Alzheimer's disease (AD) is the leading neurodegenerative disease of aging characterized by multiple cognitive impairments. Given the recent failures of disease-modifying drugs, the current focus is on preventing or mitigating synaptic damage that correlates with cognitive decline in AD patients. Transcranial Direct Current Stimulation (tDCS) is a safe, non-invasive, non-painful electrical stimulation of the brain that is shown to act as a primer at the synaptic level when administered along with behavioral therapy, mostly involving language, learning and memory. Previous studies have shown that tDCS over the left angular gyrus (AG) improves language associative learning in the elderly through changes in functional connectivity between the AG and the hippocampus. The investigators' previous clinical trial on the effects of tDCS in neurodegenerative disorders has also shown augmented effects of lexical retrieval for tDCS. In the present study the investigators will compare the effects of active vs. sham tDCS over the AG-an area that is part of the default mode network but also a language area, particularly important for semantic integration and event processing-in two predominant AD variants: probable AD with amnesic phenotype (amnesic/typical AD) and probable AD with non-amnesic (language deficit) phenotype also described as logopenic variant PPA with AD pathology (aphasic/atypical AD). The investigators aim to: (1) determine whether active high-definition tDCS (HD-tDCS) targeting the left AG combined with a Word-List Learning Intervention (WordLLI) will improve verbal learning; (2) identify the changes in functional connectivity between the stimulated area (AG) and other structurally and functionally connected areas using resting-state functional magnetic resonance imaging; (3) identify changes in the inhibitory neurotransmitter GABA at the stimulation site using magnetic resonance spectroscopy. Furthermore, the investigators need to determine the characteristics of the people that may benefit from the new neuromodulatory approaches. For this reason, the investigators will evaluate neural and cognitive functions as well as physiological characteristics such as sleep, and will analyze the moderating effects on verbal learning outcomes. Study results can help provide treatment alternatives as well as a better understanding of the therapeutic and neuromodulatory effects of tDCS in AD, thus improving patients' and caregivers' quality of life.
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Change in auditory recall accuracy based on the sum of words recalled in Trials 1-5 of semantically related - trained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory delayed recall accuracy of semantically related - trained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory recall accuracy based on the sum of words recalled in Trials 1-5 of semantically unrelated - trained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory delayed recall accuracy of semantically unrelated - trained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory recall accuracy based the sum of words recalled in Trials 1-5 of semantically related - untrained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory delayed recall accuracy of semantically related - untrained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory recall accuracy based on the sum of words recalled in Trials 1-5 of semantically unrelated - untrained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks
Change in auditory delayed recall accuracy of semantically unrelated - untrained word-lists
Timeframe: Before intervention, immediately after intervention, 1 month and 3 months post intervention, up to 31 weeks