Cortical diffusion restriction predicts individual cognitive ability and carries cellular information

Whether the cellular architecture of the cerebral cortex constrains individual cognitive ability is a century-old question, open because its two methods do not meet. Histology resolves cells in tens of specimens; imaging reaches thousands of participants without a cellular quantity to measure. Here we show that standard diffusion MRI carries one. The return-to-origin probability, a model-free measure of how tightly cortical tissue confines water at cellularscales, predicts individual cognitive ability out of sample. In 978 adults it predicts the shared variance of 22 cognitive measures ($\rho = 0.19$) above any test constituting it, while 21 self-report scales stay at chance. It reproduces in 1{,}012 participants of an ageing cohort on a routine two-shell acquisition, with accuracy still rising. The same profile recovers the layer thicknesses of a post-mortem cytoarchitectonic atlas and tracks cytoarchitecture in mouse cortex.Cellular hypotheses of cognition can now be tested in living cohorts.