There is a link between the emotional state of horses and the type of wrinkles they get above their eyes. Dr. Sara Hintz from Switzerland researched the topic and at a conference presented her results indicating that wrinkles above a horse's eye are common but differ between individuals in number and shape of wrinkles. Dr. Hintz believed that horses with positive emotions would exhibit only a few eye wrinkles and those with negative emotions would have increased wrinkles. To test her theory, she used 15 stallions and one mare and exposed them all to positive situations, such as anticipating feeding and petting. She also exposed them to two negative situations, including food competition and the fear horses have when a plastic bag is being waved around.; The horses were exposed to the different stimuli for 60 seconds and over 500 photos were taken of the horse's eyes. In these photos, researchers scored the eyelid shape, noted the amount of white, number of wrinkles, the angle between a horizontal line through the eye, and the highest wrinkle.
Although results indicated there was no difference in the number of wrinkles and eyelid shape, she did see more eye whites and a sharper wrinkle angle during positive situations compared to the negative situations She explained that the more relaxed the underlying muscle of the eyelids are, the narrower the angle would be. Therefore it seems possible that some characteristics of eye wrinkle expression could be used to indicate a horse's emotional state. Assessing animals' emotional states is difficult and involves lengthy and sometimes invasive procedures, including blood testing. Any clues to the mood of horses that can be gained by rapid visual inspection may be helpful to the horses and their handlers.