(CNN) -- In Daniel Tammet's mind, three is a dotted green crescent moon shape, one is a sort of white sunburst and four is a blue boomerang. Every number has a distinct color and shape, making the number pi, which begins with 3.14, unfold like a beautiful poem.
For math enthusiasts around the world, March 14 (3-14) is Pi Day, honoring the number pi, which is the ratio of circumference to diameter of a circle. On Thursday, Tammet is promoting France's first Pi Day celebration at the Palace of Discovery science museum in Paris.
Tammet's relationship to this number is special: At age 25, he recited 22,514 digits of pi from memory in 2004, scoring the European record. For an audience at the Museum of the History of Science in Oxford, he said these numbers aloud for 5 hours and 9 minutes. Some people cried -- not out of boredom, but from sheer emotion from his passionate delivery.
"What my brain was doing was inventing a meaning, like a story," Tammet said. "What I did was make a poem or a novel out of pi, and took those colors and those emotions and used them to perceive patterns, or at least to perceive patterns in my mind that were memorable, that were meaningful to me."
Many people around the world -- including me -- have been interested enough in this number, or in memorization itself, to see how many digits they can bank. Pi has infinitely many digits with no discernible pattern, yet it mathematically explains the shape of all circles. This makes memorizing it a difficult, yet somehow meaningful, challenge.
Serious pi memorizers such as Tammet have become fascinating subjects of study for scientists, too. They bring up fundamental questions about innate ability vs. learned skills. Are the brains of people with superior memory somehow different? Or can anyone learn thousands of random digits?
Making meaning out of numbers
Superior memorizers, according to the research of K. Anders Ericsson, professor of psychology at Florida State University, have three special skills. They use knowledge and patterns that they already know to encode information in their long-term memory. They associate that information with retrieval cues, so that they can trigger the information again. They also get faster at all this by becoming better at encoding and retrieval through intense practice and effort.
This theory appears to explain Chao Lu, who set the current world record for pi recitation at 67,890 digits in 2005, at age 23. Creating associated meanings in numbers played a big part of that. He used mnemonics relating to the sounds of numbers as well as the shapes or meanings of particular digits and images, according to a 2009 study by Ericsson and colleagues.
Strangely, if presented with one number at a time, at one digit per second, Lu's recall is no better than the average person's, Ercisson and colleagues found. With that rate of presentation of numbers, he is forced to rehearse numbers in his head just like everyone else, Ericsson said.
But with large blocks of numbers, it's a different story. Lu and a previous pi-memorizing record holder, Hideaki Tomoyori, who recited 40,000 digits of pi, have said they linked words or images to groups of two, three or four numbers. Then they created stories connecting them. Tomoyori practiced daily, spending between 9,000 and 10,000 hours total memorizing before his recitation.
A rare case was Rajan Mahadevan, who set a record at 31,811 digits of pi and did not report using any mnemonics. When tested by researchers, it appeared he sequentially memorized numbers in blocks of 10. While some argued that Mahadevan may have an innate ability to memorize, Ericsson and colleagues found in their experiments with Mahadevan that he learned his unusual methods for memorizing numerical patterns after a thousand hours of practice.
Amateurs have their own methods and perceptions. Lulis Leal, 47, of New York, is also getting in on the pi memorization action. Inspired by a CNN iReport assignment, she is up to 314 digits and recorded herself reciting them.
"It's a visual thing," she said of pi. "It's just a picture that I see -- a line going up and down."
Listening to Leal's recitation is jarring for me because I hear it in a different rhythm, as though it were a song with the down-beat on every other digit. When I was a teenager I saw a website suggesting pi could be a song, with one as middle C, two as D, three as E, etc. This helped me get up to 178 digits in college, winning me a T-shirt.
The neuroscience of expert memory
There's no straightforward control over what we remember and what we forget; we all wish we could remember some things and forget others. But it is possible to train as a mental athlete, and there are entire competitions around that, said Joshua Foer, author of "Moonwalking With Einstein" and memory championship winner.
"Why do people memorize pi? Why do people climb Mount Everest? You don't need to do it," Foer said. "There's something about wanting to see how far you can go and how much you can push yourself."
For memorization, it helps to find meaning in the information and tie it to multiple senses, such as auditory and visual, said Dara Manoach, associate professor of psychology at Harvard Medical School and researcher at Massachusetts General Hospital.
An area of the brain called the dorsal lateral prefrontal cortex gets activated when you're holding information online -- for instance, keeping a phone number in your head until you can write it down or call it. Associating a number with sensations, and integrating it with existing knowledge, engages more neural systems and helps you store the information.
