# What Horses and Humans See: A Comparative Review

Authors: Jack Murphy; Carol Hall; Sean Arkins
Journal: International Journal of Zoology
 (2009)

---

## Abstract

Adaptations of the mammalian eye have tailored each to its own particular ecological niche. On the one hand, it would appear that the horse is best served by a system that can keep “half an eye” on everything, while the human benefits from focussing on more specific aspects of the visual array. By adapting a range of techniques, originally used to assess human visual ability, it has been possible to compare the human visual experience with that of the horse. In general, the results of the majority of these comparative studies indicate that the visual capabilities of the horse are broadly inferior to the human equivalents in acuity, accommodation, and colour vision. However, both the horse and human abilities to judge distance and depth perception may be quite comparable while equine vision is certainly superior to that of human's under scotopic conditions. Individual variation in visual ability, which is routinely taken for granted in humans, is also likely to occur in the horse. Such variation would undoubtedly affect equine performance, particularly in terms of expectation of athletic competitive outcomes in modern equitation. In addition to such considerations as conformation and athletic ability, a detailed assessment of the visual ability might contribute to a more accurate prediction of future performance characteristics in the horse. Although further investigation is required in order to appreciate fully both the capabilities and limitations of the equine visual system, the information currently available should now be considered and applied more rigorously both in the design of the equine environment and in the implementation of contemporary equine training methods. This need is the greatest in areas of equestrian sport where the outcomes of either or both equine and human visual judgements can be critical, the cost of failure often high and occasionally results in fatal consequences for both parties of the horse-human dyad.


---

## Body

## 1. Introduction

Historically there has been much
discussion and debate as to the visual ability of the horse (Equus caballus).
In the earliest known work on the horse,
written twenty three centuries ago, Xenophon links the possession of a
prominent eye with the ability of the horse to see over great distances
[1]. More recently but still
over half a century ago, Walls [2] reported that horses appeared to enjoy excellent vision
based upon Arab legend concerning the ability of the Arabian horse to recognise
their masters from afar. Shortly thereafter,
a more scientific approach to the assessment of equine visual ability was
adopted by Grzimek [3] who also later pioneered a number of studies into
other aspects of equine behaviour [3]. Further advances in methodology and technology
have resulted in additional conclusive findings, but the value of the early
work should not be underestimated. For
example, despite a series of inconsistent findings [3–6], the most
recent investigation into colour vision in the horse has largely substantiated
Grzimek’s early work [7, 8].A number of different methods are now
used to investigate visual perception in animals where the structure and
adaptations of the eye provide the initial evidence of “potential visual
capabilities” within any species. Thereafter
any theories regarding such potential visual capability then need to be
demonstrated behaviourally in order to confirm or refute the predictions made. The amount of information available had been limited,
at least in some considerable part by, not only design features of the various
experimental protocols, but also the time it takes to conduct such behavioural
trials [9]. However,
a number of recently developed techniques have been shown to produce consistent
results and will allow more accurate predictions to be made in the future. One of our objectives here is to evaluate the
correlations found between the different approaches used when assessing equine
vision and then subsequently to highlight the areas that warrant further
investigation.The horse is unique with respect to
its relationship with humans. Although
this close partnership may have resulted in a number of misconceptions in
relation to the visual ability of the horse [10] it also means that
there is a requirement to find out more. While it may never be possible to view the
world through the eyes of a horse, by comparing aspects of equine and human
vision, the similarities and differences can be explored. This is particularly relevant during ridden
work when two separate visual systems have to coordinate and control what is
essentially one pattern of movement such that any resultant athletic performance
is the consequence of two different perceptual experiences. A further objective of this review is thus to
compare equine and human vision and to identify visual features that are
salient to either the horse or the human, or to both. The adaptations of the equine visual system
will be compared to human visual perception and the application of these
findings to ridden work will be discussed. Although the ultimate aim should be to gather
together enough information to allow an appreciation of the overall visual
experience of the horse, such a goal is still some way off.


## 2. Adaptations of the Equine Visual System

The equine visual system displays
adaptations to particular environmental conditions, accentuating those features
that are most important for the survival of this herbivorous prey species. Feral horses spend approximately 50–60% of their time grazing with their heads
lowered and their eyes near ground level [11]. The horse's requirement for large amounts of
fibrous herbage [12], and the open grassland that provides this,
leaves the horse vulnerable to predation. The horse is subject to little or no threat
from aerial predation, and typically, predators include wolves, lions, and
snakes such that ground level visual stimuli are particularly salient to the
horse. Furthermore the risk of predation
is not limited to daylight hours and, similar to the larger carnivores and some
other ungulates (e.g., cows and sheep); the horse is active during both the day
and night. Although feeding activity in
the horse has been found to occur throughout the day and night, this activity
peaks after dawn and before dusk [11]. Therefore this crepuscular lifestyle requires
a visual system that is sensitive in low light levels (scotopic conditions),
provides early warning of approaching predators, and can assess the immediate ground
conditions quickly for a speedy escape from imminent danger.


