Until now, however, most human layer-fMRI studies have been confined to primary motor and sensory cortex and have used basic fMRI task block-designs. There are several reasons why layer-fMRI methods are not applied more widely. One of the major limitations is that large draining veins in conventional GE-BOLD fMRI methods spatially blur the fMRI activity, imposing a cortical depth specific contrast weighting across the layers - with superficial layers being most heavily weighted (Uludag and Blinder 2018). Thus, locally specific interpretations of individual layer activity are not straightforward with GE-BOLD (Kay et al. 2019). Alternative non-BOLD fMRI sequences (reviewed in Huber et al. (2019)), such as the CBV-weighted VASO (Lu et al. 2003) method, have been shown to be locally more specific and having contrast that is more evenly weighted across the cortical depths (Huber et al. 2015).