Studies of people with exceptional memory have not found any structural differences between their brains and those of people with average memory, Ericsson said.
But researchers have found differences in the activation of certain brain areas in exceptional memorizers. The patterns reveal at the neurological level how people with "good memory performance" are associating new information with things they already know, such as familiar numbers like area codes or other semantic information.
Ericsson's take on becoming an expert at something, such as memorizing pi, is deliberate practice. His research basis for the idea, popularized by Malcolm Gladwell, is that it takes 10,000 hours to achieve expertise. But Ericsson pointed out that this was based on studies of musicians in top groups who spent an average of 10,000 hours at practice. That means some spent more time and some spent less.
"To win an international piano competition, I calculated that you need about 25,000 hours of piano training," Ericsson said.
That doesn't mean there aren't people who can just memorize numbers without connecting them with anything else -- but, said Ericsson, "so far I've yet to see anybody who is willing to be tested."
A coincidence of senses
There are, however, ways in which Tammet may inherently have a different approach to numbers than most pi memorizers.
Tammet has high-functioning autism, a developmental condition associated with impaired social communication and repetitive behaviors or fixated interests. On top of that, he has synesthesia, meaning he experiences a mixing of senses and perceptions -- specifically, he sees words and numbers as colors and shapes.
In an analysis of Tammet's case, published by Simon Baron-Cohen and colleagues in the Journal of Consciousness Studies in 2007, researchers wrote that both autism and synesthesia, while different, are associated with an excess of neural connections; it's possible they have a "common neural abnormality." They also proposed the idea that autism and synesthesia together may have increased the likelihood of Tammet's savant memory. More research is needed to support both these ideas.
Foer's book questions whether Tammet has used mnemonic methods to memorize digits of pi and other information, just like everyone else without synesthesia. But Julian Asher at the Autism Research Centre at the University of Cambridge, who studied Tammet, said synesthesia is "fundamental to his feats of memory." Recalling pi for Tammet is like "walking through a synaesthetic landscape in which different numbers are represented by geographical features, and 'reading' them off as he goes," Asher said in an e-mail.
Tammet grew up as the oldest of nine children. The stimulation he received from his large family probably helped him develop social skills, he said.
"If I hadn't had that love and that patience and that support from an early age, I might not be where I am today, certainly," he said.
As a child, he was called "private," "shy" and "timid." He also had epileptic seizures, and took medication until he grew out of them.
"When he sat on the carpet during story time, with his eyes tight shut and his fingers in his ears, picturing numbers in his mind and their shapes and colors, whilst the other children looked at each other or at the teacher and listened to the story, (Tammet) was in some sense in a world of his own," Baron-Cohen and colleagues wrote.
In high school, Tammet learned about pi and thought it was beautiful. But when he talked about it with his classmates, they didn't understand. For them, pi was just a number. It didn't have any color or beauty or emotion.
"That's when I had a perception of the world, of words, of numbers, that was different," he said. Secretly, he started memorizing digits of pi.
Finding life through pi
At age 25, Tammet finally felt confident enough to "share all those years of loneliness, all those years of battling with this condition, with my autism, all the years of also experiencing this tremendous beauty, this emotion."
He spent about three months learning thousands of digits of pi, which he saw as an emotional experience, being transported into another world.
Tammet's achievement of the European record for pi recitation wasn't about showing off, he said. First and foremost, he wanted to accomplish something that meant something to him personally. He was able to reach out to people without autism, without synesthesia, and communicate to them in a new way. The event also benefited an epilepsy charity.
"When the digits darken in my mouth — heavy eights and nines packed together — the tense distant faces grow tenser still. When a sudden three emerges from a series of zeroes and sevens, I hear something like a faint collective pant. Silent nods greet my accelerations; warm smiles welcome my slowdowns," he wrote in his book "Thinking in Numbers," which comes out in the United States in July.
At the pi recitation, Tammet realized he had a gift of communicating, and that set him on the path to becoming a full-time writer.
The pi memorization event also led Tammet to Baron-Cohen and colleagues, who made the diagnosis of high-functioning autism. It explained the difficulties he had with communication and social skills as a child.
He has not recited pi since the 2004 event, but still appreciates the beauty and wonder of the number enough to promote Pi Day in France.
"You can find a lover's telephone number. Your date of birth. Your Social Security code. You can find everything in pi," he said. "It's big enough to have all of life inside of it."
Want to memorize pi? Here are 10,000 digits.