title: Objective and subjective refractive error measurements in monkeys.
abstract: OBJECTIVE
  To better understand the functional significance of refractive-error measures obtained using common objective methods in laboratory animals, we compared objective and subjective measures of refractive error in adolescent rhesus monkeys.
  
  
  METHODS
  The subjects were 20 adolescent monkeys. Spherical-equivalent spectacle-plane refractive corrections were measured by retinoscopy and autorefraction while the animals were cyclopleged and anesthetized. The eye's axial dimensions were measured by A-Scan ultrasonography. Subjective measures of the eye's refractive state, with and without cycloplegia, were obtained using psychophysical methods. Specifically, we measured spatial contrast sensitivity as a function of spectacle lens power for relatively high spatial frequency gratings. The lens power that produced the highest contrast sensitivity was taken as the subjective refraction.
  
  
  RESULTS
  Retinoscopy and autorefraction consistently yielded higher amounts of hyperopia relative to subjective measurements obtained with or without cycloplegia. The subjective refractions were not affected by cycloplegia and on average were 1.42 ± 0.61 D and 1.24 ± 0.62 D less hyperopic than the retinoscopy and autorefraction measurements, respectively. Repeating the retinoscopy and subjective measurements through 3 mm artificial pupils produced similar differences.
  
  
  CONCLUSIONS
  The results show that commonly used objective methods for assessing refractive errors in monkeys significantly overestimate the degree of hyperopia. It is likely that multiple factors contributed to the hyperopic bias associated with these objective measurements. However, the magnitude of the hyperopic bias was in general agreement with the "small-eye artifact" of retinoscopy.
authors.affiliation: College of Optometry, University of Houston, Houston, Texas 77204-2020, USA.
authors.forename: Li-Fang
authors.initials: LF
authors.lastname: Hung
authors.forename: Ramkumar
authors.initials: R
authors.lastname: Ramamirtham
authors.forename: Janice M
authors.initials: JM
authors.lastname: Wensveen
authors.forename: Ronald S
authors.initials: RS
authors.lastname: Harwerth
authors.forename: Earl L
authors.initials: EL
authors.lastname: Smith
mesh_terms: D000818:Animals; D015350:Contrast Sensitivity; D004195:Disease Models, Animal; D007908:Lens, Crystalline / Q000000981:diagnostic imaging; D008253:Macaca mulatta; D012029:Refraction, Ocular; D012030:Refractive Errors / Q000175:diagnosis* / Q000503:physiopathology; D042262:Retinoscopy; D012189:Retrospective Studies; D014463:Ultrasonography; D014787:Vision Tests