Seeing Life (Part III)

How do my eyes stay in focus so I can see clearly when I’m moving around?

Your eyes are remarkable.  They have six muscles attached that move in tandem to allow you to look at what you want in almost any direction.  They also use the cornea and the lens to focus light rays on the macula in the retina so you can see things clearly.   If you look at something far away and then at something close and vice versa the accommodation reflex automatically changes the curvature of the lens to let you keep what you’re looking at in focus.  But to keep things in focus when you’re moving around the body has to use information from a source that is outside of your eyes.  After all, you can use the focus on a camera to get a clear image of what you want to take a picture of but if you accidentally move your hand when you click the shutter it’s going to end up being a big blur.  Well, the same thing applies to your vision.  When you stand up or sit down, walk, run, jump up and down, fall, roll over and turn around, this changes the position of your eyes with respect to what you’re trying to focus on.  Just like knowing that you have to keep your hand steady when snapping a picture so it will come out right, so too, without something to tell your body in what direction and how fast it’s moving so it can compensate, your vision would always be blurry.  That something is your semicircular canals which are located in your inner ears.

Like the three sides in each corner of a box, the three fluid-filled semicircular canals are oriented at right-angles to each other.  The lateral canal is positioned like the bottom of the box, but is oriented about thirty degrees above the horizontal plane.  The other two sides, the superior and posterior canals, are oriented vertically and at right-angles to each other like the front or back and either side of the box.  In this way they are set up to provide the brain with three-dimensional information about the angular acceleration caused by head motion. 

The lateral canal is most sensitive to spinning or rotating the head from right to left or left to right, whereas the vertically oriented canals are most sensitive to nodding the head up or down or flexing the head to the right or left shoulder and back again.  Head motion in any direction naturally moves the fluid within the semicircular canals and stimulates hair cells which are sensory receptors.  The messages sent from the semicircular canals are sent through the vestibular branch of the same nerve that carries information on hearing to the brain (vestibulo-cochlear nerve) where they are processed and analyzed so the body can maintain its balance. 

However, as noted above, another important function the information from the semicircular canals on angular head motion provides is to help stabilize the retinal image.  Think about it.  When you’re in motion, unless you focus on something, everything moves across your visual field at the same speed as you do.  If you were unable to have controlled eye movements when your head moves in any direction, everything you look at would always be blurry.  Imagine our earliest ancestors running up and down over hill and dale trying to find food or avoid becoming food, without being able to focus on anything?  So how does the body do it?

It's called the vestibulo-ocular reflex.  Look into a mirror and focus on your eyes as you rotate your head from side to side, up and down and then in any direction.  Notice how your eyes automatically move in the opposite direction of your head so you can keep them in focus.  This is also known as the doll's eye reflex and is often used by physicians (along with the corneal and pupillary light reflex) to assess brainstem function.  It is the sensory information supplied by the semicircular canals about angular head motion that allows the brain to reflexively move the eyes in the opposite direction to maintain the retinal image so we can focus on things no matter how fast or in what direction we move. 

So that’s how your eyes stay in focus so you can see clearly when you’re moving around!

Three Questions for Mr. Darwin

  1. How did my body anticipate the need for and where did it obtain the information to produce and place the eye muscles exactly where needed and how did it learn how to move them in tandem so I can look and have clear vision in almost any direction?

  2. How did my body anticipate the need for and where did it obtain the information to produce and place the semicircular canals exactly where needed so they could provide it with information on angular head motion and so know where it sits in space?

  3. How did my body anticipate the need for and learn how to use the information provided by the semicircular canals on angular head motion to come up with the vestibulo-ocular reflex to keep my eyes focused when I move around?

 


Also see Dr. Glicksman's Series on

"Beyond Irreducible Complexity"

"Exercise Your Wonder"


Howard Glicksman M. D. graduated from the University of Toronto in 1978. He practiced primary care medicine for almost 25 yrs in Oakville, Ontario and Spring Hill, Florida. He now practices palliative medicine for a Hospice organization in his community. He has a special interest in how the ethos of our culture has been influenced by modern science’s understanding and promotion of what it means to be a human being.

Comments and questions are welcome.

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