To investigate the speed of accommodative and disaccommodative responses as a factor of age, we considered peak velocities for response amplitudes from 0.5 to 3.5 D of subjects divided into two age groups: younger (age, 3–19 years) and older (age, 20–38 years). This range of response amplitudes was selected because it included most of the responses measured from all subjects and did not exceed the maximum response amplitude of either group. The results are plotted in
Figures 7A and
7B, as a main sequence of accommodation and disaccommodation with
V max plotted as a function of the amplitude for each response. For accommodative peak velocities, data were fitted with nonlinear models of
V max versus response amplitude. To derive the models, we plotted time constant (τ) as a function of response amplitude for each age group (data not shown). For the older age group, time constant increased linearly with response amplitude (
r 2 = 0.031,
P < 0.02) and the statistically significant linear relationship was used to calculate the modeled
V max from the formula
V max = a/τ. This modeling is the same as that described in previous studies of age-related changes in accommodative velocities.
13,15 For the younger age group, there was no linear increase in time constant with response amplitude, and thus mean τ was used to calculate
V max for all stimulus response amplitudes. In
Figure 7A, the 95% confidence interval for the modeled
V max was calculated for the older group from the 95% confidence interval of the linear fit to time constant versus response amplitude. Peak velocities in the older group were significantly slower than those in the younger group for response amplitudes greater than approximately 1.75 D, as evidenced by the lack of overlap between the model of the younger group and the model 95% confidence interval of the older group.
For disaccommodative responses there was a significant linear increase in peak velocity with response amplitude for both age groups (younger:
r 2 = 0.28,
P < 0.0001; older:
r 2 = 0.40,
P < 0.0001).
Figure 7B demonstrates that there were no age-related differences in disaccommodative peak velocity for the two groups which was confirmed statistically by using one-way ANCOVA analysis (
F = 0.27,
df = 2, 376,
P = 0.6).