What Does My Blind Spot Look Like?
Friday, May 20, 2011 at 6:00PM
Bob Gerber

Welcome back.  Let's do a simple visual test today, as promised.  The test may reveal some unexpected aspects of our own eyes.  For me, the test also raises new questions about how much, or how little, our eyes contribute to what we think of as vision.  

For the test, I would like you to go to the VPandA Images section of this site.  Before you do that, you may want to open a second browser window.  That will allow you to see the test pattern on that page AND still be able to read the instructions in this window.

On the VPandA Images page, please click on the image entitled Blind Spot Test Pattern.  It should enlarge on your screen.  If you are using a separate monitor or maybe something like an iMac, try taking the test directly from the screen.  If you have a laptop, I suggest that you right-click the image, save it, and print a copy for the test.

Either way, let's get started. 

1.  Cover your right eye. 

2.  Now get as close to the test pattern as possible, really close please, and stare at the topmost "+" sign.  Don't worry about focusing, that's not the point of the test; blurry works just fine. 

3.  SLOWLY, pull your head back from the test pattern.  You'll see the black dot and the horizontal line pattern but don't look directly at them; keep staring directly at the "+" sign. 

As you continue to pull back slowly, you will notice a point when the black dot disappears.  Stay focused on the "+" sign.  As long as you keep your head in about that same position, you will not see the black dot.  Part One of the test is over.  You seem to have a blind spot in the field of vision of your left eye.   Did you know that you had a blind spot?  It is perfectly normal.  

This blind spot is not an illusion; it is very real.  You may want to close the test pattern image now and open the image named "Drawing of Left Human Eye" and follow along.  

In the back of each eye there is a lining (the retina) that senses light, then it begins interpreting that information, and then it sends the information to the brain for further processing.  The retina is your only link to the visual world.  Without it, you are blind so it is hard to overstate its importance in seeing.  

The retina is the part of your eye that contains the "rods and cones," the individual sensors for detecting light.  Within the retina of each eye, there are about five million cones and 120 million rods.  Including both eyes, that is  a quarter billion sensors, each sending a critical stream of visual information to the brain.

In order for it to get out of the eye, the "bundle of wires" carrying the information must penetrate the eye lining.   The "bundle" is the optic nerve and it leaves each eye through an opening in the retina called the optic disc.  The optic disc is a small portion of the retina that has no rods or cones because that area is like a "grommet" through which the optic nerve travels.  Without rods or cones, any part of the image that falls on the optic disc, is completely lost.  The optic disc is a blind spot in each eye.  

Finally, the blind spot.  The image of the black dot is located in the upper left part of your screen.  The lens of your eye inverts the image projected onto the retina.  So the image of the black dot hits your retina (in your left eye) within the lower inside quadrant, the same quadrant that has the optic disc.  As you moved your head back in the test, the image of the dot moved along the back of your left eye wall.  When the image finally fell upon the optic disc, the black dot disappeared.

Did you notice the title of this post, what would a blind spot look like?  That seems odd because how could we ever see a blind spot?  So I don't know what a blind spot looks like.  

I do know that there is no explanation for how the eye, alone, could fill in the blind spot that we just saw with the surrounding white color.   The brain fills in the missing black dot using a mechanism that is still not completely understood.  

Here is another example with the same test pattern.  Again, move your head close to the pattern but focus your left eye on the LOWER "+" sign.  Pull your head back as before.  When the image of the break in the black bar lands on your optical disc, the bar fills in; there is no gap, just a solid black bar.

Again, we “see” things that the eye could not have seen.  We know there never was a solid bar; there was a clear gap.  Yet we “saw” the gap filled in. 

It is easy for me to forget my brain when I am using my eyes to see something.  All of the motor control is focused on my eyes while my attention is on what I am watching.  These simple experiments demonstrate the essential role of the brain in all that we see.   

Article originally appeared on Graphic Design and Photography (http://gerbergrafix.com/).
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