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SR Kasthurirangan, AS Vilupuru, A Glasser; Accommodative Dynamics in Humans . Invest. Ophthalmol. Vis. Sci. 2002;43(13):404.
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Purpose: Prior studies of accommodative dynamics in humans (over a limited range of accommodation) and monkeys (over the full accommodative range) suggest that peak velocity increases with amplitude. This study was undertaken to measure peak velocity of accommodation/disaccommodation in young adult humans for different accommodative amplitudes, spanning the entire accommodative range and to understand the relationship between peak velocity and amplitude for accommodation and disaccommodation. Methods: Step accommodative responses to targets presented at real distances were measured with a PowerRefractor (infrared optometer, 25Hz.) in 7 subjects (22 - 28 yrs; mean age: 25 yrs). Cyclic accommodative (far to near)/disaccommodative (near to far) stimuli were presented at 6m and at accommodative demands of 1D to 7D in 1D steps, in separate trials. H-letter targets with the same angular subtense and luminance (3 cd/m2) at distance and near were used for each stimulus demand. At least ten dynamic responses for each stimulus demand were recorded for each subject. The resulting dynamic responses were analyzed to obtain peak velocity of accommodation and disaccommodation as a function of accommodative amplitude. Results: For accommodation, peak velocity shows a linear rising phase for lower amplitudes and saturation at higher amplitudes. As expected, time constants were constant at lower amplitudes and increased at higher amplitudes. Peak velocity of disaccommodation was linear over the entire range of response amplitudes in all subjects (slope = -2.6281, r2 = 0.69). One way ANOVA of peak velocities of accommodation and disaccommodation to different stimulus amplitudes, shows significant difference for different amplitudes (p <0.05) in every subject. Pairwise multiple comparison procedures (Tukey Test), revealed significant differences (p<0.05) between lower (1D, 2D) and higher amplitudes for accommodation and between all amplitudes for disaccommodation. Conclusion: This study has characterized the peak velocity of accommodation and disaccommodation over the entire range of accommodative amplitudes in young humans. In a previous study on humans (Ciuffreda and Kruger, 1988) peak velocity of accommodation was linear, but only low amplitudes (0.1 to 3.25D) were tested. With central stimulation in anesthetized monkeys both accommodation and disaccommodation showed a linear rise in peak velocity with amplitude over the full range of accommodation (Vilupuru and Glasser, 2001). Interestingly, however the present study shows, in conscious humans, that peak velocity of accommodation saturates at higher amplitudes, whereas disaccommodation shows no saturation.
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