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Mei/May 2017
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
Dr Antony Goodhead &
Dr Izak Venter
Specialist Veterinary
Ophthalmologists, Johannesburg
and Cape Animal Eye Hospitals
(www.animaleyehospital.co.za)Regulars
I
Eye column
COLUMN
E
ye
What do
horses see?
Part 3
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