Blue light can both set the mood and set in motion important biological responses. Researchers at the University of Pennsylvania's School of Medicine and School of Arts and Sciences have teased apart the separate biological responses of the human eye to blue light, revealing an unexpected contest for control. Their work addresses the properties of melanopsin, a light-sensitive protein in the eye that establishes the rhythm of our day-night cycle and the familiar constriction of the pupil to bright light. They measured the pupil response to stimulation of melanopsin and of short-wave-sensitive (S) cones, the other blue light-sensing cells that operate in daylight. Surprisingly, they found that melanopsin and S-cones have opposite effects and compete for control of the pupil in blue light. Their complete results are published in the current issue of PNAS. Drs. Aguirre and Brainard and graduate student Spitschan found that melanopsin, a protein and short wave-sensitive S-cones, both in the retina have opposite effects and compete for control of the pupil in response to blue light.

"The challenge of studying melanopsin is that it is very sensitive to blue light, a short-wave light emitted by digital devices including smartphones, tablets, and computers, as are S-cones," says lead author, Manuel Spitschan, a Penn graduate student in psychology. "Previous studies in the human eye have not separately studied the S-cones and melanopsin because flashing a blue light stimulates both of these cells, so we didn't know if what a person saw or the response of the pupil was from one or both." To overcome this problem, the Penn team developed a special class of visual stimuli: they produced flickering light that stimulates melanopsin but is invisible to S-cones, and a second flickering light that stimulates S-cones but is invisible to melanopsin. The lights were created using a machine that can sculpt and switch between computer-designed rainbows of light.

The researchers had 16 people watch this flickering light while the response of their pupil was recorded. The light that stimulates melanopsin made the pupil slowly contract. To their surprise, they also discovered that stimulation of S-cones made the pupil get larger. That is, when the S-cones of the eye captured more light, the pupil enlarged, the opposite of what is generally thought of as the natural pupil response. This means that blue light sets off a tug-of-war between melanopsin and S-cones to make your pupil smaller or bigger. The melanopsin effect is stronger, resulting in the familiar shrinking of the pupil to bright light of any color.