On the presence and role of polar lipids in meibum.

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. 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. 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). This result suggests that memory of the distance chart letter subset had been inhibited or neutralized during this 30-second procedure. 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 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. 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. 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. Charles W. McMonnies