Remembering how to travel from one location to another is critical in everyday life, yet this vital ability declines with normal aging and can be further affected by conditions that disproportionately affect the elderly, such as vision loss or progressive dementia. Human and animal research has shown that two distinct memory systems interact during navigation. The first, referred to as allocentric navigation, is very flexible and uses spatial knowledge of key features or landmarks to develop and use a mental map of the environment. This approach involves brain regions that are critical for new learning and memory but that decline with age. The second, referred to as egocentric navigation, is inflexible and relies on "habit" memories that link specific features with specific directions. This approach relies on brain regions that are critical for "automatic" responses and that are relatively unaffected by age.

The main problem is that allocentric navigation declines with age and is accompanied increased dependence on egocentric navigation. This change increases the risk of becoming disoriented or "lost" when traveling in unfamiliar areas or even when traveling new routes in familiar areas. Therefore, the main goal of this project is to examine whether non-invasive brain stimulation, specifically transcranial direct current stimulation, can improve allocentric navigation in healthy older adults and patients with mild cognitive impairment.

Participants will complete two functional magnetic resonance imaging sessions while learning new environments. Before one of these sessions, participants will receive active brain stimulation over the parietal cortex. Before the other session, participants will receive sham brain stimulation over the parietal cortex. The effects of this stimulation will be evaluated using both an allocentric and an egocentric memory test. Physiologic effects will be evaluated using both task-based and resting-state MRI.

Inclusion Criteria:

General inclusion criteria (all participants):

• All medications stable for approximately 1-2 months;
• No history of severe mental illness;
• No current untreated alcohol or substance abuse/dependence;
• English as native and preferred language;
• MRI-compatible if taking part in fMRI studies
• Able to give informed consent.

MCI Inclusion Criteria:

• Diagnosis of amnestic MCI based on criteria set forth by Petersen (2004). Additionally, other potential causes of cognitive deficit ruled out by the referring physician;

Healthy older adults

• intact cognitive functioning as measured by neuropsychological testing

Exclusion Criteria:

• History of neurological disease or injury
• History of severe mental illness
• Current untreated alcohol or substance abuse
• Other conditions may exclude; please discuss with contact

duration of each participant's study related activities (~2 weeks), with monitoring ending following the second tDCS session

All participants completed a standardized side effect questionnaire after every tDCS session.

Double blinded