The Trick Your Eyes Do for Your Brain

The machine behind The Brain Tax.

Your eyes and brain do a huge amount of work without you ever noticing. Here is the plain version of how. If you read The Brain Tax, this is the machine underneath it.

Quick note. I am giving the simple version of how the eye works for this essay.

There are about 126 million light-detecting cells in each eye. About 1.2 million nerve fibers carry signals from each eye to the brain.

Here is the cool part. Each of those light cells, the rods and the cones, has a tip that does the actual catching. And that tip is built to be thrown away. Catching light literally breaks the molecules that do the catching (rhodopsin in the rods). And it is catching all day. Every one of those millions of cells, breaking and remaking those molecules, more times than anyone could ever count. Think the atoms in a room. That is how much work your eyes do without you ever feeling it.

So the tip is disposable. Every rod throws away its whole tip and grows a new one about every ten days, your whole life. One of the fastest rebuild jobs in the body. Think brake pads. You do not replace the car. You replace the part that wears.

Okay. Now here is what matters for us. The making of new tips is not the part that breaks. The cell keeps trying to rebuild. What breaks is the upkeep. Every rebuild ships its parts down one narrow neck (the connecting cilium) to reach the tip, and that neck needs a crew of proteins to keep it clean and moving.

In many forms of USH and RP, that crew is the broken part. And it goes back to our genes. A gene is the recipe for a protein. The gene for that crew has an error in ours. So the crew comes out broken, or never gets built at all. There is even a name for this whole family of disease. A ciliopathy. A disease of that one narrow neck.

For a while the cell still keeps up. Then, as we age, the crew falls further behind. The rebuild cannot keep the connection alive. That is when the cell goes dark and dies off. Slowly, over years. That is the real reason we lose our sight. That is the root of it. How it spreads from the sides inward, and why light makes it worse, is another essay.

Okay. Back to the machine while it still works. Here is the wild part. Your eyes and your brain split the work. They split it differently between your center and your side. The center cells send their signal up almost as-is, one cell to one nerve. The side cells get bundled and pre-processed first, many cells to one nerve. Two different ways to the brain. I did not know this either. The eye feeds many of those side cells into fewer output cells (ganglion cells) that work like coprocessors. Each one does a few basic tasks. Detect motion. Detect an outline. Detect a light change.

You have over a million of these coprocessors running at the same time. Like the heart, they just keep running. They are always sending little signals. The motion-detecting ones (M-cells) fire mostly when something changes. Your brain is mostly being told when something new happens. Then it decides what to do about it.

Side vision is about twice as fast at catching motion, especially in low light. That is why side catches a thing first, then your center swings over to look.

All this pre-processing happens across 95% of your view. The other 5% is your center, where the brain does the work instead.

And the side barely does color. The color cells sit in the center. So the side hands you shape and motion, and the center fills in the detail and the color.

Your side vision sends those basic signals up. Your brain matches them to patterns you have seen before. Once the brain knows what it is seeing, it stops looking closely and just watches to see if it moves. The rest of what you “see” is your brain remembering objects you have seen.

Here is something you can try right now. Close your left eye. With your right eye, stare at one spot in front of you and hold it there. Now hold your right thumb up at arm’s length and slide it slowly out to the right, toward your ear, keeping your eye locked on that spot. Somewhere out to the side your thumbnail vanishes. Gone. And you do not see a hole. Your brain paints the background right over it. Move a little more and it comes back.

That gap is your blind spot. Every eye has one, right where the nerve leaves the back of the eye. No sensors there, so there is a real hole in everything you see. You never notice, because your brain fills it in every second by guessing from what is around it.

And here is the part most people get wrong about going blind. That missing piece did not go black. You did not see a dark dot where your thumb was. You saw the wall, filled right in. You would swear you saw it all, nothing missing. Losing your sight is not a black screen dropping down. It is your brain quietly guessing to cover a hole you cannot see.

An octopus does not have this. Its eye is wired the other way, sensors facing the light and wiring behind, so it has no hole. Ours is wired backwards, and the brain hides that from you your whole life.

We like to think we are the top species. The octopus quietly got this one right. No hole. Cleaner wiring. Humbling.

But here is the twist. Some scientists say our backwards way is not a flaw at all. With the wiring in front, the cleanup crew sits right behind the tips. Right where it needs to be to feed them and swap them every ten days. So maybe backwards was the smart bet after all. Nobody fully agrees yet. I just had to share it. And hey, if we come back, maybe I get to be an octopus next time. Then I can finally tell you if their eyes really are better.

Here is the part for us. That little hole does the same job my whole eye is doing now. Filling in what it cannot see by guessing from the edges. RP just grows the hole.

You needed a finger to even find your blind spot. Mine grew until it swallowed the trick. That spot where your thumb vanished sits out in the side vision a lot of us have already lost. So if you have RP and the demo did nothing, you did not do it wrong. That is just the hole, bigger now.

I am sharing all this for a reason. All those coprocessors sit out on the side, and they run on the side light cells. The ones we lose first. As those cells die off, the pre-processing dies with them. The side goes quiet. What is left is the center, now doing the rough side-work it was never built for, on top of its own.

That is the hardware. The side did the fast, free work, and we are losing it. In The Brain Tax I walk through what that costs us when it goes.

Mark G. Hubers

Just an Engineer with USH

Part of the cabinet series, on how Usher syndrome and RP actually work.

Read the whole series here: ushengineer.substack.com/s/essays