Remembering one thing can cause you to forget another, new research has shown.
The brain archives memories a bit like a library, with a neurological card-file index that logs the location of stored memories.
To retrieve the memory you look up where to find it in the index. But as we acquire a lifetime's worth of learning and experiences, the brain faces a progressively greater challenge pulling the right memory book from the cognitive shelves.
Closely-related experiences could interfere with each other, affecting the accuracy of recall.
Scientists have speculated that the brain might resort to a form of "active forgetting" to weaken these competing memories to boost the fidelity of recall, although, in the absence of tangible evidence, this was just a theory.
Now Maria Wimber and her colleagues at the University of Birmingham in the UK have shown this process in action for the first time.
Writing in Nature Neuroscience, the team asked volunteers to memorise pairs of images, but then to recall just one of the pair, forcing their brains to suppress the memory of the other, interfering item.
By watching the subjects' brain activity patterns in a scanner, Wimber was able to see the memory pattern corresponding to the suppressed item being eroded and actively forgotten.
"The activity fingerprint corresponding to the undesired memory was actually less than activity corresponding to unrelated items, showing it was being actively suppressed and forgotten," says Wimber.
The finding has practical implications.
"Testing your memory when learning for an exam is good," explains Wimber. "But you have to make sure you test yourself comprehensively on everything you have learned. Otherwise you might be at risk of some of the information being written off as irrelevant and discarded!"