Horses are sensitive to pictorial depth cues.

The study investigates whether horses can perceive depth from flat images by using pictorial cues. It concludes that horses are indeed sensitive to these cues as they demonstrated susceptibility to the Ponzo illusion when presented with photographs including such depth indicators.

Initial Training and Discrimination Thresholds

• The research started by training two horses to discriminate between two lines of different lengths, displayed one above the other.
• The horses’ discrimination thresholds were measured using psychophysical methods. This involved determining their ability to reliably distinguish the length of two lines when viewed from around 160 cm away.
• The results showed that the horses could reliably tell a lower line that was 10 cm long from an upper one that was 14 cm long.

The Experiment with Pictorial Cues

• The second phase of the experiment involved superimposing two lines of equal length on two different photographs: one of a set of converging railway tracks with many pictorial cues indicating depth and the other of a pastoral scene with fewer obvious depth cues.
• For humans, the image with the railway tracks normally creates a Ponzo illusion, which makes the upper line appear longer than the lower one even though they’re identical in length.
• When the horses were then asked to choose between the two photographs, they overwhelmingly chose the one featuring the railway tracks. This suggests that the horses were also experiencing the Ponzo illusion, indicating a sensitivity to pictorial depth cues presented within the photograph.

Control Experiments

• Control experiments were conducted to rule out alternative explanations for the horses’ noticeable preference for the image with the railway tracks.
• These control experiments confirmed the initial conclusion: horses are susceptible to a Ponzo illusion created by depth cues in photographs.

Conclusion

• From this research, horses were found to be sensitive to pictorial depth cues. They displayed an ability to compare the relative lengths of two lines and seemed to perceive depth within flat images, as indicated by their reactions to the Ponzo illusion.