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Helena M. Culhane, Barry Winn; Dynamic Accommodation and Myopia. Invest. Ophthalmol. Vis. Sci. 1999;40(9):1968-1974.
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purpose. Accommodative effort during nearwork is thought to be a causative
factor in the development of myopia. It has been proposed that an
anomaly in autonomic control may be a precursor to the development of
myopia. In the present study the closed-loop accommodation response
after variations in fixation period was investigated in emmetropes,
early-onset myopes and late-onset myopes to determine characteristics
of reflex accommodation for each refractive group.
methods. Closed-loop accommodation responses were measured in a group of
emmetropes (n = 7), early-onset myopes
(n = 7), and late-onset myopes (n = 7)
by use of a dynamic tracking infrared optometer. A variation in
fixation period (10 seconds, 60 seconds, and 180 seconds) before an
accommodative step was used to stimulate the accommodation control
results. Group results of accommodative response times showed that late-onset
myopes were significantly affected by the duration of fixation before
the change in stimulus vergence. Accommodative response times after 3
minutes of sustained near vision were significantly longer than those
observed for other groups for the near-to-far condition. Reaction time
appears to be independent of refractive grouping, prior fixation
period, and direction of step change.
conclusions. Late-onset myopes showed significantly extended accommodation response
times after a sustained near vision task that was demonstrable under
well-controlled experimental conditions. The extended response times
observed in the present study were consistent with previous reports of
refractive shifts in late-onset myopes and early-onset myopes and
provide a corollary between reflex and adaptive components of the
accommodation response. Potential mechanisms are discussed in an
attempt to explain the resultant hysteresis under closed-loop viewing
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