Is My Pet Possessed?

Posted on May 17, 2011 · Posted in General

Bath and Body Works had a sale: buy any 2 moisturizers, get 1 cat free

I think we’ve all seen the infamous family gathering photo full of people with glowing red eyes.  The dreaded red-eye effect has spoiled many a picture.  Thankfully, most of today’s photo management software has editing features that can easily fix red-eye and salvage these important moments.

I think we’ve all also seen a much different picture from our pets.  Instead of the typical red reflection that comes from a human eye, animal eyes can produce a variety of colors including white, blue, yellow, green, and even pink.  Ever wonder why?

Eye and Retinal Anatomy. Click to enlarge.

The answer lies in the fascinating world of eye anatomy.  Light that enters the eye targets the back wall called the retina.  The retina consists of many different layers through which the light passes until it ultimately is absorbed by cells called photoreceptors.  (You might remember these from school as the “rods” and “cones”) The photoreceptors then turn the light into an electrical signal that travels out of the eye via the optic nerve, weaves through different areas of the brain, and ultimately reaches its destination in the occipital lobe (back of the brain) where the visual cortex deciphers the message and generates a picture.   It really is an amazing process.

What does all this have to do with red/white/blue/pink/green eye?  Animals (non-human) have a structure called the tapetum lucidum behind the retina.  Its job (theoretically) is to act as a reflector bouncing light that passed through the retina back toward the photoreceptors.  It’s believed that this process allows the animal to see in dimmer light than it would otherwise be able to.  The color we see (called eyeshine) depends on the properties of the tapetum lucidum in the specific animal.

Humans lack a tapetum lucidum. Therefore,  the color reflected from all of our eyes will be the same: red.  (Note: a number of eye problems can be detected by the lack or alteration of the normal red reflection)

So rest easy.  No exorcism needed.  Your pet’s eyes are just fine.  Now if we could just teach our dogs to use a litter box…