## 3. The Visual Field

The position of the equine eye and
resultant visual field varies greatly from that of the human. The frontally placed human eyes convey a
number of advantages where each eye has a visual field of approximately150°
with a large degree of binocular overlap. The consequence of receiving similar visual
information from two eyes simultaneously is to increase visual acuity (the
ability to see detail) and to provide information about the distance and
solidity of objects [13, 14]. The position of the human eye also means that
visual attention will be focussed on stimuli that are positioned directly
ahead. In contrast, the lateral position
of the equine eye provides the horse with almost panoramic vision, which
facilitates maximum detection of predators at the expense of the advantages of
binocular overlap.The equine eye is large and
prominent, being housed within enclosed orbits located laterally in the skull [15]. Together with the size and
curvature of the cornea, size and horizontal shape of the pupil, and angular
extent of the retina, this provides the horse with extensive monocular vision. In particular, from an anatomical perspective,
a marked forward prolongation of the nasal retina provides the horse with a
wide visual field [16]. The retinal field of view of each eye in the
vertical plane has been estimated (using measurements based on the position and
dimensions of the equine eye) at 178° [17], while that in the
horizontal plane is slightly greater at approximately 200° [18]. Furthermore, the elongated
horizontal pupil also allows much wider lateral vision than is possible with
the circular pupil of the human eye [19].The binocular overlap of the visual
field is located down the nose of the horse and is limited to between approximately65° [16] and 80° [18]. Two blind spots have been identified within
the equine visual field, one in front of the forehead [18, 19] and one directly behind the horse [19]. It has been noted
that variations in the settings of the eyes and head carriage, which are
characteristics of different horse breeds, can affect the extent of these blind
spots [20]. In breeds of horses where
the mane, in particular in the region of the forelock, is long and thick,
vision may also be affected, Figure 1. However,
given the length and mobility of the neck, the horse can bring such areas into
view quickly. With the head lowered, the
horse has the benefit of being able to scan the lateral horizon for potential
threats with its monocular fields, while assessing the ground surface for both
suitable food and safe footing with its binocular field [18]. Hanggi [21] reported that
horses possess interocular transfer where stimuli seen with one eye are also
recognised by the other eye.Figure 1
The position and extent of the binocular overlap in the region of the inferior visual field of the horse (approx.65°: [16]). The extent of the monocular visual field, blind spots, and angle of the optic axis (approximately 40° from the midline: [15]) are also shown.Although relative to body size some
mammals possess larger eyes than the horse, in absolute dimensions, the eyes of
the horse are the largest [22]. Total eye volume has been  reported in the
ranges 45–50.9 mL [23] and 42–62.5 mL [24] and the variation does not appear to differ as a function of
the breed of horse [24]. Andersen  and Munk [25] reported that, in addition to such large internal space
within the eye, the circumference of the equine retina was greater than that
found in the eye of even the much larger bowhead whale. The large globe size and extensive retinal
surface ultimately result in an image magnification that is 50% greater than in
the human eye [26].
Clearly, this point should be taken into consideration when features of
the retinal structure are used to make predictions about the visual acuity of
the horse.


## 4. Accommodation: The Ability of  the Eye to Focus

The refraction required to focus an
image on the retina depends upon the curvature of the optical surfaces (cornea
and lens), optical media, and the thickness of the optical component. In both the horse and human, the anterior
surface of the cornea represents the primary ocular refractive surface
[19, 27]. More
precise focusing is the result of slight changes in the shape of the lens in
response to the relaxation and contraction of the ciliary body, resulting in
changes in the refractive power of the lens. This mechanism serves to maintain a focussed
image of objects viewed from different distances and is termed accommodation
[27].Previously, horses were thought to
lack this dynamic lenticular accommodation ability [28].  Indeed there is
still some support in modern veterinary textbooks for the unique “ramp retina”
theory previously attributed to equines. This theory assumed that the distance from the
lens to the retina was variable with the dorsal retina being further from the
lens than more central regions. Static
accommodation occurred as a result of the image being focused on different
areas of the retina, as brought about by changes in the angle of the head of
the horse. Walls [2] and Duke-Elder [29] suggested that there was no evidence to support the
occurrence of lens-adjusted accommodation in studies conducted on equine
subjects.  However, it has since been
demonstrated that the equine lens is capable of limited accommodation [18, 30] and that only a small magnitude of change in refraction
(less than 2 diopters) is required to maintain a focused image on the retina
[30]. The movements
of the head that were previously associated with the “ramp retina theory” in
the horse have now been linked to the utilisation of the equine binocular field
[18].Essentially, the focal point must
correspond to the retina in order to perceive a clear image. When this occurs it is termed emmetropia, and failure
of the eye to locate the focal point on the retina is the consequence of
refractive error. Human subjects can
experience two forms of refractive error, hyperopia and myopia. In hyperopia the focal point is posterior to
the retina and in order to bring images into focus the refractive power of the
eye must be increased. This is
accomplished by continuous accommodation, which if sufficient, will result in a
clear focused image. However, such uncorrected
hyperopia will often result in eyestrain and sometimes headaches [27]. Less accommodation is required to
view objects in the distance than those close nearby and during myopia the
focal point is anterior to the retina such that the objects viewed from nearby
are seen most clearly. In humans, it has
been estimated that approximately one in five people is myopic, while one in
three is hyperopic. However, it is now
also known that the incidence of both hyperopia and myopia varies considerably
in relation to age, race, and geographical location [27]. Many common daily visual demands of the human
lifestyle require adjustments of refractive errors in order to optimise
performance in routine tasks such as, for example, reading and driving a car.The horse does not appear to exhibit
significant refractive errors, but there is a tendency towards hyperopia [22, 26]. Calculations, based on the equine eye being a
general lens system, were made in one study of the physical optics of the
equine eye. The eyes of three mature
horses were assessed and they were all found to be hyperopic [22] and therefore based on their findings, Knill et al. [22] concluded that the horse should have excellent visual acuity for distant
objects.   Farrall and Handscombe [26]
assessed the refractive errors of 30 eyes (from 15 horses) and found 7 eyes to
be myopic, 10 were emmetropic, and 13 were hyperopic.The ability to see distant objects
most clearly is an obvious advantage to prey species and it has been noted that
many wild mammals tend towards hyperopia [31] and so it is indeed likely
that the Arabian horses could easily recognise their masters from afar [2]. It is also likely that horses,
like many other mammalian species, may experience difficulty in focussing on
objects less than 1 metre away [18, 20]. As in other domestic
species there is some evidence to suggest that domestication and, perhaps
inappropriate stabling in particular, have had the effect of altering the hyperopic
vision in the horse and inducing a situation of equine myopia as a consequence [22, 31]. However, Harman et al. [18] failed
to find any evidence of this development in the horses used in their study.Another factor that should be
considered, when assessing the ability of the horse to focus on images at
different distances, is the size of the aperture (the pupil) through which the
light passes into the eye. In general,
the smaller the aperture, the greater the range of distances that will appear
focused, and as the pupil size decreases, the depth of focus increases
[32]. The human pupil can
alter in diameter and attain the optimum size for resolution of 2.0 millimetres
[33]. However, in the
horse, the horizontal pupil while allowing the horse a wide visual field, does
not enhance resolution. In fact, the
size of the horizontal pupil is reduced by relatively weak circular muscles
[15], which limits the depth of focus available to the horse. Consequently, it is thus extremely likely that
while the horse may be able to focus on specific images, other areas of the
visual array will appear more blurred.


