The length of the thalamo-cortical white matter fibers brings insight into sex differences in sleep spindle frequency

Sleep spindles (SS) are crucial to brain functions like memory and learning. SS characteristics result from the propagation of nerve impulses along white matter (WM) projections underlying an intricate loop between the thalamus and the cortex. SS amplitude and density have been associated with WM diffusion microarchitecture but physiological mechanisms underlying individual and sex-related variations in SS frequency are unknown. Here, we tested a model of traveling signals along the thalamo-cortico-thalamic projections to explain individual differences in spindle frequency. We predicted the presence of a relationship between the length of the thalamo-cortical WM bundles and a specific characteristic of this functional network, SS frequency. Thirty young participants underwent a polysomnographic recording and a 3T MRI including a diffusion sequence. The length of WM fiber bundles between the thalamus and the frontal cortex was derived from probabilistic tractography computed through constrained spherical deconvolution. Longer WM fiber bundles between the thalamus and specific regions of the frontal cortex (rostral middle frontal gyrus and anterior and middle part of the superior frontal gyrus) were associated with slower SS frequency. Moreover, the length of these WM fiber bundles statistically mediated the sex-related differences in SS frequency. By providing a neuroanatomical marker of individual and sex-related differences in SS frequency, this study is the first to highlight the association between the anatomy of a specific brain network and a specific functional characteristic of this network, the frequency of oscillations produced during sleep.