Brightness discrimination and neutral point testing in the horse

Equine brightness discrimination ability and color discrimination were measured using a two-choice discrimination task. Two Haflinger horses (Equus caballus L., 1758) were trained to discriminate 30 different shades of grey varying from low to high relative brightness. Their ability to distinguish shades of grey was poor, with calculated Weber fractions of 0.42 and 0.45. In addition, a "neutral point" test to determine the dimensionality of color vision was carried out. Three hues of blue-green were tested versus a range of grey targets with brightnesses similar to those of the blue-green targets. A neutral point was found at about 480 nm. Thus, we can conclude that horses possess dichromatic color vision.