Spatial specificity of cerebral blood volume-weighted fMRI responses at columnar resolution

The spatial specificity of functional magnetic resonance imaging (fMRI) signals to columnar architecture remains uncertain. At columnar resolution, the specificity of intrinsic cerebral blood volume (CBV) response to orientation-selective columns in isoflurane-anesthetized cats was determined for CBV-weighted fMRI signals after injection of iron oxide at a dose of 10 mg Fe/kg. CBV-weighted fMRI data were acquired at 9.4 T with an in-plane resolution of 156 × 156 μm² in area 18 during visual stimulation at two orthogonal orientations. A 1-mm-thick imaging slice was selected tangential to the cortical surface. Regions with large CBV changes in response to two orthogonal orientation gratings were highly complementary. Maps of iso-orientation domains in response to these gratings were highly reproducible, suggesting that CBV-weighted fMRI has high sensitivity and specificity. The average distance between iso-orientation domains was 1.37 ± 0.28 mm (n = 10 orientations) in an anterior-posterior direction. CBV-weighted fMRI signal change in the iso-orientation domains induced by preferred orientation was 1.69 ± 0.24 (n = 10) times larger than that induced by orthogonal orientation. Our data demonstrate that CBV regulates at a submillimeter columnar scale and CBV-weighted fMRI has sufficient specificity to map columnar organization in animals.