yet known, they raise the intriguing point that this finding could one day be used to study how the quantum properties of photons – such as their ability to exist in two places at once – affect the visual system.

^{The human visual system may be a lot more powerful than we thought. Ramona Heim/Shutterstock}