Winston Churchill called it his black dog and Billie Holiday sang of the cold old rain, but now scientists have shown that the associations of gloom and greyness with depression may be more than metaphorical.
A study by German scientists suggests that depression could impair people's ability to detect black and white contrast, and that our perception of the world could literally fade to grey when we feel blue.
They compared the phenomenon, identified through tests on the retinas of patients with depression, to turning down the contrast control on a television.
The finding could explain why grey uniformity has so frequently been used by artists to depict depression, throughout the ages and regardless of culture. If confirmed, scientists believe that the test could provide an objective way of measuring levels of depression.
The condition is estimated to affect one in five people during their lifetime.
The study, conducted by Ludger Tebartz van Elst and colleagues at the University of Freiburg, is reported in the journal Biological Psychiatry.
John Krystal, the editor, 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 40 depressed individuals, half of whom had recently begun taking medication, and 40 who were not depressed.
The retina, at the back of the eye, contains the sensitive photoreceptor cells that convert incoming light signals into electrical impulses that are sent to the visual system in the brain.
By placing electrodes on the surface of the eye and on the skin surrounding the eye, the scientists were able to record the electrical activity of the retinal cells in response to a flashing checked pattern.