Topographic maps are a fundamental feature of cortex architecture in the mammalian brain. One common theory is that the de-differentiation of topographic maps links to impairments in everyday behavior due to less precise functional map readouts. Here, we tested this theory by characterizing de-differentiated topographic maps in primary somatosensory cortex (SI) of younger and older adults by means of ultra-high resolution functional magnetic resonance imaging together with perceptual finger individuation and hand motor performance. Older adults' SI maps showed similar amplitude and size to younger adults' maps, but presented with less representational similarity between distant fingers. Larger population receptive field sizes in older adults' maps did not correlate with behavior, whereas reduced cortical distances between D2 and D3 related to worse finger individuation but better motor performance. Our data uncover the drawbacks of a simple de-differentiation model of topographic map function, and motivate the introduction of feature-based models of cortical reorganization.

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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.

AbstractTopographic maps are a fundamental feature of cortex architecture in the mammalian brain. One common theory is that the de-differentiation of topographic maps links to impairments in everyday behavior due to less precise functional map readouts. Here, we tested this theory by characterizing de-differentiated topographic maps in primary somatosensory cortex (SI) of younger and older adults by means of ultra-high resolution functional magnetic resonance imaging together with perceptual finger individuation and hand dexterity. Older adults’ SI maps showed similar amplitude, size, and levels of stimulus-related noise than younger adults’ SI maps, but presented with less representational similarity between distant fingers. Larger population receptive field sizes in older adults’ maps did not correlate with behavior, whereas reduced cortical distances related to better hand dexterity. Our data uncover the drawbacks of a simple de-differentiation model of topographic map function, and motivate the introduction of a feature-based model of cortical reorganization.