Your short-term memory can be unreliable after just a few seconds

You can misremember something just seconds after it happened, reframing events in your mind to better fit with your own preconceptions. Our brains probably do this in an effort to make sense of the world in line with our expectations, even if that isn’t helpful all of the time.

Marte Otten at the University of Amsterdam in the Netherlands and her colleagues wanted to tease out the relationship between prior expectations and short-term memories. “We already know that long-term memory is fallible, we just wanted to find out if we could determine the specific ways in which short-term memory is fallible also,” she says.

The team conducted several experiments on more than 400 people that all involved showing the participants random letters arranged in a circle on a computer screen.

In the simplest form of this experiment, the participants were shown the letters for a quarter of a second before the screen went blank. After a gap of 3 seconds, a box appeared where one of the letters had been for half a second, followed immediately by a different circle of letters for half a second.

The participants were asked to recall which letter from the original circle had been in the position held by the box on the screen. Crucially, some of the letters were flipped, which Otten calls “pseudo-letters”. The participants were explicitly warned about these flipped letters and told not to mistake them for real ones.

After recalling the letters, the participants were asked to rate their confidence in each answer. The team focused its analysis on the most confident participants, in order to weed out random guesses.

The researchers found that when asked to recall the position of a pseudo-letter, the confident participants incorrectly gave the answer as its real letter equivalent 39 per cent of the time, despite their high confidence in the answer.

Variations of the experiment revealed that this misplaced confidence is likely to be to do with how our short-term memory works and how it relies on our preconceptions.

“People seem to be sensitive to this memory illusion where they already have a preset notion of what the world should look like,” says Otten. “This is very strong for letters because we have a lot of experience with them.”

This effect appears to be due to a feature of our neural system that relies on generating predictions about the world, says Otten. We expect to see normal letters when reading, she says. “These predictions are normally quite helpful and efficient in normal life,” she says. “This is not something we have control over.”

Several studies have previously shown that long-term memory is fallible and affected by prior expectations and biases. Tracey Shors at Rutgers University in New Jersey says it has been difficult to study whether the same is true for short-term memories. “This ingenious set of experiments finds that previous knowledge can reshape short-term memories for visual perceptions,” she says.

“It is tempting to refer to these memories as ‘illusions’ or even ‘false memories’,” she says. “But in our everyday life, they likely help us better predict the future – and do so faster than we had imagined possible.”