This is not a disorder; these people do not suffer. Australian PhD student, Anina Rich considers these people to have an unusual gift. As part of a small University of Melbourne team, Rich is one of the few people in the world studying the phenomenon of Synaethesia, a condition that raises more questions than answers and generally has researchers baffled.
Rich is giving a plenary paper this week at the ninth annual meeting of the Cognitive Neuroscience Society in San Francisco, USA about her latest research from the preliminary analysis of brain images of synaesthetes. It is one of the first objective analyses of the condition and their results have begun to reveal the secrets of how the brain of synaesthetes function.
The University of Melbourne has 200 synaesthetes on their database - the largest in the world - and they estimate there could be as many as 1 in 2000 people with synaesthesisa.
"Many synaesthetes don't realise they have the condition. They are unaware that the way they perceive the world is different," say Rich.
In most people, a particular physical stimulus presents a single united perceptual experience: light gives us a visual experience, sound an auditory experience, odour an olfactory experience.
Synaesthetes, however, get an extra one or more perceptual experiences. For example, a particular sound might induce vivid experiences of color, taste or odour. Visual symbols like letters or numbers might elicit specific colors producing for the person a kaleidoscope experience.
This extra layer of information may be behind synaesthetes' excellent rote memory.
"They have color as an extra bit of information to help them remember things like names and strings of numbers," says Rich.
The literature is also replete with assertions that synaesthetes tend to be creative, artistic and highly emotional individuals.
The University of Melbourne team is focusing on the most common form of synaesthesia where digits, letters or words elicit specific colors when they are seen or heard.
"The condition doesn't interfere with color vision. Few people describe the colors they see as 'out there' in space. Most describe it as being in the mind's eye as a distinct form of mental imagery. Some people see the actual letter or word as a color, others get a wash of color, others get a feeling of color," says Rich.
Rich describes one woman's experience. She said: "If I think about the word grass, I know the object we call grass is green, but the word grass is purple because it starts with 'G' and the letter 'G' is a purple letter for me."
The experiences are involuntary, occurring without conscious effort. They are typically present from early childhood and they are highly consistent over time," says Rich.
"One lady saw most of her letters as different, but specific shades of burgundy. These shades were consistent over time. She was even able to match most of the shades she saw on a computer paint program.
"Much of the research on synaesthesia is anecdotal and has relied on self-report. All perceptual experiences are subjective, which poses a challenge for the study of synaesthesia."
Using Magnetic Resonance Imaging, the team recorded brain images from a group of synaesthetes and non-synaesthete controls as they performed a series of visual tasks to find out which areas of the brain are involved in synaesthesia.
"When colors are elicited in a synaesthete, many areas of their brain are active. Our initial results, though, identified one consistent area known as the anterior cingulate that is involved in response inhibition.
One test involved synaesthetes being shown a letter that, for them, elicits a color experience. One synaesthete saw the letter 'A' as red. When shown a red 'A' and asked what color it was, she could quickly answer red. But when shown a green letter 'A' and asked what color the letter was, her response time was significantly slower.
"The letter was eliciting a color, but it was a different color to the one she was trying to name. This is what is known as response inhibition," says Rich.
The anterior cingulate is also activated in non-synaesthate people when exposed to tests that require a response inhibition.
"Ultimately this research will help reveal how we bind perceptual information from all five senses, information that is normally processed in different parts of the brain," says Rich.
"Synaesthetes provide a way of investigating this binding process.
"Synaesthetes also have this remarkable experience that the rest of us don't have. We want to find out how and why."