Macular degeneration (MD) affects the central portion of the retina around the fovea, resulting in a scotoma. When the binocular scotoma obscures an object of interest, individuals with MD miss information and are often unaware of it, which creates difficulties in the performance of daily-living tasks. We propose a method to map precisely the binocular scotoma.
Seven individuals with MD (mean age = 75; four with binocular scotomas) and 4 age-matched controls (mean age = 68) participated in our study. We measured the peripheral fixation locus, the location of the foveal pit and the extent of the monocular scotoma in each eye using an OCT/SLO. From these monocular data, we estimated the area of the binocular scotoma using the algorithm from Ghahghaei and Walker (2016), by aligning the monocular data on the foveal pits of each eye. To compare how this estimate compared with the binocular scotoma under more realistic viewing conditions, we then mapped the edges of the binocular scotoma using an eye tracker. Participants fixated the screen center while we flashed dots, and pressed a button whenever they detected one. The dots were first presented on a coarse grid, and then we manually selected points to examine the edges of the scotoma in finer detail.
A kernel density plot of fixation distribution (Crossland, Sims, Galbraith & Rubin, 2004) allowed us to determine that all participants used mostly a single (foveal or peripheral) fixation locus during the fixation interval. Critically, monocular scotoma maps obtained with the eye tracker were very similar to those obtained with monocular SLO perimetry, suggesting that individuals use the same preferred retinal locus under both imaging conditions. More interestingly, the binocular scotoma maps obtained with the eye tracker were highly similar to the binocular maps estimated from monocular SLO perimetry using the Ghahghaei & Walker algorithm, yielding an appropriate method to assess residual functional retina in binocular vision.
Determining the size and location of the binocular scotoma with respect to the fixation locus is key for assessing and training an appropriate strategy for eye movements in age-related macular degeneration. Our results show that our eye-tracker method offers a reliable and sensitive tool for measuring both monocular and binocular scotomata, without the need of an OCT/SLO.