Human beings can generate false memories of events mere seconds after they have occurred, a new study has found. 

The phenomenon, which researchers have dubbed "short-term memory illusions," shows how easily and rapidly humans reimagine experiences to fit our preconceptions, rather than accurately recording what takes place. The researchers published their findings April 5 in the journal PLOS One (opens in new tab).

"It seems that short-term memory is not always an accurate representation of what was just perceived," the researchers wrote in the study. "Instead, memory is shaped by what we expected to see, right from the formation of the first memory trace."

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To test the accuracy of short-term memories, the researchers enlisted 534 volunteers to take part in a series of four experiments, each designed around memorizing a sequence of letters of the Latin alphabet. 

In each round, participants were shown a collection of letters, arranged in a circle. Those letters would then disappear and a box would pop up at a specific position in the circle, to indicate which letter they should remember. Participants had to remember both the letter's identity and the direction it was facing, as some had been mirrored to face backwards. 

Sometimes, participants were shown a second, irrelevant batch of letters before their memory was tested. After giving the answer, they were then asked to score their confidence, from very low to very high, that they had guessed correctly. 

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When the participants were asked to recall what they saw just a half second later, they were wrong just under 20% of the time, and this error rate shot up to 30% when asked three seconds later. When asked to recall whether a letter was facing forwards or backwards, participants who responded with high confidence had flipped the letter to its regular position 37% of the time, even though they had been explicitly warned that mirrored letters would appear in the tests and should not be mistakenly reported for real ones.

To confirm their findings, the researchers repeated the tests across three similar experiments with a cohort of 348 people not included in the original analysis, who showed the same tendency to mentally flip the mirrored letters. Across all experiments, this mental letter-flipping was the most common high-confidence error — a sign that human brains record experience based on preset notions (in this case, how a letter should appear) that enable us to generate better predictions about the world, while pruning out peculiarities that don't fit with those preconceptions.

"These memory illusions seem to be the result of world knowledge and not of visual similarities," the researchers wrote in the study. "Taken together, the results thus show that world knowledge can shape memory even when memories have only just been formed."

The researchers' next steps are to design experiments that could demonstrate similar short-term memory adjustments in real-world settings, as well as for other types of memory besides those related to visual and language-related stimuli.