Evidence for adaptive myelination of subcortical shortcuts for visual motion perception in healthy adults

Conscious visual motion information follows a cortical pathway from the retina to the lateral geniculate nucleus (LGN) and on to the primary visual cortex (V1) before arriving at the middle temporal visual area (MT/V5). Alternative subcortical pathways that bypass V1 are thought to convey unconscious visual information. One flows from the retina to the pulvinar (PUL) and on to medial temporal visual area (MT); while the other directly connects the LGN to MT. Evidence for these pathways comes from non‐human primates and modest‐sized studies in humans with brain lesions. Thus, the aim of the current study was to reconstruct these pathways in a large sample of neurotypical individuals and to determine the degree to which these pathways are myelinated, suggesting information flow is rapid. We used the publicly available 7T (N = 98; ‘discovery’) and 3T (N = 381; ‘validation’) diffusion magnetic resonance imaging datasets from the Human Connectome Project to reconstruct the PUL–MT (including all subcompartments of the PUL) and LGN–MT pathways. We found more fibre tracts with greater density in the left hemisphere. Although the left PUL–MT path was denser, the bilateral LGN–MT tracts were more heavily myelinated, suggesting faster signal transduction. We suggest that this apparent discrepancy may be due to ‘adaptive myelination’ caused by more frequent use of the LGN–MT pathway that leads to greater myelination and faster overall signal transmission.