Inclusion Criteria:  Inclusion criteria for healthy participants:  50-90 years of age, as verified via photo identification with date of birth. Should be right-handed and learned English by seven years old. Generally healthy  Inclusion criteria for participants with MCI or AD:  50-90 years of age, as verified via photo identification with date of birth. Should be right-handed and learned English by seven years old. Is legally able to sign the consent form on their own behalf, or has a legally authorized representative that is able to sign. Has a caregiver that can assist with taking health history.  Exclusion Criteria:  Exclusion criteria for healthy participants:  Significant history of psychiatric disorders or current psychosis including self-report or Geriatric Depression Scale short form score-GDS >5 if uncertain. Current excessive drug, alcohol or nicotine use defined by participant self-report. History of epilepsy, migraines, severe stroke, or traumatic brain injury. Taking medications with significant psychotropic effects. Severe sensory impairment. Severe chronic illness where participation in the study could put participants at an unusual level of risk. Chronic conditions will be evaluated on a case by case basis as they arise. Severe subjective cognitive concerns. Requires a helper animal. Has sufficient prior experience with neurostimulation that might unblind or alter the results. Has metal or electronic implants that may be sensitive to stimulation or could interfere with stimulation or be an MRI contraindication, or has any other MRI contraindication. Has sensitivity to components of tES electrodes being used (typically nickel or latex). Feels ill or have any potential COVID-19 symptoms, such as fever or chills, cough, difficulty breathing, overly tired, unusual aches or pains including headache or sore throat, recent or unusual (for them) loss of taste or smell, congestion or runny nose, nausea or diarrhea.  Exclusion criteria for participants with MCI or AD:  Significant history of psychiatric disorders or current psychosis not related to a neurodegenerative condition. Current excessive drug, alcohol or nicotine use. Significant history of epilepsy, stroke, or traumatic brain injury. Taking medications with significant psychotropic effects, not related to neurodegenerative condition Severe sensory impairment Severe chronic illness where participation in the study could put participants at an unusual level of risk. Chronic conditions will be evaluated on a case by case basis as they arise. Requires a helper animal. Has sufficient prior experience with neurostimulation that might unblind or alter the results. Has metal or electronic implants that may be sensitive to stimulation or could interfere with stimulation. Has sensitivity to components of tES electrodes being used (typically nickel or latex). Feels ill or have any potential COVID-19 symptoms, such as fever or chills, cough, difficulty breathing, overly tired, unusual aches or pains including headache or sore throat, unusual loss of taste or smell, or unusual (for them) congestion or runny nose, nausea or diarrhea.