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Roth, L. S. V., Balkenius, A., & Kelber, A. (2008). The Absolute Threshold of Colour Vision in the Horse. PLoS ONE, 3(11), e3711 EP -.
Abstract: <p>Arrhythmic mammals are active both during day and night if they are allowed. The arrhythmic horses are in possession of one of the largest terrestrial animal eyes and the purpose of this study is to reveal whether their eye is sensitive enough to see colours at night. During the day horses are known to have dichromatic colour vision. To disclose whether they can discriminate colours in dim light a behavioural dual choice experiment was performed. We started the training and testing at daylight intensities and the horses continued to choose correctly at a high frequency down to light intensities corresponding to moonlight. One Shetland pony mare, was able to discriminate colours at 0.08 cd/m2, while a half blood gelding, still discriminated colours at 0.02 cd/m2. For comparison, the colour vision limit for several human subjects tested in the very same experiment was also 0.02 cd/m2. Hence, the threshold of colour vision for the horse that performed best was similar to that of the humans. The behavioural results are in line with calculations of the sensitivity of cone vision where the horse eye and human eye again are similar. The advantage of the large eye of the horse lies not in colour vision at night, but probably instead in achromatic tasks where presumably signal summation enhances sensitivity.</p>
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Roth, L. S. V., Balkenius, A., & Kelber, A. (2007). Colour perception in a dichromat. Journal of Experimental Biology, 210(16), 2795–2800.
Abstract: Most mammals have dichromatic colour vision based on two different types of cones: a short-wavelength-sensitive cone and a long-wavelength-sensitive cone. Comparing the signal from two cone types gives rise to a one-dimensional chromatic space when brightness is excluded. The so-called `neutral point' refers to the wavelength that the animal cannot distinguish from achromatic light such as white or grey because it stimulates both cone types equally. The question is: how do dichromats perceive their chromatic space? Do they experience a continuous scale of colours or does the neutral point divide their chromatic space into two colour categories, i.e. into colours of either short or long wavelengths?We trained horses to different colour combinations in a two-choice behavioural experiment and tested their responses to the training and test colours. The horses chose colours according to their similarity/relationship to rewarded and unrewarded training colours. There was no evidence for a categorical boundary at the neutral point or elsewhere.This study suggests that dichromats perceive their chromatic space as a continuous scale of colours, treating the colour at the neutral point as any other colour they can distinguish.
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