Early temporal characteristics of cerebral blood flow and deoxyhemoglobin changes during somatosensory stimulation

The close correspondence between neural activity in the brain and cerebral blood flow (CBF) forms the basis for modern functional neuroimaging methods . Yet, the temporal characteristics of hemodynamic changes induced by neuro nal activity are not well understood, Recent optical imaging observations o f the time course of deoxyhemoglobin (HbR) and oxyhemoglobin have suggested that increases in oxygen consumption after neuronal activation occur earli er and are more spatially localized than the delayed and more diffuse CBF r esponse. Deoxyhemoglobin can be detected by blood oxygenation level-depende nt (BOLD) functional magnetic resonance imaging (fMRI). In the present stud y, the temporal characteristics of CBF and BOLD changes elicited by somatos ensory stimulation in rat were investigated by high-field (9.4 T) MRI. With use of high-temporal-resolution fMRI, it was found that the onset time of the CBF response in the somatosensory cortex was 0.6 +/- 0.4 seconds (n = 1 0). The CBF changes occurred significantly earlier than changes in HbR conc entration, which responded after 1.1 +/- 0.3 seconds. Furthermore, no early increases in HbR (early negative BOLD signal changes) were observed. These findings argue against the occurrence of an early loss of hemoglobin oxyge nation that precedes the rise in CBF and suggest that CBF and oxygen consum ption increases may be dynamically coupled in this animal model of neural a ctivation.