When most of us picture the year ahead, we have a vague image in our minds of the upcoming months, in rough, calendar-shaped grids. But for some people, that picture is a whole lot more vivid.
Around 1 percent of the population has a strange ability known as 'calendar synaesthesia', which means they're able to visualise in crisp detail an elaborate calendar of the months of the year.
For example, one woman sees the months ahead in a symmetrical 'V' in front of her. Another, sees the calendar shaped like a hula-hoop, with December always passing through her body no matter what month of the year it is.
For the first time, researchers have now shown that these mysterious images aren't just mental images - they're something much more perceptively real than that. And figuring it out could help us understand how our brains process time.
"Our experiments provide, for the first time, clear unambiguous proof for the veracity and true perceptual nature of the phenomenon," writes the team from the University of California, San Diego, in the journal Neurocase.
Synaesthesia is a pretty broad neurological phenomenon, where stimulation of one sensory or neurological pathway automatically stimulates a second sensory pathway.
For calendar synaesthetes, this means that when they try to think about the months of the year, their visual pathway is automatically stimulated as well, leading them to 'see' a calendar in front of them.
But until now, the phenomenon has been incredibly mysterious, and scientists haven't been able to figure out how it works - are these people picturing calendars in their minds eye like the rest of us? Or are they actually visually seeing them in front of their eyes?
To figure this out, the team performed a range of experiments with two synaesthetes: 'ML' is the woman who sees the months of the year in a symmetrical 'V'; and 'EA' is the woman who experiences the hula-hoop calendar.
ML's calendar is so specific that she sees each month written in Helvetica font. To see how real this was, the researchers asked ML to draw her 'V'-shaped calendar on a screen using a laser pointer, several times over the course of a month.
Incredibly, every time she drew it, the angles and lengths of the 'V' were exactly the same, and just like a real-life object, the 'V' expanded and contracted depending on how far ML was standing from the screen.
Next, the team asked ML and eight non-synaesthetes to list the months of the year backwards. Each time, they'd ask them to skip one or two months to make it more challenging.
The idea was, if ML really could see this perfect Helvetica calendar in front of her, then surely she'd be able to complete the task quicker than the control group.
And that's exactly what they found - ML could recite every third month backwards, taking just 1.88 seconds per month, while the control group took a slow 4.48 seconds.
"During the task we noticed that ML appeared to be inspecting her calendar," lead researcher Vilayanur Ramachandran told Helen Thomson from New Scientist. "She would involuntarily move her finger from month to month and shift her gaze."
The team then asked ML to stare at a contracting spiral for 20 seconds, before either looking at darkness, a static image, or visualising her calendar.
This is a common optical illusion that results in any objects you look at afterwards, but not blank spaces, appearing to expand.
When ML performed the task, nothing happened to the darkness, and the static image expanded as expected. But so did her image of the calendar. This didn't happen when she pictured something else neutral, such as an apple.
In other words, the calendar ML sees is distinct, and more real, than a simple mental image.
EA's image of her calendar is a little different - she pictures a hula-hoop-like calendar, which has December passing through her chest.
Interestingly, when she turns her head, the calendar remains where it is, and doesn't move with her field of vision.
And when EA turns her head to the right, she explained that the months on the left of the calendar become "fuzzy", and she struggled to remember what happened during those months. The opposite is true when she looks to the left.
In this case, Ramachandran suggests that EA's access to her memories are influenced by the movements of her neck and eyes.
While both these cases are incredibly unique, they could teach us a lot about how all of our brains perceive time.
It's possible that when humans evolved the ability to deal with time and numbers, our brains didn't have the capacity to evolve a separate area to deal with this, so it could have ended up packed into the same area that controls our visual cortex.
"Given the opportunistic nature of evolution, perhaps the most convenient way to represent the abstract idea of sequences of numbers and time might have been to map them onto a preexisting map of visual space, already present in the brain," said Ramachandran.
This idea is backed up by the fact that brain scans have shown connections between the areas of the brain involved in processing numbers, and those involved with mapping the world around us. For example, blind people can 'repurpose' their visual cortices to make mathematical calculations.
If that's the case, perhaps synaesthetes simply have more connections bridging these areas in their brains than the rest of us.
It's too soon to say for sure what's going on. So far, we only have these two case studies to go on, and a lot more work needs to be done - including brain scans of what's happening when calendar synaesthesia occurs.
But at least now we have some clues as to which regions of the brain are behind the phenomenon, and how real it is for those who experience it.
The research has been published in Neurocase.