“At the centre of it all, your eyes, your eyes …”

Many aspects of the life and incredible achievements of David Bowie will be considered in the weeks and months ahead following the news of his death. Yet the cryptic lyric above from the lead single on David Bowie’s new album is a reminder that the unusual appearance of his eyes was a key part of the singer’s star persona.

Indeed their iconic presence features in the advertising campaign for ★[Blackstar]. For many people it is that look – that the eyes formed a core part of – that will be an abiding memory of Bowie.

So, why were they apparently two different colours?

Complete heterochromia is a fairly rare condition (in humans) whereby each iris is a distinctly different colour, such as having one blue iris and the other brown.

But this isn’t why Bowie’s eyes looked different.

Instead, the unusual appearance of Bowie’s eyes were due to a condition called anisocoria. Anisocoria is a condition characterised by an unequal size in a person’s pupils. In Bowie’s case, his left pupil was permanently dilated.

This can create the illusion of having different coloured eyes because the fixed pupil does not respond to changes in light, while the right pupil does. So Bowie’s left eye often appeared to be quite dark, due to the blackness of his dilated pupil, when compared to the blue of his right iris.

The dilated pupil of his left eye was also potentially more prone to the effect of “red eye”. This sometimes adds to the appearance of a different colour when contrasted to his right eye.

Red eye occurs when light reflects off of the fundus (the back of the eye), through an open pupil, and captures a red coloration by picking up tonality from the blood in the choroid lining of the eyeball.

This can clearly be seen in the Aladdin Sane – Eyes Open photograph by Brian Duffy (shot in 1973 but unpublished until 2011) that was used as the lead image on the posters for the V&A David Bowie is (2013) exhibition.

David Bowie: Life in pictures Show all 30 1 /30 David Bowie: Life in pictures David Bowie: Life in pictures David Bowie in 1960s Dezo Hoffman/Shutterstock David Bowie: Life in pictures Davy Jones; life before David Bowie Getty David Bowie: Life in pictures David Bowie in 1964 Dezo Hoffman/Shutterstock David Bowie: Life in pictures David Bowie 'In Mime' at the Middle Earth Club, London, 1968 Ray Stevenson/Shutterstock David Bowie: Life in pictures David Bowie in 1969 I T N/Shutterstock David Bowie: Life in pictures David Bowie performing his final concert as Ziggy Stardust at the Hammersmith Odeon, London, 1973 Getty David Bowie: Life in pictures David Bowie in 1973 PA David Bowie: Life in pictures David Bowie, with his wife Angela (Angie) and his son Zowie, after receiving an award for his latest record "Ziggy stardust" in Amsterdam, 1974 AFP David Bowie: Life in pictures David Bowie in the 1970s Sunshine/Shutterstock David Bowie: Life in pictures David Bowie poses with a pig David Bowie: Life in pictures David Bowie's son, Duncan Jones, confirmed his death on Twitter Duncan Jones/Twitter David Bowie: Life in pictures David Bowie in the 1980s Everett Collection/Rex David Bowie: Life in pictures David Bowie gives a press conference presenting the Japanese movie 'Merry Christmas Mr. Lawrence' directed by Nagisa Oshima, during the 36th International Film Festival in Cannes, 1983 AFP via Getty David Bowie: Life in pictures David Bowie performs on stage during a concert in La Courneuve, 1987 AFP David Bowie: Life in pictures David Bowie during his concert in West Berlin, Federal Republic of Germany, 1987 EPA David Bowie: Life in pictures David Bowie shakes hands with Princess Diana, 1993 PA David Bowie: Life in pictures David Bowie autographs copies of his newest album 'Outside' at the grand opening of a Herald Square music store 26 September 1995 in New York AFP/Getty David Bowie: Life in pictures David Bowie performs at the Panathinaikos stadium in Athens during a rock festival, 1996 Reuters David Bowie: Life in pictures David Bowie and his wife, supermodel Iman smile as they pose for photos after Bowie received a star on the world famous Walk of Fame 12 February in Hollywood, 1997 Getty David Bowie: Life in pictures David Bowie getting ready to perform 'Earthling' at the Phoenix Music Festival in 1997 Pat Pope/Shutterstock David Bowie: Life in pictures David Bowie on stage performing during the Tibet House Benefit Concert in New York City, 2001 Getty David Bowie: Life in pictures David Bowie Meltdown concert at the Royal Festival Hall, London, June 2002 Rex David Bowie: Life in pictures David Bowie performing during his concert at the Stravinski hall stage of the Montreux Jazz Festival, in Montreux, Switzerland, 2002 EPA David Bowie: Life in pictures David Bowie in 'Last Call with Carson Daly' TV programme taping in New York, 2003 Startraks/Shutterstock David Bowie: Life in pictures David Bowie walks with his with wife Iman and daughter Alexandria (2) in New York, 2003 Shutterstock David Bowie: Life in pictures David Bowie performs on stage on the third and final day of 'The Nokia Isle of Wight Festival 2004' at Seaclose Park, in Newport, UK Getty David Bowie: Life in pictures David Bowie and Kate Moss at the 2005 CFDA Awards dinner party at the New York Public Library in New York City, 2005 Getty David Bowie: Life in pictures David Bowie and model Iman arrive to the Metropolitan Museum of Art Costume Institute Gala, Superheroes: Fashion and Fantasy, held at the Metropolitan Museum of Art in New York City, 2008 Getty David Bowie: Life in pictures David Bowie anf Tilda Swinton at the MoMA's 6th Annual Film Benefit in New York, 2013 BFANYC.COM/Rex David Bowie: Life in pictures Flowers are left below a mural of David Bowie on the wall of a Morley's store in Brixton on 11 January 2016 Getty

So what happened?

Anecdotally, the cause of Bowie’s anisocoria was attributed to the fallout from a lusty scrap in the spring of 1962. Bowie had come to blows with a friend, George Underwood, over a girl they were both hoping to date.

Both were just 15 at the time and their friendship seemingly remained intact.

The two performed together in various bands before Underwood turned from music to painting and graphics.

But Bowie’s left eye remained seriously damaged.

An impulsive punch had accidentally scratched the eyeball, resulting in paralysis of the muscles that contract the iris. From that day, Bowie’s left pupil remained in a fixed open position.

Over time, Bowie apparently thanked his friend for his notorious eye injury, telling Underwood that it gave him “a kind of mystique”. This mystique helped fuel some of Bowie’s greatest creations and enhance iconic images, such as the album cover for Heroes (1977).

His eyes could appear eerie and mismatched, producing a captivating or mesmeric gaze from on stage or through the lens of a camera. And the uncanny appearance of Bowie’s eyes was ideal for a performer who embraced ideas of the alien, the outsider, the otherworldly and the occult.

In an increasingly visual world seemingly preoccupied by perfection, Bowie’s damaged left pupil became an intrinsic and arresting part of his enigmatic identity.

Kevin Hunt, Senior Lecturer in Design and Visual Culture, Nottingham Trent University