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S.R. Bharadwaj, Y.S. Kim, C.M. Schor; Dynamic Control of Ocular Disaccommodation . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2325.
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Purpose: Peak velocity of disaccommodation is independent of response magnitude for a fixed starting position but increases with proximity of starting position (Kasthurirangan et al, 2003; Bharadwaj & Schor, 2003). This suggests that responses are initiated toward a constant–primary destination & switched to a desired final position. In this model, two factors could control peak velocity of the response: the distance of the starting position from a primary destination & when the switch occurs. Here, we sought to determine the role of these two factors. Methods:Disaccommodation responses to step changes in optical defocus were measured on six adults (age range: 25.2 +/– 3.7yrs) using the SRI dynamic optometer. Step stimuli (1D – 4D), each lasting for 4secs, were presented monocularly (left eye) from three different starting positions (2D, 3D & 4D). Position traces were differentiated to compute velocity & acceleration profiles. X–intercept of the linear regression equation of peak velocity as a function of starting position predicted the primary destination of disaccommodation. The time–to–peak–velocity (TPV) indicated when the switch occurs. Results: Peak velocity & peak acceleration of disaccommodation were constant with response magnitude & increased linearly with starting position (peak velocity: y = 1.39x + 1.12; peak acceleration: y = 8.84x + 22.5). Linear regression of peak velocity as a function of starting position produced negative x–intercepts (mean: –1.45 +/– 0.96D) in five subjects and a positive x–intercept (0.91D) in the sixth. The x–intercepts were correlated well with the subjects’ cycloplegic refractive state (slope: 0.69; r2: 0.95) but they were correlated poorly with the resting focus of accommodation. The TPV did not change significantly with either response magnitude or starting position (mean TPV: 206.4 +/– 82.34msec). Conclusions: Increase in peak velocity & peak acceleration of disaccommodation with starting position suggests that peak velocity is correlated with distance of the starting position from the primary destination. A constant TPV indicates that the switching time is invariant. Correlation between the x–intercepts and cycloplegic refractive states suggests that the primary destination of disaccommodation is close to the anatomical position of rest.
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