Equis ISSN 2398-2977

Neurology: vision testing - overview

Contributor(s): Keith Barnett, Dennis E Brooks, Ruth Morgan, Graham Munroe, Joe Wolfer

Equine Vision

  • Equine vision is designed to detect motion in order to avoid predators while grazing and allow fast flight.
  • To achieve this, the eyes are laterally positioned, relatively far back in the skull.
  • The binocular vision field is limited (65°) with huge uniocular fields left AND right (146°), leading to a very small blind spot (3°).
  • Further adaptations include horizontal pupils (increased lateral vision), and high numbers of rods increasing peripheral vision and response to movement.
  • Large retinal ganglion cells predominate for motion detection.
  • Night vision is improved by the presence of a tapetum and increased rods:cone ratio.
  • Color vision is present (yellow   →   green and blue) but red is poorly differentiated.
  • Refractive errors do occur in horses (slight myopia).
  • Visual acuity is very good, on the order of 20/33 with 20/20 being normal.

Vision testing

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techniques

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Further Reading

Publications

Refereed papers
  • Recent references fromPubMedandVetMedResource.
  • Farrall H & Handscombe M (2000)Equine vision. Equine Vet J31(5), 354-355PubMed.
  • Harman Am, Moore S, Hoskins R & Keller P (2000)Horse vision and an explanation for the visual behaviour orginally explained by the 'ramp retina'. Equine Vet J31(5), 384-390PubMed.


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