Dogs See Things Differently.
We just returned from a visit to our son and his family, which includes Stella and Xena, two of our "grand-dogs." They love to play fetch. Xena is obsessive about it, and had many strategies to make herself just about impossible to ignore.
Each dog has her own ball. The balls are identical except Stella's is red and Xena's is green, and they do not get them mixed up. Several times I threw the balls at the same time. The two dogs would both take off in pursuit. About half the time, one or the other would initially grab the wrong ball, but immediately spit it out and go get the other one.
Dogs are, for the most part, color blind, so Xena and Stella could not recognize their ball by color, but as soon as they got the scent or taste, they knew immediately if they had the wrong ball.
Marvin has a different kind of partial blindness. He was just a tiny puppy, two or three weeks old, when a big dog attacked him, and crushed his skull. My associate, Dr. Cumberland, was on call that night. It was an expensive long shot to try and save the puppy and the owners declined treatment. Dr. Cumberland, as many veterinarians are prone to do, felt sorry for the little guy and decided to try to save him.
She devised a treatment plan and nursed Marvin back to health, and he is now a robust and happy adult dog. He is not, however, quite normal. He has never grasped the concept that, while getting to spend the day at his owner's workplace is a perk of her job, he must stay within certain boundaries. Marvin stays on a leash, tied to a doorknob.
He also spins. Now and then, especially when excited, he will spin around like a polka dancer six or eight times in a row, always in the same direction. He has to have a harness instead of a collar and his leash has a swivel on it so it won't get twisted up. When he and Dr. Cumberland approach the door of the clinic coming to work, Marvin invariably pauses to do some spinning.
In a normal dog, or human, visual information from the right half of the field of view of each eye is sent to a specific area in the left half of the brain, and information from the left half of the field of view of each eye is sent to the right half of the brain. I think Marvin has brain damage in the visual area of one side of his brain, so he is blind in one half of his field of view. It's as though he wore glasses with the right half of each lens blacked out.
People that have brain damage from strokes or injury sometimes have this problem. It's different than blindness in one eye. They can see out of both eyes, but only the right or left half of the field of view. When walking, they tend to head for the center of what they see, which causes them to veer to right or left. It's a little like a moth circling a street lamp.
Marvin's injury was horrific and his case is more complicated than damage to a single area of his brain. But, thanks to a compassionate veterinarian, he was able to recover from his injuries and lives a happy life. (Incidentally, Dr. Cumberland also has horse with only one lung. She nursed him back to health but he will never be more than a "pasture pet.")