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What do cats see?
The cat is a nocturnal animal, thus the amount of rods (17,
Retina: No 2) is much higher than that of the cones (17, Retina: No 3).
Vision field
Vision field of a cat (200° in total) is much less when compared
to preys, because the cat is a predator (9, lens)
and has to look into forward and downward direction when hunting.
Her eyes are forward-faced and relatively large.
The vision is, compared to humans (180°
in total), larger. Horses have a vision of 305°, dogs that of 250°
in total.
Binocular overlapping in cats and humans is about 140°, in dogs
only between 30°-60°.
Acuity
Visual acuity is the ability to see the details of an object
separately and not blurred. Acuity is measured in "cycles per degree",
which means how many lines one can distinguish as being separate in
a degree of the visual field. Humans see 30 cycles per degree,
horses 18, dogs 12 and cats 6. Acuity in cats is 0.2 times as that of
humans, 0.33 as that of horses and 0.5
times as that of dogs.
If normal humans without vision impairment have an acuity of 20/20 -
i.e. an object having a distance of 20 feet can be seen sharply,
then a dog has 20/75 , and a cat has between 20/100 and 20/200. That
means: if normal humans can see an object sharply in a distance of
100 feet, a cat will see the same object sharply in a distance of 20
feet.
Calculation of acuity in feet is based on the method of Snellen:
1)
metric
meter |
decimal |
Snellen
feet |
|
|
| 6/3 |
2,0 |
20/10 |
|
|
| 6/4,5 |
1,5 |
20/15 |
|
|
| 6/6 |
1,0 |
20/20 |
humans |
| 6/7,5 |
0,8 |
20/25 |
|
|
| 6/9 |
0,67 |
20/30 |
|
|
| 6/12 |
0,5 |
20/40 |
|
|
| 6/15 |
0,4 |
20/50 |
dogs |
|
| 6/30 |
0,2 |
20/100 |
cats |
| 6/60 |
0,1 |
20/200 |
|
| 6/120 |
0,05 |
20/400 |
|
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Flicker rate
is the number of images per second that the image can be seen in
motion and not as single images following each other.
In humans the flicker rate is lower (50 Hz) than in dogs and cats
(70-80 Hz).
Usual television has a flicker rate of 60 Hz, which means dogs and
cats see separated images following each other.
Such measurements are made by an ERG (electroretinography) and
make sense to detect PRA or glaucoma.
Accommodation
In dogs: 2-3D (50 to 33 cm)
In cats: 4D (25 cm)
In children: 14D (7cm).
That means, cats have a relatively slow accommodation to see objects in
nearer distances, a reason why they also use their other senses such as touch
and smell.
Distribution of rods and cones
In
cats there
are about 25 rods to each cone.
Human eyes have four rods to one cone.
In cats rod density in the area centralis equals about 463,000
cones/mm2, and 250,000/mm2 on the periphery near the ora
serrata.
The density of cones in the area centralis is about 27,000 cones/mm2,
and 4000/mm2 in the periphery.
In humans rod density peaks at 150,000 rods/mm2 at
a distance of about 3-5 mm from the foveola.
Cone density in the foveola is about 150-180,000 cones/mm2.
In dogs cone density in the area centralis is about 10,000 cones/mm2.
Note:
Cats do not have a Fovea centralis like humans, they have a streak
called area centralis.
Which colours do cats see?
Distribution of S-cones and M-cones
There are 3 types of cones in humans:
S = short waved light: blue
M = medium to long waved light: yellow
L = long waved light: red
It is common sense that cats only have S-cones and M-cones (like
dogs).
83%-88% of the cones in cats are M-cones, those are cones
sensible for yellow.
Cats are not colour-blind, but they see their surrounding like
humans, who are colour-blind for red-green, a cat cannot distinguish
red colours.
For a cat the world is coloured in fuzzy pastels.
As the cat is a nocturnal animal it does not need an exact
discrimination of colours.
If the colour red is deleted from the spectrum, a cat
would see the world like that.
Why do the eyes of cats glimmer in the
darkness?
Cats as well as dogs have a layer behind the
retina (17) called Tapetum lucidum (16). This layer works
like a mirror and reflects the light rays back to the retina, thus
working like an amplifier.
And that makes some sense, because the cat is a predator and is
active during dawn and darkness, and must be able to recognize
objects also in weak light intensities.
The Tapetum lucidum reflects the light rays
back hitting it, for example light from reflectors, what we notice
in the dark as glimmering of the eyes.
The tapetum also reflects flash-light when we take an image of the
cat, it reflects in turquoise-blue to yellow-green and not in red as
human eyes do.
There are colours in cats, where the tapetum
might be degenerated or even missing, like in white cats (preferably
with blue-eyes and odd-eyes), but also in cats with an extreme
amount of white (like Van-cats), and also in albino-varieties which
have a Siamese-pattern.
The tapetum is an important part of the cat's eyes and enables its
night vision.
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