As stated, some coat colors in ferrets are associated with neurological
defects, including such problems as blindness or other defects in vision,
hearing loss, behavioral changes, loss or deterioration of motor skills,
epilepsy, and even mild to severe retardation.  Because the timing of
eye development takes place so early in the growth of the embryo, vision
is rarely impacted, or only to a minor scale (with exceptions).  In
contrast, hearing is more frequently impacted, with a wider range of
problems, because development takes place at a later point of embryonic
development.  In terms of health impact, because ferrets have been bred
for albinism for centuries, most of the nastier problems have long been
bred out and visual differences--although profound and represent
comparatively deteriorated vision--do not generally change the quality of
life experienced by the ferret.  There doesn't seem to be major visual
problems in ferrets having diluted coat colors or with pointing, and no
data for the melanistic ones.  Piebald colorations MAY impact vision, but
apparently not to any significant degree, and no specific experiments
have been reported.
 
The human and the ferret eye are essentially identical in all respects,
with a few minor exceptions that reflect different evolutionary
histories.  Notably among these differences are an area on the retina
of humans that allows extremely fine focus to take place (the fovea),
a different density and ratio of rods to cones, a reflective layer of
tissue behind the retina in the ferret (the tapetum), differences in lens
shape and malleability (a far-sighted eye versus a near-sighted eye), a
difference in the shape of the pupil (round in humans, horizontal slit in
ferrets), and a third eyelid in the ferret (just a fleshy pink area at
the inside corner of the human eye).  There are other differences, such
as the angle of the eye in the bony orbit, but those are influenced by
skull anatomy.  Some may make the comment that these represent a lot
of differences, however, compared to thousands of nearly identical
structures in both the human and ferret eye, the differences pale in
comparison.
 
Each one of these differences exists because humans and ferrets evolved
doing different tasks to make a living.  The ferret's eye is adapted to
locate prey in dim light (large numbers of rods, tapetum, shape of
pupil), while at the same time protecting itself from the types of
debris encountered in burrows, running through brush and grass, and from
injuries that might occur while struggling with prey or predators (third
eyelid).  The human eye evolved an area of very sharp focus (fovea),
useful in recognizing faces, making tools and finding edible plants, but
the ferret uses it's nose for identification, it's whiskers for precise
prey location, and it's eyes are adapted to see in dim light, so has
little need for such sharp focus.  Humans have a far-sighted eye, useful
in scanning the horizon, but ferrets have a near-sighted eye, designed to
best visualize objects within a couple dozen feet (objects farther away
are more noticed for movement rather than precise focus).  This is the
type of visual adaptation expected from an animal adapted to hunt prey
in the dark underbrush and within burrows.  It is probable the smell
(olfactory sensory organs) and whiskers (tactile sensory organs) are
adapted for hunting, while sight (visual sensory organs) is adapted to
detect predators, which would explain why polecats are periodically found
in large owl pellets, and the number one predator of polecats throughout
Europe (and ferrets in New Zealand) are automobiles--neither are visually
noticed until it is too late.  The proportion of rods to cones in the
ferret shows high adaptation of the eye to work in dim lighting
conditions.  Rods are very sensitive to light but (in most cases) produce
little color information and do not produce a very sharp image compared
to cones.  Cones require high levels of light, and produce color
information as well as shaper images than rods.
 
The bottom line is that the ferret sees more or less the same as humans,
but that vision is somewhat grainier, has little color information (most
reds and some blues), is nearsighted, and cannot bring objects into
super-fine focus (Gosh, expect for seeing in color, I think I have ferret
vision!).  Compared to humans, ferrets are somewhat colorblind, but since
ferrets are domesticated polecats, and polecats hunt mostly by smell, not
being able to see some colors is unimportant.
 
Bob C
[Posted in FML issue 4256]