A group of scientists has revealed structural "traces" in the brain that help to keep memories precise.
Neuroscientist Pico Caroni at the Friedrich Miescher Institute in Basel, Switzerland, led the group. The team worked with mice that went through a fear-conditioning procedure in which their paws received electric shocks in a particular room.
They then looked at changes in the synaptic structures, the structures involved in signal transmission from neuron to neuron, in the hippocampus of the brain.
It was found that after a couple of days, when the mice entered that room, they froze in fear, and in a different but similar room, they sniffed about as normal.
But after about two weeks the mice froze in both rooms. The memory had been "generalized", producing a functional response to a wide range of cues instead of the specific one that had been learned.
"The memory is still there, it might be there forever. But it changes," Nature quoted Caroni as saying.
When the team followed the structural changes in the hippocampus during the conditioning process, they found a large increase in the numbers of synaptic structures at the terminals of neurons called granule cells, which have axons called large mossy fibres after the memories were formed.
By the time the memory had been generalized, these synaptic structures had disappeared.
To confirm that the formation of the synaptic structures was involved in memory precision, Caroni's group put mice that lacked a protein necessary to form the synaptic connections through the same shock treatments.
In these mice, the memory became generalized after only one day. But when researchers introduced that protein into the mossy-fibre neurons in the hippocampus, even if into only 20 percent of the cells, the mice recovered the ability to maintain precise memories for weeks.
Caroni says that the phenomenon could improve the understanding of post-traumatic stress disorder (PTSD), in which traumatic memories are evoked by environments very different from those in which they originated.
The findings have been published in Nature1.