Staring into someone's eyes can be an emotional experience, but psychologists at the University of Pennsylvania have it down to a science: they've been measuring pupils to decipher how people perceive blue light.

Shorter wavelength, or blue light, is key for regulating our daily circadian rhythms and keeping us happy. Two photoreceptors in the eye perceive this kind of light — a cell that contains a photopigment called melanopsin, and the better-known S cone. Because both respond to blue light, no one has been able to study them in isolation in humans.

But in work published this week in the journal PNAS, the Penn team used different pulses of light to selectively active only one of those cell types at a time — revealing opposing roles on the effect of the pupil.

When they activated only the melanopsin-containing cells, the pupils of 16 volunteers shrank. When they stimulated the S cones, however, pupils widened.

"We were very surprised by this finding," said Manuel Spitschan, a graduate student in psychology. "Basically it's a paradoxical response."

Penn psychologist David Brainard said this type of tug of war is common in the visual system, and may serve an important role.

"It's possible this evolved for the purpose of circadian control, and that something about these dawn/dusk spectral signatures is really what the system is trying to pick up," he said.

The technique will help the team further investigate the role of melanopsin, which may play a role in seasonal mood disorders and photosensitivity.

"It turns out there's a lot of folks out there with neurological diseases and ophthalmological diseases who have pain from bright light," said behavioral neurologist Geoffrey Aguirre. "And we'd like to know if this melanopsin system is contributing to their discomfort."

If those connections pan out, researchers hope their technology—which they've patented—could one day make it to the clinic as a diagnostic tool.