Brightness discrimination in the dog

Dogs’ ability to discriminate brightness is about half as good as that of humans, according to a study appearing in Volume 4, Issue 3 in the Journal of Vision. In research conducted by scientists from the Veterinary University of Vienna and the University of Memphis, dogs showed a surprising lack of ability to discriminate between grey cards that varied in brightness, says researcher Ulrike Griebel of the University of Memphis.

While a great deal is known about dogs’ visual acuity and the cellular components of their eyes, there is a paucity of information about their ability to discriminate brightness, says Griebel. Furthermore, she notes that there is relatively little information on how well other animals discriminate brightness.

The researchers tested three police dogs–two Belgian shepherds and a German shepherd. The dogs faced a series of pairs of grey squares, which differed in brightness. The task required the dog to determine how much the one square differed in brightness from the other. The correct choice was rewarded with a food treat. The dogs needed a far greater difference in brightness (known as the Weber fraction) than do humans to discriminate between two squares.

For the Belgian shepherds the Weber fraction was 0.27; for the German shepherd it was 0.22. Although the researchers did not test humans in their study, previous studies found that humans need a Weber fraction of 0.14 to be able to discern a brightness difference.

While there has been little research into brightness discrimination in animals, says Griebel, what has been found is surprising. Dogs are arrhythmic animals, meaning they are active during both day and night. Consequently, it would be expected that they would have a high level of brightness discrimination. Brightness would be an important cue for such animals. Like dogs, says Griebel, sea lions, a species of manatee, and the horse–all arrhythmic animals–are quite inferior to humans in the ability to discriminate brightness. Humans are diurnal animals–active primarily during daylight.

Griebel hypothesizes that the relatively poor brightness discrimination ability of these arrhythmic animals represents a compromise. Because their visual systems have to operate under a wide range of light conditions, something has to be given up. She says this idea is an initial supposition, and that more study is needed to arrive at a firmer conclusion. This research helps to show that dogs’ perceptual world is very different from ours, and that we cannot expect the same thing from them that we expect from ourselves.