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Abstract:
Introduction
The primary visual cortex of humans contains patches of neurons responding preferentially to stimulation of one eye (ocular dominance columns) (Adams et al., 2007). The majority of previous fMRI studies reporting eye-specific activity in V1 used magnetic field strengths of 4 T and higher (Cheng et al., 2001; Yacoub et al., 2007; Nasr et al., 2016). However, there have been reports of reliable eye-selective activations at 3 T (Haynes et al., 2005). Here we present preliminary results on the ability to detect eye-selective V1 activity using high-resolution fMRI at 3 T and 9.4 T.
Methods
BOLD signal of one healthy adult volunteer was measured at 3 T and 9.4 T using 2D GE-EPI (3 T: 1.5mm isotropic resolution, TR/TE/matrix/GRAPPA = 1720/30/128x128x24/R=3, 873 volumes; 9.4 T: 0.8mm isotropic resolution TR / TE / matrix / GRAPPA=2 s / 22 ms /230×230×40 / R=5, FLEET autocalibration (Polimeni et al., 2016), 900 volumes). Each eye was stimulated separately with a checkerboard flickering at 2 Hz for 18 s with 18 s breaks, viewed through a prism stereo display (Schurger, 2009). Structural scans (1mm3 for 3 T and 0.6 mm3 for 9.4 T) were acquired in each session for cortical surface reconstruction.
All analyses were performed using FreeSurfer 6.0 and FS-FAST (Fischl, 2012). Functional data were motion-corrected, co-registered to the anatomy using boundary-based registration (Greve and Fischl, 2009) with 6 DOF for 3 T and 9 DOF locally constrained to V1 for 9.4 T, and analyzed using voxel-wise GLM with left eye, right eye and baseline regressors. Surface-based prediction of V1 location (Hinds et al., 2008) was used to label volume voxels belonging to V1 gray matter, and z-statistics of the contrast “left eye vs. right eye” were extracted from those voxels.
Results
We observed a more than two-fold increase in the percentage of eye-selective voxels and in run-to-run correlation of eye preference at ultrahigh field.
Conclusion
Increase in spatial resolution and improved BOLD point spread function at 9.4T allows for better detection of eye-selective signal related to ocular dominance columns.
