But now neuroscientists believe they can erase feelings of fear or anxiety attached to stressful events, in a breakthrough which could help treat depression or post-traumatic stress disorder.
Researchers at MIT, US, have discovered which brain circuits attach emotions to memories, and crucially, how to reverse the link.
They managed to 'switch off' feelings of fear in mice which had been conditioned to feel anxious. It is likely the same technique could be used in humans.
"In our day to day lives we encounter a variety of events and episodes that give positive or negative impact to our emotions," said Susuma Tonegawa, Professor of Biology and Neuroscience at the Riken-MIT Centre for Neural Circuit Genetics.