Human vs. Animal Ophthalmology

Why dogs and cats can actually see Spirits...

Have you ever wondered why a cat suddenly bolts from the room, or just what exactly it is that crazy dog is looking at when he stares at the wall like he sees something?

Well, chances are they do see something...or someone.

By now, you all know about my little cat, Puma. He has never had a problem in my house, because he's been here since he was just a handful of fuzz.

If he sees somebody, I guess he figures they belong here. He used  to sleep on the back of my Dad's chair when Dad was still alive, so he still does...or did, until the dog moved in.

At the time of this writing, it will be year since my dad passed away. I acquired a new puppy recently, Angel.....ok, she's 10 months old, but she's still a puppy...a white German shepherd who's as big as a couch.

The other night, I was monkeying around on the computer, and Angel was sleeping next to me on the floor. All of a sudden she gets up and stands at attention, and walks purposefully into the living room with her ears perked and forward, and the hair on her back bristled. She went straight over to Dad's chair, and let out a couple of those "I mean business" deep, low barks. She stood there in front of the chair for a few seconds, then her ears went back, her head went down, and her tail started wagging. Then she comes happily trotting back to her spot on the floor beside me like nothing was wrong.

I could just hear my dad talking to her...."Oh, that’s ok...what're you doin'?"....he used to talk to animals like they were babies...he loved dogs.

So this little episode is what gets me to thinking. We already know that animals can see in the dark and that there have been numerous cases of them going bonkers in a room for no reason. So my big question was, why?

Here is what I found out, and put the pieces together...I got this info from the internet, so I should give credit where credit is due, so they don't hunt me down and beat me up.

www.nahra.org/nahra/vision.htm

This is a Veterinarian website that deals with vision in dogs, and is comprised of excerpts from an article, titled "Vision in dogs", and was written by Paul E. Miller, DVM and Christopher J. Murphy, DVM and published in the Journal of Veterinary Medical Association (JAVMA, vol. 207, no.12, pp.1623-1634 Dec. 15, 1995)

First, lets look at how the human eye works.

To see anything, we need light. Light rays send electrical signals to the brain to be interpreted into images.

The eye sits in a cone-shaped cavity about an inch in diameter and is surrounded by soft tissue.

Here are the parts of the eye and how they work.

Cornea

Composed of six layers of tissue, the cornea has been referred to as the "window of the eye" because it focuses light entering the eye. Of those six layers, the most important is the outer layer, or epithelium which serves as a protective layer. The outer layers add strength to the eye.

Lens

The lens works like the auto-focus on a camera. It alters shape to concentrate the eye, say on a book.

Pupil

These black circular openings in the iris are where light enters the eye. Like the shutter of a camera, the iris regulates the amount of light coming into the eye.

Iris

What color are your eyes? The color is in a pigmented layer of the iris. The iris and pupil work together to control the amount of light entering the eye.

Vitreous Body

Inside of the eye is a gel like substance known as the vitreous body.

Optical Nerve

The optical nerve carries information from the eye to the brain.

Retina

Part of the information transported from the optical nerve originates on the retina. The retina acts sort of like film in a camera, to give you a better idea.

Sclera

The sclera give you "the whites of your eyes". It gives your eyes structure, protection, and strength.

So, as you're reading this, your Iris is controlling the amount of light while the Retina picks up the information and sends it along the Optical Nerve to the brain. The Retina is picking all this up by capturing images that have already been focused on by the Cornea and Lens.

Now, on to what dogs see...

Drs. Miller and Murphy began their article on the canine visual system with the basic difference between canine and human vision. Dog eyes are designed to operate well under low light conditions, while people eyes perform best in bright light. The canine visual system is able to function under a wide range of lighting conditions, but the adaptation to low light enhances their ability to function as predators. The minimum degree of light for vision in cats is six times lower than for humans, and the minimum degree for dogs is thought to be somewhat greater than cats, it is still much lower than humans.

Several methods are used to improve a dog's vision in low light. The principle method is based on the types of photoreceptor present in the canine retina. The dog's retina is made up primarily of  rod photoreceptors, while the human retina is primarily cone photoreceptors.

Rods function much better in dim light, while cones are used for color

vision and require bright light.

Humans and dogs both have rods and cones in their retinas, but the amount of each is very different.

Another method used by dogs to improve their low light vision is the use of a Tapetum Lucidum.

