The effect of different forms of monocular occlusion on measures of central visual function

The purpose of this study was to determine whether clinically used measures of central visual function were influenced by the method of monocular occlusion used. Twenty young visually normal subjects were tested both monocularly and binocularly using an Australian Vision Chart No. 5 (which includes high and low contrast letters) and the Pelli-Robson letter chart. During monocular assessments, the non-tested eye was occluded with either a translucent (frosted) diffuser, a +1.50 D (fogging) lens, or a white opaque occluder. A significant difference in monocular sensitivity between the three occluder types was found for Pelli-Robson contrast sensitivity, and the high and low contrast letter acuities, with poorest performance being recorded with the opaque occluder in all cases. The same trend was evident in the derived binocular summation ratios which were found to vary with the type of occluder used before the non-tested eye, as well as with the visual function measured. The data imply that binocular interactions occur during 'monocular' testing of central visual function and that these interactions vary with the method of occlusion used. The clinical ramifications of these results are discussed.