In the experimental situation, the rat needs to learn a rule to get a reward. It must remember which of two outer arms of a W-shaped maze it had visited previously and alternate between them visiting the opposite arm after first visiting the center arm. The ripple activity occurs when rats are inactive during breaks between trials.
Place cells associated with the maze fire in rapid succession and in synchrony with other neurons in the neighborhood. The same place cells fire in the same sequence as they did when the rat first walked through the maze suggesting that the rat is mentally replaying the earlier experience, but on a much faster timescale.
In the current study, an automatic feedback system shut down place cell firing, via mild electrical stimulation, whenever it detected ripple activity, thereby also preventing the replay of the maze memory. Without benefit of mental replay, rats' performance on the maze task deteriorated. The impairment was in the animals' spatial working memory their ability to link immediate and earlier past experience to the reward. This ability was required to correctly decide which outside arm to visit after exiting the center arm during outbound trials.
The researchers propose that awake replay in the hippocampus provides such information about past locations and future options to the brain's executive hub, the prefrontal cortex, which learns the alternation rule and applies it to guide behavior.
Even though the replay events in rats last just a fraction of a second, Frank notes that they are not unlike our own experience of memories, which tend to compress often lengthy events into snippets of just the highlights of what happened to us.
"We think the brain is using these same ripple-li
|Contact: Jules Asher|
NIH/National Institute of Mental Health