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Daria Ivanchenko, Frank Schaeffel, Ziad Hafed; Microvergence fixational eye movements. Invest. Ophthalmol. Vis. Sci. 2019;60(9):520.
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While microsaccades during fixation are highly correlated in both eyes, the literature is less coherent regarding drifts. If drifts show uncorrelated components, the positions of the images of a fixated target on the two retinae are no longer (mirror)-symmetrical. We studied inter-ocular correlations of drifts when subjects fixated a stationary target.
We built a high-resolution binocular eye tracker using two USB3 infrared monochrome cameras having 640x480 pixel resolution, and sampling images at 400 Hz (TheImagingSource, Model DMK33UX174). We merged the output of the two cameras, using custom software, into one video buffer and tracked both pupil centers and first Purkinje image centers, generated by a field of IR LEDs illuminating the eyes. Spatial resolution was < 2 arcmin. After an automated calibration procedure, fixational eye movements were recorded in 8 naive subjects fixating a small yellow target (square of 4x4 arcmin) on a black computer screen at 50 cm distance. A chin rest was used to stabilize head position.
During inter-microsaccadic fixation, the eyes often moved in opposite directions in the horizontal dimension (i.e. vergence). We quantified such “microvergence” eye movements by computing correlations of eye position between the two eyes during 1-sec epochs, and we made sure to remove microsaccades. We analyzed 400 such epochs per subject. Almost 50% of 1-sec epochs had significant negative correlation in horizontal eye position between the two eyes. On the other hand, only approximately 25% of epochs had negative correlation in vertical position between the two eyes. We also found that the occurrence of microvergence was dependent on microsaccade frequency: the more microsaccades a subject had, the less microvergence eye movements he/she made. Moreover, 500 ms before and after microsaccades, ocular drifts were more positively correlated in the horizontal direction between the two eyes. Thus, microvergence was most likely at long times away from microsaccades.
We found clear evidence for microvergence during microsaccade-free fixation: in the horizontal plane, the eyes drifted in opposite directions. Microvergence also depended on microsaccade frequency. A speculation could be that microvergence eye movements act to improve stereo acuity. We also believe that there are implications on how visual processing proceeds when the two eyes move in opposite direction.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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