VN May 2017

Regulars I Eye column E ye What do horses see? Part 3

COLUMN

Dr Antony Goodhead & Dr Izak Venter Specialist Veterinary Ophthalmologists, Johannesburg and Cape Animal Eye Hospitals (www.animaleyehospital.co.za)

Colour Vision T here are two types of photoreceptors in the retina, namely rods and cones. In the horse rods outnumber cones 20:1. Cones are responsible for vision in photopic conditions as well as colour vision. It is generally accepted that the horse possesses some form of colour vision. Most mammals have dichromatic vision meaning they possess two cone types, one having maximum absorption in the short wavelength end of the spectrum, the other in the middle to long wavelengths. In humans as well as some other primates colour vision is based on three cone types, with maximum absorption in the short, middle, and long wavelengths. This is referred to as trichromatic vision. The horse possesses two types of cone photopigment, one maximally sensitive to medium long wavelengths and the other to short wavelengths. Like most mammals horses are thus dichromatic. Values for the spectral peaks of these photopigments have been estimated at 545 nm and 429 nm. In a recent behavioural study of colour vision in the horse it was found that horses most easily discriminate the colours orange, yellow and blue from grey. It was also found that the horses failed to discriminate the colour red as easily as they discriminated the other colours from grey. Horses appear to demonstrate more problems when negotiating fences of contrasting colours where green is paired with yellow or blue, which are two of the colours that have been reported as most easily visible to the horse. 2 Trichromatic colour vision allows greater discrimination of a greater range of colours, there are however

certain visual advantages associated with dichromacy. Trichromatic vision may obscure differences between objects that are based on other features besides colour, such as texture. It is possible that certain objects may

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26 Mei/May 2017