Enhancing Sleep Dependent Consolidation by Non-invasive Brain Stimulation

Stroke, one of the most common causes for acquired adult disability, is not only a burden for the individual but also for his or her close relatives and caregivers. Functional recovery is commonly associated with the re-acquisition of lost skills. This skill (re-)acquisition is separated into different phases during which learning takes place while the skill/movement is actively performed - so called online learning - or during the time of non-performance between the training - so called offline learning or consolidation. During the initial phase of training, performance improvements are commonly steep (online learning). During the following processes of consolidation, which often depend on sleep, memory traces are being modified and stored for long-term memory retention leading to a further improvement without additional training (offline learning). Previous studies focusing on individuals after stroke could show a beneficial effect of sleep on motor skill acquisition. As an intervention, transcranial electrical stimulation (tES) with motor tasks could show beneficial effects on motor skill acquisition. tES is a method to stimulate an area of the brain non-invasively and this is done by applying low voltage current to the scalp that lies in close proximity to the target brain region. In the current study, stimulation is performed during sleep and types of stimulation resemble natural sleep physiology: slow-wave and spindles. As slow-wave and spindles are shown to be important for memory consolidation, it is hypothesized that applying physiologically-inspired stimulation could enhance memory consolidation in individuals after stroke. It is known that patterns of sleep physiology change in older individuals, thus, this population is also investigated in the current study. It is interpreted and discussed that older individuals do not benefit from sleep as much as younger individuals do. Thus, it is hypothesized that applying physiologically-inspired stimulation could enhance memory consolidation in healthy older individuals.