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Visual Acuity

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Visual acuity is the minimal detection power of the eye, or the minimal angle that can be resolved by the eye. There are a number of ways to express visual acuity, but the best known method is based on the Snellen chart. This is determined by the size of letters that a subject can read at a distance of 20 ft, or 6 m. Obviously, determination of Snellen acuity requires verbal cooperation by the test subject and therefore is not applicable in veterinary medicine. In animals, visual acuity can be determined using behavioral discrimination tests, by electrophysiological recordings to determine the smallest pattern that elicits an ERG or cortical response, or by pursuit (i.e., optokinetic) eye responses to determine the smallest stimulus that elicits a tracking eye movement. Acuity is also often determined by theoretical calculations based on cone or RGC density (Table 2.16).


Figure 2.19 A colorful dog, as seen by a normal trichromat (a). In (b), the color information from the photograph has been extracted. The photograph has been filtered to mimic how a protanope (c), a deuteranope (d), a tritanope (e), and a cone monochromat (f) would perceive the same scene. The protanope and deuteranope can distinguish between short and long wavelengths, whereas the tritanope can subdivide the middle‐to‐long wavelengths into different hues.

Table 2.16 Visual acuity in select species.a

Species Snellen resolutionb Spatial frequency (cycles/degree)b Methodc References
Eagle (Aquila audax) 20/4 140 Behavioral and anatomical Reymond (1985)
Falcon (Falco berigora) 20/8 73 Behavioral and anatomical Reymond (1987)
Macaque monkey 20/16 38 Behavioral Merigan & Katz (1990)
Human 20/20 30 Ravikumar et al. (2011)
Horse 20/26 23 Behavioral Timney & Keil (1992)
20/36 16.5 Anatomical Harman et al. (1999)
King penguin Anatomical Coimbra et al. (2012)
Underwater 20/30 20.4
In air 20/40 15.3
Alpaca 20/45 13.4 Anatomical Wang et al. (2015)
Sheep 20/51–20/43 11.7–14 Behavioral Sugnaseelan et al. (2013)
20/86–20/60 7–10 Anatomical Hughes (1977)
Camel 20/60 10 Anatomical Harman et al. (2001)
Dog 20/140–20/52 4.3–11.6 Electrophysiology Odom et al. (1983); Ofri et al. (1993); Murphy et al. (1997)
20/110–20/31 5.5–19.5 Behavioral Lind et al. (2017)
Cat 20/190 3.2 Behavioral Jarvis & Wathes (2007)
20/90 6.5 Electrophysiology Berkley & Watkins (1971)
20/33 18 Anatomical Steinberg et al. (1973); Clark & Clark (2013)
Barn owl Behavioral Orlowski et al. (2012)
20/190 3.2 (Mesopic)
20/500 1.2 (Scotopic)
Rabbit 20/200 3 Electrophysiology Pak (1984)
Cow 20/460–20/230 1.3–2.6 Behavioral Rehkämper et al. (1998, 2000)
Rat Behavioral Prusky et al. (2002)
pigmented 20/600–20/400 1–1.5
albino 20/1200 0.5
Mouse 20/1000 0.58 Behavioral Lehmann et al. (2012)
20/1500 0.39 Optokinetics Lehmann et al. (2012)

a This table mostly contains data for species commonly seen by veterinary ophthalmologists in clinical and research settings. Avian species have been included to demonstrate their high acuity (eagle, falcon) or the effects of aquatic vision (penguin) and light conditions (owl). The scientific literature reports visual acuity values for numerous other species such as giraffes, elephants, rhinoceros and marsupials that are beyond the scope of this book.

b Most articles report visual acuity in cycles/degree. These values have been converted to Snellen units by the author (R.O.) as most readers are more familiar with this latter scale. Values for Snellen acuity <20/200 have been rounded.

c Anatomical methods of assessment include counting the density or cones and/or RGCs; electrophysiological methods are based on recordings of the pattern electroretinogram or pattern visual evoked potentials.

Essentials of Veterinary Ophthalmology

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