## 5. The Perception of Depth and Distance

The frontal placement of human eyes,
while limiting the overall field of vision, provides the advantage of a wider
range of binocular overlap. Stereopsis
is the ability to see depth based on binocular disparity, and it has been cited
as one of the more beneficial advantages associated with frontal eye placement
[2]. However, notwithstanding
the lateral placement of their eyes, it has also been demonstrated that horses enjoy
a full range of stereoscopic skills [34].  While equine stereopsis occurs within a
limited field, it has also been shown that the horse can use other additional
cues to judge depth [35]. In fact, physiological evidence supports the
existence of stereopsis in other lateral-eyed animals as discussed by Hughes [17], and binocular neurons have been found in the cortices of a number of
species, including the rabbit [17], goat [19], sheep [19], and guinea pig [36].Timney and Keil [35] provided the
first behavioural demonstration of a full range of stereoscopic skills in a
lateral-eyed animal, the horse.  The two
horses in the study were trained to discriminate between stimuli on the basis
of whether or not the target stimuli protruded from a screen (in other words
either a 2-D image or 3-D image).  The
findings demonstrated that when the horses viewed the target stimuli binocularly,
their performance was better than when the stimuli were viewed
monocularly.  In fact, depth measurements
from the binocular thresholds were several times superior to those obtained
monocularly and the authors  concluded that horses would therefore use stereoscopic
information preferentially when it was available [35]. In addition to the binocular overlap within
the visual field, the horse has a relatively large interocular distance. Human subjects experienced an enhanced
sensation of depth when the disparity of their retinal images increased
and this has been cited as further
evidence of depth perception in the horse [35, 37].The horse has also been shown to be
sensitive to pictorial depth cues [34].  The two horses in
this study were trained to make a relative-line-length discrimination. They were able to distinguish between two
lines reliably: a lower line 10 cm long from an upper line of 14 cm long when
at a viewing distance of approximately 160 cm. Timney and Keil [34] also investigated
whether horses were susceptible to a visual illusion, which was generated by
pictorial depth information. During the
experimental protocol, two lines of equal length were superimposed on a
photograph of railway lines (presenting many pictorial cues to depth) and on a
pastoral scene with much fewer depth cues and the horses overwhelmingly chose
the railway track display. In human
subjects, if two lines of equal length are superimposed on two flanking lines
tilted inwards, the upper line looks longer than the lower line and is known as
the Ponzo illusion.  This effect is even
greater during a situation such as if the lines are superimposed on a
photograph containing perspective and other cues to depth. As in humans, the horses tested with the
railway track photograph and pastoral scenes were found to be susceptible to
the Ponzo illusion. Timney and Keil [34] concluded therefore that horses would utilize pictorial depth cues, as
do human observers in similar situations.The importance of the equine binocular
portion of the visual field for judging distance was discussed by Harman et al.  [18]. In order to direct
this binocular portion forwards, the horse must be allowed to raise its head. This is of particular importance in equestrian
competition such as jump racing or showjumping when the horse approaches
obstacles/fences at speed. Both horse
and rider must use and interpret visual information accurately in order to
negotiate the obstacles/fences successfully. In a study by Laurent et al. [38], it
was found that riders consistently focused their gaze centrally towards the top
of the obstacle to be jumped. This
suggests that the riders used the retinal expansion rate of the obstacle to
assess the time to contact (takeoff point). Given the evidence that other animal species
demonstrate an appreciation of size constancy [39, 40], the speed at which the size of an approaching object
increases may also provide the horse with the means of judging distance. For both horse and rider, accuracy of takeoff
would thus be enhanced by prior experience of approaching obstacles at varying
speeds. However, if the speed of
approach causes the retinal image to expand faster than can be processed by the
horse or rider, mistakes are likely to occur in judging the distance from the
obstacle and thus in assessing when to initiate takeoff.The ability to utilize visual cues to
assess spatial relationships between objects has been linked to head movements.
In humans, when the head is moved from
side to side objects that are viewed from different distances appear to change
position relative to one another. Nearby
objects are displaced to a greater extent than more distant ones and this
information is utilized to make judgements about relative distances between
them. This phenomenon is referred to as
motion parallax and may occur as a result of eye movements in addition to
movements of the whole head. Human
vision has been shown to exploit such information in addition to binocular
stereopsis in the judgement of depth [41, 42]. Motion parallax, as
evoked by head movements, has been found to contribute to distance estimation
and depth perception in insects and probably occurs in birds and rodents also. However, there is currently a lack of
conclusive evidence for the role of head movements (or how head movements
affect motion parallax) in larger mammals such as the horse [43].As in a number of other species,
including humans [44, 45], gender differences
in visuospatial ability have been reported in the horse [46]. This ability has been defined as
the mechanism by which an individual monitors changing spatial relationships
that occur as they move through their environment [47]. Such changes are inevitably
affected by the speed at which the movement occurs and the precise means by
which they are monitored may vary as a result. In all species investigated to date, including
horses, males have exhibited superior visuospatial ability compared to females
[45, 46] and this difference in
visual ability may contribute to the relative success of male as compared to
female horses [48, 49] at the highest level of competition. There is no evidence however as to which of
the possible visual mechanisms responsible for the perception of depth and
distance may be sex-linked. Further
investigation is required in order to determine whether the ability to judge
distance at speed varies between male and female horses, or indeed between male
and female riders. Regardless of any potential
differences, between males and females of either species, it is likely that the
horse, being more familiar with travelling at speed than most humans, is better
able to judge distance when approaching any obstacle rapidly than its human
counterpart.


