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S.R. Kasthurirangan, A. Glasser; Influence of Amplitude and Initial Position on Accommodative Dynamics in Humans . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2729.
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Purpose: Previously we found that peak velocity of accommodation saturated and peak velocity of disaccommodation increased linearly with amplitude. In that study, accommodation was measured as change in focus from fixed far distance to various near distances and disaccommodation as change in focus from various near distances to a fixed far distance. In the present study, accommodation and disaccommodation were stimulated by targets at 1) fixed far and various near distances and 2) fixed near and various far distances to understand the influence of both amplitude and initial position on dynamics of accommodation and disaccommodation. Methods: Step accommodative responses to Maltese cross like targets presented at real distances were measured with a PowerRefractor (infrared optometer, 25Hz.) in 5 subjects (ages 21 - 23 yrs). Two conditions were used: 1) far target fixed at 6 meters; near target presented at accommodative demands of 1D to 5D, in separate trials; 2) near target fixed at 5D; far target placed at various demands of 1D to 4D. At least ten dynamic accommodative responses were recorded for each stimulus demand and analyzed to estimate amplitude, peak velocity and time constants of accommodation and disaccommodation. Results: The cumulative slope of accommodative amplitude vs time constant was significantly different from zero for condition 1 (slope: 0.078 S/D, p < 0.01) and condition 2 (slope: 0.053 S/D, p < 0.01) but were not different between the two conditions (p = 0.38). The cumulative slope of peak velocity vs disaccommodative amplitude was significantly different from zero for condition 1 (slope: 3.65 S-1, p < 0.01) and not for condition 2 (slope: 0.67 S-1, p = 0.18) and were significantly different between the two conditions (p = 0.013). The individual slopes of peak velocity vs amplitude of disaccommodation were similar in condition 1 (3.77 + 0.63 S-1 ) and variable in condition 2 (0.26 + 1.93 S-1). For disaccommodation, peak velocity increased with amplitude (slope: 3.65 S-1 , p < 0.01) when the near distance changed, and time constant increased with amplitude when the near distance was fixed (slope: 0.059 S/D, p < 0.01). Conclusions: Peak velocity of disaccommodation, which occurs very early in a response, is influenced by the initial position of focus. Time constants of disaccommodation depend on both amplitude and initial position of focus. Speed of accommodation is similar at far range and near range. Speed of disaccommodation is faster at near range, however, the difference is amplitude dependent. This study shows that accommodation dynamics depend on initial position and disaccommodation dynamics depend on both initial position and amplitude.
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