Task-modulated coactivation of vergence neural substrates

Abstract

While functional magnetic resonance imaging (fMRI) has identified which regions of interests (ROIs) are functionally active during a vergence movement (inward or outward eye rotation), task-modulated coactivation between ROIs is less understood. This study tested the following hypotheses: (1) significant task-modulated coactivation would be observed between the frontal eye fields (FEFs), the posterior parietal cortex (PPC), and the cerebellar vermis (CV); (2) significantly more functional activity and task-modulated coactivation would be observed in binocularly normal controls (BNCs) compared with convergence insufficiency (CI) subjects; and (3) after vergence training, the functional activity and task-modulated coactivation would increase in CIs compared with their baseline measurements. A block design of sustained fixation versus vergence eye movements stimulated activity in the FEFs, PPC, and CV. fMRI data from four CI subjects before and after vergence training were compared with seven BNCs. Functional activity was assessed using the blood oxygenation level dependent (BOLD) percent signal change. Task-modulated coactivation was assessed using an ROI-based task-modulated coactivation analysis that revealed significant correlation between the FEF, PPC, and CV ROIs. Prior to vergence training, the CIs had a reduced BOLD percent signal change compared with BNCs for the CV (p<0.05), FEFs, and PPC (p<0.01). The BOLD percent signal change increased within the CV, FEF, and PPC ROIs (p<0.001) as did the task-modulated coactivation between the FEFs and CV as well as the PPC and CV (p<0.05) when comparing the CI pre- and post-training datasets. Results from the Convergence Insufficiency Symptom Survey were correlated to the percent BOLD signal change from the FEFs and CV (p<0.05).

Keywords: Convergence Insufficiency Symptom Survey; cerebellar vermis; convergence insufficiency; frontal eye fields; posterior parietal cortex; task-modulated coactivation; vergence.

FIG. 1.

(A) Schematic of the LED targets used to stimulate vergence eye movement responses. (B) The experimental block design was composed of sustained fixation (denoted as Fixation) and vergence eye movements (denoted as Eye Mvt) which modulated functional activity of the BOLD signal within the vergence neural substrates. BOLD, bold oxygenation level dependent; LED, light emitting diode.

FIG. 2.

Representation of average masks with one standard deviation for the frontal eye fields, left and right (FEF-L [brown] and FEF-R [red]), posterior parietal cortex, left and right (PPC-L [dark green] and PPC-R [yellow]), Broca's region, left and right (BRC-L [light blue] and BRC-R [light green]), and cerebellar vermis (CV [dark blue]) shown using three views within a three-dimensional model brain. The centroid of each mask is denoted as left (positive) or right (negative), anterior (positive) or posterior (negative), and superior (positive) or inferior (negative).

FIG. 3.

(A) Averaged time series for the FEF-L (red) and PPC-L (green) ROIs from one typical BNC (left) and one CI before (middle) and the same CI after vergence training (right). (B) Example of the task-modulated coactivation analysis between FEF-L (red circle), PPC-L (green circle), CV (blue circle), and Broca-L (purple circle). The Pearson correlation coefficient (r-value) and p-value are shown where the thickness of the line represents the significance. The time series from Broca-L was not significantly correlated to FEF-L, PPC-L, or CV in any of the subjects studied. (C) Averaged time series for Broca-L (purple) and CV (blue). BNC, binocularly normal control; CI, convergence insufficiency; ROIs, regions of interests.

FIG. 4.

Percent BOLD signal change for FEF-L (left), FEF-R (right), PPC-L, PPC-R, CV, Broca-L, and Broca-R for CI subjects before vergence training (blue), CI subjects after vergence training (red), and the BNC subjects (green). Bar plots are the average plus one standard deviation. Color images available online at www.liebertpub.com/brain

FIG. 5.

Pair-wise correlation analyses. Group-level correlation values are between FEFs (left and right), PPC (left and right), CV, and Broca's region (left and right) for BNCs (A); CI subjects before vergence training (B); and the same CI subjects after vergence training (C). Group-level statistical comparisons reporting p-values are shown for BNC versus CI before vergence training (D), BNC versus CI after vergence training (E), and CI before versus CI after vergence training (F).

FIG. 6.

Percent BOLD signal change versus CISS scores for CI groups (before and after vergence training) for the FEF-L (A), FEF-R (B), and cerebellar vermis (CV) (C). Linear regression analyses for the difference in the percent BOLD signal change (measurements after vergence training minus measurements before vergence training) as a function of the change in the CISS score (after minus before vergence training) for the FEF-L (D), FEF-R (E), and cerebellum (F). CISS, Convergence Insufficiency Symptom Survey. Color images available online at www.liebertpub.com/brain