When something exciting happens to a person (like winning money) their pupils dilate. Researchers found that acutely depressed people's pupils dilate less than non-depressed people's pupils.
From Bionity:
Study participants at the Max Planck Institute of Psychiatry (MPI) played a simple game while in the magnetic resonance imaging (MRI) scanner, where they could win a small amount of money. Winning money is a clear incentive that is known to cause pupil dilation in healthy people. The researchers measured the pupils of their study participants extremely accurately and at an extremely high speed: Using a special setup, they were able to take 250 images per second – for comparison, we only blink every four to six seconds.
For the first time, the MPI scientists were able to prove a correlation between pupil dilation in response to an expected reward and the severity of depression. The more severe the symptoms of depression were, the less dilated the pupils would become.