## 6. Visual Acuity

The ability of the eye to transmit
detailed images depends upon the amount of information available from the retina
and is clearly of particular importance in many human activities. The retina consists of an outer layer of
photosensitive cells and an inner layer of nerve cells. Two main classes of photoreceptor are present
(rods and cones) and both photoreceptor types synapse with bipolar cells, while
these bipolar cells in turn synapse with retinal ganglion cells. The rods are responsible for vision in
scotopic conditions, but because a large number (up to 45) synapse with each
bipolar cell they provide poor spatial resolution compared to the cones
[50]. The cones are less
sensitive to low light levels, but result in better spatial resolution as a
result of their neural connections—in the human fovea each will connect with a
single bipolar cell. The cones also
respond to light more quickly than the rods, resulting in improved temporal
resolution [50]. This in
essence is why the cones are responsible for vision during photopic (bright
light) conditions.As a result, an individual's visual acuity
can be estimated by assessing the type of photoreceptors that are present, and
their connections with bipolar cells, as well as the size and density (and, by
implication, the receptive fields) of the retinal ganglion cells. Within the retina of the human eye, the
central area of the fovea consists entirely of cones. As the number of cones decreases rapidly with
increasing distance from the fovea, so at the same time do the numbers of rods increase
rapidly [51]. Within
the fovea, each retinal ganglion cell connects with a single photoreceptor
[52] which ensures that this area is capable of the greatest
acuity. Acuity varies to some degree in
the human, but the limit of resolution of a normal observer lies between 50 and
60 cycles/degree [53].  Moreover this level of acuity is confined to the central region of the
fovea, with values of between 35 and 40 cycles per degree attributed to the greater
part of the visual field [54].  An area comparable to the human fovea exists
in the retina of the equine eye. The
distribution of retinal ganglion cells in the horse has been measured, and a
streak of high cell density exists along a straight horizontal line, dorsal to
the optic disc [55]. The cell density
has consistently been found to be greatest at the temporal end of this visual
streak [18, 55, 56],
corresponding with the area responsible for binocular vision. The equine ganglion cells were found to be
relatively small within the visual streak and of a fairly uniform size, whereas
in the peripheral retina there was a mixture of all sizes of ganglion cells
[55].However, a number of different
studies have produced conflicting evidence as to the visual acuity of the
horse. François et al. [57]
concluded that the horse would have poor vision, particularly under photopic
conditions, as a result of the predominance of rods over cones in the entire
retina. More recently Ehrenhofer et al. [58] reported the
existence of large gaps between ganglion cells in most parts of the equine
retina and that the majority of these ganglion cells were not only very large but
they were also associated with many amacrine cells in turn. (Amacrine cells lack long fibrous processes).  The speed of conduction (fast) of the axons
of these large ganglion cells and their connections with the amacrine cells
suggests that the horse would be particularly sensitive to subtle changes in
illumination and stimulus motion. It is
therefore, only in the area of the visual streak and a small area close to the
optic disc (the area centralis) where there is a well-balanced ratio of
photoreceptor, bipolar, and ganglion cells, that the horse possesses any real
visual acuity [58].In another study, anatomical data
provided an estimate of peak visual acuity in the area of the visual streak of
about 16.5 cycles/degree, with far lower acuity (3.3–3.5 cycles/degree) in other retinal regions [18]. This estimate is
very similar to the 16.4 cycles/degree that was obtained in an earlier study by Hughes
[17], but slightly higher than the best value of 12.3 cycles per
degree obtained by Ver Hoeve et al. [59]. A behavioural study that focussed on visual
acuity in the horse had resulted in a range of values, with the best acuity
obtained being 23.3 cycles/degree and the lowest being 10.9 cycles/degree
[60]. Individual
variation in both the size (of the horse) and the  horses' approach
to the task was considered to be the cause of the variation in the Timney and Keil [60] behavioural study. Furthermore Evans and McGreevy  [24] have just recently reported that ganglion cell density
within the visual streak varied between different breeds of horse and that
horse breeds with longer heads may have better visual acuity than those with shorter
heads. A similar situation has been
observed in dog breeds where a correlation has been found between nose length
and ganglion cell distribution. A
regular horizontal streak has been reported in long-nosed breeds (and in the
wolf) whereas a strong area centralis with virtually no streak was found in
short-nosed breeds such as the pug [20].At this point in time, calculations
of equine visual acuity, which have been based on both physiological and
behavioural data, have produced some variable results. This may in part be the result of individual differences
among horses, but it has been shown by Pesudovs et al. [61] that different methods of measuring visual acuity might actually generate different results too. It
is also likely that results based on calculations of a single factor, such as
retinal ganglion cell density, will be less accurate than when multiple visual
mechanisms are considered.  Despite these
reservations, it is generally accepted that visual acuity in the horse, at
least in photopic conditions, is poor in comparison with human vision. It is likely however that, perhaps unlike the
human, the horse does not necessarily require the ability to see in great
detail in a great many instances. Other less
detailed visual attributes such as motion detection and an awareness of the
surrounding environment may be far more important for the horse. On the other hand and as in the human, it is
likely that visual acuity in the horse will vary to some degree according to a
number of different factors such as breed, age, previous experience, and general
health status. However, unlike the
individual clinical routine assessments regularly conducted in humans, there
has been no evaluation of how this visual ability varies in the horse, or
indeed how such variation may relate to or influence athletic performance or
participation within equestrian competition.


