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Comparative Study
, 89 (5), 563-74

Adaptive Optics Scanning Laser Ophthalmoscope-Based Microperimetry

Affiliations
Comparative Study

Adaptive Optics Scanning Laser Ophthalmoscope-Based Microperimetry

William S Tuten et al. Optom Vis Sci.

Abstract

Purpose: To develop and test the application of an adaptive optics scanning laser ophthalmoscope (AOSLO) with eye tracking for high-resolution microperimetric testing.

Methods: An AOSLO was used to conduct simultaneous high-resolution retinal imaging and visual function testing in six normal subjects. Visual sensitivity was measured at test locations between the fovea and 5.0° eccentricity via an increment threshold approach using a 40-trial, yes-no adaptive Bayesian staircase procedure (QUEST). A high-speed eye tracking algorithm enabled real-time video stabilization and the delivery of diffraction-limited Goldmann I-sized stimuli (diameter = 6.5 arc min = ∼32 μm; λ = 680 nm) to targeted retinal loci for 200 ms. Test locations were selected either manually by the examiner or automatically using Fourier-based image registration. Cone spacing was assessed at each test location and sensitivity was plotted against retinal eccentricity. Finally, a 4.2 arc min stimulus was used to probe the angioscotoma associated with a blood vessel located at 2.5° eccentricity.

Results: Visual sensitivity decreases with eccentricity at a rate of -1.32 dB/deg (R = 0.60). The vertical and horizontal errors of the targeted stimulus delivery algorithm averaged 0.81 and 0.89 arc min (∼4 μm), respectively. Based on a predetermined exclusion criterion, the stimulus was successfully delivered to its targeted location in 90.1% of all trials. Automated recovery of test locations afforded the repeat testing of the same set of cones over a period of 3 months. Thresholds measured over a parafoveal blood vessel were 1.96 times higher (p < 0.05; one-tailed t-test) than those measured in directly adjacent retina.

Conclusions: AOSLO-based microperimetry has the potential to test visual sensitivity with fine retinotopic precision. Automated recovery of previously tested locations allows these measures to be tracked longitudinally. This approach can be implemented by researchers interested in establishing the functional correlates of photoreceptor mosaic structure in patients with retinal disease.

Figures

Figure 1
Figure 1
Microperimetric testing locations for Subject 3 plotted on an AOSLO-generated retinal image montage. Manually-selected test locations for AOSLO-based microperimetry are denoted by the white circles, the size of which represents the size of the perimetric stimulus on the retina (Goldmann I = 6.5 arcmin = ~32 μm). The subject’s preferred retinal locus of fixation is marked by the black cross.
Figure 2
Figure 2
Schematic illustrating automated retrieval of retinal loci previously tested with AOSLO-based microperimetry. (A) The reference frame for real-time video stabilization from a previous testing session with the retinal location (xo, yo) targeted for sensitivity testing indicated by the white “x”. (B) The reference frame from a subsequent testing session. The reference frame in (B) is registered to the frame in (A), and the shift (Δx, Δy) between the two images is applied to the target x- and y-coordinates of the original frame (black “x”) to retrieve the previously tested retinal location (white “x”) automatically.
Figure 3
Figure 3
Stimulus delivery contour plots for AOSLO-based microperimetry. Contour lines delineate the proportion of trials that a given pixel was stimulated during a psychophysical task (n = 60 trials). Stimulus delivery plots are shown for conditions with (left) and without (right) eye tracking at two eccentricities (top: 0.5°; bottom: 1. 5°).
Figure 4
Figure 4
Automated stimulus location recovery in Subject 3. (Left) Highlighted regions corresponding to individual reference frames used to collect stabilized retinal videos at ~2.5° retinal eccentricity. The original reference frame is outlined in green and the targeted retinal location for stimulus delivery is marked by the green “x”. To retrieve the original test location, subsequent reference frames (red, yellow, blue) are registered against the original, and the shift is used to define the targeted retinal locus in the x-y coordinates of the new reference frame. The subject’s preferred retinal locus of fixation is represented by the red cross. (Right) The Goldmann I-sized white circles are plotted at the average stimulus delivery location for a 40-trial psychophysical task performed at baseline, baseline + 2 days, and baseline + 3 months.
Figure 5
Figure 5
Results from sensitivity testing on and around a parafoveal blood vessel. (A) Fundus photograph with box indicating the blood vessel targeted for AOSLO-based microperimetry (eccentricity ~2.5°); (B) AOSLO image with perimetry test locations indicated; (C) Average sensitivity plotted as a function of stimulus location. (Note: The size of the white circles marking test location represent the size of the test stimulus on the retina.)
Figure 6
Figure 6
Visual sensitivity as a function of retinal eccentricity and number of cones stimulated. (A) Sensitivity values are plotted in dB as a function of retinal eccentricity, with the corresponding threshold in log Trolands plotted on the secondary y-axis. (B) Sensitivity is plotted as a function of the number of cones sampling the stimulus, which is estimated from measures of cone spacing.
Figure 7
Figure 7
Retinal light distribution of common perimetric stimuli. These distributions, which represent the spread of light on the retina of 4 perimetric stimuli, were generated by convolution of the stimulus with the point-spread function (PSF) of the diffraction-limited condition (top) and with that of two high-order aberration-uncorrected subjects (middle; bottom). PSFs were calculated for 680 nm light over a 6 mm pupil. Contour lines indicate normalized light intensities for each pixel. The innermost line (red) encircles the photoreceptors which will be stimulated by light at 99% of the intended intensity. Stimuli, from left-to-right: 2.5 arcmin circle; Goldman I; Goldmann II; Goldmann III. The retinal eccentricity is approximately 4 degrees.

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