Good memory means you can recall details you’d previously noted or processed. Eidetic memory means you can go back to the original sense data and note things you hadn’t noticed at the time.
For illustration, make up a pirate in your imagination. It has to be a new pirate, so remembering Jack Sparrow doesn’t count. Make up a new one.
Now, here are some questions about your pirate. What color were his shirt buttons? Did he have all of his teeth? How much did he appear to weigh?
Most of the time we have to fill in these sorts of details afterward, if we’re being honest with ourselves. We didn’t conceive a completely fleshed-out, picture-perfect human being in the first place. If you’re like me, he was a badly rendered cartoon idea of a person.
Our semantic memories are a lot like this. We notice a few important details, which we can recall later on with some effort, but we can’t go back in time and view the old sense data. Practically speaking, that sense data is lost. Eidetic memory is a different phenomenon, by which some few people can retrieve original sense data and process it later on.
I’ve heard a second-hand anecdote from an old news gal that there was once a TV anchor on channel 5 who could glance briefly at the day’s script and then pace around the office, sipping his coffee, reading and reviewing the script in his head. This is an example of eidetic memory. I experienced a small amount of this in the early portion of my marijuana trip, just as my short-term memory was short-circuiting. I was able to recall previous sense data and replay it, noting strange new details.
The hmolpedia folks cited Dylan Jones as a rare example of a math/engineering prodigy with an eidetic memory, but the anecdote they use to support their case actually discredits it:
Hereman recalls a day when Dylan came to his office to let
him know that he had memorized the first 500 digits of pi. The
professor pulled the number up on his computer screen to fact
check and Dylan proceeded to recite about the first 300 digits
without a hitch. So Hereman challenged him with another
number, e, the basis of the exponential, 2.71828…, curious
if Dylan might get the sequences confused. “Why would I get
them confused?” Dylan asked, “They are two different numbers.”
Hereman remembers Dylan returning disappointed two days later:
he had only perfected the first 100 digits.
This is merely an example of very good memory, because it seems he was trying to remember it through “brute force” verbal recitation. Longtime readers may recall (hi Zeke!), I was able to learn 50 digits of pi in five minutes by devising a better, more visual method. But it was still semantic in nature, and required conscious processing.
Contrast this with Kim Peek, a proper example of eidetic memory:
He could speed through a book in about an hour and remember almost everything he had read, memorizing vast amounts of information in subjects ranging from history and literature, geography and numbers to sports, music and dates. Peek read by scanning the left page with his left eye, then the right page with his right eye. According to an article in The Times newspaper, he could accurately recall the contents of at least 12,000 books.…
Peek did not walk until he was four years old, and then in a sidelong manner. He could not button up his shirt and had difficulty with other ordinary motor skills, presumably due to his damaged cerebellum, which normally coordinates motor activities. In psychological testing, Peek scored below average (87) on general IQ tests.
One suspects his IQ scores were extremely unbalanced. He probably would not be able to do very simple spatial rotations, or math problems whose answers he hadn’t previously memorized.
The wiki misses this detail, but Peek was able to “skim” at a rate of one page per 8-10 seconds. The hmolpedia folks point this out while explaining that the reading scene in Good Will Hunting is probably misleading:
Note (on reading at rate of one reading at a rate of one page per second): Not even Kim Peek (above) who, having is corpus callosum missing, could read the left page with his left eye and the right page with his right eye and in this way he could read two pages at a time with a rate of about 8-10 seconds per page, could read at the rate shown in the film clip and also have and eidetic memory of content down to the page number and references.
Peek wasn’t so much processing the content on each page as he was just “seeing” it, and later recalling it as if reading it off the page for the first time. If Peek were tasked with memorizing the number e, I suspect he would have memorized more than Jones’ 100 digits in less than five seconds. To Peek, remembering 100 digits would be no more difficult than reading the digits off a piece of paper in front of him.
Someone might have a “very good” memory that approaches the accomplishments of Peek’s eidetic memory, but it would still be categorically different.
Because it seems that one and the same person can be capable of both sorts of memory, I consider it likely that Sidis’ brain was accustomed to “switching gears” without much friction, so that he could commit large amounts of information to memory in the same way as Peek and then switch to semantic mode to perform his mathematics using his unusually high processing speed.
The combination of cognitive styles, and a frictionless transition between them, probably has a great deal to do with his extraordinary IQ scores. For example, he could read a dictionary in the same way as Peek, thereby maxing out the vocabulary subtest, and then switch to Dylan Jones mode for the spatial rotations subtest.