There are a number of aspects of vision that can be measured. Visual acuity describes the ability to distinguish the fine details of an object. Clearly, the nearer an object is to the eye, the finer the detail that can be distinguished. Horses’ eyes are geared to be focused largely on more distant features and, like those of many mammal species, appear to have limited accommodation, i.e. the ability to focus on very close objects less than about 1 m away. Animals that have been trained to discriminate between panels painted gray and those bearing black and white stripes can expose their species’ acuity. As panels with ever finer stripes are presented to the observer, the discrimination task becomes more difficult (Fig. 2.3). The point at which the lines blur together and appear to be gray (the limit of acuity) is marked by a failure to discriminate. Most birds that have been tested excel at this task. They can distinguish extremely narrow stripes that occupy as little as th of a degree of their visual field. Horses can perceive stripes that fill about th of a degree.

Figure 2.3 A system of vertical lines can be presented progressively closer to one another to measure visual acuity.

(Reproduced with permission of Alison Harman.)

An interesting way of expressing the horse’s acuity is in comparison with normal human vision, as 20/33. This indicates that a horse can only discern at 20 m what a human can at 33 m,⁷ and this compares favorably with the dog (20/50) and the cat (20/100).⁷ Horses’ eyes are extremely sensitive to movement in all areas of their visual field, but human peripheral vision is considered a good approximation of the visual detail horses can appreciate.⁸

The visual field is affected by the corpora nigra, which are found on the upper margin of the pupillary aperture, possibly as an anti-glare device.⁹ The corpora nigra contain an intricate network of blood vessels, which suggests that they might also be used to oxygenate the anterior chamber of the eye (Alison Harman, personal communication 2002). For horses to see through their narrow pupils, they adjust their head position up and down or side to side. Best frontal vision of the ground in front is achieved when the horse flexes slightly at the poll. Horses commonly hold their heads in this position when they are moving in slower gaits. This was thought to improve focus and enhance images of the ground ahead.⁶ However, when over-flexed so that the nose is behind the vertical, the horse cannot see the space in front of it and so, when being ridden, may occasionally collide with objects, people and other horses if not directed (Fig. 2.4).

Figure 2.4 (A) The visual field in front of a horse when allowed to carry its head naturally. (B) The blind area in front of a horse when over-bent.

((B) Reproduced with permission of the Captive Animals Protection Society.)

A functionally important blind spot is created when a horse is ridden with its nasal planum behind the vertical. The blind spot is formed to the front of the horse, and is believed to be as wide as the body. Thus, when a horse is being ridden in such a fashion it cannot see directly in front of itself. Hyperflexed horses are said to be showing signs of submission and ‘listening to their riders’, but it is possible that compromising a horse in this way makes it more reliant on the rider to avoid obstacles and such that it seems more biddable. Before using physical constraints, such as tie-downs and standing martingales to keep the head down or overchecks to keep it up, one should consider the effect of these restraints on the horse’s ability to convey itself safely over rough terrain and most especially over jumps. The effect on vision is subject to current debate since Bartos et al¹⁰ drew attention to the ability of the horse to rotate its eyeball to maintain the pupil in a horizontal position. These authors claimed that the horse maintains the ‘horizontal eyeball position regardless of head position’, suggesting that this rotation overcomes any deficit in vision that might otherwise arise from the horse having its nose pointed caudally by the rider. However, they did not report the head positions they examined, and their photos seemed to imply that the nose was always somewhat in front of the vertical. They were not concerned with forward binocular vision and did not use any technique such as ophthalmoscopy to determine visual field¹¹ or alignment of the pupil relative to the corpora nigra, so it is difficult to comment on their findings. Some recent data counter the generalization that pupil rotation can overcome all changes in head position, indicating that in extreme head positions the pupil is not parallel to the ground¹², but further work is needed to confirm this.

The horse has mostly monocular vision, i.e. each eye sees a completely different field of view. However, horses have a small binocular field at the front of the monocular fields. Therefore, the horse can adjust its view to overlap the visual fields of both eyes and achieve a binocular view (Fig. 2.5). This binocular field of view allows the horse to observe the ground in front with both eyes.

Figure 2.5 The visual field of one eye of a horse. Horse and human – (A) rear view, (B) front view – are standing looking out over Perth from a viewpoint in Kings Park. So in front of them they see Perth and to the back of them is a man taking a photo of Perth. The human view (C) is of Perth itself and not much more, since our vision is so frontal. Also we see just the very middle in high acuity. The horse (D) by contrast, sees Perth and everything else simultaneously right back to the man taking the photograph.

(Reproduced with permission of Alison Harman.)

To see objects at a greater distance, the horse rotates its nose upwards because its binocular overlap is oriented down the nose (Fig. 2.6). It is believed that when focusing on objects to the fore, horses may momentarily lose the ability to observe from the rear and to the sides.⁷ That said, when taking off for a jump, horses sometimes tilt their head sideways, using their lateral vision to get a better look at jumps as they get up close. Perhaps this is why blinkers have found little favor in show-jumping circles, the traditional source of a multiplicity of gadgets. Blinkers are most effective in preventing shying and have been favored by carriage drivers because they make horses less likely to attempt to turn in the shafts or bolt. It is also suggested that blinkers can render a horse more responsive to voice commands used to increase speed because they prevent it from seeing when the driver is neither carrying nor about to use a whip (Les Holmes, personal communication 2002). Blinkers on racing animals bring rather different benefits – especially, it seems, when used for the first time. It has been suggested that, if it is a generally low-ranking animal, a racehorse that sees another horse approaching from behind is more likely to defer to the challenger if not wearing blinkers. This assumes that more high-ranking horses are more motivated to take the lead in a galloping herd, a hypothesis that has yet to be tested.

