Laminar CBV and BOLD response characteristics over time and space in the human primary somatosensory cortex at 7T

Uncovering the cortical representation of the body has been at the core of human brain mapping for decades, with special attention given to the digits. In the last decade, advances in functional magnetic resonance imaging (fMRI) technologies have opened the possibility of non-invasively unraveling the 3rd dimension of digit representations in humans along cortical layers. In laminar fMRI, it is common to combine the use of highly sensitive blood oxygen level dependent (BOLD) measurements with cerebral blood volume sensitive measurements, such as slice-saturation slab-inversion vascular space occupancy (SS-SI VASO, henceforth referred to as VASO), which are more specific to the underlying neuronal populations. However, the spatial and temporal VASO response characteristics across cortical depth to passive stimulation of the digits are still unknown. Therefore, we characterized hemodynamic responses to vibrotactile stimulation of individual digit tips across cortical depth at 0.75 mm in-plane spatial resolution using BOLD and VASO fMRI at 7T. We could identify digit-specific regions of interest (ROIs) in putative Brodmann area 3b, following the known anatomical organization. In these ROIs, the BOLD response increased toward the cortical surface due to the draining vein effect, while the VASO response was more shifted toward middle cortical layers, likely reflecting bottom-up input from the thalamus, as expected. As an incidental finding, we also saw slightly negative BOLD and VASO responses for non-preferred digits in the ROIs. Therefore, we conducted an exploratory analysis in which we investigated the temporal signal dynamics for BOLD and VASO as a function of distance from activation peaks resulting from stimulation of contralateral digits. With this analysis, we found a triphasic response consisting of an initial peak and a subsequent negative deflection during stimulation, followed by a positive post-stimulus response in BOLD and to some extent in VASO. Similar response dynamics have been observed in the animal literature using invasive methods and in the ipsilateral somatosensory cortex in humans. Furthermore, lateral inhibition has been implicated in models of sensory aging. However, our study is the first to show these temporospatial signal fluctuations directly in human contralateral cortex upon stimulation of individual digits using sub-millimeter BOLD and VASO fMRI. In this context, our findings might offer new windows into the investigations of the potential neuronal excitation-inhibition mechanism in a center-surround architecture in the human somatosensory cortex using layer-specific CBV and BOLD measurements.