Depends on How a Horse Sees It
A study has demonstrated for the first time that a horse's emotional response to an object influences which eye they prefer using to look at it. Whether horses experience something as new, frightening or enjoyable directs a preference to seeing it with the left or the right eye.
The experiment conducted by French researchers at Université de Rennes documented how horses responded when presented with one of three different objects. Each item had a different emotional connotation. The Arabian mares studied were shown a shirt that was considered negative, and a cone that was new to them. A bucket, being associated with food, produced a positive reaction in the horses.
Researchers watched how the mares visually investigated each object, and recorded the number of glances and amount of time the horses spent looking at it with each eye. For horses, each eye sees almost completely separate scenes. Unlike humans with our binocular vision, horse eyesight has just a tiny overlapping field of view.
The data revealed that an item's emotional charge relates to which side horses favor for examining it. When encountering the shirt, horses had a slight preference for viewing the somewhat scary object with their left eye. The novel cone got investigated more with the right eye than the left. The pleasing bucket produced no side bias in how the horses viewed it.
The preference for looking at something with one particular eye is influenced by how the horse feels about what it's seeing and which side of the brain primarily processes that emotional response. The brain's two hemispheres split the work of processing the range of emotions, with each side responsible for certain ones. As well, information from each eye is processed by the opposite brain hemisphere. Thus what's detected by the right eye goes to the left hemisphere.
This study's findings show that among horses the brain's left hemisphere plays an important role in dealing with new situations. This same left brain function has been noticed in many other animals. Additionally for horses, the right hemisphere seems significant for assessing negative reactions, while both sides of the brain participate in processing positive experiences.
Alice De Boyer Des Roches, Marie-Annick Richard-Yris, Séverine Henry, Mohammed Ezzaouďa and Martine Hausberger. 2008. Laterality and emotions: visual laterality in the domestic horse (Equus caballus) differs with objects' emotional value. Physiology & Behavior. 94(3): 487-90.