Study author Jennifer Wathan, an associate professor of psychology at the University of Sussex, took life-size photos of horses whose attention was attention focused on something. She then placed the photos between two feed buckets in front of real horses. The horses were able to tell which bucket the ersatz horse was looking at, and chose to direct their attention to it, as well.
But when Wathan covered up either the ears or eyes of the horse in the photograph with illustrated hoods and blindfolds, the horses in the experiment lost their ability to detect interest in the photographed horse. In fact, their ability to pick the right bucket dropped to only 50-50, suggesting that they weren't picking up any cues from the photographed horse at all.
Apparently, it's not enough that a horse's long face be pointed in the right direction: The animals seem to rely on a much more complex read of facial features when they intuit emotion in their peers. It's easy for animal trainers -- and even researchers -- to get hung up on studying the same cues that humans use, assuming that animals use versions of the same thing. But for horses, it seems, social interaction may rely on muscles that humans have forgotten how to use.