Why does Karen taste of blackcurrant? The answer is being debated in Edinburgh this weekend by psychologists from around the world, who are gathered for the annual conference of the UK Synaesthesia Association.

Speakers include artists whose paintings are inspired by music, a pianist who composes pieces in different colours, and a man for whom swear words can literally leave a bad taste in the mouth.

Around one in 100 of us have synaesthesia - a common condition experienced as a "crossing of the senses". Famous "synaesthetes" include Vladimir Nabokov and the artist Kandinsky.

Some associate numbers and letters with colours - a is often red, while g is green. Others, such as James Wannerton, experience words and sounds as intense taste.

"I was eating potatoes the other day when someone called Karen started talking," says Mr Wannerton, of the UK Synaesthesia Association.

"Karen tastes of blackcurrant yoghurt to me, and so it was rather off-putting."

But why does "Karen" taste like blackcurrant? Is it the sound of the word, or could it be the shape of the letters?

The answer is important, say psychologists, because we all experience some underlying mixing of the senses - even those of us who are not conscious of it.

Synaesthesia is helping to shine a light on the hidden workings of the human mind.

At the conference, Dr Julia Simner, of Edinburgh University, will present the first study into a rare subtype of synaesthesia known as "tickertape".

"People with this form of synaesthesia actually see conversations projected in front of them - as if they could reach out and grab them."

When Patricia Roberts hears a train announcement, she sees the words spelled out in colour, letter by letter. It's just as if a tickertape is going through your head all the time," said Mrs Roberts, 63, a retired teacher, from Leeds. "It happens when I'm at an airport, or when I'm reading."

"The words come in from right to left, so the word I'm actually hearing is large, but the others tail out smaller and smaller into the distance.

"Of course, I thought everybody was like this. At school, when other children couldn't spell out a word, I would say - that's easy that's a yellow word'. But I got some very strange reactions, so I began to keep quiet about it."