Here’s what you’ll learn when you read this story:

• Synesthesia is a neurological phenomenon that causes multiple senses to mix.
• For some synesthetes, time isn’t just a ticking clock, but rather something that can be uniquely felt and visualized.
• Researchers say that studying synesthesia could help us better understand human perception.

What do paintings sound like? Do musical notes have color? Can a name have a taste? For some people, these questions have simple and obvious answers. They experience extraordinary sensory crossovers due to synesthesia, a neurological phenomenon that causes two or more senses to mix in complex ways and present concurrently. One of the most common types involves linking specific colors to numbers or letters, but for a subset of synesthetes, the concept of time is felt and visualized in unique ways. These curious sensations can help shed light on human perception, which remains a fundamental mystery that science cannot fully explain.

“When I was little, I didn’t understand why calendars were in a grid, because to me, the months of the year present in the shape of an oval,” says Mary Spiller, PhD, a synesthete and cognitive psychologist who studies perception as a senior lecturer at the University of East London (UEL). “I just assumed everyone thought about time that way.”

It wasn’t until Spiller was a PhD student at UEL investigating mental imagery that she realized she herself had a type of the rare phenomenon called time-space synesthesia, in which days, months, and other units of time have a spatial layout in and/or around a person’s body.

“If I’m thinking about a few days ago, then it’s a sense of space to my right. For some people, time has color and really exciting stuff, but mine’s just an empty space,” she explains. “If I’m thinking more about something that’s going to happen months in the future, then I sort of mentally think about this oval shape in front of me, and my mind automatically goes to the point on the oval where that time is.”

Journalist and time-space synesthete Emma Yeomans describes units of time represented as boxes in her mind, much like a “hopscotch square.” “I am always standing in today,” she writes in a BuzzFeed article. “The closer a date is to me, the larger it appears, so next week is bigger than next year, and if I’m reading a history book with lots of dates then I might find myself ‘zooming out’ to see it in units of decades or even centuries.”

Since the 1800s, researchers have puzzled over how time-space and other types of synesthesia develop in the brain, but experts have yet to arrive at a solid conclusion. There are currently two main camps of thought, according to David Brang, PhD, who runs the Multisensory Perception Lab at the University of Michigan. One model posits that most healthy individuals have the same connections between different sensory areas of the brain—such as the visual cortex and parietal lobe—but these pathways are inhibited to some degree in non-synesthetes and more unrestrained in people who have synesthesia.

“There’s some evidence to suggest that this is at least part of the answer, as people who do sensory deprivation or go under the influence of certain drugs, for example, can experience auditory and visual synesthetic-type hallucinations,” Brang says. “That’s consistent with the idea that everybody has the pathways in place to produce these synesthetic experiences, but they’re normally just inhibited for one reason or another.”

The second idea is that the wiring in the brains of synesthetes is fundamentally different, with an increase in connections between sensory areas of the brain. Synesthesia is neurodevelopmental, which means it emerges as an individual’s brain grows in early childhood. Spiller and other researchers believe that uncommon genetic traits give synesthetes extra connections in their brains over time. In fact, imaging studies have found that synesthetes tend to have more white and gray matter in their brains, which suggests higher structural and functional connectivity.

Brang considers himself in a less-conventional third camp. “One of the most interesting aspects to me, at least in terms of what makes somebody a synesthete, is the consistent research finding that they have increased visual imagery,” he says. “So maybe synesthesia is a little bit more about excitability in the brain that responds more strongly to external sensory inputs.”

He isn’t the only one investigating a combination of factors. A recent study of ticker-tape synesthetes—who automatically see their words as they are spoken in their mind’s eye—found that the participants had an over-activation in an area of the brain where speech interfaces with vision, and increased white matter connections in the same region.

Synesthesia is highly hereditary and researchers believe that roughly 4% of the population—more than 330 million people—experience at least one kind of synesthesia. But why has it survived evolution as a reasonable trait for people to experience?

“Enhanced creativity, increased imagery abilities, and increased artistic abilities tend to be linked with synesthesia,” Brang says. “All these can point toward why this is a phenomenon that has survived within humans. It’s an extreme version of what everybody has, which can help us better understand the continuum of sensory perception across people.”

Because Spiller is constantly surrounded by a mental picture of her calendar, she believes her time-space synesthesia gives her a cognitive boost when it comes to time management. “I’m very organized in terms of planning and thinking ahead,” she says. “With two kids and all their various activities, I can kind of see it all, and that is an advantage.”

Brang and Spiller agree that it’s unlikely one could develop true time-space synesthesia by, say, practicing the visualization of days of the week in a certain way. However, it is also tough to pin down whether someone is a true synesthete or not.

“There is no blood test for synesthesia,” Brang says. “Most individuals do not discover that they have synesthesia until they see TV shows or read news articles about it, and it’s the first time they encounter these things that they always assume that everybody else had.”

In a recent presentation at the Royal Institution in London, a nonprofit dedicated to connecting people to science, Spiller asked two participants to draw what time “looks like.” One person drew a simple straight line, while the other, who is a synesthete, drew curving lines around a body that represented the months in a year. The ability to replicate the very same physical—or mental—image over time is currently one of the best ways of “testing” for time-space synesthesia.

Synesthetes don’t need a diagnosis, but Spiller says awareness is important, so if someone’s talking about feeling shapes or seeing time, they aren’t misunderstood for having a mental disorder.

“To understand what [synesthesia] is and how it works is also a good way for people to have a bit more understanding of how our brains perceive the world,” Spiller says. “Synesthesia is a really nice way of explaining to people that we don’t all experience the world in the same way, and that’s okay.”