Download Rat Eye

The Rat Eyes
The rat's eye has the same basic structure and function of all
mammalian eyes, including the human eye.
The rat eyes look like spheres of a single color -- little black or
pink beads. Close inspection can distinguish the iris and pupil of a rat's
eye. These structures may be easiest to see on a pink-eyed rat. In both
humans and rats, light passes through the cornea, which allows both
visible and ultraviolet light (down to 300 nm) to pass through. Then light
passes through the pupil. Like the human pupil, the rat's pupil size is
highly variable. Under dim light, the rat's pupil can reach a diameter of
1.2 mm. Under bright light it shrinks to a small opening of about 0.2 mm
in diameter. Pupil diameter changes can be very rapid in the rat: a
contraction from 2 mm to 0.5 mm takes only half a second.
Next, the light passes through the lens. The lens acts as a filter to
block certain wavelengths of light: it is not equally transparent to all
colors. Which colors are allowed through differs between species. Human
lenses allow only visible light and almost no ultraviolet light to pass
through. Rat lenses, on the other hand, allow all visible light plus almost
50% of ultraviolet A light to pass through.
The human lens is flexible: the ciliary muscles pull on the lens and
thus change its shape. This change in shape causes the light passing
through the lens to bend in different ways (called refraction), which
allows the lens to focus light on the retina (a process called
accommodation). Rats appear unable to change their lenses' shape. For
one, rats have a poorly developed ciliary muscle. For another, relaxing
the lens with eye drops (atropine) does not change its focus. These
findings are consistent with the idea that rats are unable to change the
shape of their lenses, but do not conclusively prove it.
Once the light hits the retina, it is detected by photoreceptors.
Humans have two types of photoreceptors: one type that senses light and
dark, called rods, and one that senses colors, called cones. We have three
types of cones: green, blue, and red. Rats have rods and cones as well, but
only two types of cones: green and blue. Therefore, rats are unable to see
reds. In addition, the rat's blue cones are sensitive to shorter wavelengths
than our blue cones, which means that rats can see into the ultraviolet.
Behavioral
experiments
have
demonstrated
that
rats
can
discriminate between greens, blues and ultraviolets, but also that these
colors may not have much intrinsic meaning to them. Rats don't have as
many cones as we do -- 5% of the human retina consists of cones,
compared to 1% of the rat's retina, so their perception of color may be
much fainter than ours.
The rat retina has a very "coarse" neural grain. Each neural cell in the
rat retina is responsive to a much larger number of photoreceptors than
those of the human retina, which increases sensitivity at the expense of
acuity. Specifically, the receptive fields of the rat ganglion cells are an
order of magnitude larger than those of the human fovea.
Most scientists agree that rats have poor visual acuity, as
demonstrated in behavioral experiments. Rats' acuity is about 20 times
worse than that of humans. Recent experiments by Prusky et al., (2000,
2002) have shown that a normally pigmented rat has about 20/600 vision
(1 cpd), and an albino has about 20/1200 vision (0.5 cpd). The rat's poor
visual acuity has to do with poor optics combined with the coarse neural
grain of the retina.