Given that this is from the Royal Astronomical Society, I think they're literally talking about galaxies. They're then using these same scoring functions to cha

Given that this is from the Royal Astronomical Society, I think they're literally talking about galaxies. They're then using these same scoring functions to characterize the reflections on the subjects' eyes, and comparing the result for the two eyes -- real photos should have similar values, generated images have more variation between the two eyes.