We all have our blind spots. We’re born with them. It’s our blind spots that let us see. Our blind spots are somewhere in the center of the retina. They are where the optic nerve goes through the back wall of the eye, carrying light-triggered electrical impulses to the brain, where we do our actual “seeing.” There are no rods or cones at the point where the optic nerve goes through the eye, so there is nothing there to see with.

Sometimes, however, we have other temporary blind spots that are created by a burst of light. They block our vision for a short time. You’ll usually get such a spot, called an afterimage, after you’ve looked at a bright light, such as a photographer’s strobe light.

After the Flash: A Lingering Image

“I couldn’t see a thing after the flash.”

When a sudden bright light hits the eyes, the photoreceptors in the retina that registered that light go into temporary overload. For a while they won’t register anything at all. Then, when they do get back to work, they are very likely to produce a reverse afterimage of the light that overloaded them. It’s like a photographic negative.

The most common afterimage is the one you get when you stare into a photographer’s strobe light. The bright spot of strobe light turns into what appears to be an equally large spot of darkness—sometimes blue, sometimes green—that appears to get between your eyes and whatever you are trying to look at. The dark spot is produced by the overloaded rods and cones on the retina, which are temporarily out of service.

The same thing can happen when someone turns on a bright light in a dark room or lights a match in the dark.

If you are in the dark and know that a light is about to be turned on, you can prepare yourself for the change in lighting by closing one eye until after the light goes on. That will reduce the time spent waiting for the spot to go away. You can also partly shield your eyes with your hand so that they can slowly grow accustomed to the light, instead of being hit with the full force of the light all at once.

The brightness of the light is only one factor in determining how long the afterimage will last. The other one is how “open” your eyes were. If your eyes were adjusted to very dim lighting—meaning the pupils were wide open to capture as much light as possible—the afterimage will last longer because more light hit the retina. If, however, you’re in a brightly lit setting already, your pupils will be contracted to keep out the excess light and any afterimage will not last as long.

Other types of spots can be created with pressure, light, or by learning how to “look” at the inside of your eyes.

How to Find Your Blind Spot

The normal blind spot is so small that we rarely even notice it. But it is there, and it can be mapped with a machine called a perimeter. If you don’t happen to have a perimeter handy and you still want to find your blind spot, you can use a straight pin instead.

You do not stick the pin in anything. You look at it.

Take the pin—one with a white head works best—and hold it directly in front of you. While looking straight ahead, move the pin slowly from side to side. If you concentrate on keeping your eyes straight ahead, you will find that the head of the pin disappears briefly in a small area just to the outside of your straight-ahead central vision. Do the same thing while moving the pin up and down. If you concentrate, you may be able to map out your blind spot’s horizontal and vertical dimensions.

The reason you’re not usually aware of the blind spot is that the eye “fills in” the image with what surrounds it. It’s kind of like ink “leaking” out of a picture in a magazine and coloring the blank space around it.

So much for our normal blind spots, the ones we were born with. As we trudge the road of our destiny, we pick up others along the way.

Those Mysterious Floaters

Sometimes we notice spots that seem to float across our field of vision, especially if we are looking at a bright background, such as a clear blue sky. These “floaters” are usually caused by bits of debris floating around in the vitreous, the jellylike substance that fills most of the eye. The ancient Romans used to call floaters muscae volitantes, which is Latin for “flying flies.”

These “flying flies” flit between the cornea and retina, so the light entering the eye hits the spots and creates shadows on the retina itself—like a rotten tomato flying between a spotlight and the singer on stage. As we get older, the vitreous becomes more liquid and less jellylike, and the floaters become more prominent.

Floaters can also be produced when the vitreous detaches from the back of the eye. This detachment is sometimes accompanied by an occasional sensation of flashing or flickering lights and an increased number of floating spots. This on-again, off-again flickering or flashing can last for several weeks.