## 7. Scotopic Vision

Based on physiological data, [17] it is calculated that the horse, owl, dog, and grey squirrel have
similar light collecting power even in
spite of the difference in the size of their eyes.  Furthermore, these values rank below those
calculated for cat, rabbit, and rat based on the values recorded for maximum
pupil diameter. Hughes [17] also
contrasted both daylight and nocturnal vision characteristics between horses
and humans and reported that while human daylight vision is most acute,
nocturnal vision in the horse is superior to that in humans.  As we have previously stated, horses like
other herbivorous ungulates are active during both daylight hours and during
darkness at night and their visual system has subsequently evolved to function
in both scotopic and photopic conditions. However, even though equine retinae contain
both rods and cones, the former outnumber the latter by approximately 20:1 [62], although within the area of the visual streak, a
higher percentage of the photoreceptors (approx. 17%) were found to be
cones [57]. Cone
densities vary from approximately 5000 per square millimetre in the peripheral
retina to 15 000–20 000 per square millimetre in the visual
streak [63]. This
is considerably lower than the mean peak density in the human fovea of 161 900
cones per square millimetre but comparable with the cone concentration found in
the peripheral human retina where cone density has been measured as less than
5000 per square millimetre [51]. This is further supporting evidence that human
visual ability is likely to be superior in photopic conditions, at least in the
area of the fovea, whereas the horse is likely to have the visual advantage in
scotopic conditions.The ability of the horse to see in
levels of low light is also enhanced by the presence of an intraocular
reflecting structure, the tapetum lucidum [64]. This structure is present in species that are
not strongly diurnal (e.g., carnivores, rodents, sheep, cows) and represents an
adaptation for dealing with scotopic conditions. The tapetum lucidum increases retinal
sensitivity by reflecting light back through the photoreceptor layer [64], but at the expense of resolution by the scattering of this
light [55]. Moreover, the lower
margin of the choroidal tapetum fibrosum of the horse coincides with the
location of the visual streak [55]. It extends horizontally from the surface of
the optic disc almost to the equator, and to a comparable distance above the
optic disc, forming a rounded triangle in the upper half of the retina
[64]. The
position of this reflective layer provides a mechanism that increases
sensitivity to light, particularly to that reflected from the ground [10] and in contrast to the forward-directed human gaze, ground level visual
stimuli are therefore most salient to the horse [65, 66].Thus the lifestyle of the horse is
better served by visual sensitivity in low light levels at the expense of the
ability to see objects in great detail. Light
enters the equine eye through the horizontal pupil, which while allowing the
horse a wide visual field, does not enhance resolution and is slow to adapt to
differing light levels. The human pupil
can alter in diameter in response to changes in the level of light and attain
the optimum size for resolution of 2.0 millimetres [33]. Contraction of the circular muscles within the
iris quickly reduces the size of the pupil in response to bright light, thus
preventing damage to the light sensitive retina. In contrast, the size of the horizontal pupil
of the horse is reduced by relatively weak circular muscles and adapts
less quickly to changes in light levels [15]. To prevent actinic damage during grazing, the
dorsal papillary margin has specialist projections (the corpora nigra), which
act to shade the lower half of the retina and these projections are also
thought to contribute to constriction of the pupil [23]. This adaptation, by limiting the extent to
which light enters the eye from above, also serves to accentuate information
from the inferior visual field. As a
result, however, the horse is subsequently more easily “blinded” by exposure to
sudden bright light.  This presents practical
difficulties for the horse when navigating between abrupt changes from dark to
well lit areas (and vice versa) such as from shaded areas into bright sunlight
and this is likely to have implications for certain aspects of equitation. Saslow [67] demonstrated that stimuli
viewed in overcast, rather than sunny daylight conditions, are actually clearly
visible to the horse.  The external
features of the equine eye (Figure 2) include the iris, which in terms of
colour generally varies from dark brown to golden brown to yellow, occasionally
blue or white [68].  While these
features have provided beneficial adaptations to the horse in low light levels,
the presence of cone photoreceptors in the equine retina and diurnal activity
patterns would also suggest that the horse enjoys the benefit of colour vision.Figure 2
External features of the equine eye (right eye), including the horizontal pupil, corpora nigra, lateral and medial canthus (angle at either end of the eyelid aperture), and iris.


