When Hannah Morgan was three years old, she told her family that Mondays are red. Her mother, Nicola, thought little of her daughter’s comment, until some while later when Hannah made this statement again - and then again.
As time went on Morgan realised that for Hannah, the days of the week did not just have names - they had colours too. So did all the letters of the alphabet. And so did the months of the year.
"Gradually I realised that when I mentioned Thursday or November, Hannah did not just hear a word - she heard a colour too, like a filter coming across her vision.
"It seemed she had an extra way of sensing things that we did not share, and that her experience was very real. When I said Tuesday, Hannah saw blue. When I said Monday she really did see red."
Hannah, now aged 15, has a neurological condition known as synaesthesia - a phenomenon sometimes described as a cross-wiring of the senses. Derived from the Greek words for "sensation" (aisthesis) and "together" (syn), synaesthesia is a condition whereby a stimulus in one in one sensory modality gives rise to an experience in another.
For example, when a synaesthete sees a particular word they will simultaneously see a particular colour. When they experience a particular taste, they might also feel smooth or spiky taste in their mouth. When they hear the high notes of a violin they might sense a smell of lemon, or some other scent.
Morgan has used her daughter’s first manifestation of multi-sensational medical phenomenon as a key component of her first published novel, titled Mondays are Red.
The book charts the experience of Luke, a teenager who awakes from a near-fatal bout of meningitis to discover that his brain - and therefore his world - has changed.
The most obvious symptom is his ability to hear things he could previously only see, feel things he could once only hear, taste things he once only felt - he has developed synaesthesia. "Mum stroked my hand," Luke says, shortly after coming round from his coma. "Strawberry music flowed from her fingers, softening my muscles."
Although Mondays are Red is not about synaesthesia per se, Morgan has used the condition as a rationale for using a multi-sensory style of prose that gives a vivid idea of how the world must look to people like her daughter.
The novel, which is aimed at teenagers, is bound to bring to a wider audience this most intriguing of medical phenomena, the causes of which are not yet fully understood.
The first cases of synaesthesia were recorded more than 300 years ago. In 1690, John Locke, the 17th-century philosopher, knew a blind man who described scarlet, a colour he’d never seen, as " ... the sound of a trumpet". And an 18th-century English ophthalmologist noted a blind patient who claimed to "hear" colour.
The first systematic study of synaesthesia was carried out by Francis Galton, a geneticist of University College London, at the end of the 19th century, when he compiled the reported experiences of a number of synaesthetes.
But interest in synaesthesia waned as the 20th century progressed, perhaps because although unusual, it does not cause distress in most people who have it. It was only in 1980 when Dr Richard Cytowic, an American neurologist, sat down to dinner at the home of his friend Michael Watson that the condition enjoyed revived curiosity.
Cytowic’s host told his puzzled guests that the meal was a disaster because: "There aren’t enough points on the chicken." The cook experienced the insipidity of the sauce as a lack of angles, rather than as a taste alone.
Cytowic decided to look further into this phenomenon, and the results of his experiments with synaesthetes are recorded in his 1993 book, The Man Who Tasted Shapes.
For Watson, the taste of quinine was "smooth polished wood", sugar made things taste "rounder", and citrus made the taste have "points".
Cytowic concluded that synaesthesia is both diagnosable and genuine. The experience is actual - not in any way related to metaphors such as "seeing red" when we are angry or "feeling blue" when we are down.
He found that although the sensory perceptions it creates are different in each affected person, these perceptions are vivid and constant in each individual. If you are a synaesthete and you see green when you hear the word Friday, you will be seeing green when you hear the word Friday in six months’ time and in 20 years’ time. Cytowic’s conclusions were revolutionary in terms of modern medicine’s accepted understanding of the brain.
While the cortex of the brain is usually considered the home of sensory perception and consciousness, he draws attention to the importance of the limbic system of the brain, which is responsible for emotions.
Cytowic’s experiments found that the cerebral cortex shuts down almost completely during synaesthesia, leaving the limbic system to dominate brain activity.
The limbic system is a circuit of nerve centres that regulates the internal state of the body, memory, emotions and functions that are basic to survival. As the brain is bombarded with sensory data in everyday life, the limbic system selects only those which are relevant to our immediate needs.
It is believed that in most of us the cortex is dominant, so we are not aware of this multi-sensory processing. However, it is suggested that in synaesthetes the limbic system is dominant, and this data filtering is revealed.
Investigations into this most unusual of phenomena continue, but medical science has neglected synaesthesia for so long that it is not even clear how many people are affected, with estimates ranging from one in 2,000 to one in 10,000 of the population. It is believed be more prevalent in people who are left-handed and those who confuse the directions left and right.
Women are much more likely to have it than men - around 90 per cent of those who are known to have the condition are female. Despite this, famous synaesthetes from history are all male.
David Hockney, the British artist, has the condition. He is quoted as describing the sets he has created for operatic productions as "painting themselves" after he had heard the score.
The composer Alexander Scriabin created his symphony Prometheus, a poem of fire for orchestra and simultaneous light show in which each musical note is accompanied by the colour it triggered in Scriabin’s mind.
Vladimir Nabokov, the Russian author, spoke of his own experience in an interview in the Listener magazine in 1962. He described it as a "rather freakish gift of seeing letters in colour". He also wrote in detail about his own and his mother’s synaesthesia in his autobiography.
Others are unaware that they even have the condition, believing that their own multi-sensory perception of the world is shared by all. Morgan says: "At the launch party for my book, two friends said to me quite separately, ‘What do you mean, Mondays are red? Mondays are white. All the days of the week are white.’
"They have synaesthesia and had never realised it, because it is so little known and because in most people, it does not cause distress.
"In fact, many feel it enriches their lives - that the rest of us see the world in black and white."
Mondays are Red by Nicola Morgan is published by Hodder at 5.99. For more information on the condition, see International Synaesthesia Association - www.psychiatry.cam.ac.uk/isa