Reliable 3D mapping of ocular dominance columns in humans using GE-EPI fMRI at 7 T

Since the discovery of the BOLD effect, detection of ocular dominance columns (ODCs) in primary visual cortex (V1) served as a benchmark for high-precision functional magnetic resonance imaging (fMRI) (Menon et al., 1997; Dechent and Frahm 2000; Cheng et al., 2001; Yacoub et al., 2007). Although gradient-echo (GE) echo-planar imaging (EPI) is often used at lower field strengths, the applicability for high-resolution fMRI at higher field strengths is still under debate because of its inherent sensitivity to large draining veins (Polimeni et al., 2010). To counteract the loss of specificity, it was recently suggested to only sample far away from the pial surface when using GE-EPI (Nasr et al., 2016; Polimeni et al., 2017). Here, we assessed whether differential ocular dominance responses can be resolved using GE-EPI with different isotropic resolutions (0.8 mm and 1.0 mm) and how the corresponding BOLD signal is distributed across the cortex

Perceived and mentally rotated contents are differentially represented in cortical depth of V1

Primary visual cortex (V1) in humans is known to represent both veridically perceived external input and internally-generated contents underlying imagery and mental rotation. However, it is unknown how the brain keeps these contents separate thus avoiding a mixture of the perceived and the imagined which could lead to potentially detrimental consequences. Inspired by neuroanatomical studies showing that feedforward and feedback connections in V1 terminate in different cortical layers, we hypothesized that this anatomical compartmentalization underlies functional segregation of external and internally-generated visual contents, respectively. We used high-resolution layer-specific fMRI to test this hypothesis in a mental rotation task. We found that rotated contents were predominant at outer cortical depth bins (i.e. superficial and deep). At the same time perceived contents were represented stronger at the middle cortical bin. These results identify how through cortical depth compartmentalization V1 functionally segregates rather than confuses external from internally-generated visual contents. These results indicate that feedforward and feedback manifest in distinct subdivisions of the early visual cortex, thereby reflecting a general strategy for implementing multiple cognitive functions within a single brain region

High resolution quantitative and functional MRI indicate lower myelination of thin and thick stripes in human secondary visual cortex

The characterization of cortical myelination is essential for the study of structure-function relationships in the human brain. However, knowledge about cortical myelination is largely based on post mortem histology, which generally renders direct comparison to function impossible. The repeating pattern of pale-thin-pale-thick stripes of cytochrome oxidase (CO) activity in the primate secondary visual cortex (V2) is a prominent columnar system which is known to be differentiable by myelin content as well. However, depending on the applied histological method, higher myelination in both thin/thick and pale stripes were found, respectively. We used quantitative magnetic resonance imaging (qMRI) in conjunction with functional magnetic resonance imaging (fMRI) at ultra-high field strength (7T) to localize and study myelination of stripes in several humans at sub-millimeter resolution in vivo. Thin and thick stripes were functionally localized by exploiting their sensitivity to color and binocular disparity, respectively. Resulting functional activation maps showed robust stripe patterns in V2 which enabled further comparison of quantitative relaxation parameters between stripe types. Thereby, we found lower longitudinal relaxation rates (R1) of thin and thick stripes compared to surrounding gray matter in the order of 1-2%, indicating higher myelination of pale stripes. No differences for effective transverse relaxation rates (R2*) were found. The study demonstrates the feasibility to investigate structure-function relationships in living humans within one cortical area at the level of columnar systems using qMRI

Reliable mapping of columnar structures in early visual cortex using GE-EPI at 7 T

Despite GE-EPI being the method of choice in most fMRI applications, their use in high-resolution acquisitions is questioned due to their inherent sensitivity to large draining veins [1]. To examine the effective spatial resolution limits of GE-EPI, it is necessary to measure physiological structures in the human brain with known spatial extent such as ocular dominance columns (ODCs) in V1 or the stripe architecture (thin, thick, pale) in V2. We show that ODCs and thin stripes can be reliably detected using GE-EPI at 7 T