BOLD fMRI signals of visual white matter encode visuotopic information and predict effective connectivity between visual areas

The functional significance of BOLD signals in white matter (WM) remains unclear. The current study investigated whether 7T BOLD fMRI signal from visual WM tracts contains high fidelity retinotopic information and whether it correlates with the effective connectivity between visual areas. Population receptive field (pRF) analysis of the 7T retinotopy dataset from the Human Connectome Project revealed clear contralateral retinotopic representations from two visual WM bundles: optic radiation (OR) and vertical occipital fasciculus (VOF). The retinotopic organization of OR is consistent with post-mortem studies. The pRF size of WM voxels also increases with eccentricity. Based on the retinotopic maps of OR, we investigated whether BOLD signals in OR during visual stimulation are related to the resting-state effective connectivity between the lateral geniculate nucleus (LGN) and the primary visual cortex (V1). Results show that visually-evoked BOLD responses in OR correlate with the feedforward and feedback connectivity between the LGN and V1 during resting state. These findings demonstrate that WM BOLD signals contain high fidelity information such as visual field maps, and also predict the functional connectivity of brain areas. Significance statementWhite matter (WM) tracks conduct spiking activity between distant neurons. Weak fluctuations of BOLD signals in the WM can be detected with fMRI, but their functional relevance remains largely unknown. Here we characterized the visual field map properties of two major visual WM bundles: the optic radiation (OR) and vertical occipital fasciculus (VOF). Population receptive field analysis of the WM BOLD signals revealed clear visual field maps in both WM tracts. Effective connectivity analysis further showed that visually evoked BOLD responses in OR can predict the resting thalamo-cortical functional connectivity. These findings demonstrate that WM BOLD signals contain highly specific functional information and could directly index the functional connectivity between brain areas.