Abstract
Brain’s modular connectivity gives this organ resilience and adaptability. The ageing process alters the organised modularity of the brain and these changes are further accentuated by neurodegeneration, leading to disorganisation. To understand this further, we analysed modular variability—heterogeneity of modules—and modular dissociation—detachment from segregated connectivity—in two ageing cohorts and a mixed cohort of neurodegenerative diseases. Our results revealed that the brain follows a universal pattern of high modular variability in metacognitive brain regions: the association cortices. The brain in ageing moves towards a segregated modular structure despite presenting with increased modular heterogeneity—modules in older adults are not only segregated, but their shape and size are more variable than in young adults. In the presence of neurodegeneration, the brain maintains its segregated connectivity globally but not locally, and this is particularly visible in dementia with Lewy bodies and Parkinson’s disease dementia; overall, the modular brain shows patterns of differentiated pathology.
Highlights
Brain’s modular connectivity gives this organ resilience and adaptability
We compared differences in modular variability (MV) and modular dissociation (MD) between young and older adults from the Nathan Kline Institute (NKI) and the Functional Connectome (TFC) cohorts and the deviation from healthy ageing to disease with the Newcastle University (NCL) cohort
The healthy participants within the NCL neuroimaging database were classified as an independent older adults (OA) group and used as a reference for comparisons with the three dementia groups
Summary
Brain’s modular connectivity gives this organ resilience and adaptability. The ageing process alters the organised modularity of the brain and these changes are further accentuated by neurodegeneration, leading to disorganisation. It is hypothesised that the modularity of the brain is the result of evolution, where, in an always changing environment the brain developed a strategy to adapt subsystems rapidly without compromising the totality of its network[6] In this regard, recent studies on brain dynamics have reported that modules constrain dynamic communication within their boundaries without affecting other modules[5,7,8]. There is no agreement on their number, the majority of functional studies report that between three to ten modules are present in the brain[6,12] This number depends on many factors and methodologic preferences such as the number of brain regions (brain parcellation), the chosen brain atlases[13], neuroimaging pre-processing pipelines[14], connectivity measures (e.g. wavelet or Pearson correlations), and treatment of connectivity weights[15].
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