After specific groups of neurons-those associated with memory-were stimulated in mice, the animals froze in fear as they recalled frightful memories. These findings were presented at the Neuroscience 2009 Show in Chicago.

Exactly how the brain retrieves memories from its circuits is a long-explored topic of research. While there's been great progress in understanding memory retrieval, researchers still know little about how events from our past can be recalled under the right conditions.

In this case, scientists used two genetic tools to find how brain circuits are involved in memory recall. One was a gene that highlighted the particular cells associated with recent brain activity, such as learning something new. The second was a gene that produced a light-sensitive protein taken from algae that responds to blue laser light. The researchers used the first gene to mark recently active neurons, while the second gene (expressing the light-sensitive protein in the same neurons) made it possible for them to be activated by blue light.

"These tools allowed us to performed a powerful experiment: We could reactivate specific groups of cells and test whether this 'spark' was enough to recall the memory the animal had learned," says senior author Michael Hausser at the Wolfson Institute for Biomedical Research in London.

The authors discovered that stimulating neurons in the hippocampus made mice freeze with fear because they remembered prior fearful experiences. When cells unassociated with learning and memory were stimulated, the mice did not appear fearful. Results also showed that activation of only a tiny fraction of neurons in the learning brain area were needed to recall memories.

Their research was supported by the Gatsby Charitable Foundation and the Wellcome Trust.

Source: University College London and the Society for Neuroscience