An analysis of the superiority of binocular over monocular visual acuity.

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Abstract
In testing visual acuity, it is customary to test each eye separately, occluding its fellow with a blind, and then to test binocularly. Usually, the binocular acuity is distinctly superior to either monocular value. This expt. was concerned with 2 hypotheses to explain this difference: (1) that in monocular judgements, the low intensity of the occluded eye will decrease the diam. of the pupil of the nonoccluded eye, resulting in a lowered monocular acuity; and (2) that the difference in intensity and pattern in the 2 eyes will result in either rivalry or fusion of the dissimilar fields, leading to a reduction of monocular acuity. An artificial pupil was used to control pupil changes. Brilliance differences were controlled by the regulation of the intensity of light in the occluded eye. Pattern differences are inherent in monocular vision and were held constant since they could not be eliminated. Appropriate apparatus was constructed to yield both binocular and monocular judgements of acuity with 2 test objects. These were the Ives grating and the Landolt broken ring. The results may be summarized as follows: The primary variables (subjects, position, intensity), with the exception of eyes, were statistically significant beyond the 1% level of probability. The 40-ml. intensity yielded significantly higher monocular acuity than any of the other 3 intensity conditions. Binocular acuity was significantly higher than monocular acuity in all 4 expts. The artificial pupil tended to decrease the acuity for the grating and to increase it for the broken ring. The over-all effect was significant beyond the 1% level. When the artificial pupil was used, the difference between binocular and monocular acuity was decreased. The decrease was significant for grating at the 1% level and for the broken ring at the 5% level. It is concluded that pupil changes and brilliance differences contribute significantly to the binocular-monocular acuity difference. A discussion of the possible factors involved in visual interaction is included.
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