What do dogs (Canis familiaris) see? A review of vision in dogs
and implications for cognition research
&Philippe A. Chouinard
&Tiffani J. Howell
Pauleen C. Bennett
#Psychonomic Society, Inc. 2017
Abstract Over the last 20 years, a large amount of research
has been conducted in an attempt to uncover the cognitive
abilities of the domestic dog. While substantial advancements
have been made, progress has been impeded by the fact that
little is known about how dogs visually perceive their external
environment. It is imperative that future research determines
more precisely canine visual processing capabilities, particu-
larly considering the increasing number of studies assessing
cognition via paradigms requiring vision. This review dis-
cusses current research on visual cognition and emphasizes
the importance of understanding dog visual processing. We
review several areas of vision research in domestic dogs, such
as sensitivity to light, visual perspective, visual acuity, form
perception, and color vision, with a focus on how these abil-
ities may affect performance in cognition tasks. Additionally,
we consider the immense diversity seen in dog morphology
and explore ways in which these physical differences, partic-
ularly in facial morphology, may result in, or perhaps even be
caused by, different visual processing capacities in dogs.
Finally, we suggest future directions for researchin dog vision
Keywords Visual processing .Dog .Cognition .
In the last 20 years, researchers have discovered a variety of
behaviors and abilities that make dogs an important model for
studying cognition. This is largely a result of their unique
social cognitive abilities (Hare & Tomasello, 1999;Miklösi,
Polgárdi, Topál, & Csányi, 1998; Soproni, Miklósi, Topál, &
Csányi, 2001; Udell & Wynne, 2008), and began when it was
found that dogs outperform all other non-human animal spe-
cies in locating hidden food based on human-given cues
(Bräuer, Kaminski, Riedel, Call, & Tomasello, 2006;Hare,
Brown, Williamson, & Tomasello, 2002; Kaminski, 2009).
While a myriad of research has since been conducted with
dogs, many of the findings are not without controversy.
Across cognition studies there appears to be unexplained
variation in observed findings. For example, in studies of ob-
ject permanence, it remains unclear whether or not dogs are
capable of visible and invisible displacement tasks (e.g., Bell
&Fox,1997; Collier-Baker, Davis, & Suddendorf, 2004;
Fiset, Beaulieu, & Landry, 2002; Gagnon & Dore, 1992;
Gagnon & Doré, 1994;H.C.Miller,Gipson,Vaughan,
Rayburn-Reeves, & Zentall, 2009). Additionally, in gestural
communication studies, conflicting findings are often ob-
served in performance in pointing and gaze cue tasks (e.g.,
Dorey, Udell, & Wynne, 2010;Hare&Tomasello,2005;
Soproni, Miklósi, Topál, & Csányi, 2002; Tauzin, Csík, Kis,
& Topál, 2015; Udell, Dorey, & Wynne, 2008; Virányi et al.,
2008; Wynne, Udell, & Lord, 2008). While these mixed find-
ings in canine cognition studies may be due to a variety of
reasons, below we highlight two that have often gone
First, little is known about vision in dogs. However, to date,
the most common approach to assessing dog cognition is via
visual tasks. In fact, approximately 74 % of dog cognition
studies use visual tasks (Bensky, Gosling, & Sinn, 2013).
On the one hand, this is not surprising, as many cognition
tasks used in human and non-human animal (hereafter animal)
research utilize experimental paradigms that are heavily reli-
ant on vision. What is surprising is the lack of research on dog
vision and visual perception, needed to justify the use of these
School of Psychology and Public Health, La Trobe University,
P.O. Box 199, Bendigo, Victoria 3552, Australia
Psychon Bull Rev
visual paradigms, many of which require recognizing, or mak-
ing distinctions between, certain visual details.
Second, dogs represent the most morphologically diverse
species in existence (Hart, 1995; Wayne, 1986a,1986b), with
exceptional systematic variation between breeds (Wayne &
Ostrander, 2007). Dogs also differ behaviorally in systematic
ways that reflect diverse selection pressures, now encoded
genetically in different breeds. For example, sight-hound
breeds were selected to hunt primarily by sight and chasing
at speed, whereas terriers were predominantly used to hunt
ground-dwelling vermin, using odors and a powerful phy-
sique suited to digging tenaciously until their quarry is
unearthed. Considering sight-hounds and terriers differ in
height, size, body type, facial morphology, and behavior, it
is feasible to consider if these differences could be associated
with, affect, or perhaps even be caused by, different visual
processing capacities. Conceivably, selection for different
visually-directed behaviors underpinned some of the differ-
ences in morphology that arose as breeds were developed.
This makes potential morphological differences in visual
perception of enormous theoretical interest. It may also help to
explain discordant experimental outcomes that appear, poten-
tially due to the composition of the sample of dogs used in a
given study. Individual differences in visual perception have
been acknowledged, often in limitation sections of research
articles as a possible reason for differences in observations,
particularly when sample sizes are small. To our knowledge,
however, formal assessments of these differences remain
scarce (for a review, see Bensky et al., 2013).
As will be evident from this review, significant gaps exist
in our understanding of fundamental visual perceptual capac-
ities of dogs in general, and of morphologically distinct breed
types more specifically. This makes it difficult to determine
whether or not specific visual cognition tasks are appropriate
for assessing canine cognition. Is it the case that testing con-
ditions for some cognitive tasks are outside some or all dogs’
perceptual capabilities? While the answer to this question is
not known, the goal of this review is to provide an overview of
dogs’visual system, visual perception, and morphology, to
examine the effect these may have on canine cognition re-
search, and to suggest areas where the need for additional
research is urgent.
Part 1: Understanding dogs’visual system
A basic understanding of dog vision has been available for
over two decades, with an extensive review being published
by P. E. Millerand Murphy (1995). Advances in canine vision
science since then have been limited, and this remains an
excellent in-depth overview of many aspects of the dogs’vi-
sual system. However, in the last two decades, the amount of
research conducted with dogs, and the way their cognitive
skills are assessed, has drastically changed. For example, ex-
periments now involve visual stimulus presentation (e.g.,
Pongrácz, Miklósi, Dóka, & Csányi, 2003), and touch-
screens (e.g., Range, Aust, Steurer, & Huber, 2008), to name
a few. Therefore, we begin this review with a brief overview
of vision in dogs, specifically highlighting aspects of dog vi-
sion that may affect perception during modern visual cogni-
Fundamentals of dog vision
While dogs appear to be visual generalists, with functional
vision during both the day and night (Duke-Elder, 1958;
Wall s, 1942), they appear to be more scotopic than humans,
meaning that they are highly adapted to function in dim light.
In fact, they appear to have developed several ways of improv-
ing visual functioning across a variety of ambient light levels.
The retina of the dog is largelycomposed of rod photoreceptor
cells, which are extremely helpful in dim light as they can
function in less intense light conditions (Kemp & Jacobson,
1992). Only 3 % of retinal cells in dogs are cone photoreceptor
cells, which are primarily responsible for color vision (Peichl,
1992). This compares with roughly 5 % in humans (Purves,
Augustine, & Fitzpatrick, 2001).
