Project Summary/Abstract A growing body of research has found that neural representations are less distinctive in older relative to younger adults, a phenomenon known as age-related neural dedifferentiation. Neural dedifferentiation has been associated with many of the behavioral impairments typically observed in healthy aging, and evidence collected by our group during the original funding period has found a strong relationship between neural dedifferentiation and gamma-aminobutyric acid (GABA), the brain’s major inhibitory neurotransmitter. Specifically, we found that GABA levels are lower in older vs. younger adults and that older adults with less GABA exhibit greater dedifferentiation. We propose to take this line of research in two new directions. (1) We will conduct a longitudinal study and explore age-related trajectories of change in GABA, neural distinctiveness, and behavior, as well as directional relationships among these measures (e.g., Do GABA levels at age 65 predict subsequent declines in neural distinctiveness? Does neural distinctiveness at age 65 predict behavioral declines over the next decade). (2) We will also examine how dedifferentiation is related to Alzheimer’s pathology by collecting the same measures in patients with Mild Cognitive Impairment in whom amyloid beta and tau burden has already been measured. The proposed studies will provide novel insights into how the brain changes with age and with neuropathology, whether those changes can explain the observed behavioral deficits, and whether changes in GABA levels are a contributing cause. Such insights could lead to novel interventions to alleviate the behavioral impairments associated with healthy and pathological aging.