Full-field accommodation in rhesus monkeys measured using infrared photorefraction.

There is growing interest in understanding peripheral refraction as a cue for myopia progression. Studies on peripheral refraction date back to the 1930s, when refraction was measured up to an eccentricity of 60° temporally or nasally in human subjects.1–3 Peripheral refraction was later measured in a much larger population that considered the role of peripheral refractive error in the development of central myopia.4–6 The results showed a relatively higher risk for young emmetropic pilots with peripheral hyperopia to develop central myopia than those with peripheral myopia.6 Subsequently, many studies have investigated the question of whether peripheral refraction can affect the development of myopia in humans7–12 and in animals.13,14 In rhesus monkeys, if the fovea is ablated and the periphery is form deprived, peripheral refractive error can still affect emmetropization.13,14 These findings suggested peripheral refraction in both humans and rhesus monkeys may be related to progression of myopia and imply that factors affecting peripheral refraction may have an impact on myopia development. When the eye accommodates for foveal tasks, peripheral refraction must also change. Accommodation has been linked to the progression of myopia.15–17 However, it is not clear exactly how accommodation affects peripheral refraction or whether the central and peripheral refractive changes are similar. Several studies18–25 have investigated the relationship between accommodation and peripheral refraction in humans (Table 1). If there is a significant difference between the central and peripheral refraction during accommodation, this means that a large amount of defocus may occur in the periphery, even though accommodation reduces central defocus. Large defocus in the periphery might then induce foveal myopia.13 This could be regarded as a peripheral mechanism of development of refractive error. If there is little difference between the central and peripheral refraction during accommodation, accommodation is neither an exaggerating nor a damping factor for peripheral mechanism in developing central refractive errors. Table 1. Studies on the Relationship between Accommodation and Peripheral Refraction To understand how accommodation affects peripheral refraction, full-field measurement with a high angular resolution can be helpful. Most studies measured peripheral refraction at about every 10° (see Table 1). One study23 achieved an angular resolution of 0.4° using photorefraction. In myopic eyes some retinal regions of higher refractive variations have been identified compared with emmetropic eyes.26 The more irregular peripheral refraction was suggested as an early indicator of myopia progression. Full-field refraction measurement with high angular resolution can provide more detailed information across the retinal field and therefore a better understanding of peripheral refraction. Rhesus monkeys have been used to study myopia27 and accommodation,28 but no prior studies have demonstrated how peripheral refraction changes during accommodation in rhesus monkeys and whether it is similar to humans. Understanding this is of significance to demonstrate if rhesus monkeys are a good animal model for human accommodation and myopia studies13 from the aspect of peripheral refraction. In this study, a photorefraction system was developed to measure full-field refraction in rhesus monkeys to understand the relationship between accommodation and peripheral refraction. A comparison of results from the present study to the human studies listed in Table 1 was used to evaluate if drug-stimulated accommodative changes in peripheral refraction in rhesus monkeys are similar to those with voluntary accommodation in humans.