Scientists at the University of Sussex in Brighton studied 24 horses from Woodingdean livery yard, Brighton, and Sussex Horse Rescue, Uckfield in what’s termed a "cross-modal expectancy violation paradigm" by psychologists. Each horse watched a herd member being led past them before the individual went of view, and a call from that or a different associate was played from a loudspeaker positioned close to the point of disappearance. The researchers studies the horses’ reaction when the correct and incorrect sounds were played.
When horses were shown one associate and then the call of a different associate was played, they responded more quickly and looked significantly longer in the direction of the call than when the call matched the herd member just seen. According to the researchers, this was "an indication that the incongruent combination violated their expectations". In other words, the horse was surprised that the sound they heard was not the one they were expecting.
The research, titled "Cross-modal individual recognition in domestic horses", was published in the journal Proceedings of the National Academy of Sciences. The paper concludes that the work demonstrates "a clear and systematic demonstration of cross-modal individual recognition" horses.