Test–retest of a phoria adaptation stimulus-induced functional MRI experiment

Abstract

This study was designed to identify the neural substrates activated during a phoria adaptation task using functional magnetic resonance imaging (MRI) in young adults with normal binocular vision and to test the repeatability of the fMRI measurements for this protocol. The phoria adaptation task consisted of a block protocol of 90 seconds of near visual crossed fixation followed by 90 seconds of far visual uncrossed fixation, repeated three times; the data were collected during two different experimental sessions. Results showed that the oculomotor vermis, cuneus, and primary visual cortex had the greatest functional activity within the regions of interest studied when stimulated by the phoria adaptation task. The oculomotor vermis functional activity had an intraclass correlation coefficient (ICC) of 0.3, whereas the bilateral cuneus and primary visual cortex had good ICC results of greater than 0.6. These results suggest that the sustained visual fixation task described within this study reliably activates the neural substrates of phoria adaptation. This protocol establishes a methodology that can be used in future longitudinal studies investigating therapeutic interventions that may modify phoria adaptation.

Underlying neurological mechanisms associated with symptomatic convergence insufficiency

“…The MNI coordinate of the most significant T value was extracted and a 5 mm radius sphere consisting of 81 voxels was created around this peak voxel coordinate using the MarsBaR MATLAB toolbox as part of SPM 77 . Our prior studies showed that the 5 mm radius sphere produced the highest intraclass correlation coefficients within our repeatability studies 67 , 68 and has been used in a prior oculomotor fMRI investigation 78 . Additionally, the mean beta weights from 3 and 7 mm radius spheres consisting of 19 and 179 voxels, respectively were also computed for comparison to the 5 mm radius sphere results.…”

“…The fast and slow fusional vergence systems were studied using functional imaging employing established methods published by our group using these exact protocols 67 , 68 . The visual stimuli are different compared to the behavioral studied performed in the laboratory because of the physical constraints of the magnet.…”

“…Within the same imaging session, the slow fusional system was studied using a sustained fixation task to adapt the phoria. A single square centered in the middle of the screen was presented to induce the perception of far distance (far disparity) viewed for 90 s followed by two eccentric squares that required the participant to converge and fuse, to stimulate near disparity viewed for 90 s, repeated three times for a total of 270 volumes or 540 s of image acquisition as shown in our prior study 68 . The time to adapt the phoria for 90 s was choosen because that was the amount of time observed from our behavioral studies where the largest change in phoria occurred given the exponential decay of the system.…”

“…The change in disparity was 6∆ base-out, which was analogous to the 6∆ base-out prism used in the behavioral experiment. Both the fast and slow fusional vergence fMRI experimental protocols were found to be repeatable by studying BNC participants to determine how well the visual stimuli protocol elicited reliable activation within the vergence neural substrates on different days 67 , 68 .…”

Functional magnetic resonance imaging activation for different vergence eye movement subtypes

“…Functional magnetic resonance imaging (fMRI) has been used to assess cortical activity associated with convergence. 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 However, prior research has not investigated the neural substrates associated with convergence eye movements using fMRI under conditions where the stimulus to vergence is varied such that one vergence subtype is primarily expected to change. For example, previous studies have employed light‐emitting diodes (LEDs) placed at different distances as vergence stimuli.…”