Temporal characteristics of cerebral blood flow velocity changes evoked by short-term visual stimulation

Abstract

The basis of blood flow sensitive imaging techniques is the activation-flow coupling (AFC), the coupling between neuronal activation and regional cerebral blood flow (rCBF). In functional magnetic resonance imaging (fMRI) experiments investigating brain activation the termination of stimulation can a cause drop of the post-activation signal below the preactivation baseline. The nature of this “undershoot” remains controversial. Hedera et al. propose that “undershoot” is caused by a decreased oxygen content in the venous blood compared with the preactivation level and may reflect increased extraction of oxygen as a result of glycolytic metabolism (1). Functional transcranial Doppler ultrasonography (ffCD) allows to measure the cerebral blood flow velocities (CBFVs) within the basal cerebral arteries in real-time. We used fTCD to assess the temporal characteristics of the regional CBFV change evoked by short-term visual stimulation. Methods: Using a 2 MHz TCD device (Multidop T, DWL Sipplingen, Germany) the velocities within the middle cerebral artery (MCA) and the P2-segment of the posterior cerebral artery (PCA) were investigated simultaneously in a 26 year old female subject. As a visual stimulus we used a 0.1 ms light flash of moderate intensity produced by a LED goggle. The stimulus was repeated 80 times using an randomized interval of 10 to 20 s. The goggle shut tightly. For ffCD analysis data were heart cycle integrated, artifact filtered, normalized and baseline corrected using the AVERAGE software package (2). Results: The first significant (pcO.05) CBFV deviation from baseline level occurred t= 1.4 s after stimulation and peaked at t=3.5 s for the PCA. The maximum change was about 8%. Baseline level was reached 7 s after light flash. The full width at half maximum (FWHM) of the bell shaped blood flow velocity increase within the PCA segment was about 3.2 s. A CBFV reduction (“undershoot”) of about 1.5% (pcO.05) occurred from 7 s to 10 s after stimulation. No significant modulations of the CBFV could be observed within the MCA. Discussion: Functional magnetic resonance imaging based on the BOLD contrast is sensitive to blood flow as well as to blood volume modulations. FTCD is only sensitive to blood flow changes. The present study demonstrates a regional postactivation CBFV decrease within the PCA which most likely reflects a regional postactivation CBF decrease within the stimulated visual areas. The CBFV drop can not be caused by global systemic effects because the CBFV within the MCA remained constant during stimulation. The CBFV time course within the PCA was similar to time course of the BOLD signal found in fMR1 studies using a comparable stimulation (3). Conclusion: The postactivation drop in the BOLD signal below the preactivation baseline seems to reflect a regional postactivation CBF decrease.