Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

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)