Modulating Cortical Dynamics of Dual-task Standing in MCI

Inclusion Criteria: Healthy older adults: Our target population will be healthy older men and women who are cognitively intact. We will also ensure that participants are not so impaired that they cannot safely participate nor potentially benefit from the intervention. * Age 65+ without any cognitive impairment (based in MoCA or TICS) * Willing and capable to give informed consent for the participation in the study after it has been thoroughly explained able and willing to comply with all study requirements informed consent form was signed * Ability to stand and walk independently * Understanding of the informed consent form (ICF) will be assessed by asking the participant to answer the following three questions: 1) What is the purpose of this study? 2) What are the risks of study involvement? 3) If you decide to participate, are you allowed to withdraw from the study at any time? Answers will be recorded by study personnel on the "Assessment of Protocol Understanding" form. Insufficient understanding will be defined by one or more incorrect answers, as determined at the discretion of the investigator. Older MCI participants: * Age 65+ * Who have been diagnosed with cognitive impairment (based in MoCA or TICS and Clinical Dementia Rating: CDR 0.5) * Ability to stand and walk independently * Willing and capable to give informed consent for the participation in the study after it has been thoroughly explained * Cut off of MOCA score \>18 will be applied * Able and willing to comply with all study requirements informed consent form was signed * Understanding of the ICF will be assessed by asking the participant to answer the following three questions: 1) What is the purpose of this study? 2) What are the risks of study involvement? 3) If you decide to participate, are you allowed to withdraw from the study at any time? Answers will be recorded by study personnel on the "Assessment of Protocol Understanding" form (see attached). Insufficient understanding will be defined by one or more incorrect answers, as determined at the discretion of the investigator. Exclusion Criteria: Healthy older adults: Exclusion Criteria have been selected to ensure safety and optimize compliance while minimizing confounds due to overt disease or conditions that may significantly influence study outcomes. * Potentially eligible individuals will complete the Montreal Cognitive Assessment (MoCA) - score \< 26 * Major psychiatric co-morbidity including major depressive disorder, schizophrenia or psychosis * Self-reported diagnosis of Parkinson's disease, Alzheimer's disease or dementia, multiple sclerosis, history of stroke or other neurodegenerative disorder. * Self-reported active cancer for which chemo-/radiation therapy is being received. * Blindness or other disabilities that prevent task performance * Contraindications to tACS, as recorded on a standardized screening questionnaire, which include a reported seizure within the past two years, use of neuroactive drugs, self-reported presence of specific implanted medical devices (e.g., deep brain stimulator, medication infusion pump, cochlear implant, pacemaker, etc.), or the presence of any active dermatological condition, such as eczema, on the scalp * Inability to stand or ambulate unassisted for at least 25 feet * Hospitalization within the past 3 months due to acute illness or as a result of musculoskeletal injury significantly affecting gait and balance Older MCI participants: * Cut off of MOCA score \<18 will be applied * Major psychiatric co-morbidity including major depressive disorder, schizophrenia or psychosis * Blindness or other disabilities that prevent task performance * Self-reported history of stroke or neurodegenerative disorders other than MCI. * Self-reported active cancer for which chemo-/radiation therapy is being received. * Contraindications to tACS, as recorded on a standardized screening questionnaire, which include a reported seizure within the past two years, use of neuro-active drugs, self-reported presence of specific implanted medical devices (e.g., deep brain stimulator, medication infusion pump, cochlear implant, pacemaker, etc.), or the presence of any active dermatological condition, such as eczema, on the scalp.