19 Jan
How do horses see?

Horses have large complex eyes that can see the horizon and the ground at the same time.  Because of this, they don’t see clearly at a distance.  They see movements and shapes better than specific details. 

They are “near-sighted” and their depth perception is not very strong.  Rule of Thumb: Eyes on the side of the head = prey; eyes on the front of the head = predators. Having eyes set back from his nose means he can’t see directly in front of himself; he can’t  see  his  nose  and  mouth.  You  can  probably  see  your  nose  but  not  your  mouth.  

Horses’  eating  habits  dictate  that  their  heads  be  at  ground  level  most  of  the  day.    Therefore,  they  need  to  see  the  grass  they  are  eating  and  scan  the  horizon  for  predators at the same time.  Their range of vision is about 300 degrees, except for an area of about three feet directly in front of and six feet behind them.    

Horses have both monocular and binocular vision.  When a horse looks to either side, each  eye  moves  and  sees  independently.    This  results  in  lack  of  depth  perception.    When a horse looks straight ahead, both fields of vision overlap and his vision becomes binocular,  just  like  humans,  and  he  gains  some  depth  perception.  In  order  to  bring  objects into focus, the horse will move his head up or down.  

Horses  have  a  complex  eye  to  brain  structure.    Each  eye  feeds  into  one  side  of  the  brain with limited crossover, and the pictures the eye receives are not as sharp as are ours.  This is an important factor in the training process. This means that while you may cause no reaction when working on one side of a horse, you may cause a reaction on the  other.  

Horses,  with  their  monocular/binocular  vision,  may  also  see  objects  larger  than they really are.  You may appear to be ten feet tall to a horse!  

Horses  have  much  better  night  vision  than  we do.    This  is  possible  because  of  the  tapetum, a highly reflective area in the back of the eye.  This is what you see when you shine a light into a horse's eyes at night; it is reflected back as a green glow. 

Most animals, other than birds and primates, see just two colors (blue and green).  The colors these animals see best are yellowish green and bluish purple. Therefore, yellow is a high-contrast color for almost all animals.  This may be why animals react strongly to yellow turnouts and machinery. 

According  to  Temple  Grandin  in  "Animals  in  Translation",  horses  see  the  way  they  do  because of the difference in the shapes in our eyes. Human retinas have a "fovea" (foe-vee-a), a round spot in the back of the eye where they get their best vision.  Domestic animals --   and fast animals who live on the open plains have a "visual streak" instead of  a  fovea.    The  visual  streak  is  a  straight  line  across  the  back  of  the  retina.  Most  experts think the streak helps animals scan the horizon. 

Many animals see more intense contrasts of light and dark because their night vision is so much better than ours.  Good night vision involves excellent vision for contrasts and relatively poor color vision.  Animals’  sharper  contrast  seems  to  make  dark  spots  appear  to  be  deeper  than  lighter  spots; the reason cattle guards work. 

In  "An  Anthropologist  on  Mars",  Oliver  Sacks  told  about  an  artist  who  lost  his  color  vision.    It  became  very  difficult  for  him  to  drive  because  tree  shadows  on  the  road  looked  like  pits  his  car  could  fall  into.    Without  color  vision,  he  saw  contrasts  between  light and dark as contrasts in depth.

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