New Experiences Throw Neurons Into Reverse
News From The Frontier

by Elizabeth Norton Lasley

March, 2006

The sleeping brain is known to replay the day's events; recordings taken from the brain cells of busy rats show that neurons active in a given behavior-exploring a new maze, for example-fire in the same sequence after the rat goes to sleep.

According to a new study, the brain also "rehearses" its neurons while awake -only it does so backwards.

David Foster and Matthew Wilson of the Massachusetts Institute of Technology used electrodes to record the hippocampal neurons of rats doing laps in a run that had food treats at each end. In their findings, scheduled for publication in a March issue of Nature, individual "place" cells fired in response to being in a specific part of the run.

But when the rat ate its food and rested for a moment, the same neurons fired in the opposite sequence. This reverse replay was more pronounced when the rat was in a new apparatus. In a separate test, when the rat was placed at the end of the run without having done the lap, the reverse pattern did not appear, suggesting that it only reflects the animal's actual experience.

In an early online edition of Nature, the authors explain that reverse replay lets the brain evaluate the steps leading to a central, anchoring event such as a food reward-unlike the pattern seen in sleep, when the brain merely goes over recent events. "Moreover, by converting single experiences into multiple reverse events  awake replay represents efficient use of hardwon experience," the authors write.