| Summary: | The presence of ‘crowding’ features on visual acuity tests for young children are considered important for detecting amblyopia, early treatment of which is key to success. The optimum placement of ‘crowding’ features has not previously been investigated, nor has the change in magnitude of crowding with age been measured with such stimuli. Recently, contrast-modulated noise (CM) stimuli have been suggested to be potentially more sensitive to amblyopia, than standard black on white, or luminance (L) stimuli. CM stimuli also result in larger magnitudes of crowding in normal adults, but this has not been tested in children, or in adults with child-friendly CM optotypes. The first study of this thesis shows that placement of features surrounding the target optotype provide more consistent crowding across symbols, pictures and letters, when separation is specified in units of stroke width, as opposed to units of optotype width. Steeper slopes of the underlying psychometric functions, and thereby increased sensitivity, are produced by placing contour interaction or crowding features near to 1 (one) stroke width away. This separation also maximises contour interaction and crowding. In normal adults, the magnitude of contour interaction is smaller than that of crowding with L and LM, but not with CM, stimuli. The second study of this thesis shows that visual acuity develops more slowly, and becomes adult-like later with CM, compared to L and LM (luminancemodulated noise) stimuli. The magnitude of contour interaction is similar for L, LM and CM stimuli and varies very little across age group (3 to 16 years old and adults). Crowding is larger than contour interaction with L and LM, but not CM stimuli in binocularly normal participants; this is not the pattern of results found in very young children or in binocularly anomalous adults. A comparison of ‘equivalent ages’ for binocularly abnormal adults finds that CM crowded acuity predicts an earlier arrest of normal development, than do L or LM crowded, or any of the isolated optotype acuities.
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