RecruitingBehavioural interventionHigh Frequency Stimulation to Improve Cognition, Mobility, and Affect in Individuals With and Without Subjective Cognitive DeclineThe goal is to determine whether three months of at least three times / week of sensory flicker stimulation improves cognition, mobility, and affect in healthy older adults and older adults with and without Subjective Cognitive Decline (SCD). Investigators will also determine whether the intervention slows cortical thinning and declines in brain functional network segregation and changes in blood biomarkers of Alzheimer's Disease (AD).