How does normal crowding affect visual acuity?

Abstract

Purpose 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. Methods We measured acuity thresholds, with and without crowding, in central vision (i.e. at the fovea and at +/‐1 degree, +/‐1.5 degrees, and +/‐2 degrees) in 80 normal subjects with the age range of 29.3 +/‐10.7 years. The stimulus optotype was a Landolt ring of 100% luminance contrast presented either in isolation or together with four surrounding rings at a distance equal to 1.5 times the diameter of the stimulus, these parameters were selected on the basis of preliminary studies varying the spacing and the number of distracters. Results The threshold stimulus size for correct discrimination of gap orientation increased almost linearly with eccentricity, the presence of distracter rings caused a significant reduction in VA. Conclusion The statistical distribution of the differences between the two measures of VA provides the data needed to define the effects of crowding in “normal” vision. The template extracted from these data is clinically useful to identify those subjects that show abnormal sensitivity to crowding, i.e. amblyopia or early degenerative conditions/ disease.