Although a foundational understanding of rod and cone
photoreceptor cells in dogs is available, only recently have
scientists furthered our knowledge of their precise
distribution. Mowat et al. (2008) observed that the area
centralis, a region typically centrally located in the retina,
contains the maximal density of rod and cone photoreceptor
cells in dogs. Even though cone photoreceptor cells are more
numerous in the central portion of the retina (20 % of all
receptors) (Koch & Rubin, 1972; Parry, 1953;Peichl,1992),
the area centralis in dogs does not consist exclusively of cones
as it does in humans (Mowat et al., 2008). These newer find-
ings are consistent with older work, some of which is
reviewed below, suggesting that dogs may be more adapted
to dim light conditions and less sensitive to color perception
than humans. If so, this would have clear implications for
cognitive tests requiring color discrimination.
While both dogs and humans utilize rod photoreceptor cells to
function in dim light conditions, the rod photo pigment, rho-
dopsin, differs between the two species. Rhodopsin, a g-
protein-coupled receptor, is highly sensitive to light and im-
proves vision in dim light conditions. Dogs typically have a
rhodopsin peak sensitivity to light wavelengths of 506–
510nm (Jacobs, Deegan, Crognale, & Fenwick, 1993;Kemp
& Jacobson, 1992; Parkes, Aguirre, Rockey, & Liebman,
1982), whilehumans have a peak sensitivity to slightly shorter
wavelengths of 495 nm (Kraft, Schneeweis, & Schnapf,
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1993). As the peak rhodopsin wavelength sensitivity hardly
differs between dogs and humans, it appears that the dog’s
enhanced vision in dim light conditions may be due to other
attributes (P. E. Miller & Murphy, 1995). However, additional
research is required to substantiate such claims.
One attribute that increases dogs’sensitivity in dim light
conditions is the reflective tapetum lucidum. This superiorly
located layer of tissue in the eye is a biologic reflector system
commonly found in vertebrates (Ollivier et al., 2004), but not
in humans. Typically, the tapetum lucidum offers light-
sensitive retinal cells an additional opportunity for photon-
photoreceptor stimulation by reflecting light, which has al-
ready passedthrough the retina, back through it a second time.
This reflection enhances visual sensitivity in dim light condi-
tions, but typically reduces the ability of the eye to observe
details of an image due to increased scattering of light in the
eye (Walls, 1942).
Perhaps surprisingly, there is variation in dogs’tapetum
lucidum (Granar, Nilsson, & Hamberg-Nyström, 2011;
Lesiuk & Braekevelt, 1983).Astrainoflaboratorybeagles
has been observed to have hereditary tapetal degeneration
(Burns, Bellhorn, Impellizzeri, Aguirre, & Laties, 1988)and,
in a sample of 539 dogs, a tapetal area was completely present
in only 70.3 % of them, being completely absent in 1.9 %
(Granar et al., 2011). Generally, smaller sized breeds, like
Papillons, Shetland Sheepdogs, Dachshunds, American
Cocker Spaniels, Miniature Schnauzers, Miniature Poodles,
Bichon Frisé/Havanais, and Cavalier King Charles Spaniels,
have a smaller tapetal area, while larger dogs, like Border
Collies, Leonbergers, Samoyeds, Golden Retrievers, and
English Springer Spaniels, typically have a full-sized tapetal
area (Granar et al., 2011). Labrador Retrievers have smaller
than expected average tapetal size because there appears to be
increased variation within the breed. Specifically, a large pro-
portion of Labradors lack a tapetal area. Thus, it appears that
the size of the tapetal area depends largely on breed and body
size, but that marked variation may also exist within a breed.
Since the main function of the tapetum lucidum is to facil-
itate detection of small amounts of light, perhaps dogs without
a tapetum lucidum are worse at discriminating between light
conditions. We were unable to uncover evidence for such an
effect, and the previous review on vision in dogs claimed that
no functional differences had been reported (P. E. Miller &
Murphy, 1995). Regardless, as this kind of variability within a
species is uncommon, it should be investigated further using
modern, technologically advanced equipment and techniques.
We recommend future research attempt to evaluate the effects
of tapetal-area differences, or develop a method for easily
observing such differences so that researchers can be aware
of potential physiological differences within their sample.
Finally, as dogs are sensitive to a variety of lightconditions,
this can affect their recovery from exposure to bright light.
The phenomenon of photo bleaching occurs when photo
pigment becomes almost transparent following exposure to
light, after which it must regenerate to regain pigmentation
when in the dark. The regeneration from rhodopsin’sphoto
bleaching effects is longer in dogs (over an hour) than in
humans (approximately 30 min). This means that when a
dog and a human come inside after having been outside, the
recovery time from the photo bleaching effect is twice as long
in dogs as it is in humans. Because abrupt changes in light
conditions could more drastically affect dogs than humans,
this should be considered when bringing dogs from bright
outdoor environments directly into indoor laboratories for
As dogs appear to have evolved as visual generalists, it may be
assumed that their sensitivity to differences in brightness
would be quite good. However, two relevant studies provide
conflicting reports. Stone (1921) observed relatively low
brightnessdiscriminationthresholds for two fox terriers, com-
parable with thresholds observed in humans. By calculating
the smallest difference between two stimuli that the dogs
could detect, Stone (1921) determined Weber fractions of
0.12 and 0.10, a result quite comparable to the 0.11 observed
in humans (Griebel & Schmid, 1992). However, he only
assessed the dogs on one standard brightness intensity, a po-
tential limitation as psychophysical studies on humans dem-
onstrate that brightness discrimination thresholds decrease
with increasing light intensity (Craik, 1938)
More recently, Pretterer, Bubna-Littitz, Windischbauer,
Gabler, and Griebel (2004) observed that brightness discrim-
ination was about two times worse in dogs than it is in
humans, with reported Weber fractions of 0.22 and 0.27 for
three subjects, a German Shepherd and two Belgian
Shepherds. This result suggests a relatively high brightness
discrimination threshold for dogs compared to Stone (1921).
It has been previously suggested (Scholtyssek, Kelber, &
Dehnhardt, 2008) that the high threshold observed by
Pretterer et al. (2004) is likely a result of the experimental
methods and may underestimate the brightness discrimination
capabilities of dogs. In the experiment, dogs were required to
discriminate between stimuli of various intensities that were
1.1 m apart, a reasonably large distance that may have affected
the subject’s choices and, consequently, the threshold ob-
served. Additional research is required to evaluate the findings
of these two studies.
Visual acuity and spatial resolution
Visual acuity refers to the clarity of vision and is dependent on
optical and neural mechanisms (e.g., eye structure, the health
of the eye, the brain’s interpretation). While dogs’visual acu-
ity is difficult to measure, it is typically estimated to be
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considerably worse than humans’. Based on a variety of stud-
ies employing different methods, such as behavioral testing,
measurement of visually evoked cortical potentials, pattern
electroretinography, and optokinetic responses, P. E. Miller
and Murphy (1995) estimated the visual acuity of a typical
dog to be 20/75. This value suggests that, from 20 feet away, a
dog could perceive an object that a person with normal vision
could differentiate from 75 feet away.
