“Multiplying 10-Digit Numbers Using Flickr: The Power of Recognition Memory”, 2011 (; backlinks; similar):
In this informal article, I’ll describe the “recognition method”—a simple, powerful technique for memorization and mental calculation. Compared to traditional memorization techniques, which use elaborate encoding and visualization processes1, the recognition method is easy to learn and requires relatively little effort…The method works: using it, I was able to mentally multiply two random 10-digit numbers, by the usual grade-school algorithm, on my first attempt! I have a normal, untrained memory, and the task would have been impossible by a direct approach. (I can’t claim I was speedy: I worked slowly and carefully, using about 7 hours plus rest breaks. I practiced twice with 5-digit numbers beforehand.)
…It turns out that ordinary people are incredibly good at this task [recognizing whether a photograph has been seen before]. In one of the most widely-cited studies on recognition memory, 1973 showed participants an epic 10,000 photographs over the course of 5 days, with 5 seconds’ exposure per image. He then tested their familiarity, essentially as described above. The participants showed an 83% success rate, suggesting that they had become familiar with about 6,600 images during their ordeal. Other volunteers, trained on a smaller collection of 1,000 images selected for vividness, had a 94% success rate.