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Synesthesia can be broadly understood as a jumbling-up of the senses — seeing a certain color when you hear a certain sound (called chromesthesia), or feeling a sensation on your own body after seeing it happen to somebody else (mirror-touch synesthesia) — but sometimes, it gets a little more nitty-gritty than that. Take, for example, grapheme-color synesthesia, one of the most common types, in which letters and numerals are imagined in their own consistent hues (for instance, “A” is always red, or “4” is always yellow). Or tactile-emotional synesthesia, in which running your fingers over a given texture, like denim, can be enough to trigger a strong feeling of joy or disgust.

Or the case of two “calendar synaesthetes,” recently published in the journal Neurocase: When asked to visualize a calendar, the patients, a pair of women identified as ML and EA, literally saw it laid out in front of them, as if in physical form. As New Scientist reported, ML’s was arranged in a V-shape, with the months written out in a specific font along the two lines; EA’s was “like a hula-hoop in front of her chest, with December passing through her body no matter what the actual time of year.”

To ensure that what the patients were seeing was different than an ordinary mind’s-eye visualization, the paper’s authors ran a series of visual tests; in one, for instance, they asked ML to recite the months in reverse order, skipping over two out of every three; the task took her less than two seconds per month, compared with roughly four and a half seconds per month for the control group, suggesting that she really was “reading” them off some invisible chart rather than counting backward in her head.

The study authors, who called their paper the first “clear unambiguous proof for the veracity and true perceptual nature” of calendar synesthesia, estimated that the phenomenon affects roughly 1 percent of the population — but its existence, they argued, has implications for the more universal question of how our brains make sense of time.

Specifically, New Scientist explained, it offers some support for the idea that we’re “hardwired to some extent to map time in space” — even though time, evolutionarily speaking, is a fairly new concept for our species, our brains may have repurposed other areas to help us comprehend it. “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,” lead author Vilayanur Ramachandran, a neuroscientist at the University of California, San Diego, told the magazine. For most of us, that translates to the ability to imagine a calendar; for a few, it means the ability to conjure one out of thin air.