Mapping the binocular scotoma in macular degeneration

Abstract

When the scotoma is binocular in macular degeneration (MD), it often obscures objects of interest, causing individuals to miss information. To map the binocular scotoma as precisely as current methods that map the monocular scotoma, we propose an iterative eye-tracker method. Study participants included nine individuals with MD and four age-matched controls. We measured the extent of the monocular scotomata using a scanning laser ophthalmoscope/optical coherence tomography (SLO/OCT). Then, we precisely mapped monocular and binocular scotomata with an eye tracker, while fixation was monitored. Participants responded whenever they detected briefly flashed dots, which were first presented on a coarse grid, and then at manually selected points to refine the shape and edges of the scotoma. Monocular scotomata measured in the SLO and eye tracker are highly similar, validating the eye-tracking method for scotoma mapping. Moreover, all participants used clustered fixation loci corresponding to their dominant preferred fixation locus. Critically, for individuals with binocular scotomata, the binocular map from the eye tracker was consistent with the overlap of the monocular scotoma profiles from the SLO. Thus, eye-tracker-based perimetry offers a reliable and sensitive tool for measuring both monocular and binocular scotomata, unlike the SLO/OCT that is limited to monocular viewing.

Figure 1.

(A) Time course of scotoma mapping procedure. (B) Illustration of coarse-to-fine grids. (C) Steps in probing the visual field for one participant with central fixation. One panel represents one session and the dots are cumulative. A green dot represents a probed region seen twice. A red dot is a region missed twice. A yellow dot is when the participant had seen it once and missed it once. The white circle represents the position of the PRL for MD participants or fovea for control participants, at the screen center.

Figure 2.

(A) Fundus image of the right eye for a control participant (C4) in the Optos OCT/SLO. The intersection of the green and blue lines represents the foveal pit, obtained through a cross-sectional topography map. This location is represented by a yellow open circle in panel B. The blue crosses represent the fixation loci over a 10 second period with the pink ellipse indicating the BCEA. This fundus image illustrates the observed offset between the anatomical position of the foveal pit and the fixation locus (pink ellipse) for controls in our Optos OCT/SLO. (B) Illustration of the methods used to compute the d’ value between the monocular optic disc maps obtained in the eye tracker (depicted as the gray region) and SLO microperimetry (unseen and seen flash locations depicted by red and green dots), for the same control participant. The yellow open circle marks the location of the foveal pit; the black ellipse (displayed centered on the foveal pit and around the scotoma boundary) represents the BCEA of fixation stability during probe presentation in the eye tracker. The pink ellipse represents the BCEA of fixation stability measured in the SLO, and is plotted at the same relative location to the foveal pit as shown in A, as well as around two unseen SLO flashes. The overlapping black and pink ellipses illustrate two instances of the method used to determine if an SLO point at the edge of the eye tracker map was within the combined fixation error of the SLO and the eye tracker.

Figure 3.

Comparison between the monocular scotoma maps from the eye tracker and SLO microperimetry in one eye of a control participant (C2, first row, left), the affected eye of a participant with a monocular scotoma (M2, first row, right) and both eyes of a participant with binocular MD (B3, second row). The d’ value for each comparison is shown in the boxed text. For each comparison, the eye-tracker maps are on the left and the SLO maps on the right. For the eye tracker map, see legend Figure 1C. For the SLO monocular microperimetry, the red and green dots represent the locations of unseen and seen flashes. The small yellow crosses represent the fixation loci during perimetry. The pink ellipse represents the BCEA during the 10 seconds fixation stability assessment. The yellow open circle marks the location of the foveal pit, obtained through a cross-sectional topography map obtained using the OCT. Note: the SLO images have been flipped vertically to be consistent with the eye-tracker map measured in visual field coordinates.

Figure 4.

First and second columns: Eye tracker map of binocular scotoma of all participants with binocular MD (see legend Figure 1C), estimated as a polygon that envelops the set of missed points, with respect to their PRL. Their fixation stability is represented by a kernel density plot of fixation distribution. Third and fourth columns: SLO microperimetry for the left and right eyes (see legend Figure 3). When fixation stability was too poor to conduct microperimetry, the retinal image during the fixation test is provided (for participants B5 and B6, the retinal damage around the PRL is visible). Note: the SLO images have been flipped vertically to be consistent with the eye-tracker map measured in visual field coordinates.

Figure 5.

Different patterns of fixation stability across the three groups of participants (binocular scotoma in green, monocular scotoma in blue, and control in red). Fixation stability is represented by a kernel density plot of fixation distribution. The left and right columns show a representative participant from each group that has a single PRL and more than one PRL, respectively. Note the zoomed-in scale compared to Figure 4.