Abstract
Topographic maps form a critical feature of cortical organization, yet are poorly described with respect to their microstructure in the living aging brain. We acquired quantitative structural and functional 7T-MRI data from younger and older adults to characterize layer-wise topographic maps of the primary motor cortex (M1). Using parcellation-inspired techniques, we show that qT1 and QSM values of the hand, face, and foot areas differ significantly, revealing microstructurally-distinct cortical fields in M1. We show that these fields are distinct in older adults, and that myelin borders between them do not degenerate. We further show that the output layer 5 of M1 shows a particular vulnerability to age-related increased iron, while layer 5 and the superficial layer show increased diamagnetic substance, likely reflecting calcifications. Taken together, we provide a novel 3D model of M1 microstructure, where body parts form distinct structural units, but layers show specific vulnerability towards increased iron and calcium in older adults. Our findings have implications for understanding sensorimotor organization and aging, in addition to topographic disease spread.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We resubmit a revised version of our manuscript. Below, we summarize some of the major improvements to the manuscript. We performed additional analyses to investigate the relationships between measures of cortical microstructure in M1 and measures of motor behavior. This provide preliminary insight into the behavioral relevance of age-related differences in the cortical microstructure of M1. Additonally, we have ensured that full statistics (degrees of freedom, F-values etc.) are reported for all effects, along with estimates of effect sizes and 95% confidence intervals. We have also ensured that the Holm-Bonferroni correction is applied consistently across all analyses, to correct for multiple comparisons and thus reduce the risk of type II errors. Finally, we now more clearly line out the five hypotheses that motivated our analyses. While these hypotheses were also described in the previous version of the manuscript, we did not explicitly number them, which may have made it more difficult to follow them up throughout the manuscript.