Applied Vision Research Centre, Henry Wellcome Laboratories for Vision Science, City University, London,
UK
Clinical tests of visual acuity (VA) that employ multiple, neighbouring optotypes assume that visual ’crowding’ at the fovea is negligible. Findings from recent studies suggest that crowding effects can affect high contrast acuity thresholds at the fovea. The absence of data to describe the distribution of crowding effects within ’normal’ vision makes it difficult to establish when a measured reduction in VA (with crowding) can no longer be considered to be within the normal range and is therefore indicative of abnormal development or pathology. The aim of this study was to quantify the effects of crowding on VA in the normal population. We measured acuity thresholds, with and without crowding, in central vision (i.e. at the fovea and at 1∘, 1.5∘, and 2∘) in eighty normal subjects with the age range of 29.3 ± 10.7 years. The statistical distribution of the differences between the two measures of VA shows significant crowding effects at the fovea that increase linearly with eccentricity and provides a useful template to identify those subjects that show abnormal sensitivity to crowding.