Using external stimulation to 'replay' recent experiences during sleep can strengthen the memories of those events, according to a study in rats.

Daniel Bendor and Matthew Wilson at the Massachusetts Institute of Technology in Cambridge trained rats to run to either the left or the right in response to one of two sounds, while recording from the brain's hippocampus. As the animals slept, the researchers played the sounds again to see whether this would trigger the rats to recall the task. The duo observed signs of 'replay' by analysing the response of neurons in the hippocampus, the brain region in which memories are thought to be consolidated.

These findings echo recent experiments in which human sensory learning improved following exposure to task-related cues during sleep, suggesting that such cues might strengthen memories.

Nature Neurosci. http://dx.doi.org/10.1038/nn.3203 (2012)