For one in 2000 people, everyday activities like reading or listening to music can illicit taste or colour sensations.

The ground-breaking genetic study could provide "profound insight into normal perception", according to Dr Julia Simner, head of the Synaesthesia and Sensory Integration lab at the university's psychology department.

She said: "Scotland is a world leader and we've been dominating the field in many areas.

"Now we're gathering the world's largest cohort of genetic materials from grapheme-colour synaesthetes, using highly sophisticated genetic association techniques to find the genes that trigger synaesthesia.

"Although it's relatively rare, synaesthesia can give profound insights into how the average brain works because all people experience crossed sensations to an implicit degree, and often in ways that mimic synaesthetes.

"For example, rougher textures tend to be 'seen' as darker colours by some synaesthetes, but are also paired to darker colours by all people in sensory association tasks."

Scientists say the relationship with normal perception is also highlighted in how most people understand the notion of a high-pitched sound being light in colour.

There are three kinds of synaesthetes: some enjoy the sensations, some are indifferent, and some can find them very difficult, such as when experiencing foul tastes. However, even the latter rarely seek a "cure" for their condition.

Others said to be synaesthetes are Tori Amos and Stevie Wonder and also Marilyn Monroe.

Synaesthesia is caused by extra connections within the brains of synaesthetes, which scientists can see using brain imaging techniques, but it not known why.

Cross-sensations in synaesthetes vary, but the Edinburgh team, including geneticist Duncan Carmichael, has also discovered that most of them are likely to say that the letter A is red in colour.

Mr Carmichael said: "It's challenging. You can think of the brain as the final frontier in genetic research if you like.

"It's an unusual way of seeing the world. We hope to have an insight into how normal perception in the brain works."

The team have had to scan 2000 people to find 150 subjects so far but they are calling for more people to come forward.

Dr Simner added: "So far, scientists have only a rough idea about the broad regions of the genome that might contain the synaesthesia gene or genes, but our study will be the first to identify the actual genes themselves. If we can identify a gene, we know what that gene does, and by knowing what that gene does, we can link back its function to how it might be causing synaesthesia."

In 2006, Dr Simner found that taste is often triggered by thinking of a word's meaning, rather than by sound or spelling. She also identified a specific group of synaesthetes for whom words elicit tastes.

Then, in 2008, the Edinburgh researchers discovered a colour code for the letters of the alphabet, common to people both with and without synaesthesia.

CASE STUDY

JENNIFER Mankin never knew she was a synaesthete until she surprised a friend with the news that her name was a hue of orange.

The 25-year-old Edinburgh University student, who is from Seattle, Washington, said: "When I was 17 and in a German class I told my class mate she had an orange name and she looked at me as if it to say 'what?'. I can't remember her name, but it was orange.

"After I told her I could see letters as colours people were asking me what colour their names were.

"Then I went on the internet and found out what it was.

"Some people have trouble understanding (the condition), some people think it is that we have a good imagination.

"I know another synaesthete in the archery club, where I also at first had problems matching scores to the coloured coded rings on the target."

The psychology and language student, moving into her PhD this year, said she has never thought of synaesthesia as a particular asset or a hindrance.

"The only problem I've ever encountered is small memory problems.

"My brother's birthday is either December 16 or 18 but both of those numbers are green in colour and kind of the same shape so I can never remember which it is."