Measured Visual Acuity as a Function of Phenomenal Size
Abstract
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 a daylight situation favorable to the 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 a 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 measured visual acuity as a function of 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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Oct 01, 1955
- Accession Number
- AD0090914
Entities
People
- Earl A. Alluisi
Organizations
- Ohio State University