Neural activity-driven BOLD responses within the cortex occur first at synaptic input layers

To further advance understanding of brain functions, identifying the direction of information flow, such as thalamocortical vs. corticothalamic projections, is critical. Because the early hemodynamic response at microvessels near active neurons can be detected by ultrahigh field fMRI, we propose using the onset times of fMRI responses to discern the information flow. This approach was confirmed by observing the high temporal resolution BOLD fMRI responses to bottom-up somatosensory stimulation and top-down optogenetic stimulation of the primary motor cortex in anesthetized mice at ultrahigh field of 15.2 T.