Dr. Lena Ström discussed the horse's vision and methods of diagnosing visual disorders using advanced electronic equipment at the 2017 WBFSH seminar on "The Horse's Perspective" in Gothenburg, Sweden on 22 August 2017.
The WBFSH seminar was held in co-operation with the Swedish Equestrian Federation (SvRF), European Equestrian Federation (EEF), and Swedish University of Agricultural Sciences (SLU) and facilitated by Göran Dalin and Elisabeth Lundholm.
The purpose of the seminar was to ensure that horse sport will remain relevant in our society and to build a platform for future cooperation between stakeholders of the equestrian world – i.e. in sport, breeding and science. It is important that public perception of the horse and its uses are not negative and that horse welfare is respected and safeguarded.
Dr. Lena Ström, DVM, Resident in Ophtalmology
The horse’s normal vision and methods of diagnosing visual disorders using advanced electronic equipment –
Anatomy of the horse’s eye is structurally very similar to that of the human. Some noteworthy features of the horse’s eye are:
• Has the largest eye of the terrestrial animals
• It is a “visual generalist” i.e. it has good day vision and very good night vision. The large pupil has a dynamic size range and a horizontal orientation. When it contracts during the day to protect it from sunlight, it becomes slit-like and still allows the horse to scan the horizon.
• As in all herbivores, it has a corpus nigra on the upper edges of the pupil (sometimes also on the lower edges) which may help to shade the eye in sunlight, whilst allowing the pupil to be large. • Adaptation in the lens – yellow pigments that filter out blue wavelengths, therefore reducing glare.
• Fair visual acuity for day vision
- o In humans best visual acuity is 20/20. Studies in horses have shown an acuity of 20/30 up to 20/60, which means that they don’t see details as well as humans can.
- o Tapetum lucidum – reflective layer in the bottom of the eye which is good for dim-light vision but results in a loss of detail during day vision
• Vast number of photoreceptors o Rod photopigment – increased ability to adapt to dark conditions o Two types of Cones for colour vision – blue & yellow/green, i.e. horses cannot differentiate the colour red.
• Visual field – characterised by a large monocular vision, only 65° binocular vision (allows for depth perception) and blind spots. But horses have developed an ability to perceive depth to an extent even with monocular vision. Studies have shown that the position of the head does not have as much influence on depth perception as one might think, due to the large number of ocular muscles that are responsible for movement of the eye and therefore the ability to maintain the slit-like pupil in a horizontal position.
Vision tests in horses are currently not an exact science, but available techniques are:
• Eye exam including biomicroscopy, ophthalmoscopy
• Traditional cotton-ball tests and obstacle courses
• Diagnostic imaging (ultrasound, CT, MRI)
• Electrophysiologic testing – such as Electroretinograms (ERGs) and Visual Evoked Potentials. These methods allow for objective evaluation of the function of the visual pathways. Therefore, differentiation as to whether there is a problem in the eye or in the visual pathways is possible. ERGs and VEPs can also be used to study development of visual function in the young horse.
Behavioural problems and poor performance can be signs of visual impairment, so should be considered when examining a horse. However, blind horses can function very well in all disciplines (e.g. dressage, racing, reining).
Photos © Astrid Appels
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