Two eyes are better than one: binocular enhancement in the contrast domain.

The purpose of this study was to compare the improvement in visual acuity (VA) with the improvement in contrast sensitivity (CS) with two eyes as compared with one. Computer generated letter charts were used to measure VA and small letter CS (20/25 Snellen equivalent) in 13 emmetropic subjects. Letter size (for VA) and contrast (for CS) were varied in equal log steps making the task comparable for the two types of measurement. VA improved by an average of 10% (2-3 letters), while CS improved by an average of 40% (1 1/2 lines) when tested with two eyes as compared with one. Greater sensitivity in the contrast domain prevailed even when expressed relative to variability. Using this approach, binocular enhancement was identified in 8/13 subjects with CS, but in only 4/13 subjects with VA. Binocular enhancement of letter recognition occurs in both size and contrast domains. However, the effect is 4x greater when small letters are varied in contrast rather than size. Potential applications are considered.