The tapetum lucidum is a highly reflective layer of cells located behind the photoreceptors in the canine retina. This reflective layer is responsible for the bright shine of a dog's (and/or other species) eye when a bright light is shone at the eye in the dark. This reflective layer functions to improve low light vision by reflecting light back through the retina, essentially allowing the photoreceptors two chances to react to each amount of light. It has been shown that the tapetum lucidum shifts the wavelength (via florescence) of the reflected light to more closely match the optimal wavelength for sensitivity of the rod photoreceptors, thus enhancing contrast.

SENSITIVITY TO MOTION

Rod photoreceptors are better able to detect motion and shapes than cones. An early study indicated that dogs could discriminate an object in motion at 810 to 900 meters, but were only able to discriminate the same object when stationary at 585 meters or less. 

SENSITIVITY TO FLICKERING LIGHTS  (this is important)

The frequency at which rapidly flickering light fuses appears to fuse into a constantly illuminated light termed "Flicker fusion".

In most humans, flicker fusion occurs at around 50 to 60 Hz. It appears that the flicker fusion rate for most dogs may be as high as 70 to 80 Hz.

The refresh rate on our television is about 60hz, so we perceive the picture as a smooth image. But to our dogs it may appear as a rapidly flickering image.

VISUAL FIELD OF VIEW

Visual field of view is the area seen when the eyes are fixed on one point.

In dogs, this probably varies from breed to breed, because of the differences in shapes if the skulls, placement of the eyes, and the shape and size of the nose. In the average dog. the eyes are placed such that they deviate approximately 20 degrees lateral to the midline. In humans. the eyes do not deviate, but look straight forward. When both eyes were compared, two studies have indicated that the total field of view in dogs is approximately 240 to 250 degrees, which is 60 to 70 degrees greater than the normal human's field of view, 180 degrees.

Our dogs are probably more aware of the activity occurring around them than we are because of this larger field of view.

VISUAL ACUITY

Visual acuity is the ability to see the details of an object separately and clearly. Visual acuity depends on three factors.

A) Optical properties of the eye.

B) Retinal detection and processing of the image.

C) Proper interpretation of the images by higher centers in the brain.

So after learning this, here is what I surmised.

Dog eyes work more like a camera than human eyes do. It was the flicker fusion frequency thing that convinced me.

We can use a ceiling fan as an example. If you have one, turn it on and watch it for a moment. The spinning blades are one big blur. Now, blink your eyes rapidly while still looking at the moving fan.

The fan will appear to slow down to you, but it is still moving at the same rate of speed.

Now here is another use for your digital camera or Polaroid, just something that gives  you instant results. Take a picture of the spinning fan...the blades will be caught as being still. That is because the camera records images faster than the brain can interpret them.

If you can not do this experiment because you do not have a ceiling fan, then take a photo of your TV set while it's on. You'll see the retrace lines as they move down the screen. Your eyes cannot detect this, but cameras and apparently dogs can. This what brought the flicker fusion to light for me.

Since it is also due to the low light capabilities of canine eyes, added to flicker fusion, that I feel they are able to see in the different spectrum, thus actually seeing "beings of light", if even for a second or two. They will see them, recognize it as someone there, and their natural curiosity will make them go investigate further, protect their loved ones from a stranger, or frighten them out of the room.

It is like capturing a spirit on a single frame of video tape.

When people who claim to have the ability to "see" spirits, they are not actually seeing them with their eyeballs, they're seeing them with their 3rd eye, and interpreting the mental image as being actual. Whereas animals can see them due to the higher degree of visual acuity.

Ceiling Fan Experiment

This is to show you basically the difference between the way human and animal eyes see things.

I took a photo of my ceiling fan while it was on and set at the high speed.

The first photo will represent "what a dog sees"....I used the flash...i.e. more light entered the camera, thus simulating the Flicker Fusion.

What a dog sees

 What a human sees

The second photo was taken without using the flash....i.e. less light (as in human vision)

So, this all comes together in understanding why animals act oddly in a dwelling that may be "occupied"....Its the Flicker Fusion rate of light frequency that allows pets to see a moment in time that we can't. Just like a camera....

The next time your dog stares at the wall, don't think of him as being goofy...try to take a picture.

~~~Copyright 2002

None of this material may used without
the written consent of Dr. Ken.