Figure 2.18 Binocular disparity and the perception of stereoscopic depth. The green diamond is on the plane of fixation of both eyes. It is therefore seen at the same angle by both eyes and projected onto both foveas (fv). Both the purple circle and red square are outside the plane of fixation. Therefore, they are viewed at different angles by both eyes, and projected onto disparate (but corresponding) retinal regions. The purple circle is closer than the object of fixation (the green diamond) and therefore the projection lines from both eyes intersect before the plane of fixation. The red square is further away, and the projection lines from both eyes intersect after the plane of fixation. The α angles of these projection lines, and their intersection, serve as range finders in stereoscopic depth detection.

      Processing Retinal Disparity

      Retinal disparity is resolved in the visual cortex that has the unenviable task of reconstructing a three‐dimensional image from the projection of this image on two two‐dimensional retinas. The segregation between the outputs of the two eyes is still maintained at the first cortical synapse, that is, the simple cells populating layer 4 of the striate cortex. Binocular interaction begins when these cells output to adjacent layers of the striate cortex and to extrastriate visual areas, where many of the neurons receive binocular input and act as disparity detectors. The disparity‐sensitive neurons of the occipital cortex act as “low‐level” detectors of spatial disparity and process stereopsis; additional neurons in the parietal lobe, inferotemporal cortex, and other cortical areas process “high‐level” cues such as texture, shading, and motion to construct a three‐dimensional image of the visual field.

      Stereoacuity

      Stereoacuity is the measurement of the smallest detectable stereoscopic depth. Just like visual acuity, it is measured in arc minutes or arc seconds (see the following section on visual acuity). It is largely determined by the distance of the object, as obviously smaller disparities can be detected for nearby objects than for distant objects. For example, at a distance of 25 cm some humans can detect a depth of 25 μm! Of course, such fine discrimination is not possible for objects 100 m away. However, stereoacuity is also determined by other stimulus parameters such as color, contrast, orientation, size, duration of exposure, location (central or peripheral), and luminance.

Color Vision

Prerequisites for color vision are that the retina has both photoreceptors with different spectral sensitivities that are active under the same background light conditions and circuits where signals from the different photoreceptors are compared. In most mammals, two or three types of cones with different opsins provide the first step in color vision in daylight, but some amphibians have more than one type of rod and are therefore likely to distinguish between hues at night too. The central part of the absorption curve of a photopigment is bell shaped and the overlap of different photopigments' absorption curves will allow perception of intermediate hues. Photopigments are the most common opsin molecules of cones, such as L‐, M‐, and S‐opsins, most sensitive to either long (∼560 nm, greenish‐yellow light, but by convention called the L‐ or red opsin), medium (∼530 nm, green light), and short (∼420 nm, blue light) wavelengths, respectively. This means that even though peak absorbance (or maximum sensitivity) occurs at a primary wavelength, the photoreceptor can also be hyperpolarized by a relatively broad range of wavelengths.

      Humans and Old World primates possess three opsins, thus allowing them trichromatic vision. The green photopigment is the most abundant in the human retina, while the blue is the scarcest. Total color blindness, which is very rare, usually refers to rod monochromacy (or achromatopsia) where the patient has no cones at all and no color vision. Cone monochromats potentially have limited color vision under lighting conditions where both the rods and their single type of cones are active. Most color vision‐deficient human subjects have dichromatic vision, either missing or having a mutated form of the red (protanopia or protanomaly, most frequent), green (deuteranopia or deuteranomaly), or blue opsin (tritanopia or tritanomaly, least frequent). Hence, they perceive colors but, for example, a protanope will perceive red and green objects to have very similar color, but will readily discriminate between isoluminant blue and green (or red) objects.

      Some species are monochromats. Rod monochromacy is mainly found in some fish species, whereas marine mammal species and a few terrestrial mammalian species, including the owl monkey, are cone monochromats.

Most mammals, including cats, dogs, horses, cattle, goats, sheep, and swine, are dichromats, just like human protanopes or deuteranopes. Horses, for example, have cone opsins with peak absorbance in the blue and green parts of the spectrum, making their color vision comparable to human protanopes. Dogs (and cats), on the other hand, have cone opsins most sensitive to blue and greenish‐yellow, making their color vision more similar to that of human deuteranopes (Figure 2.19). When light stimulates the two opsins in a dichromat equally (a monochromatic light of a wavelength that coincides with the intersection of the absorption curves), the retina will not be able to distinguish this wavelength from an achromatic stimulus. This neutral point is reported at about 505 nm in the cat, and 480 nm in the dog and horse. Despite having fewer cones than humans and being dichromats, color vision cues seem to be important during daylight conditions for dogs, and it is likely that other mammalian species also take advantage of their ability to discriminate between different wavelengths to enhance their daily lives, and particularly their sexual and feeding behavior.

      Some dichromats, including many rodents, such as the mouse, rat, gerbil, and Siberian hamster, have a specialized short‐wavelength opsin that peaks in the UV range of the spectrum rather than in the blue. Hence, they have extended the spectral range of the electromagnetic radiation that they can perceive. Furthermore, some dichromats that have “regular” S‐ and M/L‐cone pigments, such as the reindeer and dog (and most likely the cat, too), have lenses transmitting UV light, which enables them to see in the UV part of the spectrum using their regular cone pigments.

      Many modern‐day reptilian, avian, and fish species still have all four ancestral photopigments, including an additional short‐wavelength opsin with peak absorbance in the UV or violet range (355–450 nm) that humans and most domestic mammals have lost, and have thus tetrachromatic vision.

      Birds have developed additional unique mechanisms for color vision. Their double cones are used for fine spatial discrimination (visual acuity), while single cones are used for color vision. Oil droplets found in the cones of birds contribute to color perception by filtering out different wavelengths of incoming light and shifting the wavelength sensitivity of the photoreceptor.

      Visual Acuity

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,