Mild cognitive impairment (MCI) describes an intermediate stage from normal cognitive functioning to dementia, and identifies a spectrum of diseases that includes impairment in both memory and non-memory cognitive domains. A deficient working memory (WM) plays a crucial role in normal aging, MCI, and cognitive decline associated with dementia, such as Alzheimer's disease (AD). WM refers to the on-line temporary storage and manipulation of information to be employed in ongoing processing, and is central to the execution of a variety of daily functions. The loss of WM efficiency related to either aging or neurodegenerative process is hypothesized to have various neurobiological roots, among which altered functioning of the fronto-parietal networks play an important role. The processing, exchange, and storing of information rely on intra- and interregional connectivity that allow neuronal populations to synchronize their firing. Based on this, intra- and cross frequency oscillatory interactions have been demonstrated as core basis for working memory processes, besides other cognitive functions. Any impairment preventing the adaptation and synchronization of the oscillatory activities within this network will inevitably lead to temporary or permanent loss of function.

In this project, based on the mentioned pathophysiological concepts, the investigators will develop and test novel, individualized, multifocal, physiology-inspired non-invasive brain stimulation (NIBS) approaches aimed at enhancing interregional processing in memory-related fronto-parieto-cerebello networks with consecutive behavioral modulation of WM processes impacting on daily life activities in aged healthy volunteers (HV) and MCI patients.

The investigators approach these aims in a step-wise manner: (1) The investigators will explore entrainment of oscillatory activity in the fronto-parietal network with transcranial alternative current stimulation (tACS) at frequencies relevant to memory encoding (theta) to boost WM in healthy seniors. (2) Then, the investigators will explore cross-frequency entrainment of interregional oscillatory interactions in the frontal and parietal target network with bifocal tACS to boost WM in healthy seniors. (3) Based on the current knowledge about the extensive involvement of the cerebellum in cognitive processes by its modulatory effects onto the fronto-parietal networks crucial for cognition, the investigators will leverage our experience in non-invasive modulation of the cerebellar outputs by means of repetitive transcranial magnetic stimulation (rTMS). rTMS over the cerebellum will be applied just before cortical entrainment to further enhance its physiological and behavioral effects. (4) Finally, the investigators will select the most effective approach from the previous work-packages to be applied in MCI patients and quantify the behavioral benefits. Besides the behavioral effects, the investigators will apply multimodal systems neuroscience approaches by means of neuroimaging (structural and functional magnetic resonance imaging; s/fMRI) and electrophysiological (Electroencephalography - EEG, transcranial magnetic stimulation - TMS) methods to determine the underlying network mechanisms and patterns of intrinsic connectivity changes. As there is significant spatial heterogeneity in fronto-parietal peak activity in healthy seniors and patients, stimulation targets will be individually defined based on fMRI. By leveraging the mechanistic aspects derived from individualized multifocal stimulation, the investigators expect to better understand the mechanisms through which NIBS techniques act on the brain and improve cognitive functions. The investigators will also test and quantify within a virtual reality (VR) task how well the intervention-driven cognitive improvement translates into daily life.