Professor Penny Lewis is a neuroscientist at Cardiff University where she works on ‘sleep engineering’, the science of manipulating sleep for cognitive and health benefits. Penny’s work is looking for ways in which sleep’s beneficial properties can be maximised to enhance memory, disarm negative emotions and combat cognitive decline through ageing.
Within cognition there are several different areas. The biggest one is probably memory and how some forms of memory can be strengthened across sleep whilst others are protected against decay that integrates into the knowledge we already possess. Penny also thinks that sleep works to recombine these memories which can be really important for creativity as well.
The question is what happens during sleep to facilitate this? At the moment the strongest possibility is memory replay, the neural reinstatement of recently formed memories during sleep. A simple example of this is playing a piece on the piano. It’s established that particular areas of your brain are active in controlling the finger movements and exactly where these are. Research shows that these pieces of the brain are active again during subsequent sleep and that the extent to which you’ll improve the same piano piece is on average 20% over a nights sleep. It seems therefore, that not only is the brain rehearsing these memories but that the rehearsing is changing the way they are represented in the brain, making it potentially more efficient and strengthened.
This seems to work with many types of physical and cognitive tasks. Motor or procedural tasks show the strongest actual improvement but other kinds of memories are also influenced. Episodic memories can decay across time and decline quite sharply as we forget them but they don’t decay as much across sleep where they are protected against both decay and from interference from other related memories that might cause confusion. It’s less likely to cause confusion if you slept on the episodic memory first.
Memories also get recombined and integrated together across sleep. There is growing evidence that if you learn new information, sleep helps to integrate that new information with existing knowledge. For example, the statement ‘it’s a nice sunny day in Cardiff today’. In order to make sense of this, we need to relate it to our prior knowledge about Cardiff, how often it’s sunny in April, what would you expect in springtime etc. The observation of what we’re seeing right now doesn’t mean very much until it’s integrated into what we already know and that a process of integration seems to be facilitated by sleep.
Sleep is also important for emotion. We all know that if we are sleep deprived for a night, then we can feel a pretty rough. Generally, people feel a bit fragile and are more likely to pick up on negative things. In that sense, sleep clearly impacts on emotion but it also impacts on emotional memory. If we have a traumatic memory such as a car accident, irrespective of how well we remember it, it can cause a strong emotional response that could be accompanied by physical symptoms such as an increased heart rate. Work is now being done to look at what sleep does to the emotional response that’s associated with a memory and a real debate has developed around this as there’s evidence that sleep strengthens these emotional memories so you remember them better after sleep but there is also evidence that sleep helps to decouple the emotional response from the memory itself so you are strengthening the memory but weakening the emotional response.
Penny undertakes sleep manipulations by triggering replay specific memories while people are sleeping. It is established that replay is important and what she is trying to understand is how is it important? What does it do in different situations? Penny trigger’s the memories to replay, controls when they replay, how many times they replay and what in sleep stage they replay through a simple technique called targeted memory reactivation. Learned information is paired with sounds during the day so that certain pictures are associated with certain sounds. The sounds are replayed during a chosen stage of subsequent sleep and, depending on which stage it’s played it in, different outcomes in terms of the processes would be expected.
For example, emotional memories are replayed in REM sleep, the deep stage of sleep characterised by rapid eye movements under closed lids. Typically unpleasant images have a different sound associated with it and then during REM sleep participants are exposed to half of the sounds. The next day they are shown the pictures and asked how upsetting each of them is. It’s been found in a couple of studies that if the sound is played during REM sleep, then the pictures are less upsetting than they were the previous day. The picture are remembered as much or more, but are rated less arousing.
You can listen to the full podcast at: Resilience Unravelled (Eps 075)
Find out more about Penny and her research at: