How does the brain store knowledge so that you actually remember what you have learnt the next day or even later?
In the first phase of this experiment, mice were trained to recognise that an image with a particular pattern meant that they would be given a reward in the form of a sweet drink. Two different groups of mice were then put in front of a computer screen where they were able to see several images containing different patterns. In order to demonstrate that they remembered which image led to a reward, the mice had to lick a small “nozzle” that dispensed the drink.
While the mice performed this action, researchers at the University of Oslo monitored the activity in their brain cells using a special microscope.
–?It took some time before the mice understood which pattern triggered a reward. We could see what was happening with their neurons while they mastered the task, says researcher Kristian K. Lensj? who works at the Institute of Basic Medical Sciences and the Department of Biosciences at the University of Oslo.
–?In addition, we monitored what happened in the brain of the mice during the period after the experiment, while the animals were resting.
Lensj? and his colleagues describe what happened in the experiment in a new study published in the journal Science Advances.
PV neurons ensured that the memories were replayed time and time again
It is well known that memories become stronger when we relax or sleep after having learnt something new. But it was unclear how different types of nerve cells contribute to this process.?
–?With the help of modern genetic engineering, we turned off the activity in certain nerve cells called PV neurons in a small brain region called the postrhinal cortex while the mice were resting after a learning period, explains the researcher.
We allowed these PV neurons to remain active in other mice who were in the control group. The neurons in these mice showed a great deal of activity during the hours after the experiment. The neurons were reactivated hundreds of times, while replaying the mice’s experiences again and again. The nerve cells that had been active while the “correct” image appeared continued to be active.
The following day, the researchers used the mice again in the same experiment, showing them the same images.
–?The mice that had normal neural activity, with PV neurons functioning after the first experiment, remembered what they had learnt. They licked the nozzle which released the reward. In the other group of mice, whose PV neurons had been turned off, it was as if the mice hadn’t learnt anything at all. They did not remember the association, says Torkel Hafting, professor at the Institute of Basic Medical Sciences.
Research is essential in order to understand both brain disorders that impair memory, such as dementia, and cognitive functions.
Learning involves many parts of the brain
When you learn something, the brain receives signals from the sensory organs and these are processed in many different areas of the brain. It has been shown that sleep is important, if the process involved in storing memories for future use is to function as it should.
–? What we call “memory consolidation” happens primarily while we rest and sleep, since this is when the brain processes and sorts our experiences, explains Hafting.
It has long been known that brain cells in the hippocampus – a key area of the brain when it comes to memory ?– are repeatedly activated when we are learning something. But long-term storage of memories takes place in the network of brain cells spread out over large parts of the cerebral cortex.
–?Even something as simple as learning the association between an image and a reward demands a precise interaction between different areas of the brain. This experiment revealed that a small area of the brain called the postrhinal cortex plays a decisive role in consolidating this type of memories. Up until now, it has been difficult to ascertain how long-term storage functions in such a way that you remember what you learnt afterwards. Neither was it clear how different types of nerve cells contribute to this process. So this was the focus of our study, says Hafting.
PV cells act as the conductor of the orchestra for replay of memories
In order that brain cells can recall the memory, it is essential that the right cells are activated in the correct sequence. Just as an orchestra depends on a conductor to maintain the tempo and balance between the instruments playing a symphony, so the PV cells control the activity that is needed for memories to be stored in the brain.
–?Even though all the other brain cells were active, the sequence and rhythm was disrupted when we disconnected the PV cells and the memories were then not stored in the cortex, adds Hafting.
These neurons make up less than 5 per cent of the cells in the cerebral cortex, including the postrhinal cortex.
This study has brought the researchers at the University of Oslo and the Harvard Medical School a major step further in finding out how, after learning something new, we store memories so that we can recall them later.
More information:
Local inhibitory circuits mediate cortical reactivations and memory consolidation