Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 148

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.