December 2010
Volume 51, Issue 12
Letters to the Editor  |   December 2010
Passive Letter Subset Memory and Reliability of Visual Acuity Assessment
Author Affiliations & Notes
  • Charles W. McMonnies
    School of Optometry and Vision Science, The University of New South Wales, Northbridge, New South Wales, Australia.
Investigative Ophthalmology & Visual Science December 2010, Vol.51, 6907-6908. doi:
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      Charles W. McMonnies; Passive Letter Subset Memory and Reliability of Visual Acuity Assessment. Invest. Ophthalmol. Vis. Sci. 2010;51(12):6907-6908. doi:

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      © ARVO (1962-2015); The Authors (2016-present)

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The potential for systematic bias associated with using the best of multiple measures of visual acuity as an outcome variable has been well demonstrated. 1 The point is well made that the reliability and/or validity of assessments of changes in visual acuity might be adversely affected. 1 I would like to comment on passive letter chart subset memory as a source of bias and/or reduced reliability when visual acuity is assessed. For example, during a routine clinical examination or assessment of a research subject there are usually multiple references to letter charts that are constructed using different sequences of the same letter subset. Sometimes the same letter chart and letter configuration are used multiple times throughout an examination or period of assessment. Low levels of passive letter subset memory have been found to persist for up to 10 days after a routine clinical examination of both eyes, 2 and such memory may enhance performance during assessments within that period. For example, enhancement of visual acuity was demonstrated when retesting occurred within 24 hours of a single acuity measurement, 3,4 and may have been at least partly due to passive memory of the letter subset. However, enhancement associated with passive memory could be greater when preceded by multiple measures and/or when multiple assessments occur within a shorter time frame than 24 hours. For example, when a second eye assessment immediately follows the first assessment with the same chart or the same letter subset, the finding for the second eye could be enhanced. Enhancement appears to be more likely if the two eyes share a line of threshold acuity and perhaps especially if the first eye assessed has the better acuity. In addition, routinely asking subjects to read from the top line increases the frequency of exposure to the letters and the level of passive memory that can be developed. 
Consequently, if the best of multiple measures is enhanced by passive memory, any apparent deterioration at a subsequent examination may be partly due to the loss of letter subset memory. However, a retroactive interference method for the inhibition of passive chart memory has been shown to be effective. 5 The successful technique involved a sham acuity assessment with a near-point contrachart that had been constructed from the 16-letter subset that was not involved in the construction of the chart used for distance acuity assessment. 5 Reading just 4 lines of this contrachart resulted in memory scores that were not significantly different from the scores of control subjects (who guessed which letters would be involved, not having had recent exposure to any letter chart). 5 This result suggests that memory of the distance chart letter subset had been inhibited or neutralized during this 30-second procedure. 5 Use of such a contrachart may increase the reliability of acuity scores in clinical and research settings that involve multiple measures within a single examination session and/or multiple examination sessions over a short period. Control of passive letter subset memory might be achieved if the contrachart is presented at distance; however, near presentation of a handheld version can be more convenient. 
Significant variations in individual letter readability and legibility 6,7 mean that using charts constructed from different letter subsets can introduce variations in chart and line difficulty. For example, for many chart designs involving 5 letters per line, these variations have been found to result in lines of the same nominal acuity level having different levels of difficulty, depending on the chance combinations of easy and hard letters. 6,7 Interval scaling and the potential to create alternative equivalent charts cannot be achieved using a 5-letters-per-line design, but can be achieved when a full subset of (10?) letters is used in each line. 6,7 Nevertheless, irrespective of chart design, retroactive inhibition of passive letter subset memory may eliminate memory-associated enhancement of best acuity performance and improve the reliability of the assessment of changes in visual acuity. 
Koozekanani D Covert DJ Weinberg DV . The use of best visual acuity over several encounters as an outcome variable: an analysis of systematic bias. Invest Ophthalmol Vis Sci. 2010;51:3909–3912. [CrossRef] [PubMed]
McMonnies CW . Chart memory and visual acuity measurement. Clin Exp Optom. 2001;84:26–34. [CrossRef] [PubMed]
Lovie-Kitchen JE . Validity and reliability of visual acuity measurements. Ophthalmic Physiol Opt. 1988;8:363–370. [CrossRef] [PubMed]
Ferris FL Kassof A Bresnick GH Bailey I . New visual acuity charts for clinical research. Am J Ophthalmol. 1982;94:91–96. [CrossRef] [PubMed]
McMonnies CW . Control of chart memory for retesting acuity. Clin Exp Optom. 2001;84:78–84. [CrossRef] [PubMed]
McMonnies CW Ho A . Analysis of errors in letter acuity measurements. Clin Exp Optom. 1996;79:144–151. [CrossRef]
McMonnies CW . Chart construction and letter legibility/readability. Ophthalmic Phys Opt. 1999;19:498–506. [CrossRef]

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