A systematic review on 'Foveal Crowding' in visually impaired children and perceptual learning as a method to reduce Crowding

Abstract

Background: This systematic review gives an overview of foveal crowding (the inability to recognize objects due to surrounding nearby contours in foveal vision) and possible interventions. Foveal crowding can have a major effect on reading rate and deciphering small pieces of information from busy visual scenes. Three specific groups experience more foveal crowding than adults with normal vision (NV): 1) children with NV, 2) visually impaired (VI) children and adults and 3) children with cerebral visual impairment (CVI). The extent and magnitude of foveal crowding as well as interventions aimed at reducing crowding were investigated in this review. The twofold goal of this review is : [A] to compare foveal crowding in children with NV, VI children and adults and CVI children and [B] to compare interventions to reduce crowding.

Methods: Three electronic databases were used to conduct the literature search: PubMed, PsycINFO (Ovid), and Cochrane. Additional studies were identified by contacting experts. Search terms included visual perception, contour interaction, crowding, crowded, and contour interactions.

Results: Children with normal vision show an extent of contour interaction over an area 1.5-3× as large as that seen in adults NV. The magnitude of contour interaction normally ranges between 1-2 lines on an acuity chart and this magnitude is even larger when stimuli are arranged in a circular configuration. Adults with congenital nystagmus (CN) show interaction areas that are 2× larger than those seen adults with NV. The magnitude of the crowding effect is also 2× as large in individuals with CN as in individuals with NV. Finally, children with CVI experience a magnitude of the crowding effect that is 3× the size of that experienced by adults with NV.

Conclusions: The methodological heterogeneity, the diversity in paradigms used to measure crowding, made it impossible to conduct a meta-analysis. This is the first systematic review to compare crowding ratios and it shows that charts with 50% interoptotype spacing were most sensitive to capture crowding effects. The groups that showed the largest crowding effects were individuals with CN, VI adults with central scotomas and children with CVI. Perceptual Learning seems to be a promising technique to reduce excessive foveal crowding effects.

Figure 1

PRISMA 2009 Flow Diagram.

Figure 2

Full extent of the contour interaction area. Figure 2 presents the results of three studies that have measured the full contour interaction area in children and adults with NV. Differences between the studies can partially be explained by the different optotypes used. The study by Semenov used Landolt C’s with flanking bars and the study by Jeon et al used E-gratings surrounded by gratings. E-gratings are more difficult to identify than C-rings for children, which might explain the larger contour interaction areas when E-gratings are used [20,23]. Error bars ± 1 s.e.m.

Figure 3

The magnitude of contour interaction effects at 1 and 2 MAR. Figure 3 presents the results of two studies which have measured the magnitude of the contour interaction effect in adults with normal vision, adults with congenital nystagmus (CN) and adults with albinism. As can be seen, the magnitude of the effect (defined by the decrease of visual acuity in log units) is the largest in adults with CN in both studies. Standard errors of the mean were not provided.

Figure 4

Crowding ratios measured with charts with 100% interoptotype spacing. Figure 4 presents the results of four studies which measured crowding ratios in different populations: children and adults with normal vision (NV), children with cerebral visual impairment (CVI), and visually impaired (VI) adults. Children with CVI and adults with VI showed higher crowding ratios than respectively children with NV and adults with NV. Error bars ± 1 s.e.m.

Figure 5

Crowding ratios measured with charts with 50% interoptotype spacing. Figure 5 presents the results of four studies which have measured crowding ratios in children and adults with normal vision (NV) and children with cerebral visual impairment (CVI). Line means that the crowding ratio was calculated by dividing the single through the line acuity score and circular means that the crowding ratio was calculated by dividing the single acuity through the acuity score that was measured when a target symbol was surrounded by 6 symbols surrounding the target in all directions. A clear age related reduction of the crowding ratio was observed in children with NV. Error bars ± 1 s.e.m.