Essentials of Veterinary Ophthalmology. Kirk N. Gelatt

to an 80‐fold increase in humans.

      Dark Adaptation

Dark adaptation is a process in which sensitivity of photoreceptors increases, and their threshold decreases. It takes place in darkness following prolonged exposure to bright light, which causes bleaching of a substantial portion of the photopigment and includes several processes. One mechanism of dark adaptation is biochemical and revolves around the re‐synthesis of rhodopsin from free opsin and from recycled (or newly available) 11‐cis‐retinal (Figure 2.16).

      Photopic Vision

      Light Adaptation

Light adaptation is a process in which cone (and rod) sensitivity decreases, and threshold increases, in response to increased background light intensity and the resulting increased photopigment bleaching. This is a much faster process than dark adaptation. Our eyes begin light adaptation within seconds to a sudden increase in background light intensity, such as that experienced when exiting a dark room. Indeed, it is suggested that the reason for the initial photophobia exhibited when exiting a dark room is an attempt by the eye to preserve the dark‐adapted state of the retina. Several mechanisms account for light adaptation. One is the increased activity of phosphodiesterase, resulting in shorter turnover time for cGMP and accelerating the response kinetics of cones.

      Pupil

      As with scotopic vision, the pupil also contributes to photopic vision because miosis protects the retina from excessive and harmful amounts of light. It is proposed that the large corpora nigra found on the superior (and often the inferior pupil also) border of the iris in some species provides additional protection as it decreases the amount of light entering the eye from the superior visual field (where the Sun is located), further reducing glare and improving vision in bright light. Moreover, because a miotic slit pupil can block light more efficiently than a miotic circular one, it is suggested that slit pupils have evolved in nocturnal or crepuscular species such as cats and geckos that need to function in daytime.

Figure 2.16 In a comprehensive canine ERG protocol, following preparation of the animal in ambient light, the light is turned off. During the next 20 min, the retina is stimulated with a dim flash every 4 min, thus generating a dark adaptation curve. In a normal animal, signal amplitude will increase from one flash to the next as the retina dark adapts (black, red, green, pink, and yellow traces represent the respective responses recorded after 4, 8, 12, 16, and 20 min in the dark). Failure of the signal to increase with time in the dark may be an early sign of rod dysfunction.

Flicker Detection

      Temporal responsiveness of the retina has two aspects: motion detection and flicker detection. The retina responds to flashes of light as long as there is sufficient interval between two consecutive flashes, allowing the retina to recover from one response before the next flash is presented. However, as the frequency of these flashes increases, a point is reached at which the retina does not have enough time to recover between flashes and therefore it can no longer distinguish the individual flashes. At this point, which is termed the critical flicker frequency (CFF) or flicker fusion frequency, the eye perceives a steady light even though this light is made up of numerous flickers. There are separate CFFs for rod‐driven responses and signals generated from the different cone types, just as there are separate adaptation mechanisms for the rod and cone systems.

Motion Perception

      Moving through the environment produces a flow of images projected onto the photoreceptors, resulting in huge amounts of visual information necessary for navigating through a complex environment. Perception of motion is required for both directing visual attention to a certain location in the visual field and segmenting moving objects from their background. Psychophysically, there are various ways of discerning motion: (i) movement can be perceived if an object is moving across our visual field while our eyes and head are stationary; and (ii) motion can also be perceived when either the head or the eyes are moved to pursue a moving object.

Visual Fields, Binocular Vision, and Depth Perception

      It is rather extraordinary to note that all vertebrate species, and many invertebrates, have two eyes. Having more than one eye is required for a large visual field, virtually 360° in some species, and for stereopsis, or depth perception. However, no more than two eyes are required to achieve these aims.

      Visual Fields

The extent of the visual field depends largely on placement of the orbits within the skull (Figure 2.17). Many mammalian prey, avian, and fish species have lateral eyes, providing almost 360° field of view. These animals have a small, frontal binocular field; two large peripheral monocular fields; and a small blind spot behind their head.

      On the other hand, most primate and predator species have frontal eyes. In these species, most of the frontal visual field is covered by a binocular vision; there are two small, peripheral monocular fields and a large blind spot behind the skull. Therefore, enucleation will cause a cat, for example, to lose approximately 30° of one visual field, while in a horse the same procedure will cause a loss of about 145°.

      Large (monocular) visual fields are typically associated with prey species that need to detect predators. This is why these species usually have a pupil and a visual streak whose shapes are aligned with the horizon, allowing them to identify predators and conspecifics.

      Stereopsis

Many associate stereopsis with predatory behavior, as depth vision is required to pounce on prey with precision. However, stereopsis is just as important for the prey that uses it to distinguish between a camouflaged predator and its surroundings. Though monocular stereopsis is possible thanks to various visual cues including grain, texture, brightness, contour, size, and relative motion, in most cases stereopsis is the result of binocular vision, and the extent to which the visual fields of the two eyes overlap. In addition, optimal stereopsis requires normal visual function and refraction, oculomotor control to maintain fixation, and sensory and neuronal mechanisms to extract and process important visual cues.

Figure 2.17 (a) The visual field of the horse showing a frontal binocular field (65°) comparable to that of a dog but with much larger panoramic monocular fields (each spanning 146°) and a very small posterior blind area (3°). (b) Visual field of a cat showing a large frontal binocular field (140°) with relatively small monocular fields (each 30°) and a relatively large posterior blind area (160°). (c) Monocular and binocular visual fields in a typical mesocephalic dog. The dog has a modest frontal binocular visual field (60°) with relatively large monocular visual fields (each 90°) and a posterior blind area of approximately 120°.

      Geometry and Retinal Disparity

If an object is located in the plane of fixation of both eyes, it is viewed with the same angle by both eyes (Figure 2.18). However, objects outside the binocular plane of fixation are