Dependence of the fMRI Time Course of the Hemodynamic Response Function in Visual Cortex on Global Cerebral Blood Flow

Introduction: Event-related fMR1 (1,2) is increasingly being used to assess the temporal dynamics of activation related signal changes. In contrast to conventional studies using block designs this class of experiments enables more flexible task design and quantification of regional differences in response onset (3). However, this approach requires exact knowledge of the intrinsic hemodynamic response characteristics (delay and dispersion) which may vary between subjects (4) and between brain regions. The physiological factors which influence these response characteristics are still insufficiently characterized. We and others have shown that the amplitude of activation related signal changes in visual cortex is strongly dependent on global cerebral blood flow (gCBF) (56). In this study we demonstrate that the dispersion of the hemodynamic response function (HRF) in visual cortex increases with gCBF. A wide range of pC0, values was investigated using high speed EPI and visual stimulation in conjunction with hyperventilation and CO, breathing.