Effects of flanker type and position on foveal letter recognition

Mohd Izzuddin Hairol,Qazi Mohamad Omair,Sharanjeet Kaur

doi:10.12688/f1000research.8572.1

Abstract

Our ability to identify a foveally viewed letter at resolution threshold reduces when flankers are placed at a certain distance from it, compared to when it is presented on its own. In this study, we investigated how type and position of flankers influences foveal letter recognition. We measured participants’ performance to identify unflanked Sheridan-Gardiner letters (A, H, O, U, T, V or X), using a seven-alternative-forced-choice paradigm with the method of constant stimuli to obtain 80-90% correct response. Performance was measured again to identify a target letter in the presence of different flanker types. Flankers were bars and letters (placed either to the left and right of the target, above and below the target or on four sides of the target), and a surrounding box. Separation between the target letter and flankers varied between zero (abutting) and ten stroke widths (two letter sizes). For all flanker types, separation between the target ad the flankers significantly influenced performance (all p < 0.0001). Flankers abutting the target and at separation of one stroke width caused the largest reduction in performance, regardless of flanker type. For bar flankers, the largest drop in performance (up to 50 percent) occurred with bars placed on all four sides of the target. For letter flankers, flankers placed above and below the target reduced performance as much as four surrounding flankers. At separation of one stroke width, flanking letters and bars caused a further 10% reduction in performance than a surrounding box. Our results would have significant implications on the design of crowded visual acuity charts, especially those that are available on the market, which have different crowding features between them.

Highlights

Crowding is the reduction in visual acuity for a target letter presented together with other crowding features, such as other letters, a surrounding box or contours, compared with acuity measured with single letters (Flom et al, 1963; Stuart & Burian, 1962). Flom et al (1963) reported that performance for localising the gap of a Landolt C, at acuity level, is reduced by flanking bars that are placed less than one optotype width away from the Landolt C
Knowledge on crowding and contour interaction has been applied in the design of crowded acuity tests, based primarily on the results reported by Flom et al (1963)
When a letter is surrounded by only two letter flankers, crowding magnitude differ significantly, depending whether they are in vertically- or horizontally-arranged configurations

Summary

Introduction

Crowding is the reduction in visual acuity for a target letter presented together with other crowding features, such as other letters, a surrounding box or contours, compared with acuity measured with single letters (Flom et al, 1963; Stuart & Burian, 1962). Flom et al (1963) reported that performance for localising the gap of a Landolt C, at acuity level, is reduced by flanking bars that are placed less than one optotype width away from the Landolt C. Flom et al (1963) reported that performance for localising the gap of a Landolt C, at acuity level, is reduced by flanking bars that are placed less than one optotype width away from the Landolt C. This effect has been named as contour interaction, to describe the effects of surrounding contours on acuity. Other studies have reported that contour interaction occurs over a larger distance in children than in adults (Jeon et al, 2010; Semenov et al, 2000) This suggests contour interaction (and crowding), and their associated factors including accuracy of fixational eye movement and attention are attributable to age

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