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KB Baker, DL Nickla, D Troilo; Accommodative Stimulus-Response Functions are Changed Following Experimental Myopia in Marmosets . Invest. Ophthalmol. Vis. Sci. 2002;43(13):195.
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Purpose: Accommodation may indirectly influence visually guided eye growth by affecting the blur signal experienced and used to guide growth. This view is suggested by work in humans where it has been shown that the accommodation stimulus-response functions of children developing myopia have lower slopes (gains) than emmetropic children (Gwiazda et al., 1993). Lower accommodative gains have also been reported in marmosets with induced myopia (Troilo et al., 2000). It is unclear from either study whether the change in gain is a cause or an effect of the refractive change. In this study we examined accommodation in awake, free-viewing marmosets both before and after induction of experimental myopia to determine the causal relationship between changes in accommodation and refractive state. Method: Accommodation stimulus-response functions were measured monocularly or binocularly in 5 marmosets trained to view targets. Visual stimuli consisted of video images displayed at different distances from the marmoset. A dynamic IR video refractor was used to monitor eye position and refractive state continuously. Accommodative demand (0-15D) was calculated from the target distance and the subjects' cycloplegic refraction (measured on another day). The magnitude of the accommodative response was calculated as the myopic shift in refraction relative to the cycloplegic refraction. Myopia was induced by form deprivation (n=2) or negative power spectacle lens wear (n=3). Results: In all marmosets used in this study to date statistically significant relative myopic shifts ranging from -3 to -8 D (p<0.01) were observed. The accommodative gains measured either binocularly or monocularly in these eyes were significantly reduced from pre-manipulation levels (means: binocular 0.80 to 0.59, monocular 0.83 to 0.54; p<0.05). The amount of refractive change is weakly correlated with the change in accommodative gain (binocular: r=0.66, p=0.07; monocular: r=0.57, p=0.11). The accommodative gain measured before myopia induction is not well correlated with the degree of myopia induced (binocular: r=0.46, p=0.22; monocular: r=0.22, p=0.58). Conclusion: Although still inconclusive, these preliminary results support the hypothesis that the observed reduction in accommodative stimulus-response gain is a result of the myopia induced. The alternative hypothesis - that low accommodative gain increases susceptibility to the development of myopia - is not supported by the poor correlation between the pre-manipulation accommodative gain and the magnitude of the myopic shift.
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