An investigation of colour discrimination with horses (Equus caballus).

This research studied the colour discrimination abilities of horses, confirming that they can clearly differentiate between blue, yellow, and grey, but may have difficulty in distinguishing red and green.

Research Objective

• The researchers aimed to unravel how well horses (Equus caballus) can discriminate between different colors of stimuli (blue, green, red, and yellow) against a neutral shade of grey.

Methodology

• Four horses were subjected to a two-response forced-choice procedure, which is essentially a form of behavioural training.
• The horses were trained to push the lever in front of a coloured screen (an action that was randomly reinforced) and not to push the lever in front of the grey screen (an action never reinforced).
• Each colour was randomly paired with a grey that was either brighter, dimmer, or approximately the same brightness.
• A new colour was introduced to the horse only after it displayed 85% correctness of responses over five consecutive sessions or for cases where the accuracy showed no trend across multiple sessions.

Findings

• Every horse could distinguish blue from grey with at least 85% accuracy.
• Three of the horses achieved the same level of accuracy for yellow and green compared to grey.
• All horses performed above random chance for red and grey distinction, but none of them reached the 85% threshold.

Implications

• The slight overlap of stimulus wavelengths between green and yellow suggested a limitation in the horse’s ability to separate these two colours.
• This study corroborated previous research that implies horses are dichromatic (can perceive only two of the primary color).
• The results deviated from earlier data by suggesting that horses could be capable of distinguishing yellow and blue, but potentially struggle with discrimination between red and green.