Brain scans reveal why your brain forgets details

Our brains are much better at recalling vague pieces of information than precise details, according to two studies. One possible evolutionary explanation is that abstract ideas could be more helpful than specifics for getting through daily life. “Imagine you get bitten by a dog in the park. If you want to prevent being bitten again, it doesn’t help to just remember that this particular cute, little, white dog in this particular park has bitten you. What you want to memorise is that if you meet a free running dog in a park, it can bite,” says Maria Wimber at the University of Birmingham in the UK, who presented her teams’ findings in November at the Society for Neuroscience annual meeting in San Diego. To find out how information is prioritised when we reconstruct a memory in our brain, she and her team set up studies to determine where in the brain we store different information, and in what sequence we recall bits of memories from those different places. Wimber’s colleague Catarina Ferreira found that our brains emphasise categorical information. She asked 22 people to memorise 128 pairs of objects they were shown on a screen, consisting of a scene and an unrelated object – such as a mountain paired with a kiwi fruit. Two days later, the team showed the participants just the images of the scenes and asked them to recall the associated object, whilst having their brains MRI scanned. On average, participants could correctly recall the general category the associated object fell into 79 per cent of the time. For instance, in the example above people often remembered that they had seen a fruit, but not precisely that it was a kiwi. If participants could remember the category correctly, they were sometimes able to recall the specific object – for instance, kiwi – but only between 75 and 88 per cent of the time over four trials. When participants were being asked to recall the objects, brain activity in their neocortex increased in a similar way that it does when we sleep – a process that is linked to memory formation and consolidation. This makes sense, says Wimber. When you see something, the visual processing centre in your brain first processes the low-level features – line orientations, shapes and colours. Then the information is sent to the neocortex, where it is consolidated in a more abstract form, says Wimber. When we remember, this process happens in the reverse order, suggesting it’s easier and quicker to recall memories in their abstract form. This would explain why participants in the study could recall objects they had seen belonged to general categories – fruit – even if they struggled to remember their precise identity. This idea was backed up by another experiment, where Wimber used photographs and line drawings to sort out how the brain processes visual memories. “We know your visual brain responds completely differently to a coloured image than a drawing, but the higher-level meaning remains the same. A camel is a camel, so this lets us test just the perceptual processing,” she says. This time, 24 participants memorised pairs of words and unrelated images – either drawings or photographs – and when later cued with the word, they were asked to recall the object and categorise it as living or non-living. Their brain activity was tracked with an electroencephalogram (EEG) cap while the team asked the participants to say precisely when they were able to recall a visual memory of the object. When they were memorising the pairs, their brain activity peaked in the visual processing centre 100 milliseconds before the categorical information was actually encoded in the neocortex – a process that the researchers could see happening by monitoring the EEG readout. But when they were recalling an object from memory, it was the reverse – peaks in activity in the neocortex related to the category were found 300 milliseconds before the perceptual information came to mind.

New Scientist, 7 December 2018 ;