What’s Behind Door Number One?

What’s Behind Door Number One?

Aug 27, 2012

Another curious patient came up with a great question the other day.

She asks, “Why can’t we see the back of our eyelids when we close our eyes?”

This again, is a terrific question.  You’d think, with our eyelids being literally right in front of us, that we should be able to see behind there.  But no, we can’t.

But this does bring up the topics of night vision and how our rods and cones work in the dark.

We’ve talked before about how our retina is made up of photoreceptor cells, both rods and cones.  Those photoreceptor cells need light to do their thing.  They can’t send a message to our brain of what’s being presented in front of them unless light is included.  As soon as you close your eyes, the admission of light stops.

“Then how do we see at night?” you may ask.

We humans don’t actually see very well at night.  For this very reason; there is not enough light.  Our best , sharpest vision utilizes the cones in our retinas.  Cones work best in daylight. The rods do their thing mostly in lesser light, called scotopic vision. But they are not as precise as our cones in interpreting things sharply.  They do okay though.  For example, what happens when you walk into a darkened movie theater?  You are immediately thrown into a sort of blindness and you need to stand there like a goof while you wait for your eyes to adjust a bit so you can start hunting for a seat, right?  What your eyes are actually doing is dark adapting. When you are in the lobby throwing down a twenty for popcorn, your retinal cones, being in a fairly well lit room, are helping you see just fine.  But once in the theater, your cones are not happy and can’t do well.  They are momentarily unable to assist you and the rods must wake up and help.  This takes anywhere from a few seconds to a minute or so.  The older we get, by the way, the longer this process takes. You can speed up the process by looking down at the floor and not at the screen while you adapt.  This way, your cones, not having any light to feed on, have to take a break and let your rods kick in.  The longer you stare at the previews, the longer it will take to dark adapt and you’ll be standing there for awhile! Once your “night vision” has kicked in you are able to find a seat, but you may notice your vision is not totally clear.  You can’t, for instance, tell blues from reds, or if there’s a cola spilled on the seat you chose. Thankfully once the movie begins there is plenty of light and both your rods and cones are hard at work telling your head what you are seeing.   The rods in your retina are also responsible for your peripheral vision. Way out in the periphery of your retina its pretty much rods only, and they help you to see things coming up alongside you.  For example, when you are driving, you need to be able to see a child running into the street from the side.  Also, when you are changing lanes, your peripheral vision is what’s telling you if there is a car in that lane.  A fighter pilot also needs to know as soon as possible if the enemy is approaching from behind him!

Keep those questions coming!

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