Eye-Selective fMRI Activity in Human Primary Visual Cortex: Comparison between 3 T and 9.4 T, and Effects across Cortical Depth
The primary visual cortex of humans contains patches of neurons responding preferentially to stimulation of one eye (the ocular dominance columns). Multiple previous studies attempted to detect their activity using fMRI. The majority of these fMRI studies used magnetic field strengths of 4 T and higher. However, there have been reports of reliable eye-selective activations at 3 T as well. In this study we investigated the possibility of detecting eye-selective V1 activity using high-resolution GE-EPI fMRI at 3 T and sub-millimeter resolution fMRI at ultrahigh 9.4 T magnetic field strengths with acquisition parameters optimized for each field strength. High-resolution fMRI at 9.4 T also allowed us to examine the eye-selectivity responses across the cortical depth, which are expected to be strongest in the middle layers.
We observed a substantial increase in the percentage of eye-selective voxels, as well as a doubling in run-to-run consistency of eye preference at ultrahigh field compared to 3 T. We also found that across cortical depth, eye selectivity increased towards the superficial layers, and that signal contrast increased while noise remained nearly constant towards the surface. The depth-resolved results are consistent with a distortion of spatial specificity of the GE-EPI signal by ascending venules and large draining veins on the cortical surface. The effects of larger vessels cause increasing signal amplitude, but also displacement of the maximum BOLD signal relative to neural activity.
In summary, our results show that increase in spatial resolution, reduced partial volume effects, and improved sensitivity at 9.4 T allow for better detection of eye-selective signals related to ocular dominance columns. However, although ultrahigh field yields higher sensitivity to the ocular dominance signal, GE-EPI still suffers from specificity issues, with a prominent signal contribution at shallow depths from larger cortical vessels.