Abstract
Functional magnetic resonance imaging (fMRI) using blood-oxygenation-level-dependent (BOLD) contrast is a common method for studying human brain function non-invasively. Gradient-echo (GRE) BOLD is highly sensitive to the blood oxygenation change in blood vessels; however, the signal specificity can be degraded due to signal leakage from the activated lower layers to the superficial layers in depth-dependent (also called laminar or layer-specific) fMRI. Alternatively, physiological variables such as cerebral blood volume using VAscular-Space-Occupancy (VASO) measurements have shown higher spatial specificity compared to BOLD. To better understand the physiological mechanisms (e.g., blood volume and oxygenation change) and to interpret the measured depth-dependent responses we need models that reflect vascular properties at this scale. For this purpose, we adapted a “cortical vascular model” previously developed to predict the layer-specific BOLD signal change in human primary visual cortex to also predict layer-specific VASO response. To evaluate the model, we compared the predictions with experimental results of simultaneous VASO and BOLD measurements in a group of healthy participants. Fitting the model to our experimental findings provided an estimate of CBV change in different vascular compartments upon neural activity. We found that stimulus-evoked CBV changes mainly occur in intracortical arteries as well as small arterioles and capillaries and that the contribution from venules is small for a long stimulus (~30 sec). Our results confirm the notion that VASO contrast is less susceptible to large vessel effects compared to BOLD.
Competing Interest Statement
The authors have declared no competing interest.
