Have you ever forgotten items when trying to recall a shopping list? Or dialled the wrong phone number when attempting to memorise one? The brain mechanisms that cause us to draw a blank in such situations have been identified by scientists in a surprising discovery.
Our so-called working memory retains small pieces of information that are readily accessible for planning, understanding and solving problems in scenarios like these. But it is prone to “swap errors”, when we mistakenly attribute a feature from one object to another. For example, if we are shown a red square and a blue circle, and are then asked what colour the circle was, we might say red.
To understand why we make these errors, at Columbia University in New York and his colleagues used electrodes to record the neuronal activity of the brains of two macaque monkeys while they performed different versions of a working memory task.
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“We use monkeys because their working memory is very similar to humans,” says team member , also at Columbia University. “Both monkeys and humans can hold a few items in working memory at a time and just like monkeys, humans will make swap errors, where they mistakenly report the wrong item and will sometimes forget items and have to guess.”
In one version of the task, the monkeys were shown two differently coloured squares, one above the other, for half a second. After a short delay, a black spot appeared on the screen in the same location as one of the squares, then disappeared. The animals were trained to report the colour of the square they were supposed to be remembering, based on the spot’s location, by fixing their gaze on the matching colour and location on a wheel, which was rotated to a random orientation between each go.
The second version of the task was the same, aside from the spot appearing before the coloured squares. These tests required the monkeys to hold two items in their working memory – the squares’ locations and their colours.
When doing this for around 3 hours over multiple sessions, the monkeys performed both tasks correctly between 60 and 82 per cent of the time, but occasionally made swap errors.
During these tasks, the researchers recorded the activity of small populations of brain cells across the monkeys’ frontal, parietal and visual cortices, which are known to encode neural representations of items held in our working memory.
The researchers examined the recordings to identify patterns of activity that are associated with correct responses and swap errors. They then compared these patterns with those that would be expected, based on previous studies, if the colours and locations of the squares were remembered correctly and encoded accurately in the monkeys’ working memories.
Their analyses suggest that the brain cell responses associated with swap errors emerged before the animals decided which colour to report. Rather than occurring simply as a result of them forgetting or a failure to correctly encode items to working memory, they appeared to arise during selection, when certain items stored in working memory are enhanced at the expense of others.
“Everyone assumed there were simpler explanations like failure to encode or forgetting, but this very cool study shows that working memory errors come from a previously unknown source,” says at the Massachusetts Institute of Technology.
“These results suggest an alternative to established views on the neural origins of swap errors, and highlight selection from and manipulation in working memory as crucial – yet surprisingly brittle – neural processes,” the researchers write.
The team is planning further experiments to gain a better understanding of the mechanisms underlying swap errors and the neurology behind guess responses, says Johnston.
bioRxiv
