Crowding in visual acuity tests: unravelling the relative roles of optotype separation, gaze control and attention in children and adults

The measurement of visual acuity in children is important to detect visual anomalies including amblyopia. The use of visual acuity tests that induce ‘crowding’ are often recommended despite little standardization of the features in such tests. In addition, crowding in children’s foveal vision is known to be greater in extent than in adults and to be influenced by the nature of the flankers. This thesis presents new evidence that foveal crowding in children and amblyopic adults with strabismus is greater for letter acuity tests which require accurate gaze control and where the similarity of target and flankers imposes a greater attention demand. A slower maturation of crowded than single optotype acuity in young children is also shown. Using commercially available children’s acuity tests, the first study of this thesis showed that greater foveal crowding occurred with smaller inter-optotype spacing and with letter rather than picture optotypes. A decrease in crowding, resulting in improved visual acuity between the ages of 4 and 9 was also shown. In order to follow up these initial results, custom-designed visual acuity tests were produced to disentangle the contributions of contour interaction, eye movements and attention to the overall crowding effect. The second study in this thesis showed that crowding is greater with linear rather than single letter presentation and with letter rather than bar flankers in young children (aged 4-6), but not in adult controls. In a further study using a sample of amblyopic adults with strabismus more crowding was observed with linear presentation of letters and letter rather than bar flankers, a result consistent with the results seen in young children. These findings improve our understanding of crowding in children and in strabismic amblyopia and can be used to improve the standardizing of crowded acuity measurement and have the potential to increase the sensitivity of visual screening for amblyopia.