A lack of visual acuity in dogs is not surprising, as there is
likely a trade-off that exists in dog vision. Considering the
physiology of the dog eye discussed above, and the dim
light conditions to which it is adapted, dogs may be more
sensitive to light at the expense of being able to discriminate
smaller details. Whether the exact value of 20/75 reported by
P. E. Miller and Murphy (1995) is correct or not, however, has
proved difficult to determine.
Murphy, Mutti, Zadnik, and Ver Hoeve (1997) assessed
visual acuity in three young adult Beagles using sweep
visual-evoked potentials. This method allows for visualacuity
estimates by measuring the cortical response to a sequence of
gratings of increasing spatial frequency. Visual acuity esti-
mates were between 20/45 and 20/85. More recently, a behav-
ioral assessment of visual acuity has been conducted by
Tanaka, Ikeuchi, Mitani, Eguchi, and Uetake (2000). These
authors reported visual acuity estimates of 20/60 to 20/85 in
three Shiba dogs.
Even taking into account the fact that differences in acuity
estimates are also highly variable in other species, with those
obtained through behavioral investigations often indicating
lower acuity than electrophysiological acuity measures
(Murphy et al., 1997), these disparate estimates suggest that
further work is required. It would be of great interest to know
whether all dogs have similar levels of visual acuity or vary
systematically according to breed, morphology, or other fac-
tors. Such research would allow cognition researchers to more
accurately create appropriately sized stimuli and viewing dis-
tances, such that dogs would be capable of perceiving them.
Binocular overlap refers to the overlapping portion of a visual
scene that is viewed by both eyes. Due to our forward facing
eyes, and relatively unobtrusive noses, humans have a degree
of binocular overlap of roughly 140° (Walls, 1942). In dogs,
various estimates of binocular overlap exist and these vary
based on the immense variation in facial morphology types
as well as the methodology used to calculate the estimates. P.
E. Miller and Murphy (1995) report that, in behavioral studies,
binocular overlap has been estimated to be roughly 30–60°.
However, it can be anywhere from 35° to 40° when calculated
based on ganglion cell density (Peichl, 1992), and 80–116°
when calculated based on optical considerations (Duke-Elder,
1958; Walls, 1942).
Depth perception, or stereopsis, represents the visualability
to perceive the world in three dimensions (3-D) and is en-
hanced in regions where both eyes have overlapping fields
of view. This occurs when both eyes view the external world
from different vantage points and the information is merged to
create a single image. It is this fusion of image that allows the
eye to accurately perceive depth (Bishop, 1987). In a visual
cliff experiment, young puppies demonstrated outstanding
monocular (single eye) and binocular depth perception
(Walk & Gibson, 1961). Considering the canine eye does
not completely develop until they are juveniles or young
adults (a few months of age), P. E. Miller and Murphy
(1995) suggested that adult dogs likely have even better visual
depth perception. Additional research is needed to substantiate
these claims, however, as studies of retinal ganglion cell to-
pography suggest that depth perception in dogs may be im-
paired. Dogs lack alpha, also termed BY, ^ganglion cells, in
both the right and left portions of the 15° of the peripheral
binocular overlap (Peichl, 1992). Thus, it is possible that there
is a smaller area of binocular overlap where the retina per-
ceives high quality depth perception. Additional research is
clearly required to confirm available information, and also to
determine if depth perception varies systematically by breed.
The dog’s ability to distinguish different colors remains con-
troversial. Humans have three types of cone photoreceptor
cells (long-wave (red), medium-wave (green), and short-
wave (blue), at spectral peaks of 558 nm, 531 nm, and 419
nm, respectively). Dogs have only two, which almost identi-
cally correspond to short-wave and long-wave sensitivities
(blue at a spectral peak of 555 nm and yellow at 429 nm)
(Jacobs et al., 1993; Neitz, Geist, & Jacobs, 1989). This has
been used to suggest that dogs may be unable to perceive
differences between green, yellow, and red color cues.
Accordingly, early studies suggested dogs lacked good color
vision (Neitz et al., 1989). However, there is some even older
evidence that suggests dogs may be able to perceive these
colors (e.g., red and green) even without possessing the cone
photoreceptor cells believed to be responsible for this ability
(Rosengren, 1969). More research is needed to understand the
extent to which dogs perceive color, and how similar dog
color perception is to that of non-color-blind humans.
Even with this limitation in mind, dogs appear to be atten-
tive to the colors they can perceive. Two Shibadogs were able
to appropriately identify a positive stimulus (red, blue, or
green compared to grey) in a two-choice discrimination task,
where the light intensity on the cards was 450–500 lux
(Tanaka, Watanabe, Eguchi, & Yoshimoto, 2000). The authors
of this study suggested that color vision is relatively well
developed, considering the dogs were able to discriminate
between all three primary colors and grey. Another study
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showed that, under natural photopic lighting conditions, dogs
might preferentially use color over brightness cues, when pre-
sented with yellow and blue stimuli in a discrimination task
(Kasparson, Badridze, & Maximov, 2013). Eight dogs were
observed to use color over brightness cues when discriminat-
ing and recognizing visual objects (Kasparson et al., 2013).
These findings suggest that color may be a fundamental fea-
ture of visual objects as perceived by dogs. However, addi-
tional research is needed to assess their performance in various
In addition to their preference for color cues, it appears that
dogs may have a capacity to perceive ultraviolet light
(Douglas & Jeffery, 2014). In a cross-species assessment of
ultraviolet (UV) sensitivity in mammalian eyes, dogs were
identified to have lenses transmitting significant amounts of
UV rays (335 nm). This suggests that even though dogs do not
have a specific UV visual pigment, they may be sensitive to
ultraviolet light (Douglas & Jeffery, 2014). If this is the case,
cognition researchers must begin to determine the UV light
levels that dogs can perceive, and consider the effects this may
have on the stimuli and conditions in which they are being
Finally, there is evidence to suggest that dogs may have a
magnetic sense associated with their visual system. A recent
study observed the presence of cryptochrome 1, a flavoprotein
located in the canine eye that is sensitive to blue light.