## 8. Colour Vision

The perception of colour requires at
least two spectrally different classes of receptor, together with neural
pathways capable of comparing their output [69].  An animal is said to possess colour vision if
it can discriminate between stimuli that differ in their distributions of
spectral energy, independently of any differences in total energy [70].  The ability of the horse to
perceive colours has been a matter of debate for quite some time and of the
two main types of photoreceptor cell present in the retina of the horse, the
cones are responsible for both photopic vision and the ability to see colour
[71]. Notwithstanding the array of inconsistent
findings among behavioural studies [3–6] and the studies that have
investigated the spectral sensitivity of the cone photopigments of the equine
retina [8, 54, 59, 72–75], the overall and generally accepted conclusion is that the
horse possesses, at least, a form of colour vision.The most common form of mammalian
colour vision is based on the possession of two cone types (dichromatic), one
having maximum absorption in the short wavelength end of the spectrum, the
other in the middle to long wavelengths [70]. In contrast, however, human colour vision (and
that of Old World monkeys, apes, and one New World group, the howler monkey) is
based on three cone types (trichromatic), with maximum absorption in the short,
middle, and long wavelengths [76]. Values of approximately 420, 534, and 563 nm
have been obtained for the spectral peaks of these three cone types in the
human [77], although some individual variation in the
long wavelength peak has also been reported [78]. Essentially, identification and classification
(naming) of different colours relate to the appearance of the visible spectrum
to these trichromatic humans. The
specification of these colours is typically universal, with eleven  named
colours currently in common use in all fully developed languages for
the labelling of surface colours [79, 80]. (The eleven named colours
are as follows: red, orange, yellow, green, blue, purple, black, white, brown,
grey, and pink [80].)  A small percentage of the human population
has defective colour vision and this so-called “colour blindness” is more
common in males than females, with approximately two percent of the male
population lacking one class of photopigment, resulting in dichromacy [81]. Human dichromacy results
from the absence of one of the three cone photopigments such that individuals
with defective vision are described technically as protanopes, deuteranopes and
tritanopes.  These individuals are lacking
of long, middle, or short wavelength photopigments, respectively [82].As a consequence, the perception of colour
based on the output of only two types of cone photoreceptor results in the division
of the visible spectrum into two hues, instead of the range of spectral colours
perceived by the trichromat [83]. The perception of neutral colours (colours
that are neither bright nor intense and have low saturation) is thought to be
the same for both trichromats and dichromats [81],
where the ratio of output from the different cone types results in an
achromatic perceptual experience. This phenomenon
is termed the neutral point and essentially divides the spectrum into the two
hues as perceived by the dichromat [83].  The horse possesses at least two types of
cone photopigment, one maximally sensitive to medium-long wavelengths, the
other to short wavelengths and the horses is thus termed dichromatic. Values for the spectral peaks of these
photopigments have been estimated at 545 nanometres (nm) [59, 72, 75]) and 429 nm [72]. These values vary slightly from those obtained by Campbell and Green  [54], but they are both consistent
with the range found in other ungulates [70]. The dichromatic neutral point has also been
located in the horse at about 480 nm [84]. Behavioural evidence is then required in order
to determine whether or not these findings are accurate predictors of the
ability of the horse to actually perceive colour.  The neutral point test has been used to
assess colour vision in animals and evaluates the subject's ability to
discriminate between equiluminant monochromatic and achromatic lights. The test essentially determines if there is a
band of wavelengths at which an animal’s  performance drops to levels of chance
alone.Behavioural studies into the ability
of animals to see colours have generally involved training the subjects to
discriminate between chromatic and achromatic stimuli, where all other cues (in
particular differences in lightness, olfactory, and spatial cues) have been controlled
or typically made irrelevant. Until quite
recently only four colours had been tested on the horse in this way, with
inconsistent results [3–6]. Although
all of these studies concluded that the horse had the ability to discriminate
blue from grey, the results for red, green, and yellow were variable. For example, Grzimek [3] found that the
horses in his study could discriminate yellow best, followed by green and then
blue, but had some difficulty with red. The
subjects tested by Pick et al. [4] could reliably discriminate blue
and red from grey, but encountered difficulty with the colour green. Similar results were obtained by Macuda and Timney [5] whose horses could also discriminate blue and red from grey, but could
not discriminate either green or yellow from the grey colour. Smith and Goldman [6] found some variation
in the ability of the horses that they tested and although all the horses in
this study successfully reached the criterion for learning (85% correct
responses) with red and blue—one of the subjects performed
at chance levels only for yellow and green.One of most recent behavioural studies
of colour vision in the horse [7] investigated the
correlation between the predicted effect of various colours on the
photopigments of the horse and their ability to discriminate the colours from grey
colours. Using the values for the peak
sensitivities of the cone photopigments of the horse (429 and 545 nm: [72]), the effect of fifteen colours from across the spectrum on the equine
visual system was assessed using modelling techniques of Govardovskii et al. [85]. The findings indicated correlations between
the predicted effect of the colour and the ability of the horse to discriminate
the test colour from grey. The six
horses used in the study were able to use chromatic information across the
spectrum, including those wavelengths that would be perceived as green and
yellow. The colours orange, yellow, and
blue were the colours that the horses most easily discriminated from grey—as was consistent with the predictions made [7]. Moreover as was
the case with Grzimek [3], Hall et al. [7] also found that the
horses actually failed to discriminate the colour red as easily as they
discriminated the other colours from the grey control.  In more recent studies with respect to equine
colour vision [8, 73], the authors have
also concluded that horses appear to have dichromatic capability only, much
like the red-green colour deficiencies found in some humans.Based on a synthesis of all the work
to date, it would appear that the ability of the horse to perceive colours may
be somewhat comparable to that of human dichromats. Although wavelengths from across the spectrum are
likely to be perceived by the horse as coloured (apart from that at the neutral
point) these colours will be seen as variations of only two main hues. In essence, the horse would fail to discriminate
between colours that lie on the same side of the neutral point—at least this will not be possible on the
basis of wavelength information. Similarly human dichromats commonly confuse
reds and greens and fail to detect particular dot patterns comprising of these
colours in simple “colour blindness” tests using Isihara cards. Jameson and Hurvich [86] reported that human
subjects, with such dichromatic vision, when asked to arrange colours according
to their degrees of similarly would routinely include mixed reds and greens in
their chosen sequences. It now seems likely
that the horse would experience similar differentiation difficulties, in
particular with the colours lime green and orange, and, equally likely, with
the colours blue and purple [7], although this phenomenon
has yet to be conclusively tested or demonstrated in behavioural experimental
work in the horse.In addition and despite differences
in colour perception, visual acuity in both the horse and the human appear to vary
according to the colour of the actual stimulus. For example, Grzimek [3] found that equine acuity
with blue targets was poorer than with other colours, including yellow.  Although the horse could detect a
perpendicular yellow line 5 millimetres wide from a distance of 3.3 metres
(visual angle of 3′15′′ and equivalent to 18.46 cycles per degree), a blue
line had to be 20 millimetres wide to facilitate detection from the same
distance (visual angle of 20′41′′ and equivalent to 2.9 cycles per
degree). The most plausible explanation
for this difference in equine acuity is the low population of short wavelength
photoreceptors within the visual streak of the horse including the irregular
spacings associated with them [63]. Indeed, Harman et al. [18] concluded
that the equine visual acuity value obtained for blue stimuli corresponded
closely to that obtained for the peripheral retina (2.7 cycles per degree),
while the values recorded for the yellow equivalent were closer to the value
obtained for the visual streak (16.5 cycles per degree).  A similar finding has been reported in relation
to the human retina, where foveal tritanopia (absence of short wavelength
cones) and the spatial distribution of these short-wavelength cones in the rest
of the retina also have the effect of limited resolution with short-wavelength
colours [87].It should be noted, however, that
while trichromatic colour vision allows greater discrimination of a greater
range of colours, there are also certain visual advantages associated with
dichromacy. Whereas colour generally
enhances the visibility of objects, in some cases it may actually obscure
differences between objects that are based on other features. Although human trichromats cannot detect
objects that differ in texture once camouflaged by variegated colours, any such
colour camouflage does not reduce their visibility to dichromats [88]. Thus, it may well be
that certain objects may actually be more visible to the horse than to the
trichromatic human—at least in some instances. Within the sport of horse-trials, for example,
where most cross-country fences are often of a similar colour to their immediate
surroundings, the ease with which the majority of horses negotiate such
obstacles would suggest that the fences are clearly visible and readily interpreted
by the equine visual system. This would
suggest that the horse probably relies less on colour and more on other visual
features such as textural differences to distinguish such objects from their
surrounding environments.


