¹Department of Radiology, Center for Magnetic Resonance Research, University of Minnesota Medical School, Minneapolis, Minnesota, USA

Correspondence: Dr E Yacoub, Department of Radiology, Center for Magnetic Resonance Research, University of Minnesota Medical School, 2021 6th Street SE, Minneapolis, MN 55455, USA. E-mail: yacoub@cmrr.umn.edu

Received 8 July 2005; Revised 12 September 2005; Accepted 19 September 2005; Published online 12 October 2005.

Abstract

The hemodynamic response to neural activity consists of changes in blood flow, blood volume and oxygen metabolism. Changes in the vascular state after sensory stimulation have different spatial and temporal characteristics in the brain. This has been shown using imaging techniques, such as BOLD functional magnetic resonance imaging (fMRI), which monitor vascular changes once the stimulus is turned on, and the eventual return to baseline levels, once the stimulus is turned off. The BOLD fMRI signal during sensory stimulation has been well characterized and modeled in terms of the spatial and temporal characteristics of the vascular response. However, the return of the signals to baseline levels after sensory stimulation is not as well characterized. During this period, a poststimulus undershoot in the BOLD signal is observed. This poststimulus undershoot has been modeled and investigated to characterize the physiological mechanisms (cerebral blood flow (CBF), cerebral blood volume (CBV), and cerebral oxygen consumption) associated with the response. However, the data in the literature, which lack any spatially dependent information, appear to be contradictory in terms of the mechanisms associated with this poststimulus response. With a high spatial resolution cat model at 9.4 T, we show that CBV changes in the tissue persist once the stimulus is turned off, while CBV changes in the surface vessels quickly return to baseline levels, despite a concurrent undershoot in the BOLD signal in both the tissue and surface vessel areas. In addition, the BOLD data alone indicate that different physiological mechanisms regulate the poststimulus response in the tissue versus the surface vessel regions.