Development of Crowding Distance in Children Is Faster and Later Than Visual Acuity and Better Predicts Reading...

Visual crowding is a failure of object recognition due to nearby clutter. Crowding distance (CD) is the threshold spacing for identification of a target among flankers. Visual acuity (VA) is the smallest recognizable letter size. In normal adult vision, CD worsens more than VA with increasing eccentricity and is worse than VA in central vision of strabismic amblyopia. CD is not measurable in mature central vision with standard optotypes but is with tall, skinny optotypes. We reveal developments of CD and VA in children and consider their impacts on teacher-assessed reading performance.

VA (isolated Sloan letter size threshold) and CD (Pelli optotype spacing threshold) were measured in 227 normal, healthy children aged three to 11 years and 40 adults. CD was measured for trigram and repeated arrangements. Teacher-assessed reading indicators for 200 of these children were converted to a study reading indicator (SRI).

From age three years, VA improves 1.4×, reaching near-adult levels at six years (P > 0.05). CD reduces 4.8×, reaching near-adult levels at eight years (P > 0.05). Correlations between vision measures and SRI were higher for CD than VA (CD-trigram: r = -0.68; CD-repeated: r = -0.67; VA: r = -0.37; all P < 0.0001). Removing age shows CD, but not VA, matters for reading (CD-trigram: r = -0.24, P = 0.00065; CD-repeated r = -0.25, P = 0.00049; VA: r = -0.13, P = 0.075).

Crowding develops more quickly and matures later than acuity and is significantly linked to children's reading performance, unlike acuity. Crowding distance measures may be more sensitive diagnostically than acuity for detecting exaggerated crowding found in strabismic amblyopia and may help identify children who struggle to read.