  We use entropy to characterize intrinsic ageing properties of the human
brain. Analysis of fMRI data from a large dataset of individuals, using resting
state BOLD signals, demonstrated that a functional entropy associated with
brain activity increases with age. During an average lifespan, the entropy,
which was calculated from a population of individuals, increased by
approximately 0.1 bits, due to correlations in BOLD activity becoming more
widely distributed. We attribute this to the number of excitatory neurons and
the excitatory conductance decreasing with age. Incorporating these properties
into a computational model leads to quantitatively similar results to the fMRI
data. Our dataset involved males and females and we found significant
differences between them. The entropy of males at birth was lower than that of
females. However, the entropies of the two sexes increase at different rates,
and intersect at approximately 50 years; after this age, males have a larger
entropy.
