MURIEL still remembers when her father painted the family house. The paint was white. But it smelled unmistakably blue. Then there's Kristen, who tastes words, complete with texture and temperature. Often the spelling affects the flavour. The name 'Lori', for instance, tastes like a pencil eraser, but the name 'Laun' tastes lemony.
These two women have synaesthesia, which means that their senses mingle. A stimulus to one sense gives rise to entirely unexpected responses in others. A scent, in Muriel's case, triggers not only smell but colour. Grass might smell purple - or roses grey. The taste of chocolate might bring on a prickly feeling, of pins jabbing into flesh. Sounds, too, evoke colours and shapes. One synaesthetic can picture the green loops of a helicopter's drone; another is assaulted with red daggers on hearing a bell.
Most people with the condition - there are an estimated one in 25,000 - simply hear letters, numbers and words in colour. To them, it is a natural, unobtrusive part of life, like seeing. Synaesthetics aren't impaired, don't 'suffer' and for the most part enjoy their little gift. Amusing and quaint though it is, synaesthesia, neurologists agree, is rather unimportant - at least, it would be if it hadn't fired a debate that goes to the core of how the brain works.
The problem is that synaesthesia is not easily explained. How does this sensory mix-up happen? And where in the brain does it take place? According to the widely-accepted view, the various functions of the brain, senses included, are handled in specialised neural 'modules'. Each module consists of a number of areas of the brain wired together by neural 35 pathways and centred on the cortex, the sheet of tissue that forms the outer shell of the brain and is the seat of rational human thought. Information to do with hearing, for instance, is processed in one module, while vision is processed in another.
But synaesthesia seems to defy this segregation: at least two senses are interacting. When Muriel 'smells' the colour of paint, does the activity take place in the brain's 'smell centre' or in the module for vision? Or somewhere else? Is there something special about her brain or could any old brain do this?
A team of psychologists and neurologists in London, headed by Simon Baron-Cohen of the Institute of Psychiatry, has been looking at the condition for six years. They have come to the tentative conclusion that the brains of synaesthetics are biologically distinct. 'In people with synaesthesia,' suggests John Harrison, a neurologist at Charing Cross Hospital and a leading member of the team, 'perhaps unusual pathways link centres of the brain that process what we hear to areas processing what we see.' In other words, their brains are cross-wired. 'It's a perfectly good explanation.'
But not according to an American neurologist, Richard Cytowic of Capitol Neurology, a private clinic in Washington DC. The brains of synaesthetics may not be fundamentally different at all. Far from being a hierarchy, dominated by a 'rational' cortex where functions such as sensory perception are localised and separated, the human brain may be a much more integrated unit. It may be that the neural structures beneath the cortex co-ordinate communication in the brain and decide on the importance of the information being processed. And the quest to explain synaesthesia, he says, has helped to unveil it.