Helen Pearson in Nature:
The idea stems from experiments in which neuroscientists eavesdropped on the brains of rats as they explored their environments. They found that the rats’ brains ‘replay’ their experiences in reverse when the animals pause briefly to rest.
The scientists, David Foster and Matthew Wilson working at the Massachusetts Institute of Technology in Cambridge, inserted a pincushion of fine wires into the animals’ skulls. These allowed the team to simultaneously monitor the electrical activity of around 100 individual brain cells in the hippocampus, a brain region involved in learning and memory.
The researchers placed each wired-up rat in a straight 1.5-metre run. They recorded brain-cell activity as the rats scurried up and down, pausing at each end to eat, groom and scratch their whiskers.
As the rats ran along the track, the nerve cells fired in a very specific sequence. This is not surprising, because certain cells in this region are known to be triggered when an animal passes through a particular spot in a space.
But the researchers were taken aback by what they saw when the rats were resting. Then, the same brain cells replayed the sequence of electrical firing over and over, but in reverse and speeded up.