Some people are able to remember intricate visual details such as the architectural features of a landmark building or entire pages from books, which they can later reproduce from memory without error. This impressive memorization ability is often described as "photographic memory."
When we think of "photographic memory", there's this impression that people who have this ability can record visual snapshots just like a photograph. They can then retrieve the snapshot from memory, zooming in and out on different parts. However, no study has ever been able to prove that true photographic memory exists -- at least in this sense.
Memory is more a jigsaw puzzle than a photograph
Our eyes might work, to some extent, like a lens, but our memory isn't like some camera that captures every detail -- we'd all probably go mad, if that were the case. Instead, the things we're likely to remember are those things that we pay close attention to. This is why you're very unlikely to remember what you had for breakfast a month ago, unless it was something particularly eventful. This selective attention allows us to focus and record only the important bits. Later, upon recalling, the mind fills in the blanks.
Alright, that's the case for most people. But, surely there are exceptional people out there who can remember things in such vivid and excruciating detail, one might remark. You've read about them in the papers and you've seen them in movies. However, while there are people in the world with phenomenal memorizing abilities -- whether ingrained thanks to genetics or acquired through intense training -- their memory doesn't function like a camera.
That's not to say that there aren't remarkable people with a gifted memory. Teddy Roosevelt could recite entire newspaper pages—not just articles—as if they were sitting in front of him. Kim Peek -- the real person Dustin Hoffman's character was based on in the Oscar-winning movie Rain Man -- memorized every word of every book he had ever read, estimated at around 9,000 books. Arturo Toscanini conducted operas from memory after his eyesight became too poor to read the music. And Lu Chao from China recited the first 67,890 digits of pi by employing memorization techniques.
But even people who claim to have a true 'photographic memory' haven't stood up to scientific scrutiny. For instance, while they may be able to recite pages upon pages from a book without error, they often fail to do the same in reverse. If their memories were like photos, they should have been able to easily reproduce the text in reverse order.
Instead, what's often called "photographic memory" can be more accurately described as "eidetic memory." People with eidetic memory can form a mental image of what they just saw for up to several minutes, after which it is gone. They can describe the image with an unusual level of accuracy and detail.
Eidetic memory is controlled primarily by the posterior parietal cortex in the brain. This is the part of the brain through which visual stimuli are processed, and images retained. For most people, these images are only stored for a few short seconds before being discarded or transferred to short-term memory.
Between 2% and 10% of children have an eidetic memory, but this ability gradually fades that virtually no adult retains it.
But, even if eidetikers have phenomenal memories, they still can't capture all the details. What's more, like all people, eidetikers also invent details that were never really there in the image -- so-called false memories.
How to train your memory
People like Lu Chao, who holds the record for the longest string of pi digits a person ever recited from memory, use mnemonic techniques to help them record information. Although he could remember 67,000 digits in the right order, Chao is no genius. In 2009, researchers gave Lu, along with several other people that matched his age and education, a 'digit span' test -- how well they could remember a sequence of random digits which were presented a rate of one digit per second. Lu had a digit span of 8.83 while the average for the rest of the group was 9.27.
Lu doesn't have an innate ability to encode vast amounts of information. He knows a good trick to make up for it, though. In order to remember thousands of digits, Lu used a memory technique called loci, which is Latin for 'places'. The method, also known as the memory palace method, employs spatial or environmental cues to help learning and memory.
The method works something like this: you use a familiar environment, such as your home, and walk through the environment associating information (like words or digits) you want to remember with various objects or scenery. In order to recall the digits in the right order, you simply have to do a mental walk through your mind palace. In Lu's case, he devised an intricate story, and assigned images such as a chair, a king or a horse to two-digit combinations of numbers ranging from "00" to "99."
So, even people who can barely remember where they put their car keys can perform seemingly superhuman memory tasks -- if given the proper training.