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D.W. Kline, G. Jung, R.W. C. Gagnon; Older Observers' Defocus Tolerance: Senile Miosis and or Experience? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1203.
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Older observers are better than their younger counterparts at identifying familiar and novel word messages when they are optically blurred (Kline et al., 1999), but not when they are low–pass filtered using image processing (Bartel & Kline, 2002). This study determined the relative contributions of pupil size and stimulus novelty in explaining older observers’ superior identification of optically degraded words.
Twelve young (21–36 years, M = 26.9) and 12 old (61–80 years, M = 72.7) healthy observers with good far acuity participated in the study. The stimuli were 6 words in uppercase san–serif font (THAT, WITH, HAVE, FROM, SOME, and YOUR) and 6 novel line drawings of common objects (Car, Chair, Cup, Key, Scissors, and Telephone). Word (letter height in minarc) and object (area in minarc2) legibility thresholds were determined at the test distance (6.1m), with both natural and artificial pupil (3mm), for three levels of induced acuity: best–corrected, 20/30 (1.5 minarc), and 20/40 (2.0 minarc).
For both age groups, legibility size thresholds for words and objects were better with natural than artificial pupils, a benefit that increased as acuity was degraded. Age did not affect object legibility. Artificial pupils improved object thresholds significantly in the 20/40 condition for both young (M = 681.7) and old observers (M = 686.4). Word legibility thresholds were lower for young than old observers in the best–corrected acuity condition with both natural (3.5 vs. 4.7) and artificial pupils (3.3 vs. 4.3). No age difference was seen when acuity was degraded to 20/30 in either pupil condition, or in the 20/40 condition with artificial pupils. With natural pupils in the 20/40 condition, however, the word legibility thresholds of the old observers were actually superior to those of their younger counterparts. This reversal at 20/40 reflected a significant benefit of artificial versus natural pupils for young (M = 7.9 vs. 10.2), but not old observers (M = 8.1 vs. 8.4).
Older observers’ superior identification performance in the presence of optical defocus appears to be confined to text materials viewed with natural pupils. Our findings support a two–factor explanation for the age–related acquisition of this ability. Specifically, a combination of senile miosis and greater experience may allow older observers’ to use the blur profile of individual letters to out–perform their younger counterparts, as well as their own acuity level when identifying optically defocused text.
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