The effect of aging on crowded letter recognition in the peripheral visual field

Abstract

Purpose: Crowding describes the increased difficulty in identifying a target object when it is surrounded by nearby objects (flankers). A recent study investigated the effect of age on visual crowding and found equivocal results: Although crowded visual acuity was worse in older participants, crowding expressed as a ratio did not change with age. However, the spatial extent of crowding is a better index of crowding effects and remains unknown. In the present study, we used established psychophysical methods to characterize the effect of age on visual crowding (magnitude and extent) in a letter recognition task.

Methods: Letter recognition thresholds were determined for three different flanker separations in 54 adults (aged 18-76 years) with normal vision. Additionally, the spatial extent of crowding was established by measuring spacing thresholds: the flanker-to-target separation required to produce a given reduction in performance. Uncrowded visual acuity, crowded visual acuity, and spacing thresholds were expressed as a function of age, avoiding arbitrary categorization of young and old participants.

Results: Our results showed that uncrowded and crowded visual acuities do not change significantly as a function of age. Furthermore, spacing thresholds did not change with age and approximated Bouma's law (half eccentricity).

Conclusions: These data show that crowding in adults is unaffected by senescence and provide additional evidence for distinct neural mechanisms mediating surround suppression and visual crowding, since the former shows a significant age effect. Finally, our data suggest that the well-documented age-related decline in peripheral reading ability is not due to age-related changes in visual crowding.

Keywords: aging; critical spacing; crowding; reading; visual acuity.

Figure 1

Example of stimuli used to measure crowding. Participants were required to identify the central (target) letter (N in this example). In the crowded acuity tasks, flanking letters were separated from the target letter by fixed proportions of 1.7, 2.0, and 2.6 letter heights. The size of the letters varied on each trial. Uncrowded acuity was measured with a single target letter presented in isolation. In the spacing threshold task, letter size remained fixed while the separation of the flanking letters and the target letter varied.

Figure 2

Isolated letter acuity (letter size) as a function of age. The slope of the linear regression curve was 0.001 and did not differ significantly from zero. Isolated letter acuity did not vary significantly with age. Error bars represent SEM.

Figure 3

Crowded letter acuity as a function of age when the target-to-flanker separation was a multiple of (a) 1.7, (b) 2.0, and (c) 2.6 times isolated letter acuity. Thresholds were higher when flankers were closer to the target but did not change significantly with age, for each of the target-to-flanker separations. Error bars represent SEM.

Figure 4

Crowding ratios calculated for each separation level of the crowded acuity test. (a) Individual crowding ratios for each separation level, as a function of age. The slope of the linear regression lines plotted through the data did not differ significantly from zero. (b) Mean crowding ratio for each separation level for younger (<29 years) and older (>29 years) participants. The crowding ratio decreased with increasing flanker-to-target separation. There was no statistical difference in the crowding ratio between the younger and older groups at each separation. Error bars represent SEM.

Figure 5

The effect of age on critical spacing/eccentricity. Critical spacing to eccentricity ratio for individuals plotted as a function of age. (a) Critical spacing measured directly with the spacing task, where flanker-to-target separation was varied while letter size remained constant. (b) Critical spacing determined from results of the crowded acuity task using Equation 1. There was no significant effect of age on the critical spacing-to-eccentricity ratio, which was in the region of 0.5 and thus agrees with Bouma's law (dashed lines). Error bars represent SEM.