Vergence Stress Significantly Affects Reading Rates

Abstract

Purpose: To quantify the effect of vergence stress and binocular visual coordination on reading performance. Methods: Reading rate in words per minute (wpm) was recorded via infrared eye tracker (Visagraph) with 10 subjects in grades 3-11 (mean = 5.6). Prior to measurements of reading rate each subject’s binocular status was evaluated quantitatively and by administering the Convergence Insufficiency Symptom Survey (CISS), and tracking ability was recorded using the Developmental Eye Movement test (DEM). The twelve experimental conditions for each subject included: normal baseline reading using both eyes, monocular reading with one eye occluded (monocular), and binocular reading while wearing horizontal prisms of equal value over both eyes (2, 4, 8, 16 and 20 prism diopters (pd) base in, and the same values base out ). Results: For all subjects, reading rate with one eye was statistically indistinguishable from binocular reading rate with no prisms (p=0.24, one-tailed t-test). Four subjects were classified as having abnormal binocular coordination. Subjects with normal binocular function had reduced reading rates by an average of 15% while wearing prisms of 8 to 16 pd, both base in and base out. The effect of prisms was generally robust and symmetrical, with base in and base out prism levels having correlated effects on reading rate (r=0.75, p<0.01). However, for subjects with binocular dysfunction, reading rate was not significantly affected by prisms of similar value. Conclusions: Although reading rates with one eye occluded were similar to normal binocular reading rates, inducing vergence stress by wearing prisms over both eyes resulted in slower reading rates in children with normal binocularity. The results provide the first evidence that disruptions of binocular vision can interfere with reading.