Functional activity within the frontal eye fields, posterior parietal cortex, and cerebellar vermis significantly correlates to symmetrical vergence peak velocity: an ROI-based, fMRI study of vergence training

Abstract

Convergence insufficiency (CI) is a prevalent binocular vision disorder with symptoms that include double/blurred vision, eyestrain, and headaches when engaged in reading or other near work. Randomized clinical trials support that Office-Based Vergence and Accommodative Therapy with home reinforcement leads to a sustained reduction in patient symptoms. However, the underlying neurophysiological basis for treatment is unknown. Functional activity and vergence eye movements were quantified from seven binocularly normal controls (BNC) and four CI patients before and after 18 h of vergence training. An fMRI conventional block design of sustained fixation vs. vergence eye movements stimulated activity in the frontal eye fields (FEF), the posterior parietal cortex (PPC), and the cerebellar vermis (CV). Comparing the CI patients' baseline measurements to the post-vergence training data sets with a paired t-test revealed the following: (1) the percent change in the BOLD signal in the FEF, PPC, and CV significantly increased (p < 0.02), (2) the peak velocity from 4° symmetrical convergence step responses increased (p < 0.01) and (3) patient symptoms assessed using the CI Symptom Survey (CISS) improved (p < 0.05). CI patient measurements after vergence training were more similar to levels observed within BNC. A regression analysis revealed the peak velocity from BNC and CI subjects before and after vergence training was significantly correlated to the percent BOLD signal change within the FEF, PPC, and CV (r = 0.6; p < 0.05). Results have clinical implications for understanding the behavioral and neurophysiological changes after vergence training in patients with CI, which may lead to the sustained reduction in visual symptoms.

Keywords: Convergence Insufficiency Symptom Survey; cerebellar vermis; convergence insufficiency; frontal eye fields; posterior parietal cortex; vergence; vergence training; vision therapy.

Figure 1

(A) Schematic of the LED targets showing visual stimuli. (B) The experimental block design was composed of sustained fixation (denoted as Fixation) and vergence eye movements (denoted as Eye Mvt). This experimental block design modulated functional activity of the BOLD signal within the vergence neural substrates.

Figure 2

Series of axial images showing the average masks with one standard deviation used within the analysis. The cerebellar vermis (yellow), Broca-Right (red), Broca-Left (dark blue), Posterior Parietal Cortex–Right (black), Posterior Parietal Cortex–Left (pink), Frontal Eye Field-Right (green), and Frontal Eye Field–Left (medium blue) are shown. The masks did not overlap. Broca's Region served as a control ROI to study the variability within a non-stimulated ROI.

Figure 3

Convergence eye movement responses stimulated from 4° symmetrical convergence step stimuli from a BNC (A), a CI subject before vergence training (B), and the same CI subject after 18 h of vergence training (C). Each colored trace is a single eye movement response denoted in degrees of rotation as a function of time (s).

Figure 4

BOLD time series from the FEF (red), PPC (green), Broca's Region (Purple), and CV (blue) from a BNC (A), one CI before vergence training (B), and the same CI after vergence training (C). The BOLD signal is plotted as the percent signal change as a function of volume. The time of repetition was 2 s. Hence, the time series lasts for a duration of 140 s.

Figure 5

Group-level analyses showing the average with one standard deviation of convergence peak velocities (°/s) evoked from 4° symmetrical convergence step stimuli (A) for CI subjects before vergence training (blue bar), CI subjects after vergence training (red bar), and BNC subjects (green bar). Group-level averages with one standard deviation for the BOLD percent signal change for the FEF-L, FEF-R, PPC-L, PPC-R, Cerebellar vermis, Broca-L, and Broca-R (B).

Figure 6

Linear regression analysis of the convergence peak velocity (°/s) as a function of the percent BOLD signal change for the FEF (A), PPC (B), Broca's region (C), and Cerebellar Vermis (D). The left and right percent BOLD signal changes were averaged. The blue diamonds are from the CI subjects before vergence training, the red diamonds are the same CI subjects after vergence training, and the green diamonds are from the BNC subjects. The dashed black line is the linear regression. Significant correlation is observed between the convergence peak velocity from 4° symmetrical convergence steps and the BOLD percent signal change in the FEF, PPC, and Cerebellar Vermis.