This study will evaluate the effects of a form of non-invasive brain stimulation on brain functioning and memory in cognitively intact older adults (healthy controls, HC) and in those with mild cognitive impairment (MCI).

The goal of this study is to learn important information about the effects of weak electrical stimulation (known as high-definition transcranial direct current stimulation or HD-tDCS) on brain functioning in those with mild cognitive impairment (MCI). The findings will help determine how stimulation affects brain's activity and metabolism (in particular, the neurotransmitter glutamate). Ultimately, this information may help develop new treatments for those with Alzheimer's disease. The study will use different forms of brain imaging to see whether stimulation changes how the brain responds during a memory task. Functional magnetic resonance spectroscopy (fMRS), functional magnetic resonance imaging (fMRI), and functional near-infrared spectroscopy (fNIRS) will be used. The study also uses cognitive tests and questionnaires.

Inclusion Criteria:

• Normal cognition for HC and diagnosis of Mild Cognitive Impairment (MCI) for MCI participants
• Right-handedness
• Magnetic resonance compatible, criteria that also apply for high definition transcranial direct current stimulation (HD-tDCS; e.g., absence of metallic or electronic implants in the upper body or head)

Exclusion Criteria:

• History of other contributing neurological or medical conditions known to affect cognitive functioning
• Significant mental illness
• Sensory impairments that limit ability to participate
• History of alcohol or drug abuse/dependence