US scientists studying sleep processes in rats found that aging tended to impair the brain’s ability to “replay” memories during sleep, a consolidation process that is important for converting recent memories into long term ones. However, impaired memory replay was not found in all the older rats tested in the study.
The study is published in the 30th July issue of The Journal of Neuroscience and is the work of Dr Carol Barnes and colleagues at the University of Arizona in Tucson.
During “slow-wave” sleep, the hippocampus, which is thought to play a key role in learning, memory and spatial awareness, repeatedly replays brain activity (“pyramidal cell activity”) from recent awake experiences. It also does this during wakeful time when the brain is not processing new input, suggested the authors.
Humans and other mammals have two hippocampus regions, located in the medial temporal lobe in the forebrain, one on the left and one on the right.
Replaying brain activity is thought to be important for consolidating recent memory into long term memory. Barnes and colleagues compared replay activity during sleep between old and young rats and found it took longer in the young rats. The rats who showed the least replay activity during sleep also performed less well in tests of spatial memory.
For the study, Barnes and her team used 11 young and 11 old rats and recorded their hippocampal activity as they navigated several mazes to find pieces of food. Then, when the rats were sleeping, the researchers recorded the rats’ hippocampal activity again.
The results showed that the sequence of neural activity of the young rats while they looked for food in the mazes was repeated while they slept. But in most of the old rats, the sequence of neural activity recorded while they slept did not match the sequence recorded when they were in the maze.
In a second experiment, Barnes and colleagues tested all 22 rats again using spatial learning and memory tasks and found that as before, the young rats remembered the solution to the spatial tasks faster and more accurately than the old rats. However, they also found in the group of old rats, that the ones that showed the best sleep replay were also the top performers in the memory and spatial learning test, in that group.
In other words, regardless of age, the rats whose sleep replay most faithfully reproduced the awake neural sequence recorded in the maze, performed better on the spatial learning and memory tests.
The researchers concluded that:
“The novel finding that weak replay of temporal patterns has behavioral consequences, strengthens the idea that reactivation processes are integral to memory consolidation.”
Barnes said:
“This is the first study to suggest that an animal’s ability to perform a spatial memory task may be related to the brain’s ability to perform memory consolidation during sleep.”
Dr Michael Hasselmo, an expert in brain behaviour and cognition based at Boston University, and who was not involved in the study said:
“These findings suggest that some of the memory impairment experienced during aging could involve a reduction in the automatic process of experience replay.”
Hasselmo explained that these findings could:
“Inspire the development and testing of pharmacological agents designed to enhance memory replay phenomena.”
However, other experts have not been so hasty to draw this conclusion.
Dr John Groeger, from the Sleep Research Centre at the University of Surrey, UK, told the BBC that we don’t know enough about the role of the hippocampus in memory processing:
“It would be extremely difficult to test this theory in humans, because there are wide differences in the sleep patterns of older and younger people,” he said.
Groeger said that older people tended to sleep between 1.5 and 2 hours less each night than younger people, and they also had a different “sleep architecture”, suggesting there could be other reasons for why memory consolidation might be impaired in older people.
He said he would expect hippocampal activity to be different in older people because of natural brain shrinkage.
“Sequence Reactivation in the Hippocampus Is Impaired in Aged Rats.”
Jason L. Gerrard, Sara N. Burke, Bruce L. McNaughton, and Carol A. Barnes.
The Journal of Neuroscience, July 30, 2008, 28(31):7883-7890
doi:10.1523/JNEUROSCI.1265-08.2008
Sources: Journal abstract and news release, BBC.
Written by: Catharine Paddock, PhD