Object crowding in age-related macular degeneration

Abstract

Crowding, the phenomenon of impeded object identification due to clutter, is believed to be a key limiting factor of form vision in the peripheral visual field. The present study provides a characterization of object crowding in age-related macular degeneration (AMD) measured at the participants' respective preferred retinal loci with binocular viewing. Crowding was also measured in young and age-matched controls at the same retinal locations, using a fixation-contingent display paradigm to allow unlimited stimulus duration. With objects, the critical spacing of crowding for AMD participants was not substantially different from controls. However, baseline contrast energy thresholds in the noncrowded condition were four times that of the controls. Crowding further exacerbated deficits in contrast sensitivity to three times the normal crowding-induced contrast energy threshold elevation. These findings indicate that contrast-sensitivity deficit is a major limiting factor of object recognition for individuals with AMD, in addition to crowding. Focusing on this more tractable deficit of AMD may lead to more effective remediation and technological assistance.

Figure 1

Locations of PRL (dots) and absolute central scotoma (shaded regions) in the visual field of each eye of the three AMD participants obtained monocularly using the SLO. The preferred eye for each AMD participant is indicated by a surrounding rectangle.

Figure 2

The threshold versus spacing function described in terms of four parameters (E_ceiling, E_floor, s_sat, s_critical), which, in turn, provides two key characterizations of crowding: critical spacing (s_critical) and threshold elevation (E_ceiling/E_floor). In addition, the floor threshold (E_floor) is used to compare peripheral form vision between AMD and normally sighted participants.

Figure 3

Threshold contrast energy as a function of center-to-center spacing. Each plot contains the data for an AMD participant and the two controls tested at the same retinal location as the AMD participant's PRL. The dotted and dashed vertical lines delineate the minimum and mean target–flanker spacings, respectively, when the target and flanker overlap. The solid vertical line specifies half the distance to the PRL (Bouma's scaling factor of 0.5). The dotted horizontal lines indicate the contrast energy of the flanker. The data points on the far y-axis are contrast threshold for single target objects in the absence of flankers (baseline performance). The target eccentricity and size for each AMD participant and the paired controls are specified in the upper right of each plot.

Figure 4

Critical spacings. The data are grouped according to the PRL/target eccentricity (the bracketed numbers) at which the AMD participants and the paired controls were tested. Error bars present bootstrapped 95% confidence intervals, which are generally asymmetric. Compared to the paired controls within each group, AMD participants had numerically smaller critical spacing, but this difference did not reach statistical significance except for S3 versus RC.

Figure 5

Floor threshold contrast energies. The data are grouped according to the PRL and target eccentricity (the bracketed numbers) at which the AMD participants and the paired controls were tested. Error bars present bootstrapped 95% confidence intervals on the fitted floor values, which are generally asymmetric. Floor (i.e., noncrowded) threshold level is significantly worse for the AMD participants at all eccentricities compared to both the elderly and young controls.

Figure 6

Threshold contrast energy elevations. The data are grouped according to the PRL/target eccentricity (the bracketed numbers) at which the AMD participants and the paired controls were also tested. Error bars present bootstrapped 95% confidence intervals, which are generally asymmetric. Error bars were omitted from RM and RB, who did not show saturation within the tested range of spacing (see Methods). Compared to the paired controls within each group, AMD participants had significantly higher threshold elevation (by about half of a log unit in terms of contrast energy) than the normally sighted controls regardless of the age of the controls.

Figure 7

Illustrations of the key findings with the AMD participants. Different eccentric locations were used by the three AMD participants (S1, S2, and S3), corresponding to their PRLs in their preferred eyes. The dark patches represent the estimated absolute scotoma of the preferred eye of each AMD participant to illustrate the general visibility of the stimulus. In each panel are the depicted target and flanker contrasts, stimulus size and spacing that correspond to the target threshold contrast at critical spacing, the fixed flanker contrast, the stimulus size used, and the critical center-to-center spacing, respectively, for each of the three AMD participants. For the AMD participants, target and flankers lightly overlapped at critical spacing. By design, a target always occluded the flankers whenever they overlapped.