Purpose
We have recently shown that the stability of fixational eye movements (FEM) was decreased by suppression resulting from strabismic amblyopia. To further investigate the impact of suppression on fixation stability (FS), we measured FEM during periods of binocular rivalry in normal individuals.
Methods
FEM were measured using an infrared video based eyetracker at 500 Hz in 15 normal participants (age: 28±4 yrs). Stimuli were sinusoidal gratings (3.6° diameter, 1.1cpd, central fixation target 0.5° at 40cm). Binocular rivalry was created by dichoptically presenting orthogonal gratings using a haploscope. There were three control conditions: (1) grating to the left eye (LE) and mean luminance to the right eye (RE) (monocular grating); (2) identical gratings to both eyes dichoptically viewed through the haploscope (dichoptic fusion); (3) binocular viewing of a single grating without the haploscope (binocular fusion). Note that the stimulus presented to the LE was the same in every condition whereas the RE stimulus varied across conditions. Each trial took 40 seconds and grating orientation was varied every 4 seconds to minimize adaptation in all conditions. There were 6 trials per condition. Fixational stability (FS) was quantified using bivariate contour ellipse area (BCEA).
Results
The effect of viewing condition (rivalry vs. control conditions) differed significantly between the two eyes (p=0.003) [Fig-1]. This effect involved a significant increase in LE FS for the monocular condition compared to the rivalry condition (p=0.03). However, the FS of the right eye did not differ in any of the dichoptic conditions. Further, the FS in both eyes was significantly better for the binocular fusion condition than the three dichoptic conditions (p=0.0001).
Conclusions
We conclude that the FS in the fixing eye (LE) was significantly improved when the fellow eye was suppressed (monocular grating) compared to when the suppression was alternating (binocular rivalry). Further we found that the stability of FEM was improved with “normal” binocular fusion compared to dichoptic fusion achieved through the haploscope.