How The Brain Changes As We Get Older

 

Dr Simon Cox and colleagues have published a paper in Nature communications ‘Ageing and brain white matter structure in 3,513 UK Biobank participants‘ under a Creative Commons 4.0 license. Highlighting below is mine.

This is a structural MRI study, using data from the UK Biobank.

‘Here we examine brain white matter magnetic resonance imaging (MRI) data in 3,513 generally healthy people aged 44.64–77.12 years from the UK Biobank‘

The researchers intended to characterise how aging affects the brain.  The authors describe the disconnection hypothesis

‘White matter is highly relevant to ageing: later-life cognitive decline may partly be caused by cortical disconnection, a microstructural deterioration of the brain’s connective pathways through processes such as axonal demyelination that reduces information transfer efficiency‘

as well as the de-differentiation hypothesis

‘An additional gap in our understanding relates to the de-differentiation hypothesis: the suggestion that interindividual differences in microstructure across tracts become increasingly related in older age‘

The researchers investigates these hypotheses using several parameters from the MRI data amongst which were

1. Fractional Anisotropy (FA): ‘the directional coherence of water molecule diffusion‘
2. Mean Diffusivity (MD): ‘the magnitude of water molecule diffusion‘
3. Intra-cellular volume fraction (ICVF): ‘estimates of neurite density‘
4. Isotropic volume fraction (ISOVF): ‘extra-cellular water diffusion‘
5. Tract complexity/fanning (OF).
6. Neurite orientation dispersion and density imaging(NODDI): This encompasses terms 3-5 above

The results showed that

‘Older age was significantly associated with lower coherence of water diffusion (FA; β≥−0.275), lower neurite density (ICVF; β≥−0.382) and lower tract complexity (OD; β≥−0.277), and with a higher magnitude of water diffusion (MD; β≤0.496) and ISOVF (β≤0.343) across the majority of tracts‘

The researchers conclude that

‘We identified MD as the diffusion parameter most sensitive to age and, although sex differences exist, neither sex appeared to exhibit stronger age effects. Tract-specific associations with age were strongest in thalamic and association pathways, and weakest in projection fibres. We also show that there are tract-specific and white-matter-wide correlations with age. Finally, we found novel evidence for brain microstructural de-differentiation for FA, MD, ICVF and ISOVF‘

The researchers have thus provided evidence for the de-differentiation and disconnection hypotheses. Each of these hypotheses generates sub-hypotheses linked to individual pathways. This study generates a number of sub-hypotheses that can be utilised as the primary hypothesis in future studies.

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