## 9. Visual Perception and  Performance in Equitation

Regardless of the differences between
equine and human visual perception, this modality is of prime importance to
both species. Behavioural observations
of feral horses would suggest that visual cues are highly important in the
recognition of other herd members [89]. During reproductive behaviour and when
assessing whether a mare is in oestrus and ready to cover, mature stallions
utilize visual stimuli to a greater extent than olfactory cues [90]. Indeed visual
recognition of offspring by the mare is also important and the mare typically remains
in visual contact with the foal for at least six months following parturition
[91].
Furthermore, it seems certain that the dam can identify the foal visually
from distances of approximately ten metres [92]. Among groups of adult horses, at least, visual
information appears to be very important for recognition and in determining
behavioural responses to other individuals.The importance of understanding how
the horse perceives the world becomes clearly significant within equitation
where both the horse and human are working together (ridden, driven, or handling
situations).  Any failure on the part of
the handler to appreciate the differences between equine and human visual
experience may result in the misinterpretation of visual raw data on the part
of the horse, which could then lead to inappropriate behaviour with undesirable
consequences. For example, a horse that
throws its head up and down when approaching a fence may well be struggling to
see the obstacle, rather than trying to go faster or “misbehaving” or exhibiting
other forms of conflict behaviour. Similarly any differences in colour perception
between horse and human could easily result in situations where some individual
colours that appear bright to human visual systems may be linked to adverse and
undesirable reactions in the horse or vice versa. The colour red, for example, will appear less
noticeable to the horse, while small movements (that may be imperceptible to
humans) are much more likely than other visual features, such as colour, to
elicit an avoidance response in the horse. In the same way there has been a longstanding
but fallacious belief in relation to bullfighting that the bull reacts
aggressively in response to the red cape.
It has long been proven, however, that the movement of the bullfighter's
cape is the primary cause of this response [93].  Equally, other similar misinterpretations of
equine behaviour and colour vision are also commonplace.When moving, both humans and horses
use several aspects of visual information to negotiate and avoid collisions
with obstacles within the immediate environment. Although Saslow [10] has suggested that
horses might actually rely on senses other than vision primarily during some
activities, vision is certainly of paramount importance when the horse is galloping
and jumping at speed. There have been
several anecdotal reports of horses with unilateral eye injury appearing to
have less visual information available from the remaining visual field, particularly
when negotiating enclosed or confined areas at speed. In one such set of circumstances, while a
horse with unilateral eye injury was loose schooled within the outer confines
of an indoor riding arena, there was no apparent alteration in kinematic
behaviour while the injured eye was oriented towards the inside of the direction
of travel. However, when the horse was
galloping in the opposite direction (with the injured eye towards the outside),
the horse was forced to slow considerably in order to negotiate the turns at
the ends of the arena (Somers, personal communication). Similar adjustments in motor behaviour have
been recorded in experimental studies of horses that were performing jumping
trials under conditions where unilateral visual information was controlled by a
specifically modified visor or blindfold apparatus [94].  The clear indication from such studies
is that the visual information about the position of boundaries is important to
the horse when moving at speed in a restricted space.


