SPATIAL AND TEMPORAL DIFFERENTIATION OF FMRI BOLD RESPONSE IN PRIMARYVISUAL-CORTEX OF HUMAN BRAIN DURING SUSTAINED VISUAL SIMULATION

The blood oxygenation level dependent (BOLD) response during sustained visual stimulation has been studied by several groups using fMRI with controversial conclusions. This issue was investigated for the human brain at high (4 Tesla) magnetic field strength using a flashing goggl e at 8 Hz. The results demonstrate that the overall BOLD response in t he primary visual cortex has an initial overshoot after the onset of v isual stimulation and an undershoot after the termination of visual st imulation. A significant and positive BOLD response, however, remains constant between the initial and terminal transient responses. The tem poral BOLD responses in the primary visual cortex were spatially depen dent. The regions identified as draining veins in images displayed pro portionately larger initial and terminal transient responses, whereas regions devoid of such vessels and associated mainly with parenchyma e xhibited a more time-independent BOLD response. These results reveal t hat the BOLD effect and, presumably, the uncoupling between cerebral b lood flow and cerebral metabolic rate of oxygen consumption, are maint ained in the primary visual cortex during sustained visual stimulation , and the temporal characteristics of the BOLD effect are spatially de pendent.