Age related changes in accommodative dynamics in humans

Age related changes in accommodative dynamics in humans Dear Editor Using the PowerRefractor, Kasthurirangan and Glasser (2006) conclude that: ''For accommodation, time constant increased and peak velocity decreased with age.'' For subjects less than 40 years of age, I find that the authors' data does not support this conclusion. Examination of the authors' Figs. 5a–c for their subjects of 14.7, 23.3, 31.3 years of age reveals that the mean time constant for amplitudes of accommodation between 0 and 4 diopters, was 0.4 s, 0.22 s, and 0.12 s, respectively. For these three subjects, there is a progressive decrease in the time constant with their increasing age. Examination of the authors' Figs. 7a–c reveals an increase in peak velocity of accommodation with aging for these same subjects in the range from 0 to 2 diopters. This data indicates an effect opposite to that expressed in the authors' conclusion. In another publication from their laboratory (Baumei-ster, Wendt, & Glasser, 2006), the peak velocity of accommodation from adolescent and middle aged rhesus monkeys was shown to increase from 13.83D/s to 15.59D/s for a 5.75 diopter increase in accommodation. This primate data is consistent with my evaluation of the authors' human data and is also opposite to their conclusion. I am concerned that authors may have introduced a bias in the selection of subjects used for their analysis. Additionally , the difference in the age range between the members of each of the compared subgroups may have confounded their analysis. 1. The authors indicate that they arbitrarily selected 5 subjects between the ages of 22.33 and 22.75 years and 5 subjects between the ages of 38.33 and 41.25 years to determine the effect of age on the peak velocity and time constant of accommodation between 0 and 3.5 diopters (Fig. 8a and b of the authors paper). It is not at all clear how this selection of subjects for the analysis was made, nor whether comparisons of other subgroups would result in the same conclusion. Selection bias must be carefully addressed in the comparison of the two subgroups. Age range is another confounding variable between these two subgroups. The maximum difference in age between the subjects in the younger subgroup was 0.42 years, while that between subjects in the older subgroup was 2.92 years. As shown in Fig. 3, a similar decline in maximal accommodation was found in both age groups, approximately 0.26 diopters/year. …