fMRI in Fully Myelinated White Matter: Evidence of Absent Signal

Motivation: Recent studies report evoked white matter BOLD signals in the human brain. However, their origin is not clearly established. Goal(s): Determine whether both myelinated and unmyelinated axons are responsible for BOLD-fMRI responses measured in white matter. Approach: Investigate whether visual stimulation induces vascular responses in the rodent optic nerve, which contains only myelinated axons, using a multiscale imaging approach, combining BOLD-fMRI and two-photon microscopy. Results: Visual stimulation does not induce changes in blood flow or oxygenation in the rodent optic nerve, indicating that fully myelinated structures do not contribute to white matter BOLD-fMRI signals. Impact: This study contributes to our understanding of brain imaging techniques that rely on blood flow, including BOLD-fMRI, serving as a prerequisite for interpreting such imaging in both healthy and pathological conditions.