False memories can form in the human brain in just a few seconds

Even though a reasonable number of respondents misremembered the highlighted letters (about 10%), many more respondents reported seeing the highlighted, reversed letters in their correct orientations, as many as 40% in one experiment.

Many of the respondents misremembering the mirrored letters did so with a high level of confidence as well, and would do so within 0.3 seconds of having seen the mirrored letter less than 20% of the time. That figure rose to nearly 30% of the time when misremembering the mirrored letter three seconds after they'd seen it.

This is a very strong indication that our memories are heavily shaped by our preconceptions and expectations more often than we might like to admit; that is, we're sometimes remembering what we're expecting to see, not necessarily what actually happens.

"Intuitively," lead author on the study, Marte Otten, a neuroscientist at the University of Amsterdam, told Gizmodo, "we would think that these memories are pretty reliable,” given the short amount of time that had elapsed. Long term memory has long been known to get fuzzier with time, but most people generally have very high confidence in their short term memory.

“As a second unique feature, we explicitly asked people whether they thought their memories are reliable—so how confident are they about their response?” Otten said.

A large part of this is tied to the circumstances surrounding the memory formation. If there was a additional visual information or enough time has elapsed that the brain has to work harder to pull specific details about an event, it might insert our original expectations in place of the actual thing itself without skipping a beat, leading us to "clearly" remember something that didn't actually occur.

“It is only when memory becomes less reliable through the passage of a tiny bit of time, or the addition of extra visual information, that internal expectations about the world start playing a role,” according to Otten.

Fallible memories aren't just for childhood or long-ago events

While a single study isn't enough on its own to cast all our memories into question (the vast majority of respondents remembered the highlighted letters correctly, after all, regardless of whether they were mirrored or not), it does have implications for how we engage with each other in society.

Criminal justice systems around the world are built on witness testimony of crimes to convict the accused, and while it is rarer in many countries today to convict on a single witness's testimony, the testimony of a witness can influence that of others, who might be using the testimony of others to substitute for gaps in their own recollection, especially if its unconscious.

The question of bias also looms large as expecting certain behavior from a person based on prejudice might inform a false memory very quickly after an event occurs. And it doesn't even need to be a criminal matter for a false memory to have major implications for how we interact with people. Getting into an argument with a friend or loved one can be a difficult and traumatic experience in its own right without adding in the prospect of misremembering what was just said.

"I am personally very interested in finding a way to test the effects of social knowledge, such as prejudice or stereotypes and individual beliefs, on short-term memory," Otten said. "Do the expectations that we have about people based on, for example, their gender, almost immediately start shaping what we remember about, say, their voice or facial expression? Or do I after only a few seconds start slightly misremembering certain data-representations, because it does not fit my beliefs about, say, climate change?"

All of this is to say that memory is proving more and more to be an imperfect guide to the past, even the immediate past, so we should always keep that top of mind and not rely too much on what we think we remember as ironclad.