When depressed people are rewarded, their pupils don't dilate as much as non-depressed people's

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.