Submillimeter-resolution fMRI: Toward understanding local neural processing

Functional magnetic resonance imaging (fMRI) measures the hemodynamic response to active neurons. The most prevailing fMRI contrast, blood oxygenation level-dependent (BOLD) contrast, specifically results from a complex interplay between oxygen metabolism, blood flow, and blood volume reactions. Therefore, because fMRI indirectly measures brain function, the spatial accuracy of these blood-borne signal changes to the true neuronal activity comes into question, especially at high resolutions. To better interpret the neural basis of fMRI, these properties have been studied in localized neural circuits of the brain using fMRI with blood flow, blood volume, and BOLD contrasts, as well as optical intrinsic signal imaging (OISI), which shares similar signal sources as fMRI. Here, we review how submillimeter-scale high-resolution fMRI and OISI in the visual cortex columnar and olfactory bulb laminar models have advanced our basic knowledge of the spatial localization of the individual hemodynamic signals and neurovascular coupling mechanisms.