Veterinarians are frequently confronted by questions such as 'Why do cats see better at night?', 'Is it true that dogs are color blind?', or 'How sharp is my dog's eyesight?' Vision is a very complex sense that is affected by numerous factors, varies greatly between species and can be evaluated in numerous ways, so there is no simple answer to these questions. This talk will not provide a comprehensive and detailed discussion of the subject, but instead will focus (pun intended) on some of the significant differences in vision between humans, dogs, cats and horses.

Why Does My pet Seem to be Uninterested in Watching TV?

Responses to rapidly flickering lights are generated by cones. In between flickers, cones undergo a brief process of recovery that enables them to generate the response to the next flicker. When the flickers become too rapid, the cones are unable to recover sufficiently between flashes. At this point, the responses of the cones 'fuse', and they generate just one response to a series of rapid flashes. In humans, cone responses fuse at 45 Hz. Therefore, pictures generated by computer or TV screens, which flicker at 50 or 60 Hz, are perceived as one continuous image. However, in dogs and cats, cone responses fuse at 70-80 Hz. Therefore, when watching television, pets can perceive individual flickering images, which probably has a dramatic effect on their interest in the program! Similarly, pets can detect the flickering of fluorescent lights, a fact that may be taken into account when designing the lighting of your clinic.

Does My Pet Have Color Vision?

Color vision is the domain of the cone photoreceptors. Based on wavelength sensitivity of the photopigment contained in their outer segments, four types of cones have been identified, with animals having anywhere from one to all 4 populations. Species that have just one cone population are limited to perceiving different shades of that one color (e.g., rats, with cones sensitive to yellow light). In species with more than one population, 'richer' color vision is possible through activation of different proportions of the various populations.

Contrary to prevalent public opinion, dogs and cats do not 'see in black and white'. Dogs have two populations of cones. One population absorbs light in the blue-violet spectrum (peak absorption--432 nm), while the second population absorbs light in the yellow spectrum (555 nm). This contrasts with humans who have a third population of cones, absorbing light in the green spectrum. Therefore, dogs can be likened to "color blind" (dichromatic) people who lack the green cone population, a condition known as deutranopia: they can see colors, but are unable to distinguish between red and green shades. This means that guide dogs do not distinguish between red and green traffic lights, and rely on changes of illumination to cross streets! Similarly, horses and cattle have cones absorbing in blue and in yellow wavelengths, which means that bulls do not perceive the color of the red cloth used by bullfighters.

Cats, on the other hand, have 3 cone populations, with peak absorptions at 450, 500 and 550 nm. However, numerous behavioral studies failed to reveal rich color vision in felines. In this context, one should remember that dogs and cats have far fewer cones than humans, so one can assume that color vision in these species is not as 'rich' as it is in humans. It is hypothesized that during evolution the number of cones in the retinas of nocturnal species was reduced to allow an increased number of rods, thus enabling more sensitive night vision.

Night Vision

Both dogs and cats have very sensitive night (scotopic) vision. Studies show that the threshold light intensity needed to elicit vision in humans is 6 times the threshold intensity in the cat. Several physiological and anatomical mechanisms account for this improved visual performance in the dark. The first is the amount of light entering the eye. The corneal diameter in the cat is 16.3 mm, and the diameter of the dilated feline pupil is 10.1 mm. In humans, the respective figures are 11.1 and 6.0 mm. Therefore more light can pass through the cat cornea and pupil and reach the retina. Obviously, these differences are inconsequential at daytime, when there is sufficient illumination for vision. However, at night, when "every photon counts", the ability of the cat eye to admit more stray light is very important. It is estimated that the larger corneal and pupillary diameters of the cat cause a 5.2 fold increase in the amount of retinal illumination, compared to humans.

Furthermore, the cat is more capable of exploiting this light, thanks to the tapetum lucidum. This structure, located in the choroid, gives the fundus of most mammals (with the notable exception of primates) its rich color variety. It also has an important functional role, acting as a mirror that reflects light back to the retina. Photons that are not absorbed by photoreceptors are 'wasted' in the eye, as they do not contribute to vision. The tapetum reflects these photons back towards the photoreceptors, thus doubling the probability that they will be absorbed. Once again, such reflectance is of little importance at daytime (and indeed even causes some blurring of vision), but is extremely important at night.