Additionally, it is involved in responding to light-dependent
magnetic orientation based on the earth’s magnetic field
(Nießner et al., 2016). These authors suggest that this does
not likely act as an additional pigment for the perception of
color, but instead likely functions to perceive the earth’smag-
Sensitivity to monitors
Sensitivity to flickering lights has become relevant in the
study of dog cognition due to the frequent use of screen-
presented stimuli in cognition tests. The flicker fusion rate is
the point at which rapidly flickering light appears to meld into
a constantly illuminated light. This is important when present-
ing videos as these rely on presenting a rapid succession of
static images. If the frame rate, in Hertz (Hz), is below the
threshold of sensitivity, the flicker will be viewable and the
film will appear jerky. Therefore, studying flicker fusion rates
offers insight into the functional qualities of dog
Originally, electroretinographic studies of anesthetized
dogs suggested they could detect flickering up to a max-
imum of approximately 20 Hz (Gustavo Aguirre, 1978;
GD Aguirre & Rubin, 1975). However, behavioral para-
digms using unanesthetized dogs suggest a more sensitive
flicker detection, approximately 70–80 Hz. More recently,
Healy, McNally, Ruxton, Cooper, and Jackson (2013)
observed flicker fusion frequencies to be 80 Hz in dogs
compared to 60 Hz in humans. This is potentially a major
problem for dog cognition studies, as these findings sug-
gest that dogs are more sensitive to flicker than humans,
and more sensitive than could easily be accommodated by
some screens currently in use. What appears as a fluidly
moving video image to humans may appear as a flickering
image to dogs, making it difficult to determine if their
performance on relevant tests is a genuine indicator of
their cognitive abilities.
Furthermore, it is possible that the presentation of static
images on a monitor is also affected by dogs’greater
sensitivity to flicker-fusion rates. Refresh rates, the num-
ber of times in a second that a display renews an on-
screen image, may also be perceived differently by dogs
than by humans. For example, cathode ray tube (CRT)
monitors are typically set to present at 60–70 Hz for
humans, in order to avoid viewing of flickering. While
CRT monitors are no longer common, they may be used
in some research contexts depending on the aims of the
study. For example, CRT monitors tend to maintain a
more stable brightness than modern liquid-crystal display
(LCD) monitors. LCD monitors typically exhibit no re-
fresh rate-induced flicker, and are commonly set to pres-
ent at 60 Hz (although given technological advancements
there is increased variation in this value). Unlike CRT
monitors, pixels on LCD monitors do not necessarily
flash on/off between frames. Therefore, the flicker effect
often observed on older screens may no longer be a po-
tential confound as long as the flicker-fusion rate is above
the threshold observed in dogs. Considering these physi-
ological differences (compared to humans) it is important
to utilize proper technological tools, as well as to be
aware of how differences in physiology may indirectly
affect experimental outcomes.
Part 1 conclusions
In summary, while the visual system of dogs may be consid-
ered to be worse than that of humans in many ways (P. E.
Miller & Murphy, 1995), it is evident from our review that,
in some ways, their vision is superior, or at least different. It
seems that the visual acuity and color perception capabilities
of domestic dogs are less sensitive than those observed in
humans, but the observed flicker-fusion rate, and their ability
to function in dim light, surpass those of human capacities. Of
most relevance here is that many aspects of dog vision remain
substantially understudied. Moreover, many studies have used
extremely small sample sizes, often comprising a single breed,
and, as a result, individual, breed, and morphological differ-
ences have rarely been considered (for a review see Arden,
Bensky, & Adams, 2016).
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Part 2: Visual perception in dogs
When perceiving visual stimuli, the brain processes retinal
information, allowing humans and animals to interpret the
external world and act upon it (Haber & Hershenson, 1973).
Visual perception reflects the brain’s interpretation of the
physical properties of an object, but may not always accurate-
ly reflect the physical properties themselves. For instance, the
mind may adapt the information due to preconceptions based
on experience and neurological connections, thereby permit-
ting the viewer to successfully navigate the world in a more
Badaptive^way (Kandel & Schwartz, 2000). But how do we,
and other species, perceive the world, and which mechanisms
are used to extract meaning from a visual scene? The question
is far from simple, as many different physiological and neu-
rological components are involved in visual processing.
Currently, Bayesian explanations for visual perception sug-
gest that the brain creates an optimal combination ofincoming
sensory information and prior knowledge in order to make
sense of an organism’s surroundings (Knill & Pouget, 2004).
Recently, researchers have attempted to replicate the way in
which dogs perceive visual scenes. Pongrácz, Ujvári, Faragó,
Miklósi, and Péter (2017) altered images according to hypoth-
esized dog visual perceptive conditions, producing a dichro-
matic scene with poor resolution and low brightness. The
researchers noted that, when a scene is altered in this way,
humans who perceive the scene perform poorly at
distinguishing certain contextual information relayed in the
image. While these findings are intriguing, they primarily
highlight that, when given blurry and less colorful photo-
graphs, humans find it more difficult to draw out relevant
information. What has yet to be determined is whether the
criteria and the perceptive conditions imposed on the images
are, in fact, an accurate representation of what dogs actually
see, as well as how they process and utilize visual information.
Given the lack of research currently available on the funda-
mentals of dog vision, drawing additional conclusions seems
unwarranted. Below we discuss research that has assessed
visual perception in dogs, in an attempt to better understand
how dogs perceive incoming visual information.
Visual discrimination and form perception
Visual discrimination abilities of domestic dogs have tradi-
tionally been tested using two-choice discrimination para-
digms. These typically require extensive training. From these
studies, it has been observed that dogs can easily learn to
discriminate between stimuli on the basis of form. For exam-
ple, dogs can discriminate between different objects
(Milgram, Head, Weiner, & Thomas, 1994), black/white stim-
uli (e.g., Araujo, Chan, Winka, Seymour, & Milgram, 2004;
Burman et al., 2011; Frank, 2011), and between objects of
different sizes (Byosiere et al., 2016; Milgram et al., 2004;
Tapp et al., 2004).
Research has also demonstrated that dogs are able to
discriminate between differences in the quantities of stim-
uli displayed. Quantity discrimination tasks, where sub-
jects are presented with food rewards of different sizes,
have found that dogs preferentially select the larger quan-
tity of two rewards. However, performance varies as the
ratio between quantities is reduced (Baker, Morath,
Rodzon, & Jordan, 2012; Petrazzini & Wynne, 2016;
War d & Smuts , 2007). While mixed findings have been
observed, it appears that dogs are only capable of discrim-
inating quite large (e.g., 5 vs. 10) rather than quite small
(e.g., 2 vs. 3) quantitative differences.
More recently, Byosiere et al. (in prep.) assessed visual
discrimination capabilities in Lagotto Romagnolo dogs by
assessing their size sensitivity thresholds. All subjects
successfully discriminated between circles that differed
in diameter by 20 % (42 pixels, approximately 12.6
mm). However, variation in sensitivity was observed; four
of the eight dogs were able to discriminate between cir-
cles 10 % different in diameter (21 pixels, approximately
6.3 mm) but none were able to discriminate circles 5 %
different in diameter (10 pixels, approximately 3 mm).
These findings are comparable to those observed in pri-
mate species; humans, chimpanzees, bonobos, olive ba-
boons, and long-tailed macaques have all demonstrated
successful discrimination of 3-D cubes varying in volume
by 20 % (Schmitt, Kröger, Zinner, Call, & Fischer, 2013).
