  Purpose: Functional magnetic resonance imaging (fMRI) and functional
connectivity (FC) have been used to follow aging in both children and older
adults. Robust changes have been observed in children, where high connectivity
among all brain regions changes to a more modular structure with maturation. In
this work, we examine changes in FC in older adults after two years of aging in
the UK Biobank longitudinal cohort. Approach: We process data using the
Power264 atlas, then test whether FC changes in the 2,722-subject longitudinal
cohort are statistically significant using a Bonferroni-corrected t-test. We
also compare the ability of Power264 and UKB-provided, ICA-based FC to
determine which of a longitudinal scan pair is older. Results: We find a 6.8\%
average increase in SMT-VIS connectivity from younger to older scan (from
$\rho=0.39$ to $\rho=0.42$) that occurs in male, female, older subject ($>65$
years old), and younger subject ($<55$ years old) groups. Among all
inter-network connections, this average SMT-VIS connectivity is the best
predictor of relative scan age, accurately predicting which scan is older 57\%
of the time. Using the full FC and a training set of 2,000 subjects, one is
able to predict which scan is older 82.5\% of the time using either the full
Power264 FC or the UKB-provided ICA-based FC. Conclusions: We conclude that
SMT-VIS connectivity increases in the longitudinal cohort, while resting state
FC increases generally with age in the cross-sectional cohort. However, we
consider the possibility of a change in resting state scanner task between UKB
longitudinal data acquisitions.
