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Kavitha Ratnam, Norick R Bowers, Austin Roorda; The role of fixational eye movements in maintaining a stable fixation locus. Invest. Ophthalmol. Vis. Sci. 2018;59(9):656.
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© ARVO (1962-2015); The Authors (2016-present)
To measure cone-sampling-limited acuity at sub-foveal intervals and to measure the functional role of microsaccades (MS) when presented with at-threshold stimuli at varying sub-foveal distances from the preferred fixation locus (PRL).
An adaptive optics scanning laser ophthalmoscope (AOSLO) was used for retinal imaging, eye tracking and stimulus delivery. Subjects were 3 males and 3 females, ages 25-29 years, with no known visual issues. The PRL was the mean retinal location used to fixate a 5-arcmin target presented for 10 sec. Acuity was measured using a retinally-stabilized tumbling-E stimulus presented at 0 to 10-arcmin intervals horizontally and vertically from the PRL. A QUEST adaptive staircase procedure was used to find the 72%-correct performance threshold on a 4-alternative-forced-choice task. The smallest at-threshold stimulus from the acuity task was used for the foveal relocation experiment, in which the ‘E’ was initially presented at targeted retinal locations 0 to 15 arcmin from the PRL but thereafter allowed to move according to subjects’ fixational eye movements (FEM). FEM statistics were subsequently analyzed to assess if FEM frequency, amplitude, and orientation were dependent on initial stimulus eccentricity from the PRL.
Threshold acuity values across tested locations were excellent but were all within 1 SD of each other (MAR = 0.55 +/- 0.18 arcmin (SD)), suggesting that acuity for an ‘E’ stimulus was relatively homogenous within 10 arcmin from the PRL. During the relocation task, 4 subjects showed increased MS frequency and amplitude with greater stimulus distance from the PRL, with MS orientation biased in a direction that placed the image onto the PRL. The remaining 2 subjects, who exhibited MS in fewer than 20% of trials, showed drift patterns independent of stimulus start location.
AOSLO-mediated acuity was excellent and remained relatively flat across a zone spanning a 10-arcmin radius from fixation. Despite the homogeneity in visual performance, the majority of subjects employed MS to relocate peripherally-presented images closer to the original PRL, even when stimuli were initially presented as close as 5-arcmin from the fixation locus. These findings suggest that MS serve to relocate stimuli to a precise location during fixation, even when visual performance is similar across a larger foveal region.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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