Changes in human regional cerebral blood flow and cerebral blood volume during visual stimulation measured by positron emission tomography

The hemodynamic mechanism of increase in cerebral blood flow (CBF) during n eural activation has not been elucidated in humans. In the current study, c hanges in both regional CBF and cerebral blood volume (CBV) during visual s timulation in humans were investigated. Cerebral blood flow and CBV were me asured by positron emission tomography using (H2O)-O-15 and (CO)-C-11, resp ectively, at rest and during 2-Hz and 8-Hz photic flicker stimulation in ea ch of 10 subjects. Changes in CBF in the primary visual cortex were 16% +/- 16% and 68% +/- 20% for the visual stimulation of 2 Hz and 8 Hz. respectiv ely. The changes in CBV were 10% +/- 13% and 21% +/- 5% for 2-Hz and 8-Hz s timulation, respectively. Significant differences between changes in CBF an d CBV were observed for visual stimulation of 8 Hz. The relation between CB F and CBV values during rest and visual stimulation was CBV = 0.88CBF(0.30) . This indicates that when the increase in CBF during neural activation is great, that increase is caused primarily by the increase in vascular blood velocity rather than by the increase in CBV. This observation is consistent with reported findings obtained during hypercapnia.