Furthermore, absolute size of the stimuli appears to affect
performance in dogs. While using identical percent size
differences across stimuli of varying absolute size, dogs
were more successful at discriminating between stimuli
that were larger than stimuli that were smaller.
Considering dog’s diminished visual acuity, it is possible
that these findings further reflect a difficulty in discrimi-
nating fine details.
In addition to perceiving size differences, it has also been
reported that dogs can perceive and discriminate shapes
(Duke-Elder, 1958). Karn and Munn (1932) observed that
dogs quickly learned to discriminate between horizontal and
vertical lines; however, they learned more slowly when dis-
criminating between upright and inverted triangles.
Interestingly, once the dogs learned the discrimination task
they appeared to be able to generalize their performance to
triangles of decreasing size. While it remains unclear how the
dogs discriminated between stimuli, perhaps due to the orien-
tation of the triangle rather than the shape, they appeared to
discriminate based on cues presented in the lower portion of
Byosiere et al. (2017a) evaluated if dogs could generalize a
previously learned rule to novel stimuli. After learning a two-
choice size discrimination task, where selecting the larger
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circle stimulus over the smaller resulted in a reward (or vice
versa), the subjects were presented with various novel shapes.
The results demonstrated that the dogs were capable of gen-
eralizing the previously learned size discrimination rule to
novel situations; however, there were differences in the dogs’
generalization capabilities. Specifically, dogs were highly suc-
cessful at generalizing their Brule^to ovals, squares, rectan-
gles, diamonds, triangles, and stars, but not to vertical or hor-
izontal lines. These findings suggest that perhaps lines are
perceived differently from the other shapes, possibly due to
differences in overall size.
Taken together, these findings suggest that, given appropri-
ate training, dogs can discriminate between stimuli based on
shape and size. However, how they perceive these differences,
and the extent to which this is similar to humans, remains
unknown. Therefore, additional, more complex assessments
of psychonomic phenomena have been conducted, which we
discuss in detail below.
Many visual scenes contain both global (e.g., a bouquet of
flowers) and local (e.g., individual flowers within the bou-
quet) features and, in humans and other animals, there is divi-
sion of labor within the visual system for processing these
features (Laycock, Crewther, & Crewther, 2007). Certain spe-
cies, such as pigeons, demonstrate a preference for identifying
local features (Cavoto & Cook, 2001). In contrast, Navon
(1977) observed that humans identify global (e.g., the bou-
quet) faster than local (e.g., the individual flowers) features
using a task in which a large letter is made up of smaller letters
(see Fig. 1). Typically, humans more quickly recognize the
larger global letter depicted in the figure (i.e., the H) than the
multiple smaller local letters (i.e., the X).
One study has assessed global and local processing indogs.
Using two dimensional (2-D) hierarchical compound images
created using letters and shapes, Pitteri, Mongillo, Carnier,
and Marinelli (2014) trained dogs to identify a consistent stim-
ulus, either an BO^made up of circles or an BX^made up of
Xs, in a discrimination task. After an extensive training pro-
cess, dogs were presented with two incongruent stimuli in a
test phase. One displayed the global element they were trained
on, and the other displayed a local element they had never
seen (depending on the training stimulus, an BO^made up
of BXs^or an BX^made up of BOs^). The researchers ob-
served that there was a trend for dogs to process stimuli in a
globally oriented fashion, although there appeared to be much
individual variation (Pitteri, Mongillo, Carnier, & Marinelli,
2014). A re-test of the same dogs after 2 years suggested that
individual global/local preferences are stable over time and
that there was still a trend for an overall global bias in dogs
(Mongillo, Pitteri, Sambugaro, Carnier, & Marinelli, 2016).
It is clear from a number of experiments in humans,
however, that global processing should not be considered
a singular construct, invoked by the same cognitive oper-
ations across all types of global tasks. Rather, it is an
umbrella term denoting multiple independent mechanisms
(Chouinard, Noulty, Sperandio, & Landry, 2013;
Chouinard, Unwin, Landry, & Sperandio, 2016).
Specifically, it is possible to demonstrate local precedence
on some tasks, while demonstrating an overall global pre-
cedence across a variety of tasks.
Furthermore, considering dogs have reduced visual acuity,
is the task an appropriate assessment of precedence? In Pitteri,
Mongillo, Carnier, and Marinelli (2014), the dogs were posi-
tioned 1.3 m away from the stimuli and, while the exact dis-
tance is not specified in the manuscript, the apparatus was 1.4
m wide, with the edge of each stimulus 14 cm from the edge.
Based on the evidence discussed in Part 1, it seems likely that
dogs are unable to perceive small details from such a distance.
If dogs do not have the capacity to process detail, they could
not behave in a way demonstrative of local precedence, or
detail processing. To fully determine whether dogs are Bglobal
processors,^additional precedence tasks should be conducted
and the results compared.
These results again highlight the need for more foun-
dational research, such as development of reliable visual
acuity measures, to assess whether such large distances
between stimuli, or between the dog and the stimuli, are
appropriate. In the absence of such instruments, one way
forward might be to assess dogs’performance on other
Fig. 1 Example of a Navonfigure where the global and local features are
incongruent. The global feature is represented by the letter "H" and the
local feature is represented by the letter BX^
Psychon Bull Rev
tasks of visual perception, such as tests assessing suscep-
tibility to geometric illusions.
Visual perception and misperception
One method used to study perception is assessment of misper-
ception. Geometric illusions represent misperceptions of
physical realities in the external world (Gregory, 2015).
Under experimental circumstances, the induced illusion is
the result of mechanisms that are normally helpful for seeing
the external world in the most adaptive way, but which trick
the brain into seeing something different by correcting when a
correction is not necessary.
To our knowledge, three studies have assessed geometric
illusion susceptibility in domestic dogs (Byosiere et al., 2016;
Byosiere et al. 2017b; Miletto Petrazzini, Bisazza, & Agrillo,
2016) (see Fig. 2for detailed descriptions of the illusions
assessed). Using a two-choice size discrimination task,
Byosiere et al. (2016) trained eight Lagotto Romagnolo dogs
to indicate the larger (or for some dogs smaller) stimulus.
After an extensive training process, the dogs were presented
with control and illusory stimuli and had an opportunity to
nose-touch the stimulus perceived as being larger or smaller.
They found that dogs perceived two different versions of the
Ebbinghaus-Titchener illusion in a manner opposite to the
direction in which humans and most other animals perceive
the illusion. These findings suggest that dogs perceptually
rescale the target circles in the display so that they appear more
like the other inducer circles. In the Delboeuf illusion, thought
to be similar to the Ebbinghaus-Titchener illusion in terms of
its underlying mechanisms (Sherman & Chouinard, 2016),
both Byosiere et al. (2016) and Miletto Petrazzini et al.
(2016) found that dogs were not susceptible (Fig. 2).
