Right-left asymmetry in corticospinal tract microstructure and dexterity are uncoupled in late adulthood

Abstract

Ageing leads to a decline in white matter microstructure and dexterous function of the hand. In adolescents, it has previously been shown that the degree of right-left asymmetry in the corticospinal tract (CST) is linearly related with right-left asymmetry in dexterity. Here, we tested whether this association is also expressed in older adults. Participants completed a simple circle drawing task with their right and left hand as a measure of dexterity and underwent whole-brain diffusion weighted imaging at 3 Tesla ( n = 199; aged 60–72 years). Fractional anisotropy and mean diffusivity of right and left CST were extracted from a manually defined region-of-interest. Linear regression analyses were computed to replicate the analyses in adolescents. Frequentist analyses were complemented with a Bayesian analytical framework. Outcome measures were compared with those previously reported in adolescents (aged 11–16 years). Asymmetries in white matter microstructure of the CST were evident and comparable to the degree of lateralisation observed in adolescence. Similarly, asymmetries in dexterity were evident, but to a lesser degree than in adolescents. Unlike in adolescents, we found no evidence of a linear relationship between asymmetries in CST microstructure and dexterity. Complementary Bayesian regression analysis provided moderate evidence in favour of the null hypothesis, pointing towards a lack of association between the structural and functional measures of right-left asymmetry. Our findings are compatible with the notion that, by late adulthood, a diverging impact of age on white matter structure and dexterous hand function dilutes the structure-function relationship between CST microstructure and manual proficiency that has been reported in adolescents.

Structural and functional asymmetry of the neonatal cerebral cortex

“…Namely, the somatosensory and lateral motor networks showed left-ward asymmetry in the upper limb region of the somatotopic map 69 . This functional asymmetry might be associated with microstructural asymmetry (increased fractional anisotropy) of the left corticospinal tract compared to the right, which has previously been demonstrated in neonates 29 , adolescents 72 and adults 73 . It has also been shown that leftward lateralisation of motor network connectivity was associated with better motor performance in children 74 , and that motor circuit connectivity in children diagnosed with autism spectrum disorder was more rightward asymmetric compared to children without autism spectrum disorder 75 .…”

Structural and functional asymmetry of the neonatal cerebral cortex

“…Namely, the somatosensory and lateral motor networks showed leftward asymmetry in the upper limb region of the somatotopic map 69 . This functional asymmetry might be associated with microstructural asymmetry (increased fractional anisotropy) of the left corticospinal tract compared to the right, which has previously been demonstrated in neonates 29 , adolescents 72 and adults 73 . It has also been shown that leftward lateralisation of motor network connectivity was associated with better motor performance in children 74 , and that motor circuit connectivity in children diagnosed with autism spectrum disorder was more rightward asymmetric compared to children without autism spectrum disorder 75 .…”

Relationship between manual dexterity and left–right asymmetry of anatomical and functional properties of corticofugal tracts revealed by T2-weighted brain images

“…As the results of the present study apply to adolescents, it is necessary to consider the effects of aging. In late adulthood, right–left asymmetry of corticospinal tract microstructures measured as FA value did not show a significant linear relationship with manual dexterity during circle drawing 32 . Furthermore, aging individuals may experience neurodegeneration associated with white matter lesions, at grade 2 or higher on the Fazekas Scale 33 , that may make elevated signals from T2WI of the left and right CFT difficult to distinguish from elevated signals generated by aging.…”