Project Summary/Abstract For almost 100 years, a large number of electrophysiology studies have reported on resting-state (RS) neural activity in humans. This research often focuses on RS alpha-band activity (8-12 Hz in adults), as RS alpha activity it is commonly interpreted as reflecting the brain's readiness to process information, predicting task performance and processing speed, and being integral to coordinating local- and long- range functional connectivity. Infant RS studies seek to understand infant RS neural processes and the maturation of these processes, with the first two years of life a peak period of neural reorganization, and with variation in the maturation of RS activity contributing to normal variation in development as well as clinical disorders. In infants and toddlers, this line of research is constrained by difficulty obtaining prototypical high SNR eyes-closed RS electrophysiology measures, with almost all infant studies obtaining RS measures while infants view visual stimuli, and with an eyes-open condition not generating a robust infant `alpha' response. During the current cycle of this R01 (locally referred to as Babies' Brains Change (BBC)), we developed methods that overcome barriers that have to date limited progress. Our intention is that findings from this continuation R01 will provide direct tests of theories of infant maturation of brain function, structure, and chemistry and their relationship to each other and behavior, change the way infant RS electrophysiology studies are conducted, and show the way forward for further discoveries about those relationships. Progress will be made via (1) the use of an eyes-open dark-room (DR) task advanced by the PI and which provides RS measures with a high-SNR dominant oscillation response in awake infants, (2) using recently developed algorithms to parameterize the RS power spectrum to obtain estimates of RS periodic activity (e.g., the canonical RS dominant oscillation) and RS aperiodic activity (i.e., the `background' neural activity), (3) assessing RS activity in brain space so that regional differences in RS neural activity can be identified, and so that hypothesized associations between RS neural activity and brain structure (gray and white matter), brain chemistry (glutamate (Glu) and gamma-aminobutyric acid (GABA)), and behavior can be optimally evaluated, and (4) obtaining temporally dense brain measures as understanding brain development requires assessing maturational processes. At Time 1, evaluable data (MEG measures of infant RS activity, MRI measures of gray and white matter, MRS measures of Glu and GABA, and developmental milestone measures) will be obtained from 70 infants, with a 4-month interval between visits, and with a total of 4 visits. It is predicted that the project findings will identify regional differences in the maturation of RS neural activity, show that brain structure and chemistry predict the structure of the RS activity, and that RS measures...