| Casual observation of horses indicates that they are highly visual animals. Despite numerous anecdotal reports of their colour vision capabilities, few systematic studies exist. While evidence indicates the presence of colour vision, a more complete description of equine colour vision is required. To investigate horse colour vision three fundamental questions were asked: (1) Is colour vision present? (2) What are the spectral sensitivities of the underlying cone photoreceptors? (3) What is the acuity of horse colour vision? These questions were addressed in the three separate experiments.; In the first experiment, the presence of chromatic vision was determined by testing horses' ability to discriminate between achromatic and monochromatic stimuli. The animals successfully discriminated red and blue stimuli, but showed weak discrimination ability in the green to yellow regions of the spectrum, indicating that horses possess colour vision.; Increment-threshold spectral sensitivity functions were assessed in an attempt to investigate the spectral sensitivities of the underlying cone photoreceptors. Horses were required to discriminate a circular patch centered on a dim background from a blank field. The results indicated that horses possess two cone classes with predicted spectral peaks at 429 nm and 545 nm, respectively.; The final experiment examined the acuity of colour vision by deriving the wavelength discrimination function. Animals were taught to discriminate a circular test field illuminated with a particular wavelength from a test field illuminated with a different wavelength. For most wavelengths Δλ exceeded 25 nm; however a single region of best discrimination was observed at 480 nm. These results suggest a wide range of chromatic confusability, except in the blue region. Also, these findings confirm the presence of two photopigments with spectral peaks at short and middle wavelengths respectively.; In summary, the present results showed that the horse possesses an S-cone photoreceptor and an M/L cone photoreceptor with peak sensitivities located at 429 nm and 545 nm, respectively. Thus, horses possess the necessary photopigment basis for dichromatic colour vision. These findings are consistent with those in studies of other ungulates. |
