Fig. 4. Experiments I and II. The results bars are located corresponding to the chromatic space shown at the top, and the black vertical line corresponds to the neutral point in horses. Different shades of grey in the bars signify different horses (binominal tests; ^*P<0.05, ^**P<0.01). (A–F) In Experiment I, two horses were trained to positive grey and yellow stimuli (red `+'). Blue was the negative stimulus (black `–'). (A,B) Both horses reached high choice levels in both training combinations. In tests (C) and (D), they treated the novel green stimulus (red circle) as a positive colour and showed a significant preference for green when it corresponded to the longer wavelength of the stimuli presented. In test (E), the horses again showed preference for the stimulus corresponding to the longer wavelength. In a final test (F), one horse chose the novel green test colour as if it was a positive stimulus. (G–L) In Experiment II, blue and green were positive training stimuli (red `+') and yellow was the negative training stimulus (black `–'). (G,H) All three horses reached high choice frequencies for both training combinations. In tests (I,J) with one positive training stimulus and the novel intermediate stimulus, grey, corresponding to the neutral point, grey was treated as a positive stimulus in the experiment. In test (I), a significant preference for the colour corresponding to the shortest wavelength was seen. (K) When both positive colours were presented to the horses a strong preference for blue was again seen. (L) Two horses tested with the negative yellow stimulus and the novel colour grey significantly preferred grey.