Visual acuity has been a research focus at Arlene R. Gordon Research Institute of Lighthouse International for several years. For research purposes, Drs. Aries Arditi and Lei Liu use a computerbased acuity test system specifically designed to overcome problems inherent in using printed letter charts.

In an early study, Arditi and Cagenello (1993a) determined boundary values of test-retest reliability and sensitivity to acuity change that are unlikely to be exceeded in any clinical study using a popular visual acuity chart (Lighthouse/ETDRS). The results showed that, using this chart with the recommended letterby- letter scoring, visual acuity may, with 95% confidence, be ascertained only within +0.1 log unit. In order to detect a significant change in visual acuity, about +0.14 log unit was required to achieve the same degree of confidence. These measurements may be usefully viewed as approaching the upper limit of reliability of this letter chart, and provide guidelines to clinicians who use the test as an aid to refraction and to monitor the progression of eye disease.

In a second study, Cagenello et al. (1993) determined the conditions under which binocular acuity is better than monocular acuity. They compared binocular and monocular acuity in one or both eyes under a variety of conditions of contrast differences. When the same contrast level was presented to each eye, binocular acuity was found to be better than the best monocular acuity by an average of 0.045 log MAR, which is nearly half a line on a standard acuity chart, or 11%. When the contrast level presented to each eye differed, binocular acuity remained better in most cases than the monocular acuity of the eye receiving the higher contrast. These results are likely explained by the contrast sensitivity enhancements of high spatial frequency, or "fine detail" components of the retinal image. These results alert clinicians to the possibility that letters viewed with two eyes may be more legible than letters viewed with one, even when the eyes differ from one another in acuity or effective contrast.

A few years ago, Drs. Arditi, Liu, visiting colleague Darren Albert, M.D. and research assistant William Lynn evaluated the efficiency of adaptive computerized methods of acuity testing, in which the letter size presented depended on both the size previously presented, and the observer's (or patient's) response. Their data indicated that a computer can obtain, in just minutes, visual acuity estimates that are twice as accurate as those obtained with a traditional eye chart.

Lighthouse work on acuity testing is closely related to our work on typography and legibility. Most of the Lighthouse legibility studies use acuity as a criterion; the most legible text can be seen at the smallest retinal sizes.

Dr. Liu recently studied how the contours of a square C contributed to the acuity in identifying the position of the gap. The interactions between the gap and the contours were determined by measuring acuity to a rectangular C of various aspect ratios. It was found that larger aspect ratios generally produced more correct responses than smaller aspect ratios. At the same aspect ratio, acuity was higher when the gap was on the longer side of the rectangular ring than when it was on the shorter side. A preliminary report of these findings appeared in abstract form (Liu, 2000).

In recent years, many studies of visual acuity have been conducted using computers and CRT displays. Software for computerized clinical tests has also become increasingly available. However, it is known that there are significant differences between the actual image shown on a CRT and the nominal image. One cause of the discrepancy is the anisotropy of pixel interaction along and across raster lines. Pixels appear more blurred along the horizontal raster line than across. Dr. Liu, with the help of Research Assistant Jianna Cho, evaluated the effect of this anisotropy on a computerized visual acuity test. Their results indicated that left and right gaps of a Landolt C produced better acuity than up and down gaps. Although the difference was not large, it was sufficient to introduce uncertainty in the computerized test results. These results were presented at the 2001 annual meeting of the American Academy of Optometry. A full report appeared in the journal of Optometry and Vision Science (Liu & Cho, 2002).

Most recently, Arditi (in press) performed a study that validates the letter identification task as one that is particularly resistant to lapses, which are errors that are independent of stimulus visibility, for use in letter acuity testing and letter contrast sensitivity testing.

Further Reading

Arditi, A. (1994). On the relationship of letter acuity to reading acuity. In A. C. Kooijman, P. L. Looijestijn, J. A. Welling, & G. J. van der Wildt (Eds.), Low vision: Research and new developments in rehabilitation (pp. 38-45). Amsterdam, The Netherlands: IOS Press.

Arditi, A. (1996). Typography, print legibility, and low vision. R. Cole & B. Rosenthal (Eds.), Remediation and management of low vision (pp. 237-248). St. Louis: Mosby.

Arditi, A., & Cagenello, R. (1992). A computer-based optotype acuity test system suitable for evaluation of acuity charts. Noninvasive assessment of the visual system technical digest 1992, 1, 88-91. Washington, DC: Optical Society of America.

Arditi, A., & Cagenello, R. (1993a). On the statistical reliability of letter chart visual acuity measurements. Investigative Ophthalmology and Visual Science, 34 (1), 120-129.

Arditi, A., & Cagenello, R. (1993b). Why reading acuity is worse than and is poorly predicted by letter acuity. Investigative Ophthalmology and Visual Science, 34 (4), 1417.

Arditi, A., Cagenello, R., & Jacobs, B. (1995). Letter stroke width, spacing, and legibility. Vision science and its applications. OSA technical digest series (pp. 324-327). Washington, D.C.: Optical Society of America.

Arditi, A. (2006). Lapses in the verbal letter reporting task. Vision Research. 46(8-9) 1327-1330.

Cagenello, R. B., Arditi, A., & Halpern, D. L. (1993). Binocular enhancement of visual acuity. Journal of the Optical Society of America A, 10, 1841-1848.

Liu, L., & Cho, J. (2002). The effect of pixel nonlinearity on CRT based visual acuity tests. Optometry and Vision Science, 79, 724-734.

Liu, L. (2000). Contour interaction within visual acuity targets. Investigative Ophthalmology & Visual Science (Suppl), 41 (4), S439.



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