The world literally fades to grey when we feel blue, scientists have discovered.
Depression has an effect on the eyes that makes it harder to detect the black and white contrasts.
Scientists in Germany carried out tests on the retinas of patients which showed the effect - similar to turning down the contrast control on a TV.
It could be one reason why throughout the ages and regardless of culture or language, artists have consistently depicted depression using symbols of darkness or grey uniformity.
The effect was so marked that scientists believe the test could provide an objective way of measuring depression levels.
The study, conducted by Dr Ludger Tebartz van Elst and researchers at the University of Freiburg, was reported today in the journal Biological Psychiatry.
Dr John Krystal, who edits the journal, said: "These data highlight the profound ways that depression alters one's experience of the world.
"The poet William Cowper said that 'variety's the very spice of life', yet when people are depressed, they are less able to perceive contrasts in the visual world. This loss would seem to make the world a less pleasurable place."
The German team measured electrical responses to gauge the activity of the retina in two groups of depressed and non-depressed individuals.
The retina, at the back of the eye, contains the sensitive cells that turn light signals into nerve messages, making it possible to see.
Depressed patients were found to have dramatically lower retinal contrast "gain" than the volunteers who were not suffering from depression. It made no difference whether or not they were receiving antidepressant medication.
There was also a significant correlation between the level of contrast gain and symptom severity.
Patients with the most depression had the lowest retinal responses.
The pattern was so consistent it was possible to distinguish highly depressed patients from healthy volunteers simply by looking at the test results.
With further work, "electroretinogram" tests could provide a better way of assessing a patient's mood than simply asking: "How do you feel?", the scientists believe.
Dr Tebartz van Elst said: "This method could turn out to be a valuable tool to objectively measure the subjective state of depression, having far-reaching implications for research as well as clinical diagnosis of and therapy for depression."Reuse content