High-resolution mapping of iso-orientation columns by fMRI

Blood-oxygenation level-dependent (BOLD) functional magnetic resonance imag ing (fMRI) is an important tool for localizing brain functions in vivo. How ever, the ability of BOLD fMRI to map cortical columnar structures is highl y controversial, as the ultimate functional specificity of BOLD remains unk nown. Here we report a biphasic BOLD response to visual stimulation in the primary visual cortex of cats. In functional imaging, the initial BOLD sign al decrease accurately labeled individual iso-orientation columns. In contr ast, the delayed positive BOLD changes indicated the pattern of overall act ivation in the visual cortex, but were less suited to discriminate active f rom inactive columns.