People who see vivid mental calendars could help us understand memory, says Caroline Williams.
FOR Emma, this time of the year has special significance, and not just because of all the gifts and food. It’s also the only time of year when the date in her mental calendar lines up perfectly with her body.
Emma is a calendar synaesthete, one of a handful of people who see time: not as a vague conceptual timeline, but as a vivid calendar that feels so real they could almost touch it. This is a little-known variation of synaesthesia, in which the brain links one kind of sensation to another. Some people associate shapes with certain sounds, or colours with numbers (see “Crossed wires”). Emma sees time as a hula hoop, which anchors 31 December to her chest and projects the rest of the year in a circle that extends about a metre in front of her.
Around 1 in every 40 people has some form of synaesthesia, in which sensations of one kind evoke another – so they might taste colours or hear flavours, for instance. Up to 15 per cent of synaesthetes experience vivid perceptions that seem to be “out there” in the world, rather than in their mind’s eye.
Synaesthesia runs in families and is thought to be caused by extra wiring between adjacent sensory areas of the brain. People with one type are likely to have another unrelated kind as well. It is still unknown which gene or genes are involved.
Heidi, another calendar synaesthete, sees the year as a backwards C hovering before her, with January at one end of the horseshoe and December at the other. When she thinks of a date she feels herself travel along the calendar to the right spot. She has a separate, hoop-shaped calendar for days of the week. Both have been part of her life for as long as she can remember.
The fact that certain people can vividly conjure number lines and calendars was first noted by Victorian polymath Francis Galton in 1880, but we have only recently begun to figure out how – and why. It’s not just a matter of idle curiosity. Understanding how calendar synaesthesia works may help unravel the way we all keep track of our memories as we move through space and time.
That’s because calendar synaesthetes experience a supercharged version of the way everyone else experiences time. Studies of different cultures around the world have shown that our perceptions vary slightly – most people in the West perceive time as a straight line running through their bodies, with the future ahead of them, while in parts of Papua New Guinea time flows uphill and for some Chinese people it flows downwards. But we all compute the abstract concept of time in the same way: in our brains, “time is always mapped onto space,” says V. S. Ramachandran, a neuroscientist at the University of California, San Diego.
The mapping job falls largely to the hippocampus, a pair of curved structures towards the centre of the brain that contain specialised neurons. Some, called grid cells, plot locations, while others, known as place cells, become active when we arrive on the scene. The basic circuitry seems to have evolved about 300 million years ago in a fish-like common ancestor, and similar systems are found in most other animals, from lizards to birds. At some point in human evolution, though, the hippocampus gained a second role: storing autobiographical memories, each with a time stamp recorded by specialised time cells.
Heidi’s year is a backwards C hovering in front of her, with the days of the week shaped like a hoop
“As you live your life, place cells keep track of your location in the world, and time cells keep track of stimuli receding into the past,” says neuroscientist Marc Howard at Boston University. “When you vividly remember a specific event from your life – say lunch last Tuesday – the hippocampus recovers the activity of time cells and place cells that were active during that event.”
Whether any other animals have this kind of autobiographical memory is hotly debated, but we know for sure that no other species makes calendars. Around 10,000 years ago, we began to notice the natural cycles of the sun and moon and record them for future reference, first in stone circles, and today on paper and computer screens.
But calendar synaesthetes don’t need to. They can call up their mental versions at will, something most are surprised to learn is unusual. Heidi first realised in a psychology class in high school. “My teacher was talking about synaesthesia and how some people see calendars. I said, ‘doesn’t everybody see a calendar? How can you not?’”.
Ramachandran wanted to know how they do it, and if they were really seeing calendars or summoning something from memory. So he asked a 20-year-old synaesthete called ML to recite alternate months between January and December, first forwards and then backwards. For most people, it takes three times as long to go backwards, because we have to construct the calendar from memory as we go. But ML was equally fast in both directions. She also unconsciously moved her eyes and finger as she went, suggesting her calendar was always in front of her.
To find out more, Ramachandran also used visual illusions, including the “motion after-effect”. If you stare for 30 seconds at a contracting spiral and then look at another picture, it will appear to expand, because the brain’s prediction outpaces our perception. But the illusion doesn’t happen if you look at a blank wall or just imagine a scene. “The brain needs something to attribute it to,” says Ramachandran.
When ML looked at her calendar after the spiral, it expanded in the same way as a real image. When asked to imagine an object in her mind’s eye, it stayed still. That means that, as far as her brain is concerned, the calendar isn’t a figment of her imagination, it is actually there.
What is going on? Ramachandran points to an area of the brain that we rely on to make sense of symbols and numbers and order events into sequences. The angular gyrus is found above and behind the ears on each side of the brain at the junction of several sensory areas, including the visual cortex. It also connects directly to the hippocampus. We all probably use this bit of circuitry to imagine the layout of time, but Ramachandran believes this is where calendar synaesthetes have the extra connections that make their visions so very real.
There are many open questions, not least whether this vivid calendar helps memory. There’s reason to think so. “If you ask them about a specific memory, then they’ll conjure up the calendar and put the memory in the appropriate slot,” says Ramachandran.
That might be a trick worth learning. Daniel Bor at the University of Sussex, UK, has found that people can teach themselves to experience synaesthesia by repeatedly associating colours with certain letters. It might be possible to do something similar with calendars.
But they may not be a universal boon. One synaesthete Ramachandran met finds her calendar confusing, and another says hers is missing August, which can be frustrating – not least for making plans for a summer break.
For Heidi, it’s a mixed bag. “It helps me sometimes because I can picture things better, but I do get mixed up.” Her horseshoe-shaped calendar has a big gap after December, which means January always comes sooner than she expects it to. “It feels really abrupt, like a whole month was in between them and it just went all of a sudden,” she says. With the return to the office after Christmas looming, that’s probably something we can all relate to.