# An eLORETA Longitudinal Analysis of Resting State EEG Rhythms in Alzheimer's Disease
## 1 Introduction
It belongs to the high temporal resolution neuroimaging method and provides a 3D reconstruction of the cortical electrical activity. Over the years, LORETA has been widely employed for studying the spectral power density of EEG signals in AD. [PERSON] carried out several studies about this topic. In [26], the results showed that the power in alpha 1 declined in AD in the central, parietal, temporal, and limbic lobes as compared to Nold. Alpha 1 power in the occipital lobe was lower when the disease was at a more severe stage. There was also an \"anteriorization\" of alpha 1 rhythm, as the power decreased much more in the parieto-occipital area than in the frontal one. As for the theta band, there was no significant difference between mild AD and Nold. This study was extended in [27], as it included also MCI. The findings revealed that, in comparison to Nold and mild AD groups, MCI group had intermediate power in delta (occipital area) and alpha 1 (parietal, occipital, temporal, and limbic areas) bands and greater power in alpha 2 (occipital area) band. In [28], the correlation between the disease and the hippocampal volume was explored. For the alpha 1 band, the activity was maximum in MCI with greater hippocampal volume, intermediate in MCI with smaller hippocampal volume, and minimum in AD. The study conducted in [29] showed that for MCI and AD subjects, posterior delta and alpha EEG rhythms do not decline when white matter vascular lesion increased. In [30], it was found a positive correlation between the gray matter volume and the alpha 1 power and a negative correlation with delta power in MCI and AD. [PERSON] and colleagues [31] reported a significant reduction of alpha 1 activity in the parieto-occipital regions for AD as compared to healthy subjects. In [32], the authors found a positive correlation between the severity of the disease and delta power and a negative correlation with alpha 1 power. The study in [33] showed a greater power in delta band in large areas, including the frontal lobe, in AD in comparison to controls. All the above-mentioned papers refer to cross-sectional studies. In the literature, there are only three longitudinal studies that applied LORETA. In [34], the EEG power of a group of MCI patients was evaluated during a follow-up of approximately 14 months. Twenty-four subjects converted to AD, whereas 45 subjects remained stable. The results showed that the EEG power for delta (temporal area), theta (temporal, parietal, and occipital areas) and alpha 1 (central, temporal, parietal, occipital, and limbic areas) bands was significantly higher in MCI who converted to AD, as compared to MCI stable. The remaining two works were carried out by [PERSON] and colleagues. In [35], a group of MCI patients were examined about 1 year after the first EEG recording. The subjects were divided into three groups, according to their MMSE score variation, as \"stable\", \"decreased\", and \"increased\". The only significant difference was detected for the alpha 1 band: the temporal, parietal, and occipital power density was greater in the stable group than in the decreased and increased groups, suggesting that preserved resting cortical activity at alpha band is related to a long-term stable cognitive function in MCI subjects. Resting state EEG of a mild AD group was investigated in [36]. The second recordings were carried out after 1 year. The alpha rhythm was characterized by a widespread power decrease as well as the temporal, parietal and occipital beta 1 band. It was also shown an increased power of delta sources in all brain areas except for the central region. Motivated by the encouraging results of the above-mentioned works, at least at group level, we propose a preliminary study to evaluate whether resting state EEG sources can be markers for the prediction of the disease progression, from MCI to AD, in a single subject. In particular, the purpose of our work is to explore the power current density in eight patients, who were diagnosed with MCI at time T0 and mild AD at time T1 (four months later).