Horses are sensitive to pictorial depth cues.

In an attempt to demonstrate whether horses could make use of pictorial cues to depth, two were trained initially to make a relative-line-length discrimination between two lines placed one above the other. Psychophysical measurement of their discrimination thresholds showed that from a viewing distance of approximately 160 cm they could reliably distinguish a lower line of 10 cm from an upper one of 14 cm. In the second phase of the experiment, two lines of equal length were superimposed on a photograph of a set of railway tracks with many pictorial cues to depth, or a photograph of a pastoral scene with fewer obvious depth cues. To humans, the railway tracks created a Ponzo illusion, making the upper line-appear longer. When the horses were allowed to choose between the photographs, they overwhelmingly chose the display containing the converging railway tracks. Control experiments ruled out alternative explanations, leading to the conclusion that horses are susceptible to a Ponzo illusion created by depth cues in photographs.