The phantom songs that strike the wrong note

For more than a year Cath Gamester has been hearing a tenor constantly singing the same five songs – "Silent Night", "Abide with Me", "You'll Never Walk Alone", "Happy Birthday "and "Land of Hope and Glory".

Imagine a loop, usually of the same songs, accompanying you constantly, playing so loud that the only way to dim it is to put the vacuum cleaner on or shout louder than it. Just to be clear, this isn't like having a tune in your head (although that's precisely where it is), this is experienced as music being played externally.

I met 84-year-old Cath at her home in Liverpool while making a piece about a condition called musical ear syndrome for the BBC's Inside Out programme. She's anxious about discussing it with anyone – "in case they think I'm daft".

Cath waited weeks before telling anyone, fearing it might be the start of dementia but her doctor has ruled this out. It's estimated that 10,000 people a year are affected by musical ear syndrome and, while the vast majority are elderly, it can show up in people who are isolated, or under extreme pressure – Cath's sister had recently died.

Dr Nick Warner is a psychiatrist who, over a 15-year period, studied 30 patients with musical hallucinations. Some of whom had other conditions – deafness, tinnitus, depression – but for many the constant music was the only abnormality. "Abide with Me" and Christmas carols came up again and again. His theory as to why so many hear the same songs is that for older people most of their exposure to music would have been through church, school and a small number of traditional local songs.

Studies of auditory and visual hallucinations show that total deprivation of sights or sounds isn't needed to produce them. In his book Hallucinations, Oliver Sacks, professor of neurology at the NYU School of Medicine, points out that monotony of activity has been known to accompany them. The brain not only needs "perceptual input", it requires "perceptual change". So tales of sailors on watch, pilots with only the sky for company and isolated prisoners seeing and hearing things that aren't really there run through human history like a thread.

The composer Robert Schumann claimed he heard music that he then wrote down to produce the Ghost Variations. Paul McCartney has said that he woke up one night and simply heard the tune for "Yesterday". Brian Keenan, the former Beirut hostage, told me he'd experienced musical hallucinations when he was blindfolded and held, alone, in an underground cell before being joined by John McCarthy. "It wasn't a tune out of memory. It was this kind of kaleidoscope with such eloquent and exquisite harmony and it was as if it had all come into this tiny black hole under the earth that I was locked in and was playing just for me, and it was very moving, very enriching, and then it became very frightening because I knew it didn't exist but the power of it was bigger than my capacity to resist it".

In some cultures, musical hallucinations are regarded as a privilege, a blessing, but in our own they've been associated with fear of losing one's mind and huge stigma remains. Cath feels this deeply.

Sacks believes hallucinations – of sight, sound, and smell – are an essential part of the human condition, perhaps responsible for some of our most deep-rooted folk tales and even religious belief.

Is Cath a religious woman? "No!" she laughs. "I wouldn't have these songs in the house – I'd prefer Dean Martin!"

'Inside Out North-West' is on today at 7.30pm on BBC1 and Sky channel 958. Shelagh Fogarty presents the weekday lunchtime show on BBC Radio 5 Live