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Age-related cortical plasticity reveals insights into the mechanisms underlying the stability and flexibility of neuronal circuits. Classical parcellation has long demonstrated the importance of microstructural features yet 3D approaches have rarely been applied to human brain organization in-vivo. We acquired functional and structural 7T-MRI and behavioral data of living younger and older adults to investigate human primary motor cortex (M1) aging, employing 3D parcellation techniques. We identify distinct cortical fields in M1 based on quantitative tissue contrast, which are, along with the myelin-poor borders between them, stable with age. We also show age-related iron accumulation, particularly in the output layer 5b and the lower limb field. Our data offers a new model of human M1 with distinct cortical fields, a mechanistic explanation for the stability of topographic organization in the context of aging and plasticity, and highlights the specific vulnerability of output signal flows to cortical plasticity.

The functional topography of the human primary somatosensory cortex (S1) hand area is a widely studied model system to understand sensory organization and plasticity. It is so far unclear whether or not the underlying 3D structural architecture also shows a topographic organization. We used 7T MRI data to quantify layer-specific myelin, iron and mineralization in relation to population receptive field maps of individual finger representations in Brodman area 3b (BA 3b) of human S1 in female and male younger adults. This 3D description allowed us to identify a characteristic profile of layer-specific myelin and iron deposition in the BA 3b hand area, but revealed an absence of structural differences, an absence of low-myelin borders, and high similarity of 3D microstructure profiles between individual fingers. However, structural differences and borders were detected between the hand and face areas. We conclude that the 3D structural architecture of the human hand area is non-topographic, unlike in some monkey species, which suggests a high degree of flexibility for functional finger organization and a new perspective on human topographic plasticity.SIGNIFICANCE STATEMENT:Using ultra-high field MRI, we provide the first comprehensivein vivodescription of the 3D structural architecture of the human BA 3b hand area in relation to functional population receptive field (pRF) maps. High similarity of precise finger-specific 3D profiles, together with an absence of structural differences and an absence of low-myelin borders between individual fingers, reveal the 3D structural architecture of the human hand area to be non-topographic. This suggests reduced structural limitations to cortical plasticity and reorganization, and allows for shared representational features across fingers.

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