From Comparative Physiology of Vision

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General Anatomy

As laterally positioned orbits animal (one on each side of the head),horses have remarkable peripheral vision, but reduced binocular vision and depth perception. The orbit which contains the eyeball, blood vessels, fat, muscles, nerves,and connective tissue is a large boney cavity. The eyelids are divided into the skin and contain many blood vessels. Horses have long eyelashes in their upper eyelid margins and none in their lower eyelid.[1]

Anterior Anatomy:

The horse’s anterior eye includes the eyeball along with the surrounding muscles and structures around the eyeball, known as the adnexa. The eyeball of horses has flattened shape. The eye is made up of three layers.[2]

The first layer that makes up the wall of the eye is the nervous tunic or retina.This wall is made up of cells which are an extension of the brain including cones and rods. Cones are less sensitive and provide color vision and rods are more sensitive and provide night vision.

The second layer of the wall of the eye contains vascular tunic or uvea including the choroid, the ciliary body and the iris. The choroid is made up of blood vessels that form the tapetum lucidum when it crosses over the fundus of the eye which causes the yellowish-green eye shine when the light hits the animal’s eyes at night. In the dark, tapetum lucidum reflects light back into the retina. The iris which lies between the corneas and lens gives the eye its color and allows the light to pass through the pupil.

The third layer is called fibrous tunic and protects the eye. It consists of the sclera and cornea. The components of sclera are elastin and collagen. The cornea is made up of connective tissue and has four layers. The first layer is outer epithelium, the second is the storma, the Descement’s membrane makes up the third layer and the fourth layer of cornea is a narrow layer secreted by the endothelium. [2] The structure of the lens of the eyes is gelatinous and lies behind the iris. The lens changes its shape and size by contraction and relaxation (accommodation) to focus on objects in different distance.[3]

Unique Visual Optics

Horses have the largest eyes among the land mammals and the strengths and weaknesses of their visual abilities are related to the animal’s behavior. Horses have a third eyelash which is located in the middle and contains the third eyelid gland that produces tears. It also has a cartilage that supports and covers the conjunctiva (the delicate membrane that lines the eyelids).[2] Due to tapetum lucidum, they are able to see very well in dim light. In general, horses have brown color, but blue or white iris colors are also common in some horses. Another peculiar aspect of their vision is that horses have monocular and binocular vision. The unique position of their eyes in their heads, make them see independently using one or both of their eyes. Horses can see about 360 degree around them and they have blind spots in front of their muzzles and six feet behind their tail. image: Horse1.png image: Horse2.png

Photo Transduction

Color Vision

Horses are dichromate which means they can see only two colors. Horses naturally see the blue and green colors. image: Horse4.png A study on the spectral sensitivity on horses shows that there are two distinct spectral mechanisms which consist of short-wavelength-sensitive (s) and a middle- to long-wavelength-sensitive (M/L) cones.

Scientists have been studying on the question how horses see the world over the decades, but the finding from recent study suggests that horses are capable of seeing colors but some of the colors are confusing to them. [4]

Motion Detection

Similar to people horses, maintain images on the retina. Compared to stationary objects, they easily detect the moving objects due to the greater peripheral visual field.

Neuronal Processing

The reflected light from an object enters the eye and hits the cornea. Then it passes through the aqueous humor and iris, which make the iris contract or dilate the pupil depending on the amount of light. This contracting or dilation of pupil allows the further entering of light. After traveling through the pupil it hits the lens, which focus on near or faraway objects through changing its shape. Then the light reaches to the retina where it transforms to electrical signal through a biochemical process.

In this process photoreceptors convert the captured lights into electrical signals. The photoreceptive pigments absorb the photon and interact with the rhodopsin which is 11-cis-retinal, a G-protein – coupled receptor, and opsin. The 11-cis-retinal isomerizes to all-trans-retinal. This change of structure activates the opsin, hydrolysis cGMP by activating phosphodiesterase. This results in closure of cation channels (CNG) and the membrane hyperpolarizes. The hyperpolarziation of the membrane results in release of neurotransmitter which signals to the brain through optic nerve. The brain interprets the signal into images.

Based on the animal’s unpredictable behavior in seeing things, it is hypothesized that horses are incapable of intraocular transformation. While the animal has a corpus callosum connection of the two sides of its brain. A study done in California showed the high level of transferring visual tasks in animal and proved that this hypothesis is incorrect.

Higher Order Visual Perception

Visual cues are more important to the horse than olfactory cues. This is especially true in the interactions of the horse and human. There are misconceptions between the horse’s behavior and what the human perceives, and this is based on the vision of the horse. For example, when a horse throws its head up in front of an obstacle, it might be because it cannot see it, but the human assumes it is misbehaving.

The horses’ perspective is high but they also need to have a vision for eating grass. In order to do that, they narrow their pupils in a horizontal‐shape. They don’t have the natural ability to stay focused on a single target such as predators.[5] image: Horse3.png

Evolutionary Significance

Horses followed a distinct evolutionary path and their eyes believed to have evolved about 540 million years ago. Their adaptation was based on their different way of need.

The position and adaptation of horse’s head has evolved because they were once subject to predation. In this circumstance the position of its eyes in both sides and a lateral vision is more advantageous. This evolutionary adaptation also has disadvantages of its own. For example, a horse is unable to focus on near objects and has difficulty with visual accommodation. [6]

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