## 10. Vision and Jumping Kinematics within Equitation

During ridden work, certain
disciplines of equitation and riding styles advocate the adoption of unnatural
positioning of the head and neck of the horse. Although flexion of the neck (bending in
various directions) may result in improved movement of the equine back [95], there may be much less advantageous outcomes for equine vision
as a consequence [96]. Under conditions of extreme poll
flexion, although the horse would still be able to see the ground, the area directly
ahead is certainly obscured. In these
situations, the ridden horse is then dependent upon the rider for directional
guidance and is in effect working “blind” [20]. Harman et al. [18] declared that when
approaching a fence, the horse should be allowed to alter the angle of its head
and neck to optimise the assessment of depth and distance and thus affect takeoff
in the correct place. It is fairly
common for the ridden horse (when working “on the bit”) to be subjected to
visual restriction in some form and any such restriction of head and neck
movements, either by the use of equipment or directly by rider intervention, may
have the effect of limiting the visual information available to the horse. These restrictions of the equine visual system
may affect the horse in ways in which we are as yet unaware and their impact not
only on athletic performance but also on equine welfare warrants further detailed
investigation.Stachurska et al. [97] assessed the kinematic motor behaviour of horses when jumping
a variety of different obstacles and increases in the height of the fence resulted
in more frequent knockdowns.  Stachurska et al. [97] also reported that horses exhibited refusals and “runouts”
when approaching walls and that the second elements of combination fences (two
or three related fences) prove more problematic than the first or third
obstacle of combination fences. Moreover,
fences with only a single colour (in particular, white) also caused more difficulties
for jumping horses than those fences displaying two contrasting colours. Horses also appear to demonstrate more problems
when negotiating fences of contrasting colours where green is paired with
yellow or blue, which are two of the colours that have been reported as most easily
visible to the horse [7]. Since to date it
has not been possible to separate rider influences from these apparent visual
shortcomings in the performance horse, such problems could also be the
consequence of the human visual perception rather than that of the horse or
some combination of both.  Recently, eye
movements in humans have been studied to identify the visual features that are
important in the performance of different tasks. In general, skilful performers learn particular
visual fixation patterns that are required by the task undertaken [98]. Similarly, eye movements, gaze studies,
and attempts at more sophisticated eye tracking studies in the horse have the
potential to provide so much more elucidation regarding not only functional and
behavioural but also interpretative characteristics of the equine visual
system [9].In terms of rider eye movements
during equitation, Laurent et al. [38] reported that horse riders
consistently focused their gaze centrally on the target obstacle during the
intermediate and final approach strides.
Furthermore, more experienced riders tended to deliver their focus towards
the top of the target obstacles and also tended to maintain their head in a
fixed position.  This approach is
regularly employed to train other less-experienced riders in terms of precision
and accuracy with stride kinematics to optimise takeoff position from in front
of fences during jump training.  When
technological advances are sufficient to permit the mapping of the eye
movements of successful jumping horses (when approaching different obstacles),
much more detail will be available to address our incomplete knowledge of the
equine visual system. Furthermore,
studies of this nature are likely to provide us with vital information that could
be used in the selection and training of other horses for various specific
disciplines of equitation.In addition, it may be possible to
minimise dangerous falls of horses and riders so often seen in some equestrian
sports [99] as a
result of improved safety standards in equestrianism through more appropriate
fence/obstacle design. The lateral
placement of the equine eyes, horizontal pupil, and panoramic visual field [100], however, presents serious challenges in designing
eye-tracking testing apparatus for the horse.
Some scientists suggest that it might even prove virtually impossible to
assess the direction that any horse is looking and even more difficult to judge
exactly what visual feature is being fixated upon as the horses tend to keep
“half an eye on everything” rather than focusing on specific targets [100]. Motor accuracy
and precision when jumping requires adaptive behaviour on the part of the horse
and it may take some specialist practice to develop the visual skill and
locomotory coordination that is required for such activity.In terms of jumping kinematics, a
study by Visser et al. [101] indicated that personality traits of young horses, such as
emotionality and reactivity, might be used to predict their subsequent
performance as show-jumpers. While
vision per se was not examined by Visser et al.
[101], a link between personality and visual ability has been
suggested in humans. In one personality study, Trevor-Roper
[102] observed a tendency for many hyperopic individuals to be
extroverted outdoor-type personalities while myopic individuals were much more
inclined to be introspective and contemplative.
While no such evidence exists for horses, it does suggest the
possibility of links between some characteristics of vision, personality, and
athletic or performance ability. The
study of animal cognition and emotion has become increasingly more important in
terms of our understanding of animal behavioural responses and performance-related issues [103].  Equine
conformation, motor coordination, and athletic fitness are vital ingredients ultimately
related to equine performance. Additionally, individual variations in visual ability are
equally likely to be major contributory factors in determining the success or
otherwise of competition horses within the elite echelons of the various disciplines
of equitation. As with the selection of
all desirable traits in animal breeding programmes, it would be
advantageous to be able to test for features of visual ability that were
related to performance and utilise these as part of the selection processes.The aim of the standard ophthalmic
examination currently carried out by veterinary surgeons is to identify any
abnormalities of the eye that could result in visual disability. It does not however take account of any individual
differences (acuity, colour vision, depth perception) in terms of overall visual
ability that could influence or contribute to the future success of the horse. Sex effects with respect to visuospatial
ability have already been demonstrated in the horse [46]. As is the case in all other species investigated to date; male horses
enjoy the benefit of superior visuospatial ability compared to female counterparts.  There is an adaptive advantage associated
with greater visuospatial ability in terms of depth perception, interpretation
of three-dimensional perspective, and elements of navigation and visuospatial
ability is likely to be critically important during the jumping phases of equestrian
competition.  Fence and obstacle designs within
the various disciplines of equitation may well take account of the athletic
ability of the horse, but the aesthetics and visual presentation of these
structures are regularly based on human rather than equine visual perception
and may cause difficulty and risk of injury for the horse (and rider) as a
consequence, Figure 3.  Fence designs that
challenge not only the athletic prowess but also more importantly (and perhaps
unfairly) test the visual ability of horses within equitation may contribute to
the unacceptable levels of wastage and welfare concerns within equestrianism
[104].Figure 3
Some fence designs may challenge not only the athletic prowess of the performance horse but also the visual capability of the animal, which may have safety implications for both horse and rider.Within equine competitions, course
and fence designs continue to diversify at the discretion of course designers
who may not be aware of how these various elements  actually appear to or are
perceived by the horses that have to negotiate them. Some fence elements and fence designs have
proven to be problematic for horses during the demands of competition [99].  The visual techniques and behavioural characteristics
of successful competition horses should be considered when course designers
attempt to create innovative course and fence designs for competitive
equestrianism.  Even outside of
competition, while the information regarding what horses see is by no means
complete, what is available should be considered more carefully when designing
the various element's of the equine environment such that horses are not expected
to see the world through the eyes of the human.In one recent and very relevant
study, the effect of different floor colours on the behaviour of the horse has
been demonstrated [66] and this fundamental issue has
implications for equine husbandry.  For
example, the impact of these findings in a routine management situation was
then highlighted  successfully by varying the colour of the matting on trailer
ramps. Horses are regularly required to
load into transport systems such as trailers or floats and the evidence now
suggests that horses loaded more easily and without hesitation when the ramp mats
were green compared to loading attempts when the ramps were covered with the
more commonly used black matting [105]. For the most part, the environments in which
horses are maintained are designed and constructed with human visual perception
(including aesthetics and limitations) as the basis for how things should look
and be presented to the horse. This
strategy may be inherently flawed, but by also attempting to view the world
through the eyes of the horse, much more appropriate environmental
infrastructure might be possible as a result, which would lead to improved
conditions in terms of equine husbandry, welfare, and performance within equitation.


## 11. Conclusion

Although there is a growing body of
information relating to the capabilities and limitations of equine vision, the
overall picture is still far from complete. Useful comparisons with specific features of
human visual ability have been made, generally by adapting techniques that have
been developed to assess human visual perception. While we can assess the visual ability of the
horse using these parameters, little is known about other features that are not
an important part of the human visual experience. The horse may respond to certain visual
stimuli that the human eye even fails to notice, which would suggest that some
visual capabilities of the horse, at least, may be superior to our own. Given the importance (and demands for accuracy
and precision) of contemporary equitation, there is also a lack of information
regarding individual differences in the visual ability of different horses and
how this may affect any individual horse's athletic performance. Clearly, the development of more comprehensive
methods that are capable of deciphering the complete extent of the equine
visual system is warranted and would assist equestrians to optimise performance
in the athletic or competition horse. A
better understanding of the visual strategies employed by successful horses might
prove extremely useful too when designing specific “schooling” routines that could
assist in the training of the less-experienced horse. In terms of equine competition within the
various disciplines of equitation, every effort should be made to reduce the
risks associated with the various sporting activities that involve elements of
motor accuracy, jumping kinematics, and speed by ensuring that the horse can
“see” what is expected of it.  In the high-end
competition horse, this need is paramount given the athletic (including visual)
challenges set by humans as they expose these horses to many potential risks
associated with participation in the equestrian sports.



---

*Source: 721798-2009-04-22.xml*