The condition can be create a barrage of sensory experiences
Imagine if you could see time laid out in front of you, or surrounding your body. And you could physically point to specific dates in space.
Important dates might stand out - birthdays, anniversaries. And you could scan a visible timeline - to check if you were available - whenever you made plans. No actual diary necessary.
According to Julia Simner, a psychologist from the University of Edinburgh, there is a reasonable chance you can. And that you may use the experience, unconsciously, every day.
Dr Simner studies synaesthesia - a condition caused by an unusually high number of connections between two areas of the brain's sensory cortex, making two senses inseparable.
Synaesthetes, as they are known, have experiences that might seem extremely strange to any non-synaesthete.
The extra connections might be between the brain area that processes colours and the area that processes language.
"One of the most common variants is called grapheme-colour synaesthesia," says Dr Simner.
"People with this variant know the colour of letters of the alphabet. So they know that the letter 'A' may be red. But not just any red, it's a certain shade of crimson. And B is turquoise-blue."
These colours are different from person to person, but for one synaesthete they are very consistent.
"If you are a synaesthete with a red A, your A has always been red and will always be red. And it's so intrinsic, that many synaesthetes never question whether this is unusual."
But synaesthetic experiences are not only triggered by a sensory experience - hearing a sound or reading a word that starts with at coloured letter - they can also triggered simply by thinking about things.
See the future
The shape of each synaesthete's year is different. (Image based on an original illustration by Carol Steen)
In the case of time-space synaesthesia, a very visual experience can be triggered by thinking about time.
"I thought everyone thought like I did, says Holly Branigan, also a scientist at Edinburgh University, and someone with time-space synaesthesia.
"I found out when I attended a talk in the department that Julia was giving. She said that some synaesthetes can see time. And I thought, 'Oh my god, that means I've got synaesthesia'."
So what exactly does she see?
When I'm making plans I can look at my mental calendar
Holly Branigan, synaesthete
"For me it's a bit like a running track," she says.
"The track is organised around the academic year. The short ends are the summer and Christmas holidays - the summer holiday is slightly longer.
"It's as if I'm in the centre and I'm turning around slowly as the year goes by. If I think ahead to the future, my perspective will shift."
There are at least 54 different variants of synaesthesia and Dr Simner thinks this might be one of the most common ones.
"If you ask all the people at your work, or in your family, you're likely to find at least one person who has it," Dr Simner says.
Synaesthesia is a strange condition that has attracted a great deal of research. But its variety and complexity mean that the time-space variant is one of three types that has only been properly described within the last few years.
Two others are much rarer and perhaps even more bizarre.
Dr Simner explains: "There is one called ordinal-linguistic personification. So letters or numbers trigger, not colour, but the impression of a personality or gender.
"So, you don't know that number seven is green, but you know that it's a maniacal husband who comes home from work and shouts at his wife.
"You might not have a colour for Thursday, but you know that it's a young girl who has spent too long kept in the house and wants to break out into the world."
Another variant recently come to light is called mirror touch synaesthesia. This causes people to experience sensations of touch when they see other people being touched.
"So if I sat in front of you and scratched my nose, you would feel a scratch on your nose," explains Dr Simner. Psychologists have linked this to a greater sense of empathy.
Brain imaging allows scientist to see what causes synaesthesia
And this is not the only emotional aspect of synaesthesia.
Synaesthetes often report that they actually enjoy things that match their own experiences.
"So if you have a red A, and I show you a picture of the letter A in red, you really like it," explains Dr Simner.
"But if I show you a green A, you hate it. I've had to change the colours of fonts on my power point slides in the past when giving presentations to synaesthetes."
"If you want to define synaesthesia in a purely neurological sense, it's just the predisposition to have extra pathways between areas of the brain," says Dr Simner. "And we can see those connections."
With FMRI scans (functional magnetic resonance imaging), researchers can watch activity in the brain - for example, seeing colour- and language-processing areas "light up" at the same time in a grapheme-colour synaesthete's brain.
And with a newer imaging technique, called diffusion tensor imaging (DTI), Dr Simner says you can "almost count the extra pathways".
"This tracks the movement of water molecules in the brain," she explains.
"We're still inferring the pathways from the image, but it's pretty clear. Where there are lots of pathways the water molecules, which normally move randomly, stop moving quite as much."
Power of the mind
Some types of synaesthesia interfere with everyday living. As one synaesthete told me recently, if someone says a word that tastes of roast beef whilst you're eating your strawberries, it can ruin a tasty treat.
But for those who have it, time-space synaesthesia can be useful, even fundamental to everyday life.
"When I'm making plans, I can look at my mental calendar," says Holly. "I always find it odd that other people don't have that."
And one particular case study caused Dr Simner to wonder whether time-space synaesthesia might be an advantageous thing to have.
She and her team became interested in a type of savant with a condition known as hyperthymestic syndrome.
"This is a savant whose amazing ability lies in their ability to recalling dates and events in time," she explains.
Researchers in the US wrote an article about their patient - a woman who displayed the condition.
"This person can tell you exactly what they were doing on any particular day of any year of their life," says Dr Simner.
"She can tell you which of her shoelaces she tied up first in 1974 on a Tuesday afternoon, what clothes she was wearing when she first ate a hamburger."
In a very small section of the study, the American team mentioned that this savant described how she could see time in space.
The average person recalled about 39 facts; the average synaesthete doubled that
Dr Julia Simner, Edinburgh University
"That jumped to me as someone with time-space synaesthesia, and I immediately asked: is the time-synaesthesia a necessary component of the savantism?"
Dr Simner and her colleagues Neil Mayo and Mary Spiller started to investigate this by testing and comparing the memories of synaesthetes and non-synaesthetes.
They designed a series of question and answer tasks that aimed to unravel the connection between the two conditions.
One of the tasks was to recall events from a given year in your life. Each participant was presented with a series of nine years from within their lifetime. For each year, they were asked to list things that they remembered from their own lives.
"Across all nine years, the average person recalled about 39 facts," says Dr Simner. "The average synaesthete doubled that."
One of the synaesthetes recalled 123 different facts within the time period.
"So the average person might remember that they went on holiday to America when they were seven," says Dr Simner. "This person would recall the name of the guesthouse, the name of the guesthouse owner and the breed of the owner's dog."
Synaesthetes also outperformed non-synaesthetes in tasks where they were asked to recall the dates of significant cultural or political events: What year did Nixon go to China? What year was Nelson Mandela released?
This remarkable difference showed the researchers that the average person with time-space synaesthesia does have an advantage when it comes to recalling events in time.
The team's next task is to find out if all savants - those with hyperthymestic syndrome - also have time-space synaesthesia.
But they are also testing synaesthetic tendencies in the general population.
They have already established that most people associate texture and shape with shades of colour. And most people have an intrinsic sense of the shade of different pitches of sound.
Through the research team's website, you can take part in a series of tests to find out if you are in fact a synaesthete.
If you find that you can see time, you might find it gives you impressive powers of recall. You could be a big asset for a pub quiz team, but you will have no excuse for forgetting your anniversary.
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