Ever had your picture taken by a camera with a flash? If so, you probably noticed a perfect little negative of the flash in your vision for a while after the photo. It’s less noticeable now than it used to be. Back in the good old days, a flash wasn’t a light and temperature-specific moment of brightness from your phone’s flashlight. No, in those days a flash was a stack of cubes and each one was only usable once. Each burst of light was bright as the sun, and sounded like it was probably dangerous for it to be that close to the picture-taker’s head. Before that, it was just a pile of some mystery powder that exploded when a flame touched it while the photographer wisely hid his head under the black blanket attached to the back of his huge camera. But I digress. This blog isn’t about the history of camera flashes. That would be boring. It’s about retinal physiology.
The residual spots in your vision created by bright objects are a type of “after image.” The retina has a lot of work to do recognizing the different patterns of light entering your eye. Each photoreceptor fires when a photon of light hits it, and there are a lot of photons around. And luckily, that load is shared over a lot of photoreceptors. There’s well over 100 million of them lining the back of your eye. But that includes all of your retina, and a whole bunch of that is just peripheral vision that doesn’t get a lot of use. The part of the retina we use for most every visual task is called the macula. It’s right in the center of our vision, and it’s home to around 10 million of those photoreceptors. There’s a lot of work required to process the details of every image you look at, so most of the high intensity effort gets focused on those (relatively) few photoreceptors.
The reason that matters is because each photoreceptor is akin to a Civil War era long gun. It can fire, but it’s going to take a minute to reload before it can fire again. When a photoreceptor in the macula fires, it spends its light-ready molecules and has to refresh to get ready for the next photon that’s going to hit it. That’s normally not an issue when you’re looking at words on a page, a friend’s face, or your TV. The light intensity is low enough, and the image is shifting quickly enough that those millions of photoreceptors can keep up. Many hands make light work, as the saying goes.
Then, in the middle of peacefully reading a book, you look up and someone takes your picture with a camera from 1986. Pop! The blinding flash leaves a dead spot right in the middle of your vision leaving you unable to see the face of whoever took your picture. You’ll have to wait until they come back with the developed photo when it’s ready a week from now. Back to the question at hand, why did the spot in your vision persist for so long? It’s because the intense light, focused on a small patch of photoreceptors, made all of them fire at once. There’s extras waiting and none of them could be spared with tiny blindfolds. So they have to wait, re-polarize (which I’ve left out except for this one mention because it is insanely complicated and doesn’t have any Civil War analogy), and finally come back to functional.
Once everybody has had time to recover and reload, the vision returns to normal. Until then though, that spot where your brain is trying to see what’s happening is left spent and useless. Your brain can’t see nothing, which is a fact that also happens to sound like poor grammar. It fills in the blank spot with the now comparatively over-active, untouched neighboring photoreceptor noise. It causes you to see that spot, with it’s weird anti-glowing signature. No harm done, because the retina is resilient and reloads every time. The flash bulb wins the battle, but loses the war.