Figure 2.6 (A, B) Show jumper approaching a jump with the horse’s head either (A) up or (B) down. Visual field is indicated. (C–E) A view of what a horse sees when approaching a jump with head (D) up or (E) down. (D) shows that the horse sees the jump and also a lot of other things out to the side. (E) shows that if you hold the head down it sees its foot and knee and a bit of the world out to the side. Perhaps this is why a horse may be more compliant when its head is held down, since it is unlikely to make much sense of the visual world. (C) Shows what a human sees over the jump (remember we only really see the middle of our visual field in high acuity).

(Reproduced with permission of Alison Harman.)

It has been shown that the horse’s eye has a visual streak (Fig. 2.7). This linear retinal region contains high concentrations of ganglion cells, while low concentrations appear in the peripheral regions. Concentrations in the visual streak reach 6100 cells/mm², with the peripheral regions ranging between 150 and 200 cells/mm².¹¹ It is worth noting that the distribution of ganglion cells within the visual streak of horses depends on skull shape. Long-skulled (dolichocephalic) breeds, such as the Standardbred, have less centralization of their ganglion cells than the short-skulled (brachycephalic breeds), such as the Arabian, and are therefore thought to have better peripheral vision¹³ (Fig. 2.8).

Figure 2.7 The distribution of ganglion cells on a flat-mount of a horse retina. High concentrations are represented by the peaks.

(Reproduced with permission of Alison Harman.)

Figure 2.8 Retinal ganglion cell density maps from horses of three breeds showing the dorsal part of the retina is in the background, ventral is in the foreground, nasal is to the left, and temporal is to the right. Each shaded band represents 400 cells/mm². Studies of the retinae of horses with different skull shapes have shown that (at least some of) the neural tissue of morphologically diverse breeds differs.

It was long believed that animals with laterally placed eyes and extensive monocular visual fields did not have stereopsis – the ability to see in stereo and perceive depth. However, studies¹⁴ have demonstrated that the horse’s binocular field is an arc of approximately 60° in front of the head, affording good stereopsis and thresholds of depth detection comparable to those of cats and pigeons. These findings indicate that larger interocular distances, as found in the horse, may provide useful depth judgment. This may have arisen because the horse has evolved to make judgments over a range of several meters, whereas ground-feeding birds, such as pigeons, with an extremely small interocular distance, have to focus at a distance of only a few centimeters. Horses also use monocular depth cues to judge distance.⁴ This makes sense because they spend so much of their day with their heads to the ground, a position that makes stereopsis redundant.

Harman et al¹¹ suggest that when a horse lifts its head the binocular area of vision is directed at the horizon, enabling scanning and depth perception. In this position monocular lateral vision is compromised. However, when the head is lowered and the binocular vision is directed at the area directly in front of the head, the lateral monocular fields afford good lateral horizon vision.

Effective use of the binocular field is required when a horse attempts to discern an object that is close and low. The horse is best able to use its binocular field of view by arching the neck and rotating the head. It can focus on the object by simultaneous rotation of the eye downward to optimize orientation of the visual streak (see p. 37).

Factors that affect the visibility of a stimulus for a horse include size of the object, contrast and environmental illumination. When a moving horse spots something underfoot, it not only looks at the stimulus, but is also likely to change speed.¹⁵ The level of arousal plays a part in the recognition of stimuli, an outcome that may be influenced by the horse’s age and training because recognition of distant stimuli on the ground is facilitated by carrying the head at a lower angle. Saslow¹⁵ found that younger animals tend to carry their heads higher and therefore may not notice stimuli as readily as older horses, especially those that have been trained not to carry their neck straight and head high.

Saslow¹⁵ also found that horses were able to discern stimuli better in overcast rather than bright sunny conditions. This suggests that the equine rod-dominated eye may not find bright conditions as favorable as dull conditions. That said, naïve horses are often reluctant to make the transition from a well-illuminated area into relative darkness. This is part of the challenge that trailers (floats in the USA) represent. The high proportion of rods to cones (generally 20:1)¹⁶ gives the horse excellent night vision but insufficient to make them innately fearless of areas that are poorly lit. As we will see in Chapter 4, horses will work to keep a stable illuminated, and this further explains the aversiveness of small dark spaces, including trailers (Fig. 2.9). A reflective layer of cells behind the retina, the tapetum, enhances vision in poor light. Acting like a mirror, the tapetum reflects light back on to the retina, enabling further light to be gathered. The downside of this arrangement is that image quality is somewhat compromised. Because several receptors may be stimulated by incoming light, acuity can be reduced and the image becomes fuzzy. This effect has been likened to the pixelation of a low-resolution digital image.⁴

Figure 2.9 (A) Human and (B) horse visual fields when looking into a trailer (float in the USA).

(Reproduced with permission of Alison Harman.)

Horses have a small degree of ciliary accommodation,¹⁷ which relies on the contraction of ciliary muscles and contractile fibers that extend into the corpora nigra. However, despite weak lens muscles, horses’ optics allow them to see, on the whole, between about 1 m away and infinity, with little need to vary the lens. Accommodation is required if there is a need to see even closer than that with high acuity. Horses rarely need to do that; indeed because the eye’s proximity to objects is generally limited by the length of the nose,¹⁸ things very close are felt via the skin and vibrissae of the muzzle, so highly focused vision is not essential.

According to what was originally referred to as the ‘ramp retina’ theory, it was believed that the distance from the nodal point of the eye to the retina varied so that the dorsal retina was farther away than the more central and ventral regions. This theory was supported by the observation that horses are likely to exhibit characteristic head-moving behavior when looking at things. For example, the head may be raised unusually high and the nose pointed forward when observing an object of interest to the fore. The horse may arch its neck sideways (cock its head) to look at an object of unusual interest beside it (Fig. 2.10).

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