Mirrors have been shown to decrease the weaving in horses. This study investigated whether it was the horse’s reflected image that caused the reduction or if it was some other property of the mirror. In order to answer this question, six weaving horses were shown three different posters and their responses were recorded. The posters included a life-size image of a horse’s face, the same image cut into 54 squares, which were randomly rearranged (pixilated image) and a white poster with no image. The results indicated that when horses were shown the poster of the horse’s face, weaving was significantly reduced compared to when they were shown the other two posters. Exposure to the poster of the horse’s face also resulted in a reduction of nodding compared to when they were shown the pixilated image. Horses were more alert and spent more time looking at the poster, when the poster of the horse’s face was presented. Greater activity and anticipation of specific events, which may or may not be associated with increased social contact, caused an increase in the intensity of stereotypic head movement. The authors concluded that the timing of stereotypic head movements might be associated with acute frustration as opposed to boredom/lack of stimulation.