Local and global stereopsis in the horse.

This research article investigates the ability of horses to perceive depth (stereopsis) using both of their eyes (binocular) compared to using one eye (monocular), with the findings indicating that horses perform better when utilizing both eyes. The article then further discusses evidence of global stereopsis in horses as observed through their interaction with random-dot stereograms.

Investigating Local Stereopsis in Horses

• Despite horses having laterally-placed eyes, there is enough binocular overlap (two eyes seeing an object at the same time), suggesting that they may possess an ability for depth perception, known as stereopsis.
• The study conducted an experiment to measure local stereopsis by testing monocular (one eye) and binocular (both eyes) depth thresholds using renal depth stimuli.
• The results showed that horses’ binocular performance was better than their monocular performance, which means they were better at perceiving depth with both eyes than with one.
• When depth thresholds were measured, the binocular thresholds were significantly superior to monocular ones, providing clear evidence that horses could use stereoscopic information when available.
• The average binocular threshold was measured at about 15 minutes of arc (a unit of angular measurement).

Evidence of Global Stereopsis in Horses

• In the second part of the study, the researchers worked on proving the presence of global stereopsis (depth perception abilities across a wide field of view) in horses.
• They tested horses’ ability to distinguish between random-dot stereograms (images with pattern disparities that can be perceived in three dimensions with stereoscopic vision) that had consistent disparity information and those that did not.
• When presented with these stimuli, horses showed a preference for the images that had real depth, demonstrating their ability to perceive depth across a wide field of view.
• These results are considered as the first behavioral evidence of a comprehensive range of stereoscopic skills in a lateral-eyed mammal such as a horse.