Twenty years ago, William Styron, the American author of Sophie's Choice, wrote what many regard as the definitive account of severe depression in his memoir Darkness Visible.

Today researchers give scientific credence to his choice of title by demonstrating that people in the grip of despair do indeed see the world in shades of grey. Depression not only drains life of its pleasure and its purpose; it also drains the visible world of its contrast. This "greying" effect may even be a factor in causing, or maintaining, the depression, the researchers suggest.

The findings may help explain why artists consistently depict depression using darkness. Scientists at the University of Freiburg, Germany, who previously showed people with depression struggled to detect black-and-white contrast differences, have now carried out tests on the retina which show the impact of the illness is similar to turning down the contrast control on a TV.

The effect was so marked that they believe the test could provide a way of measuring the severity of depression. The study, conducted by Ludger Tebartz van Elst and colleagues, is published in the journal Biological Psychiatry.

John Krystal, who edits the journal, said: "These data highlight the 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 researchers measured electrical responses to gauge the activity of the retina in 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 impulses which, when interpreted by the brain, make 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 the severity of their symptoms. Patients who were most severely depressed 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, "electro-retinogram" tests could provide a better way of assessing a patient's mood than simply asking: "How do you feel?", the scientists say.

Dr 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."

Darkness may also cause depression. Many people succumb in winter, when the light is low and sunshine in short supply. Seasonal Affective Disorder (SAD) is often treated with bright artificial light, emitted by special lamps. They are thought to stimulate the body's biological clock but they may also improve contrast vision and thus improve mood in two ways.

Off-colour artists whose work described their mood

Claude Monet He may be famed for his colourful water lilies but when Claude Monet sat down in 1879 to paint his dying wife Camille, the art he produced was distinctly sombre in tone. Camille had been both Monet's model and mistress before they married, sharing his poverty as the French artist struggled to make a name for himself.

Vincent Van Gogh Many of Van Gogh's final paintings lack the bright intensity that dominate his early work. 'Wheat Field with Crows' is the most famous example. The bottom half of the painting, done just a month before the artist committed suicide, shows a bright yellow wheat field. But the sky above the field is a menacing grey-blue. Many have speculated whether that sky was an indication of the artist's increasingly suicidal tendencies.