Port Hueneme - Oxnard Optometrist   Dr. Don Steensma 465 W. Channel Islands Blvd, Port Hueneme, CA 805/486-3585

Sometimes, you just can’t believe your eyes!

The human visual system consists of two separate visual systems.  The majority of the retina (the light sensitive part inside the eye) consists of 90 million cells that are called rods.  Many rods are connected to a single nerve, making them very sensitive in light detection but they lack detail and color vision.   This is our night-time vision system.  We easily see the approaching headlights but as the car passes, you may not be able to tell its color.  In the dark, we see in shades of gray.

There is a 3/16" diameter spot on the central retina called the macula.  It is jam packed with 4 million cells called cones.  Each cone connects to only one nerve so the cone cells are very sensitive to detail.  They have color perception but do not function in poor light. To see something clearly during daytime, we point our eye so that the image is on the cone cells in the macula. To see something clearly in the dark, we point our eye slightly to the side so that the image is not on the cone cells in the macula.

During the daytime, we look around us and we see everything in great detail with a wealth of information.  We see the scenes detail, the color variations, the textures, and the movement of objects.  It all appears to be clear and constant.  But, the macula is very small.  When I look at a scene 20 ft away, I am really only seeing a 6” diameter circular portion of the scene in true clarity.  When we look at a scene, our eyes continually shift around to different fixation spots within the scene.  The image that we see in our mind appears constant and is in great detail.  The mind sews all those individual 6” areas of clear vision into an apparently clear overall scene.  It is impossible for us to actually see the blurred areas because as soon as we consider a spot, we fixate on it, receive the clear image the mind is looking for, and incorporate its details into our mental picture of overall clearness.

Where the optic nerve exits the back of the eye, there are no rods or cones, so this part of the eye is blind and it is referred to as our “blind spot”. We have one in each eye.  The optic nerve is 15 degrees away from the macula (with straight ahead vision), so there is a spot that is blind 15 degrees to the right in the right eye and 15 degrees to the left in the left eye.  But, our mind does not see it and simply fills it in with nothingness of the same color and pattern as the adjacent color and pattern.

As our fixation rapidly shifts from one spot to the next (a saccade movement) we perceive a smooth transition.  The scene does not move as our eye moves because there is no visual input accepted by the mind during that saccade movement.  We are totally blind for that split-instant, but our mind preserves the pre-existing mental image as it was prior to the saccade until the saccade ends and then allows new visual input.  If our eyes slowly move from one fixating point to another (a pursuit movement) the image is continuous.

In a now famous video (http://www.youtube.com/watch?v=2pK0BQ9CUHk), subjects watched a video of two teams, one in white shirts and one in black shirts, passing basketballs (one ball per team) back and forth. The subjects were instructed to count the number of successful passes made by the white team. Afterwards, subjects were asked whether they saw anything else unusual.  During the video, a person in a bear suit moonwalks through the middle of the scene, but 35% of the test subjects did not see the bear.  As long as our eyes are bouncing around watching the basketball (a saccade movement), we will not see the bear.  If we lose fixation on the ball for a momentary fixation on something else attracting our attention, like a moon walking bear, our mind will perceive it and we consciously “see” that object.  

The next time you look at something, stop and reflect on just how wonderful your vision really is.  Take care of your eyes and get an eye exam every year.

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