Quiescent dependent memory consolidation: memory-trace replay during REM sleep

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Memory is a crucial brain function for humans and other animals. Our everyday activities are entirely dependent upon memory. For instance, when we drive a car, we use multiple forms of memory including memory of the destination, memory of the current location (which keeps updating with movement), and memory of how to operate a motor vehicle. In addition, the ability to maintain good memory becomes increasingly important because our life expectancy is getting longer. Understanding how memory is encoded and maintained in the brain is, therefore, an important and challenging problem of neuroscience. To explain the consolidation of memory in the brain, I propose a theory of Quiescent Dependent Memory Consolidation: Memory is consolidated during sleep when both motor output and sensory input are shut down. The quiescence of sleep provides a permissive environment for memory consolidation. I use multiple-electrode recording technology to record multi-neuronal brain activity in behaving animals, both during awake and sleeping periods. I investigate how brain activity is modified through sleep by comparing the activity during a task (animals are trained to perform a certain behavioural task), pre-task sleep and post-task sleep, focusing on a phenomenon called memory-trace reactivation whereby behaviourally induced multi-neuronal patterns are repeated during subsequent sleep. Additionally, by comparing the change in neuronal activity and the improvement of task performance, I investigate the mechanisms by which sleep contributes to learning. Furthermore, different contributions of REM and non-REM sleep to memory consolidation are also investigated. I test the hypothesis that REM sleep contributes to a specific form of memory such as motor-skill learning (e.g. how to drive a car), whereas non-REM sleep contributes to another form of memory such as spatial learning (e.g. recognizing location). All the results of this research program will provide an insight into the process of memory consolidation during sleep and, thus, will contribute to our understanding of the mystery of sleep.