Although many who experience negative events eventually recover, some go on to develop post-traumatic stress disorder and continue to be severely affected, experiencing anxiety and depression for months or even years after the event.
Until recently, it was thought that all memories were initially collected in the hippocampus as short-term memories, then slowly converted into long-term memory through a transfer to the brain's cortex.
But this theory of memory creation was changed this year, after research published in the journal Science showed that rather than moving from one part of the brain to another, memories are, in fact, formed in both regions of the brain simultaneously.
In pursuit of new ways to treat PTSD, scientists have been studying the pathways in the brain responsible for long-term memory formation.
A technique called optogenetics has given researchers unprecedented access to the workings of the brain, opening up new ways to observe precise neural circuitry, and even to control behaviour by directly manipulating individual brain cells.
By implanting a virus into the brain, individual brain cells can be made responsive to light - scientists then activate these modified cells by exposing them to a flash of light. At this stage, due to the unknown risks of injecting viruses into living patients' brains, this research is only being carried out in small animals such as mice.
The new research involved a study of mice using this virus-implantation technique.
By playing a certain high-pitched sound to the mice, at the same time as delivering a "mild" electric shock to their feet, the mice quickly learned to associate the sounds with the shock - they then exhibited fearful behaviour every time they heard the sound, standing absolutely still, even without the electric shock being delivered.
With this fear response memory created, the scientists then exposed the memory-creating neurons in the mice's virus-affected brains to low-frequency light.
The virus responded to the light, and weakened the connections between the neurons involved in the memory formation. After treatment - a process now known as "fear extinction" - the mice showed no further signs of fear when exposed to the same high-pitched sound, implying that the old fear memory was no longer being retrieved.
Although ethically, optogenetics cannot yet be used on humans, this new understanding that fear-invoking memories can be removed selectively from the brain could help in the development of new treatments that strengthen the connections between neurons in forming positive memories, and perhaps weaken those in negative memories, helping those suffering with PTSD to lead happier lives.