However, certain individual dogsdid demonstrate susceptibil-
ity, once again in the opposite direction to that observed in
More recently, Byosiere et al. (2017)assessedcaninesus-
ceptibility to the Ponzo illusion in four varying presentations
Two equally sized target circles are presented,
with one surrounded by small inducer circles
and the other surrounded by large inducer
circles. For humans, this juxtaposition gives the
impression that the target surrounded by the
small inducers is larger than the other target.
Two equally sized target circles are presented,
with one surrounded closely by an inducer ring
and the other surrounded by a more distant
inducer ring. For humans, this juxtaposition
gives the impression that the target surrounded
with the closer ring is larger than the other
Ponzo Illusion Two equally sized targets (e.g. circles or lines)
that appear unequal when superimposed over
converging lines. For humans, this
juxtaposition gives the impression that the
target closest to the convergences of the lines is
larger than the other target.
Fig. 2 Schematic depictions and explanations of the Ebbinghaus-Titchener, Delboeuf, and Ponzo illusions
Psychon Bull Rev
(for an example of the Ponzo illusion, see Fig. 2). While the
dogs demonstrated above-chance performance as a group in
one presentation (56 %), no individual dog performed signif-
icantly above chance. In a re-test of the same presentation, and
in two different presentations of the same illusion, no signif-
icant results were observed at the group level, although one or
more dogs did demonstrate a small but significant effect (some
in the same direction and others in the opposite direction to
Taken together, then, there is limited evidence for dogs’
susceptibility to the Ponzo illusion. While this may be due to
their poor visual acuity and spatial resolution, we believe these
findings could also be due to the way the stimuli were pre-
sented. Stimuli in the study were presented on a horizontal
(landscape) plane. However, in humans, vertically (portrait)-
presented stimuli, where the apex is at the top, produce greater
perceived depth and size than stimuli presented in any other
orientation (R. J. Miller, 1997). Therefore, one potential ex-
planation for these findings is that the horizontally presented
stimuli may not have induced the Ponzo illusion in dogs, due
to the fact that the illusion may have been weak. Furthermore,
it is currently unknown whether or not dogs perceive the min-
imum size difference required in humans in order to perceive
the Ponzo illusion. Additional research is required to address
These unexpected observations of canine illusion suscepti-
bility have implications for theoretical explanations of under-
lying visual processing mechanisms. Seeing as most animals
tested previously have demonstrated susceptibility in the same
direction as humans (for a review, see Feng, Chouinard,
Howell, & Bennett, 2016), it is possible that the mechanisms
underlying perception of illusory stimuli differ across species,
leading only some to experience the illusion. Additionally,
marked individual differences in illusion susceptibility were
observed. It may be that in these instances a difference in
individual visual acuity would explain the variation across
subjects, but other explanations may also exist. Future re-
search should attempt to assess illusion susceptibility across
a variety of different dog breeds, as well as across different
stimulus presentations,in order to provide additional evidence
for these findings. Additionally, other psychonomic assess-
ments of perception should be conducted in order to further
understanding of the mechanisms underlying visual process-
ing in dogs. If dogs do, in fact, process visual stimuli differ-
ently from humans, certain cognitive assessments may not
appropriate, as they may not induce the hypothesized effect.
In recent years, the majority of research in form perception has
focused specifically on face processing. Dogs have been test-
ed in various experimental paradigms, where they are asked to
look at images of dog and human faces (e.g., Huber, Racca,
Scaf, Virányi, & Range, 2013; Pitteri, Mongillo, Carnier,
Marinelli, & Huber, 2014; Racca, Guo, Meints, & Mills,
2012). Outcomes from these paradigms suggest that dogs
are able to differentiate familiar and unfamiliar faces presented
as 2-D representations. These abilities even extend to ob-
scured faces, where only certain areas of the face are shown.
Racca et al. (2012) attempted to address whether dogs were
able to distinguish between different human emotional expres-
sions; however, the results were inconclusive. While they ob-
served differential gaze biases depending on whether a friend-
ly or threatening expressionof a dog was shown, there was no
such difference for human stimuli.
Nagasawa, Murai, Mogi, and Kikusui (2011) addressed a
similar question: whether dogs could first learn to distinguish
between smiling and neutral images of their owner and then
transfer the ability to novel stimuli of unfamiliar humans.
While the dogs were able to choose the owner’s smiling face
significantly above chance, their success rate was significantly
lower when presented with an unfamiliar face. Not only has
this research found that dogs can distinguish differences in
human faces, but also that dogs can successfully discriminate
dog faces from non-dog faces (Autier-Dérian, Deputte,
Chalvet-Monfray, Coulon, & Mounier, 2013), with a prefer-
ence for facial images of conspecifics over human faces, toys,
and alphabetic characters (Somppi, Törnqvist, Hänninen,
Krause, & Vainio, 2012).
Taken together, these findings suggest that dogs can pro-
cess visual information cognitively at quite sophisticated
levels, and raise questions about what kinds of cognitive strat-
egies they use for this purpose. However, it should be noted
that these studies assess quite complex processing abilities
without first assessing foundational visual processing compo-
nents. For example, many of these studies make the assump-
tion that dogs visually perceive 2-D stimuli in much the same
way as humans. The photographic presentations used are of-
ten large and not life-size, highlighting potential confounds
such as the fact that these stimuli may not be perceived iden-
tically to actual 3-D human faces. Furthermore, considering
dogs have reduced visual acuity, it is possible that small dif-
ferences in emotional expression may not actually be per-
ceived. Future research should first establish whether or not
dogs process basic 2-D stimuli in the same way as3-D stimuli
before making broader generalizations about dog social cog-
nitive abilities (for a case study on iconic representation in
dogs see Kaminski, Tempelmann, Call, & Tomasello, 2009).
Part 2 conclusions
In summary, many unique and interesting studies have been
conducted on visual perception in dogs. While these studies
shed light on how dogs perceive their external environment,
many are affected by potential confounds that exist primarily
due to a lack of fundamental vision research in dogs. These
Psychon Bull Rev
confounds make it extremely difficult to disentangle actual
results from physiological limitations and may result in inap-
propriate conclusions. Furthermore, due to the extensive train-
ing process required in many of these paradigms, subject num-
bers are often small, meaning that assessing individual, breed,
and morphological differences is often impossible. Due to the
marked variation observed in dog morphology, it seems ur-
gent to consider whether such differences may affect visual
Part 3: Morphological variation and its effects on eye
structure and visual perception
Neurobiological investigation suggests there are similarities in
visual systems across vertebrates (Lamb, Collin, & Pugh,
2007) as well as in the neural circuitry underlying vision in
humans, non-human primates, and other mammals (Masland
&Martin,2007). However, it is also clear that evolutionary
pressures have led to differences in perceptual processes
(Feng et al., 2016; Lamb et al., 2007). This is largely due to
the fact that different species have different physiological fea-
tures and functions, likely specialized to be adaptive for a
given environment. Thus, while the same information that
enters the eye may be available, it may be processed and
interpreted differently by different species. Even the most pre-
cise descriptions of physiological and neurological processes
underlying vision in a species may not provide a reliable rep-
resentation of their visual processing capacities. Behavioral
data are required to substantiate any conclusions drawn about
the subjective visual experience of animals.
