Background: Animals' ability for cross-modal recognition has recently received much interest. Captive or domestic animals seem able to perceive cues of human attention and appear to have a multisensory perception of humans.
Methodology/principal findings: Here, we used a task where horses have to remain immobile under a vocal order to test whether they are sensitive to the attentional state of the experimenter, but also whether they behave and respond differently to the familiar order when tested by a familiar or an unknown person. Horses' response varied according to the person's attentional state when the order was given by an unknown person: obedience levels were higher when the person giving the order was looking at the horse than when he was not attentive. More interesting is the finding that whatever the condition, horses monitored much more and for longer times the unknown person, as if they were surprised to hear the familiar order given by an unknown voice.
Conclusion/significance: These results suggest that recognition of humans may lie in a global, integrated, multisensory representation of specific individuals, that includes visual and vocal identity, but also expectations on the individual's behaviour in a familiar situation.