Moving Memories

22 Mar Moving Memories

This is not going to be a wistful story about sentimental moments of years gone past but something much more fascinating. Scientists have recently watched memories moving from one region of the brain to another whilst sleeping!

It is well known that sleep has many health benefits, both physically and mentally, but it is also very important for consolidating memories newly acquired during the day. Understanding how our brains accomplish this during sleep could be helpful in a number of ways, and possibly lead to treatments for insomnia and similar sleep disorders. The study conducted by scientists at Stanford University in California is the first time memories have been observed being filed away in humans during sleep.

Just what this means and how they achieved it is nicely summarised in as follows:

Memories are thought to persist as altered connections between neurons, often referred to as memory traces. When we practice a skill, we activate the neurons encoding that skill over and over again, strengthening the connections between them. However, if this process were to continue unchecked, eventually the connections would become saturated and no further increases in strength could occur. One possible solution to this problem is that sleep enhances skill learning by down scaling connections across the brain as a whole, thereby freeing up capacity for further learning. Alternatively, sleep may reorganize an initially unstable memory trace into a more robust form with the potential to last a lifetime.

To test these possibilities, the scientists asked healthy volunteers to practice a finger-tapping task while lying inside a brain scanner, and then to sleep inside that scanner for 2–3 hours. When the volunteers returned to the scanner the next morning and attempted the task again, they performed better than they had the previous night. Their brains also showed a different pattern of activity when performing the task after a night’s sleep.

So what had happened overnight? As the volunteers lay awake inside the scanner, their brains reactivated the memory trace formed during learning. However, as they entered a stage of non-dreaming sleep called non-REM sleep, this activity became weaker. At the same time, a new pattern of activity – the one that would dominate the scan the next morning – began to emerge. Whereas the post-learning activity was mainly in the brain’s outer layer, the cortex, the new pattern included other areas that are deeper within the brain. The activity of one deeper region in particular, the putamen, predicted how well the volunteers would perform the task the next day.

Non-REM sleep thus strengthens memories via two complementary processes. It suppresses the initial memory trace formed during learning, and reorganizes the newly-learned information into a more stable state. These results might explain why people who are sleep-deprived often have impaired motor skills and memories. The findings also open up the possibility of enhancing newly learned skills by manipulating brain circuits during non-REM sleep.