As sufferers of post-traumatic stress syndrome know all too well, frightening experiences can be strong, long-lasting and notoriously difficult to erase. Now, we’re starting to understand why. Far from trying to purge these memories, the brain actively protects them by hiring a group of molecular bodyguards called CSPGs (or chondroitin sulphate proteoglycans in full).

By studying the brains of rats, Nadine Gogolla from Harvard University found that CSPGs – large chains of sugars and proteins – accumulate in the space around nerve cells and form defensive nets around a select few. Dissolve these nets, and the rats’ fearful memories were more easily erased.

The nets start to form round about the time when rats reach adulthood and their fearful memories become