Figure 1.

Schematics for spatial profiles of fMRI signals in response to 0° orientation stimulation when multiple 0° iso-orientation columns are activated. BOLD signal (red lines) depends on changes in dHb amounts resulting from alterations in CMRO₂ (green lines) and CBF (blue lines) in response to increased neuronal activity (white regions). If a CBF PSF is rather narrow (solid blue line) and/or if the amplitude difference between active and inactive columns for CBF is much larger than that for CMRO₂ response, the highest change of the BOLD signal (solid red line) will mark the sites of increased neural activity. In contrast, if the CBF response is rather broad or has relatively small amplitude difference between active and inactive columns (dashed blue line), the highest change of the BOLD signal (dashed red line) could mark the sites of no neural activity as seen in dHb-weighted OIS and dHb signals obtained from optical spectroscopy during the hyperoxygenated phase. Thus, the assignment of orientation preference to BOLD fMRI maps is dependent on both PSF and magnitude of CBF responses. CBV PSF may be similar to or a little broader than CBF PSF, but that is not shown here.