However, the most important factor in determining sensitivity to low light levels is the proportion of rods and cones. Rods are very sensitive to low light levels, and can function in intensities that are 10^-5 of those required by cones. Furthermore, this sensitivity can be increased through neuronal and biochemical mechanisms, in a process called 'dark adaptation'. As Table 1 demonstrates, cats have a much higher concentration of rods than humans throughout the retina, thus contributing significantly to their night vision, while detracting from their visual acuity.

Table. Concentrations of rods and cones.

How Sharp is My Pet's Eyesight?

Sharpness of vision, or visual acuity, is determined by a number of factors.

How Well Does My Pet Focus?

Incoming light must be focused on the retina in order to generate a sharp image. The active focusing process is called 'accommodation'. In mammals, accommodation takes place in the lens. In humans, it is accomplished through changes in the lens curvature. To view distant objects, sympathetic stimulation causes relaxation of our ciliary muscle, resulting in a flatter (discoid) lens; an opposite process, resulting in spheroid lens, takes place when viewing nearby objects. Due to differences in anatomy and physiology of the lens, cats and dogs are incapable of changing the shape of their lens. Instead, they change its position in the eye. When viewing distant objects, the lens is retracted (towards the retina), and it is moved forward for viewing nearby objects. This results in a diminished accommodative capability. The accommodative power of a human teenager is around 15 diopters (D), compared to 3-4 D in the dog and cat.

Does This Mean That My Pet Requires Glasses?

No. Accommodation is an active process that changes the refractive power of the eye, but other anatomical and physiological mechanisms ensure that light will be focused on the retina (emmetropia). Large surveys show that the majority of both dogs and cats are within 0.5 D of emmetropia; even in humans, glasses are rarely used to correct such a small refractive error. It is interesting to note that the refractive error of our pet's changes is affected by habitat, breed and other factors. For example, outdoor cats tend to be near-sighted (myopic), while indoor cats tend to be far-sighted (hyperopic). Similarly, small breed dogs tend to be myopic, and large breed dogs tend to be hyperopic.

The Effect of Retinal Anatomy on Visual Acuity

As noted previously and demonstrated in Table 1, the 'evolutionary price' for improved night vision is a reduction in the number of cones and the resulting visual acuity. Furthermore, the acuity of feline cone responses is only 25% of the human cones. And the tapetum, which is so helpful for night vision, causes scattering of light and visual blurring in daytime.

So My Pet Sees Quite Poorly?

In terms of visual acuity, the answer is 'yes'. Visual acuity is typically expressed as a 'Snellen fraction'. The acuity of normal humans is 20/20 (or 6/6 under the metric system). Reported values in animals vary greatly, as there are numerous methods of determining visual acuity (behavioral, electrophysiological and optokinetics being the main ones). However, on average it is estimated that the visual acuity of the horse is 20/32 (or 6/10), meaning that a horse needs to be 20feet from an object in order to see it as well as a person standing 32 feet away (or 6 and 10 meters, respectively). The estimated acuity in the dog and cat is worse, and is reportedly 20/75 (or 6/22) in the dog and 20/150 (or 6/45) in the cat. This means that a cat has to be more than 7 times closer to an object to see it as sharply as we do.

Conclusions

Compared to humans, animals have inferior color vision, binocular vision, accommodative capabilities and visual acuity. In cats, binocular and color vision may be more similar to humans than to dogs. However, animals have superior night vision and flicker detection. They are also likely to have better motion detection and low contrast vision. These properties enable dogs and cats to see well at night, while we are left groping in the dark.

References

1.  Ofri R. Optics and the Physiology of Vision. In: Gelatt KN. ed. Veterinary Ophthalmology, 4th ed. Ames, Iowa: Blackwell Publishing, 2007; 183-220.

2.  Miller PE. Structure and Function of the Eye. In: Maggs DJ, Miller PE, Ofri R. eds. Slatter's Fundamentals of Veterinary Ophthalmology. St Louis: Saunders Elsevier, 2007; 1-19.

3.  Miller PE, Murphy CJ. Equine Vision: Normal and Abnormal. In: Gilger BC. Ed. Equine Ophthalmology. St Louis: Saunders Elsevier, 2005; 371-408.

4.  Gegenfurtner KR, Kiper DC. Color vision. Ann Rev Neurosci2003;26:181-06.

5.  Ofri R, et al. Visual resolution in normal and glaucomatous dogs determined by pattern electroretinogram. Prog Vet Comp Ophthalmol 1993;3:111-6.