Sleep is essential for embedding several forms of memory. A new study has looked inside the brain during sleep to show how memory is stored. The research has found that reactivating memories triggers electrical activity and this signifies improvement in memory storage. This is borne out in studies where previous learning was reactivated during sleep, resulting in improved memory.
The research comes from neuroscientists based at Northwestern University and the University of Chicago Epilepsy Center. The institutions considered brain electrical activity in five of the centre’s patients in response to sounds administered by the research team as part of a learning exercise.
Each subject had electrode probes implanted into the brain, which enabled the researchers to record electrical activity from inside the brain (and opposed to most studies which take readers from the outside of the brain).
For the study, while each patient slept in a hospital room, the researchers recorded electrophysiological responses to 10-20 sounds that were repeatedly presented. Each sound was played very quietly so as to avoid arousal. Half of the sounds were associated with objects and their precise spatial locations that patients learned before sleep using a laptop computer, such as the jingling sound of car keys. This was designed to help the volunteers to recall their location.
The research revealed that the patients significantly improved their performance in a recall test the next morning. Specifically, the subjects demonstrated systematic improvements in spatial recall, and each subject more accurately indicated the remembered locations on the laptop screen.
Each of the five patients showed the same patterns of memory improvement and electrical activity. This infers that how we remember the things we have learned has a strong association with sleep.
In terms of what was happening in the brain, the data from the implanted brain electrodes revealed that object sounds presented during sleep elicited increased oscillatory activity, including increases in theta, sigma, and gamma EEG bands. The electrophysiological activity occurred in the hippocampus and the adjacent medial temporal area of the cerebral cortex. In particular, gamma responses were consistently associated with the degree of improvement in spatial memory exhibited after sleep.
The findings will aid future inquiries into how memory storage works and with understanding better the areas of the brain engaged in the process of overnight memory storage. Sleep stands as something particularly relevant for newly formed memories for facts and events (declarative memories).
The research has been published in the journal Proceedings of the National Academy of Sciences, titled “Electrophysiological Markers of Memory Consolidation in the Human Brain When Memories are Reactivated during Sleep.”