“Seeing Stars”—And Other Special Effects

If you close your eyes and rub them hard, you’ll probably see dots, spots, and flashes and dashes of colors. These images are called phosphenes. They are produced by pressure on your eyes. Your optic nerve translates that pressure into all sorts of bizarre patterns. That’s why being socked in the eye or hit on the head will make you “see stars.”

While phosphenes are really physically induced hallucinations, there are a number of other things you can see on the inside of your eyeballs that actually do exist—like the blood and blood vessels inside your eyes.

If you stare at a brightly lit sheet of white paper or at a clear, bright blue sky for a while, you might see luminous points or spots of light darting around in front of you, just out of reach. Sometimes these spots appear as very bright circles with darker centers. They often appear to have tails, like comets.

While no one is absolutely certain what it is you are seeing, the general consensus is that you are watching your own blood cells moving through the capillaries in your retina.

Sometimes, if the light is right, you can actually see the blood vessels running through your retina. This might happen in a doctor’s office while your eyes are being examined through a special lamp that shines a light on the back portion of the surface of the eye. The “tree branch” pattern you see corresponds to your retinal blood vessels.

In the same way that your brain “fills in” for your blind spot, it also fills in for the shadows that fall on your retina from the blood vessels inside your eye. But it only fills in for them when they fall in their normal place.

When the eyes are lit from a different angle and the shadows fall on a portion of the retina that doesn’t normally “see” them, your brain actually lets you see it, too.

A vitreous detachment can look like an insect, a tree branch, or a doughnut being wagged back and forth in front of your eye. The peculiar shape is actually the ringlike attachment of the vitreous around the optic nerve. As the vitreous body contracts with age, this attachment is often pulled loose and floats inside the eye indefinitely. Sometimes it floats out of the visual axis. Sometimes it breaks up and goes away. Usually the brain adapts to its presence and we are able to ignore it.

As a rule, a vitreous detachment is nothing to worry about. Only rarely does it create a hole or tear in the retina that may cause tiny blood vessels to break and bleed. But the flashing lights it produces could be tied to a migraine—with or without the headache.

If the flashing lights are accompanied by a large number of new spots, or a decrease in your vision, you may have a detached retina, and you should see your ophthalmologist as soon as possible.

Not All Migraines Ache

Flashing lights that appear as jagged lines or “heat waves” in both eyes and last for about 10 or 20 minutes sometimes accompany or precede migraines. They are usually caused by a spasm and dilation of blood vessels in the brain. If they are accompanied by a headache, you have a migraine headache.

But not all migraines are accompanied by headache pain. These painless migraines are referred to as ophthalmic migraines. They may be associated with peculiar visual phenomena such as light sensations and defects in the field of vision. Doctors can’t say for sure if painless migraines will lead to regular migraines or any permanent visual field loss.

If you have smaller floaters, you can even stir them up by moving your eyes around swiftly in all directions for a few seconds. This creates a “current” in the liquid inside the eye so that the floaters are moved around much like flotsam or jetsam in the ocean. After you’ve shaken them up, look at a plain, bright background for a while and watch as gravity “settles” the floaters. It’s a lot like one of those glass balls with a winter scene inside that is filled with liquid and plastic flakes that “snow” when you shake it.

Regardless of whether the floaters you see look like tree branches, insects, doughnut holes, or snow, they are usually just condensed pieces of vitreous or other particles that the eye cannot dispose of through the blood system. No matter how annoying they may be, they are quite harmless, which is nice, because there is nothing we can do about them.

While large floaters can persist for months—or even years—they usually do disappear eventually. If you have floaters, the odds are that over a period of time you will get so used to them that you will literally see right through them. You will unconsciously adjust to their presence in much the same way that you have adjusted to the natural blind spot that each eye has.

Floaters might also be a symptom of an inflammation, such as uveitis. In these cases, the floaters are usually clumps of white blood cells that are cast off by the choroid or ciliary body, the pigmented tissues connected to the iris.

Inflammations, like uveitis, or infections can increase the number of floaters dramatically. This may be an indication of a sight-threatening condition.

So while most floaters can be ignored, if they persist, get worse, or interfere with your vision, check with your doctor.