| THE
DOG'S WORLD OF COLOR
By
Dana K. Vaughan, Ph.D., Dept. of Biology, University of Wisconsin,
Oshkosh
email:
Vaughan@uwosh.edu
Vision is just one of several senses that dogs use to scan their
world for important information. Any visual scene has multiple
aspects to which dogsÕ brains are sensitive: brightness, shape,
contrast, and motion are a few of these. But what about color?
Many authorities have stated that "dogs are colorblind",
with the implication that dogs perceive only black, white,
and shades of grey. In fact, this was not known for certain,
but became "folklore". Then, in the late 1980s,
a definitive set of experiments was done at the University
of California, Santa Barbara, by what may well be the worldÕs
foremost research program on comparative color vision. I played
a very small part in this study, in that I volunteered my
two Italian Greyhounds, "Flip" and "Gypsy",
for the study. A third dog, a toy Poodle named "Retina",
was volunteered by her owner, who was also one of the researchers.
All three dogs were beloved family pets, both before and after
the studies.
To define canine color vision, each dog was placed into a
box facing a display of three round light panels in a horizontal
row. Beneath each light panel was a cup. A computer provided
combinations of different colored lights in the three panels.
Two were always the same color, while the third was different.
Over a series of some 4,000 trials, each dog was taught to
"find the one thatÕs different". Every correct choice
was rewarded with a treat in the cup beneath the correct choice.
Every wrong choice meant that no treat was forthcoming, from
any cup. All three dogs were so food-driven that they werenÕt
even starved for the tests. Mine loved their "lab days"
and would practically drag me onto the elevator to get to
the box!
When it was clear that each dog understood the test, some
200-400 tests were run per session, over a period of weeks.
This provided a large data set on which statistics could be
run. If the dog couldnÕt tell two colors apart and
was just guessing, s/he would be right, on average, only 33%
of the time (one out of three possible choices). If the dog
could tell two colors apart, s/he would be right much
more often.
To ensure that the dogs were identifying the different panel
based on color alone, the position of the different colored
panel was randomly moved around (left, middle, or right) and
the relative brightness of each light was also equalized,
so that brightness couldn't be a hint as to which panel was
which. The key publication describing this classic work
is: J. Neitz, T. Geist, and G.S. Jacobs (1989), "Color
vision in the dog", Visual Neuroscience Vol. 3,
pages 119-125.
These experiments showed that dogs do see color, but in a
more limited range than that seen by normal humans, who see
the rainbow of colors described by "VIBGYOR": Violet,
Indigo, Blue, Green, Yellow, Orange, and Red (plus hundreds
of variations on these shades). Instead, dogs see "VIBYYYR"
(Violet, Indigo, Blue, Yellow, Yellow, Yellow, and Red). The
colors Green, Yellow, and Orange all look alike to dogs; but
look different from Red and different from the various Blues
and Purples. Dogs are very good at telling different shades
of VIB apart. Finally, Blue-Green looks White to dogs.
The simple explanation for these differences in color vision
is this. The retinas of normal humans have three (3) types
of color receptors, called "cones". Each cone type
is particularly sensitive to light of a narrow limit within
the entire VIBGYOR range. That means that three different
"cone lines" of communication run back to the visual
part of the brain, which then compares the weight of the signals
coming in from each of cone "line". Different weights
produce a perception of different colors. In dogs (and in
"green-blind" humans), there are only two (2) types
of cones, so there is less basis for comparison by the brain,
and thus the perceived color range is more limited. In sum,
dog color vision is "color-limited", not "color-blind".
To better understand the difference between VIBGYOR and VIBYYYYR,
consider what a dog would see in each of the following color
combinations.
HUMANS SEE: VIBGYOR
DOGS SEE: VIBYYYR
How
can we put this information to use as dog trainers? First,
we have to recognize that color is just one aspect of a
visual scene as perceived by a dog. There are two other
aspects that are just as important, if not more so.
The first of these is
motion.
Think of a green lizard motionless on a leafy branch in the
tropics. Both you and your dog would have a hard time
seeing it Š until it moved! Think back to your dog chasing
a green tennis ball against green grass. He can follow it
fine, even though there is no color difference on which to
cue. Objects moving at a moderate rate of speed are, in general,
easier to spot than motionless objects.
The second of these is
contrast
(that is, outline against background). A stationary object
that has a very distinct and very different shape from its
background is easier to spot, while one that is patterned
like its background will blend in. ThatÕs the principle of
the camouflage clothing worn by hunters. Yes, itÕs the color
of the woods, but more importantly, camo breaks up the human
bodyÕs outline against the patchy background scene of branches,
leaves, and underbrush. Even small movements are harder to
see when an objectÕs outline blurs with its background.
The third is
color.
Back to hunters for a moment; ever wonder how we get away
with wearing screaming hunterÕs orange hats with our camouflage
jackets and pants? Deer are probably like dogs (VIBYYYR):
to them, orange is not different from the green or brown undergrowth,
and a hat doesnÕt give a terribly distinctive shape. As a
color, safety orange sticks out like a sore thumb to humans,
but not to dogs or deer.
So, to maximize an objectÕs visibility to a dog, we should
strive to combine: (1)
moderate motion (when feasible); (2)
maximal contrast
(a dark and white pattern unlike that of the background scene);
and (3)
stand-out color
(one that is identifiable by dogs and does not occur very
often in the background scene). The best of all possible worlds
for most background scenes would be boldly-patterned Bright
Purple and White objects.
Having said all that, letÕs never forget the other keen senses
the dog has at his disposal (and we do not): smell and hearing.
These are minimally helpful when a dog is working bumpers,
but play powerful roles when a dog is working live or shot
game. |
