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."