Dogs normally assess the environment around them by first using their superior sense of smell rather than by sight, although breeds in the sighthound category have been bred to depend more on sight than other breeds but also use this superior sense when in chase.
Dogs that are born blind or those that loose their sight later on in life all manage to cope far better than humans would due to their accute sense of smell.
They do this by making a memory map out of environmental photos so that the dog can relate its whereabouts by the scents it detects in various different directions. This is why blind dogs are far less stressed if they are given the opportunity to explore areas that they visit on a regular basis, rather than new walks in strange and alien environments.
Human Vision v Dog Vision
In a normally functioning healthy human eye, an image is produced in the brain by the stimulation of light sensitive cells on the retina at the back of the eyeball. Here, there are 2 varieties of light-sensitive cells, Cones (for colour) and Rods (for black & white vision, night vision, movement). These stimulations are then directed along the optic nerve to the brain.
There are 3 types of Cones depending on which part of the visible spectrum (see below) that they are sensitive to - red, green or blue light, providing colour vision in daylight. It is a lack of one or more of the cone types that causes individuals to have deficiencies in colour vision or various kinds of colour blindness.
These individuals are not blind to objects of a particular colour but experience the inability to distinguish between groups of colours that can be distinguished by people with normal vision. Recent research on colour perception in dogs has shown that our pets' eyes are structured similar to those of red-green colour-blind people, whose eyes either lack or have a much reduced number of the third kind of cone normally present in humans.
Under daylight conditions, rods are often saturated and so are not able to contribute to form pictures or patterns. The rods respond well in low-light conditions, producing monochromatic vision or detection of movement. They are located on the periphery of the eyeball, therefore when staring straight ahead, movement at the extremes of our vision is registered by our brains but not a full picture until we turn our heads to look.
Picture credit: Jay Neitz
Picture credit: Jay Neitz
The information on this page is based on an article published by livescience.com in June 2012 called "How do dogs see the world?" written by Natalie Wolchover to show how a dog sees the world in comparison to how a normal human eye sees the world.
Field of Vision
Copyright - based on Sevice Dog Central article - How well do dogs see? Kirsten, August 2009
As a dog's eyes are placed on either side of its head, this gives a visual field of 250 degrees whereas the human field of vision is about 190 degrees, giving the dog an advantage of about 60 degrees more peripheral vision than humans.
The binocular field, where the visual field of both eyes intersect, is approximately half of that of humans. This is where we focus and judge the distance of an object. Where only one eye can see, the focus isn't so good and judging distance becomes difficult but we can still see movements, colours and shapes. Humans accommodate focusing on objects at different distances better than dogs because of the shape of the lens in our eye and also the greater number of cones (ie colour receptors in our eye).
This web site has been written by Sean Hopkins (unless the author of the web page is stated otherwise).
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