et al. 2008), none of the layer-fMRI sequences (including VASO) is expected to capture meaningful layer-specific functional signal from layer I. In other layers (II–VI), the VASO bias of layer-dependent vascular density is expected to be negligible for the majority of cases. In contrast to the large diving veins that bias the GE-BOLD signal, the VASO-relevant microvascular density is rather homogeneous across cortical depth in most brain areas. As such, in the extra striate cortex, variations of the vascular density are usually smaller than 15% across layers II-VI (Kennel et al. 2017; Weber et al. 2008). There are exceptions, however. In V1 and S1, the microvascular density can be as much as 25% higher in layer IV compared to other layers (II, III, V, and VI), which can eventually introduce biases in VASO profiles.
