Eye selectivity peaks in middle layers of human visual cortex: evidence from 7T fMRI data

Poster No:

W500 

Submission Type:

Abstract Submission 

Authors:

Kimberly Weldon1, Cheryl Olman1

Institutions:

1University of Minnesota, Minneapolis, MN

Introduction:

The improving technology of high-resolution functional magnetic resonance imaging (fMRI) enables investigations into the laminar specificity of the hemodynamic response in humans (e.g., De Martino, 2013; Kok, 2016; Muckli, 2015; Olman, 2012). In this study, participants freely viewed a dichoptically-presented animated movie, which reliably produced robust neural activation. By utilizing a conservative analysis pipeline that targeted cortical areas with both high registration accuracy and minimal contributions from pial veins, we present evidence for eye selectivity in the middle of the cortical depth in human visual cortex.

Methods:

Data acquisition
Six healthy adult subjects were scanned with a custom-made 4-channel transmit 32-channel receive head coil on a 7T Siemens scanner equipped with a SC-72 body gradient set (70 mT/m; 200 T/m/s). Functional data were acquired with 0.8 mm isotropic T2*-weighted EPI images (matrix size = 168 x 200; multiband = 2; TR = 2 s). Data were acquired with a parallel imaging acceleration factor of 3 (6/8 Partial Fourier; echo-spacing = 1.0 ms). T1-weighted MP-RAGE anatomical volumes with 0.8 mm isotropic resolution were acquired for each subject on a 3T system in the same facility.

Experimental design
Subjects were allowed to freely view dichoptically-presented clips of an animated movie (12s stimulus on/12s stimulus off per cycle, 10.5 cycles per scan). Each participant completed 4-8 scans during a scanning session.

Data pre-processing
Functional data were motion-compensated, corrected for distortions caused by B0 inhomogeneities, and corrected for known gradient non-linearities using AFNI's 3dvolreg tool, AFNI's 3dQwarp tool, and the grad_unwarp toolbox (https://github.com/Washington-University/gradunwarp) respectively. Functional data were aligned to anatomical scans with AFNI's 3DAllineate tool. Cortical grey matter (GM) segmentation was calculated in Freesurfer.

Data analysis
Dichoptic movie-watching produces robust activation throughout visual cortex. We employed a multi-faceted approach to node selection for the purposes of performing depth-dependent analyses. First, we targeted areas where gray matter from functional data was well-aligned with GM that was independently defined in the anatomical volumes. Then, on the assumption that neural activation is selectively present in GM, we targeted areas where activation was present throughout the cortical depth. We further restricted our analyses to areas were at least 1 mm away from identifiable pial surface vessels, which are known to contaminate the BOLD signal.

Results:

Following these careful selection criteria, we show significant eye selectivity in middle cortical layers that was absent in deep or superficial layers.

Conclusions:

T2*-weighted functional MRI has a known bias toward pial responses, and spatial specificity can be degraded by large veins. However, these limitations can be circumvented by appropriate experiment design and analysis, providing functional imaging signals with sub-millimeter resolution both tangential to the cortical surface and through the cortical depth.

Imaging Methods:

BOLD fMRI 1

Perception and Attention:

Perception: Visual 2

Keywords:

Cortical Columns
Cortical Layers
Data Registration
FUNCTIONAL MRI
HIGH FIELD MR
Perception

1|2Indicates the priority used for review

My abstract is being submitted as a Software Demonstration.

No

Please indicate below if your study was a "resting state" or "task-activation” study.

Task-activation

Healthy subjects only or patients (note that patient studies may also involve healthy subjects):

Healthy subjects

Are you Internal Review Board (IRB) certified? Please note: Failure to have IRB, if applicable will lead to automatic rejection of abstract.

Yes

Was any human subjects research approved by the relevant Institutional Review Board or ethics panel? NOTE: Any human subjects studies without IRB approval will be automatically rejected.

Yes

Was any animal research approved by the relevant IACUC or other animal research panel? NOTE: Any animal studies without IACUC approval will be automatically rejected.

Not applicable

Please indicate which methods were used in your research:

Functional MRI

For human MRI, what field strength scanner do you use?

7T

Which processing packages did you use for your study?

AFNI
Free Surfer

Provide references using author date format

De Martino, F. (2013), 'Cortical Depth Dependent Functional Responses in Humans at 7T: Improved specificity with 3D GRASE', PLOS ONE, vol. 8, no. 3, e60514.

Kok, P. (2016), 'Selective Activation of the Deep Layers of the Human Primary Visual Cortex by Top-Down Feedback', Current Biology, vol. 26, no. 3, pp. 371–376.

Muckli, L. (2015), 'Contextual Feedback to Superficial Layers of V1', Current Biology, vol. 25, no. 20, pp. 2690–2695.

Olman, C. (2012), 'Layer-Specific fMRI Reflects Different Neuronal Computations at Different Depths in Human V1', PLOS ONE, vol. 7, e32536.