We have cells in our brains – and so do rodents – that keep track of our location and the distances we’ve travelled. These neurons are also known to fire in sequence when a rat is resting, as if the animal is mentally retracing its path – a process that probably helps memories form, says Rosa Cossart at the Institut de Neurobiologie de la Méditerranée in Marseille, France.

But without a way of mapping the activity of a large number of these individual neurons, the pattern that these replaying neurons form in the brain has been unclear. Researchers have suspected for decades that the cells might fire together in small groups, but nobody could really look at them, says Cossart.


To get a look, Cossart and her team added a fluorescent protein to the neurons of four mice. This protein fluoresces the most when calcium ions flood into a cell – a sign that a neuron is actively firing. The team used this fluorescence to map neuron activity much more widely than previous techniques, using implanted electrodes, have been able to do.

Observing the activity of more than 1000 neurons per mouse, the team watched what happened when mice walked on a treadmill or stood still.

As expected, when the mice were running, the neurons that trace how far the animal has travelled fired in a sequential pattern, keeping track.

These same cells also lit up while the mice were resting, but in a strange pattern. As they reflected on their memories, the neurons fired in the same sequence as they had when the animals were running, but much faster. And rather than firing in turn individually, they fired together in sequential blocks that corresponded to particular fragments of a mouse’s run.

“We’ve been able to image the individual building-blocks of memory,” Cossart says, each one reflecting a chunk of the original episode that the mouse experienced.

The neurons corresponding to each piece of the memory were not all located next to each other, but mixed up throughout the hippocampus. However, when they fired, they showed a clear, strong association with the other neurons that helped record that part of the run.

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