As discussed above, dogs represent an extremely variable
species in terms of their physical appearance and behavior
(Hart, 1995;Wayne,1986a,1986b). Reports from the
American Kennel Club and the International Cynologic
Federation indicate that approximately 400–500 dog breeds
are currently registered. These breeds differ in many ways,
such as size, weight, color, hair type, and length, ear and tail
positions, body and facial shape. Considering this immense
variation in morphology, it seems logical to contemplate
whether even simple differences, such as height, could affect
individual visual experiences (Fig. 3).
Furthermore, in dogs, varying facial morphologies exist,
which further complicates interpretation of even the most ba-
sic physiologic measures assessing perception. For example,
in brachycephalic breeds, the eye position is often more later-
ally directed than in dolichocephalic breeds (Fig. 4).
Therefore, one would expect differences in the amount of
binocular overlap, based on differences in eye-position, as
well as the presence or absence of a muzzle obstructing the
field of view (Evans & De Lahunta, 2013). Accordingly,
Kerswell, Butler, Bennett, and Hemsworth (2010) demon-
strated that variation in morphological features affected
communication in young dogs. Specifically, communicative
social signals may be used differently depending on whether
or not they are performed by a dog with a short or long snout.
Additionally, it has been observed that larger dogs tend to
perform better than smaller dogs when following human ges-
tural pointing cues (Helton & Helton, 2010). While a meta-
analysis assessing response to human cues found no effect of
breed (Dorey, Udell, & Wynne, 2009), this may be partly
because only a limited number of breeds have been included
in available studies.
These studies suggest that morphological variation may
affect outcome in performance, and it appears that, in dogs
at least, this diversity may also affect visual processing
(McGreevy, Grassi, & Harman, 2003; Roberts, McGreevy,
& Valenzuela, 2010). McGreevy et al. (2003) observed that,
while dog eye size is variable, it is strongly correlated with
skull dimension. In addition, certain morphological variations,
such as nose length and face shape, appear to affect eye
structure. Even the total number of retinal ganglion cells
appears to correlate with skull measurements and eye size.
Perhaps most interesting is the finding that retinal ganglion
cell distribution is highly variable and correlated with nose
length. In some dogs, retinal ganglion cells were
concentrated in a horizontal visual streak across the retina,
which presumably provides more sensitivity to movement
along a horizontal field of view. In others, they were
concentrated in a strong area centralis, with virtually no
streak being evident. Dogs with longer noses had obvious
visual streaks, but those with short noses, such as pugs, had
virtually no visual streak. Roberts et al. (2010) later observed
that human selection for diversity in domestic dogs' body
shape and size seems also to have resulted in artificial human
selection pressures affecting the organization of the dog’s
However, do such differences affect the way in which dogs
visually process their external environment? Gácsi,
McGreevy, Kara, and Miklósi (2009) observed that, while
dogs, regardless of breed group, appeared to be successful in
ahuman gestural communication task, brachycephalic dogs,
with flat faces and more forward-facing eyes, were signifi-
cantly better in using the human pointing gesture than doli-
chocephalic dogs, with longer noses. These authors suggested
that morphological characteristics might be indirectly associ-
ated with performance in cognition tasks, due to differences in
Considering the drastic variation in size, body type, facial
morphology, and behavior, it seems possible that such differ-
ences could be associated with, affect, or perhaps even be
caused by different visual processing capacities. Perhaps se-
lection for different visually directed behaviors underpinned
some of the differences in morphology that arose as breeds
were developed. The old adage of form following function
may apply here; dogs may have been selected because of
Psychon Bull Rev
small differences in visual perception that made them better or
worse at performing visually-dependent tasks, which then
may have led to gradual, breed-specific changes in morphol-
ogy and more exaggerated breed differences in visual percep-
tion (Fig. 5).
Part 3 conclusions
It is possible that morphological differences, such as nose
length and the placement of the eyes, limit or aid certain
dogs when perceiving their surroundings. We must clarify
here that we are not suggesting that differences in morphol-
ogy directly affect cognitive ability. We simply wish to
highlight that current research suggests that, in certain cog-
nitive tasks, some breeds and morphological types perform
better than others. Additionally, in many studies assessing
similar aspects of cognition (such as object permanence
and gestural communication tasks discussed in the
introduction), variation is observed in findings. Given the
diverse variation in canine morphology, it seems reason-
able to assume that, due to selective pressures, some dogs
maybebetterabletoperceivecertain cues during cognitive
tasks. This may be due to differences in visual processing
abilities or simply differences in size, height, and nose
length. Sight-hounds, for example, often characterized par-
tially by having relatively long snouts, may be quite adept
at perceiving movement from far away and potentially may
perceive the world very differently from pugs, which are
much more flat-faced and therefore have increased binoc-
ular overlap. It will be important for future research to
investigate whether these morphological differences are
associated with variation in cognitive performance in
visually-presented tasks. In the interim, because it is pos-
sible that morphological variation in the eye may result in
differences in visual processing, thereby indirectly affect-
ing performance outcomes in visual cognitive tasks, we
Fig. 3 A photographic example of the effect of visual perspective on
vision. Above are four perspectives of an identical visual scene as
viewed from 8 in. (20.32 cm) above the ground (A), 21 in. (53.34 cm)
above the ground (B), 34 in. (86.36 cm) above theground (C), and 66 in.
(167.64 cm) above the ground (D). The sizes were chosen to represent
small, medium, and large dogs and humans, respectively, but assume
identical Bhuman-like^vision in all cases
Fig. 4 Photographic representations of brachycephalic (A), mesocephalic (B), and dolichocephalic (C)dogbreeds
Psychon Bull Rev
must ensure that any task used is not only relevant for
assessing the cognitive component in question, but also
that the methodology is appropriate for all subjects in the
Part 4: Avenues for future research
In the last two decades, researchers have documented a variety
of behaviors and abilities that make dogs a unique model for
studying cognition, especially their social cognition.
Tabl e 1 . Key components discussed in this review, examples of specific questions that still need to be assessed, and whether any research has been
Topic Questions Has research been conducted?
Sensitivity to light How does photo bleaching affect dog vision, and is there
a concern for researchers when assessing cognition
in conditions where the light has abruptly changed?
Do variations in the tapetal-area (either a complete lack
or partial lack) pose a concern for assessments such
as visual acuity? If so, how could these variations
affect the results of cognition studies?
Brightness discrimination What is the brightness discrimination threshold in dogs? Stone (1921); Pretterer et al. (2004)
Visual acuity What is a typical dog’s visual acuity? Are there breed
differences, or differences that are dependent on
facial morphology type (i.e. brachycephalic,
mesocephalic and dolichocephalic)?
P. E. Miller and Murphy (1995); Murphy
et al. (1997); Tanaka, Ikeuchi, et al. (2000)
Depth perception Does stereopsis increase in adult dogs? Does depth
perception vary based on facial morphology type?
