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Am J Vet Res. 1997 Apr;58(4):414-8.

Effect of optical defocus on visual acuity in dogs.

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Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin-Madison 53706, USA.



To determine the effect of optical defocus (such as what develops in spontaneous myopia and subsequent to cataract extraction) on visual acuity in dogs.


3 young adult male Beagles.


The effect of optical defocus on visual acuity was determined by sweep visual evoked potential, using a within-subjects/repeated measures design in which each dog served as its own control. Dogs were positioned so that the eye being tested was 60 cm in front of the video display, and the target was centered on the area centralis. To create ametropia relative to the video screen, a series of concave and convex spherical lenses were placed 1 cm in front of the eye, and sweep visual evoked potential acuities were obtained.


Maximal acuity was 7.0 to 9.5 cycles/degree. Defocusing by 2.0 diopters reduces Beagle grating acuity approximately 1 octave. Mimicking aphakia resulted in a marked depression of acuity to 0.7 cycles/degree or less.


Even mild degrees of ametropia have appreciable impact on the resolving power of the canine visual system.


Spontaneous myopia is encountered in dogs and may be associated with impaired visual performance attributable to a reduction in visual acuity. Previous reports indicate the possibility of myopia in dogs to have a heritable component. On the basis of our results, refractive correction of aphakia is advisable, and refractive screening of dogs with demanding visual tasks (eg, service dogs, field-trial Labrador Retrievers) is recommended.

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