Figure 1C illustrates the manual coarse-to-fine grid when probing the binocular visual field for participant B3. Each session is represented in one panel; the dots are plotted cumulatively in each panel. Across sessions, the mapping of the binocular scotoma improves, and within a few sessions, we observe a precise map of binocular field loss for this participant.
Figure 4 represents the binocular scotoma of all B participants with respect to their fixation stability, represented by a kernel density plot of fixation distribution (
Crossland, Sims, Galbraith, & Rubin, 2004; see
Figure 5). The bounding area for the binocular scotoma is estimated by a polygon that envelops the set of missed points (using MATLAB's alphaShape function) with a scotoma area of 3.73, 26.43, 108.69, 281.99, 159.43, and 430.40 degrees
2 for participants B2, B4, B3, B1, B5, and B6, respectively. We calculated the rate of false alarms in the binocular scotoma map by considering any button press with a latency shorter than 325 ms following target onset (based on manual reaction times for elderly people in simple detection tasks;
Der & Deary, 2006;
Nebes, 1978), as well as any that occurred in the intertrial interval. On average, the rate of false alarms was 1.87% (SD = 1.57) of total trials, indicating that participants very rarely responded before seeing the flash. Alongside the binocular scotoma maps for each participant in
Figure 4 are the monocular SLO perimetry maps for the left and right eyes, respectively. For participants B5 and B6, fixation in one or both eyes was not stable enough to do SLO microperimetry, so the panels show just the retinal image with the fixation locus superimposed. Aligning the monocular SLO maps on the respective fovea of each eye (yellow circle) provides an estimate of the overlap of the two eyes’ scotomata. Importantly, the binocular scotoma estimate for all participants with bilateral field loss is consistent with this overlap region, no matter whether the two eyes’ scotomata overlap minimally (B2 and B3) or substantially (B1, B4, B5, and B6). Therefore, our method for binocular scotoma mapping was highly effective for a large range of sizes and shapes of binocular scotoma from small (B2 and B4) to extensive (B1, B5, and B6). It is also noteworthy that the eye tracker enabled us to map the binocular scotoma for two participants whose monocular scotoma, in at least one eye, could not be estimated with the SLO. Indeed, for B5, we could only get the monocular map for the right eye and for B6, we could not locate intact retinal locations in the SLO. Unsurprisingly, individuals with a monocular scotoma and control participants had no measurable binocular scotoma under conditions of binocular viewing.