Wal k and G ibso n (1961)
Color vision Can dogs behaviorally discriminate between what
most humans see as red/green?
Rosengren (1969); Tanaka, Watanabe, et al.
(2000); Kasparson et al. (2013)
If dogs have the ability to perceive the UV spectrum,
what kind of stimuli should and should not be
Visual discrimination and
How do dogs visually process shapes and forms? Byosiere et al. (2017a)
Byosiere et al. (2017a)
Duke-Elder (1958); Karn and Munn (1932)
Global vs. Local processing Are dogs in fact global processors? Are there additional
tests of precedence that can be conducted?
Mongillo et al. (2016); Pitteri, Mongillo,
Carnier, and Marinelli (2014)
Visual Perception Do dogs perceive the world differently than humans
and other animals, or is it relatively the same?
Byosiere et al. (2016); Byosiere et al. (2017b);
Byosiere et al. (2017a)
Face processing How do basic differences in vision and visual perception
affect how to interpret the findings of face processing
in dogs? How do differences in brightness thresholds,
and visual acuity affect these outcomes? Additionally,
do dogs process face-like outlines like human faces,
and does the size of the stimulus matter?
Huber et al. (2013); Racca et al. (2010);
Racca et al. (2012); Nagasawa et al. (2011);
Autier-Dérian et al. (2013); Somppi et al. (2012)
Morphological variation In addition to individual differences, and breed differences,
are there broader differences due to facial morphology
type? Research is needed to assess whether these kinds
of differences are potential confounds across studies on
canine cognition. If such differences do play a role, it is
important we begin to evaluate their extent and re-assess
certain cognitive tests.
Evans and De Lahunta (2013); Kerswell et al.
(2010); Helton and Helton (2010);
Dorey et al. (2009); McGreevy et al. (2003);
Roberts et al. (2010); Gácsi et al. (2009)
Are current cognitive assessments actually capable of
assessing dog cognition accurately? How do differences
in vision (within dogs and between humans and dogs)
affect the kinds of paradigms researchers use?
Fig. 5 Theoretical diagram of the proposed relationship between
selective breeding, morphology, visual processing, and performance on
Psychon Bull Rev
However, it is surprising how much remains unknown about
their visual-processing capacities. This is problematic, consid-
ering that the vast majority of cognition tasks utilize experi-
mental paradigms relying heavily on visual processing. Well-
established paradigms and protocols, found in human and
primate comparative cognition literature, have been applied
to comparative dog cognition studies, often without critical
review. As human and primate visual capacities are quite com-
parable (Jacobs, 1996), many of these visual tasks were cre-
ated with human visual-processing capabilities in mind.
Studies that aim to adapt human/primate-designedvisual tasks
to dog cognition should, therefore, carefully consider the
visual-processing skills of dogs, in order to avoid inappropri-
ate experimental paradigms leading to inaccurate conclusions.
The findings observed in dog vision research are particu-
larly relevant to researchers studying canine cognition, in
which dogs are typically presented with visual scenes.
Individual and breed variations on cognition tasks have yet
to be fully assessed; many studies focus on studying a single
breed, while others include mixed breed samples. While the
latter may provide more generalized results, reflective of mul-
tiple dog breeds, there are clear differences in visual physiol-
ogy between and within breeds. Before general conclusions
about dog cognition can be made, therefore, we must assess
whether or not Ba dog, is a dog, is a dog,^or are we, as
cognition researchers, creating false conclusions by not
assessing the effects morphological differences have on
Below we provide Table 1as a quick guide to some of the
key issues discussed in this review. The table includes exam-
ples, by no means exhaustive, of the questions that still need to
be answered, as well as an indication of studies that have
already been conducted on each topic. We also discuss more
general recommendations for future studies on dog vision and
In general, there is a strong need to assess basic visual
capabilities across a wider range of dogs. While a foun-
dational understanding of the canine visual system is
available, due to small sample sizes and/or the use of a
limited number of breeds, it is not yet clear whether these
findings are representative of all dogs. In any other spe-
cies, small sample sizes are sufficient for foundational
studies, because all members of a species can be assumed
to be relatively homogeneous. This is not the case with
domestic dogs, however, which provide an extreme exam-
ple of just how flexible a species’phenotype can be. Until
the extent to which these differences affect visual process-
ing is known, researchers should perhaps attempt to assess
cognitive performance in dogs using subjects with similar
visual physiology. However, we also recommend that fur-
ther investigation of dogs’visual processing skills is ur-
gently needed. It is important that we continue to study
dogs’unique cognitive abilities, but also that we assess
the effects of additional factors, such as vision and/or
morphology, on canine cognition.
More research is also needed on individual, breed, and
morphological differences, and there is a strong need to better
understand links between physiology, morphology, and cog-
nition. Future studies should attempt to increase sample sizes
as well as discern whether or not there are visual processing
differences between mesocephalic, brachycephalic, and doli-
chocephalic dog breeds. Considering the unequalled variation
in morphology between dogs of different breeds or breed
types, they present a unique opportunity to better understand
any effects these may have on vision. These studies should
extend to visual acuity measures, as acuity has been estimated
in only a few breeds. Considering the wider visual streak
observed in longer nosed-breeds, it is possible that this char-
acteristic enhances their ability to detect stimuli across a wider
field of view at the cost of reducing the ability to discriminate
fine details. Additionally, while some studies highlight a po-
tential for color discrimination capabilities in dogs, since this
is difficult to explain in neurophysiological terms, additional
research should be conducted to ensure dogs are not using
other cues on which to base their decisions. Considering dogs
may have additional visual capacities to those most commonly
tested, like the ability to see UV light, it is possible that their
behavioral decisions are based on cues other than color. This
requires further research.
Replication studies should be conducted to observe if there
is consistency across findings. While many of the studies ad-
dressed above have observed consistent results, discrepancies
in topics like visual perception, form perception, visual acuity,
and color vision warrant further investigation. While many of
these discrepancies may be due to differences in sample char-
acteristics and methodology, a significant difficulty in
assessing visual perception is the human-centric biasin exper-
imental design. Often, behavioral oddities or performance
faults in tasks are attributed to visual and cognitive inabilities.
While an absence of evidence is not evidence of absence, such
findings are rarely attributed to differences in sensory percep-
tion. This is particularly evident in the findings observed on
geometric illusion susceptibility, and underlines the impor-
tance of careful experimental design. Due tothe physiological
differences underlying perceptual processing, researchers
must keep in mind the differences underlying visual process-
ing of dogs and the effects this may have on their cognition
Finally, many of the studies described above have assessed
vision in dogs by means of assessing their physiology.
However, as noted in some instances, differences between
physiological and behavioral measures have been observed.
Considering it is important for any researcher utilizinga visual
paradigm to understand the appropriate stimuli to use, future
research should attempt to better understand why there might
be differences between physiological evidence and behavioral
Psychon Bull Rev
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