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Measured Visual Acuity As A Function Of Phenomenal Size

Report Number: WADC TR 55-384
Author: Alluisi, Earl A.
Corporate Author: Laboratory Of Aviation Psychology, Ohio State University
Laboratory: Wright Air Development Center
Date of Publication: 1955-10
Pages: 20
PB Number: PB124728
AD Number: AD0090914

The usual definition of visual acuity implicitly assumes that acuity is independent of phenomenal size as long as the test object subtends a constant visual angle at the retina. Evidence from some previous studies indicates, however, that measured visual acuity might be bettered when the test object is made to appear larger even though its objective size on the retina remains constant. The present study was designed to test this notion. In the daylight situation favorable to operation of size constancy (condition S), 36 observers read a retinally projected visual acuity chart at three convergences, and, thus, at three apparent distances and three conditions of phenomenal size of the chart. An additional 36 observers read the same retinally projected chart under similar conditions, but in at stimulus-reduced night situation (condition R); for this group, phenomenal size of the test chart remained constant. Acuities measured at the three convergences differed significantly among themselves in condition S, but not in condition R. This seems to establish measuered visual acuity asa function of the phenomenal size of the test chart. The effect was so small (less than one Snellen rating), it appears to be of little practical or clinical importance. It does, however, offer statistical support to those theories allowing for central factors in measured visual acuity.

Hunt Library, Embry-Riddle Aeronautical University
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