Babies have well known reactions (for example they react differently things that are “new” to them vs “old”) and by using these well known reactions we judge what they can and can not experience.

You know that machine the eye doctor makes you look into when you first start an eye exam? You look at a little circle with a picture in it (e.g. a picture of a hot air balloon), and the machine goes whrrrr whrrrr while the picture goes in and out of focus?

That's called an autorefractor, and it's able to (mostly) figure out how bad your eyes are all by itself. All the "which looks better, 1 or 2?" stuff afterwards is just fine-tuning the prescription from a starting point that the machine figured out.

So with babies, they can't do the fine-tuning part of the exam, but they can still look into an autorefractor.

This is actually very important for babies. The fine-tuning isn't super important, but if the baby has a very different prescription in one eye than the other, then the baby will start favoring their good eye at the expense of their bad eye. Their brain, as it grows, will literally stop paying attention to their bad eye. If it's not corrected in time, it can cause lifelong vision impairment in the bad eye.

An MRI can do an excellent job of mapping the shape of an eye in 3D, and computers are great at calculating how far away the focal point of the eye lens is from the optic nerve. Our eyeballs start out about 1.5 cm and grow slightly until they reach about 2.5 cm in our teenage years, with much of that growth coming in the first year of life.

How does the brain not paying attention to one eye work? Does the baby just go blind in that one eye?

That's what happened to my little brother. One of his eyes was "weaker" than the other and his brain mostly gave up with it. Around 10 or 11, the optometrist suggested he wear an eyepatch on his good eye to force his brain to re-engage his bad eye. It mostly worked. Could've helped more, but he wasn't very consistent with it because he obviously had a hard time seeing.

Depends on the semantics of "blind". The eye doesn't just completely turn off, but it will have severely decreased vision, and the person may only perceive stuff with it when the good eye is closed.

Brain science is still new and not fully understood, but there are groups of neurons in the brain called ocular dominance columns that light up when one eye or the other is stimulated. In most people there are equal numbers of ocular columns for each eye. But in brains that had a bad eye during infancy, most of the ocular columns will be linked to the good eye. In other words, the brain literally rewires itself to be essentially one-eyed.

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Ah, amblyopia. I was diagnosed with this at age 6. Patching somewhat helped, but my right eye is still dramatically stronger than my left.

My eye doctor told me this is what happened to my left eye. My vision in my right eye is very good, but the vision in my left eye is terrible. If I close my right eye, I can only make out letters if they're several inches tall. Is there any way of correcting this as an adult?

Sorry. The traditional textbook answer is that no, it's not possible after childhood.

However, there's a blurb on the wikipedia page that says that may be outdated knowledge. There is tentative evidence that it might be possible. If I were you, I'd look into the linked sources on that wiki page and talk to your doctor about your options.

Yes. Normally, your brain takes two separate but similar images from your two eyes and fuses them together to make your "vision". When the two images aren't similar however, like if the picture from one eye is much blurrier or you have a lazy eye that's pointing in a different direction, they're too different for your brain to fuse together. Instead of confusing you with two dissimilar overlapping pictures, your brain says "well, this is probably the right one and it'll be good enough" about the good eye and literally just stops displaying the bad eye to you.

After a while, it stops even checking to see if the bad eye has something useful to display since it never ends up using it, and the pathway from that eyeball to your brain atrophies. All brains are plastic, meaning they can create new pathways, but babies brains are way more plastic than adults, so it's critical to correct this issue as soon as possible. You can try to fix it when someone's older, but you're unlikely to be as successful.

If you don't fix it in time, the person will literally stop being able to see out of that eye because the path that sends the image from the eye to the brain just doesn't work anymore. Nothing happened to the eye itself, but without that connection the image won't get to your brain for you to be able to see it. You'll mostly see out of just one eye when you have both eyes open although if you close your good eye you may still be able to see something out of the bad eye depending on how atrophied that pathway between the eye and brain are. However, you'll probably never be able to completely successfully fuse two images together, which can result in lifelong vision problems.

Thanks for the link. I'll check it out.

We have machines that can measure how well an eye sees. You might have used one if you've been to an opthalmologist, you know that big one you rest your chin on and look into a hole and you can see an image inside that may shift as the doctor makes adjustments. Last one I used had a picture of a hot air balloon in a field. However what that machine does is send light into your eye and see how it's received and sent back at the machine. We know what a good eye is supposed to be like, how the lens is supposed to focus light on a specific spot inside the eye and how our iris is supposed to move to compensate for different light levels. If your eye ball has the wrong shape, or your eye lens has the wrong shape, or your iris is not functioning properly, the light is not hitting the right spot inside your eye and your vision is impaired.

Given that this is a completely non invasive procedure, and it doesn't really require feedback from the patient despite the fact that opthalmologists may ask questions anyways, it's possible to use it on infants as well, with probably the hardest part being holding their attention long enough to make proper measurements.

I'm 40 and havent worn any eyeglasses up until about a year ago. My (general) doctor decided to do one of those eye chart exams during an annual physical and found that I was having a semi difficult time.. and so I was referred to the actual eye doctor. They found that I needed very minimal adjustments-- so much so, that the actual lenses look like I'm wearing fake/fashion glasses lol. Anyway... It was amazing how much clarity I was missing out on!

During the first few months of wearing them, I didn't notice too much of a difference with them off. But now, a year later, without them, it's very blurry. Did my brain do that favoring thing you talked about? instead of favoring a good eye over the bad eye, it's choosing glasses over no glasses?

From what I've gathered (lifelong glasses user) your brain can adjust for your crappy vision. It refines and fixes based on what it thinks should be there and is usually pretty good at it. When you get glasses and it no longer has to do that work (because the eye is transmitting a clearer image) it gets lazy and stops working on that refine and fix for the blurred vision. If you kept your glasses off long enough it may stay the same or you may adjust to it and think the blurs are normal because your brain is "erasing" them in real time based on what it knows something should look like.

The refine/ fix thing is a perfectly normal thing for the brain to do and it does it constantly in ways we mostly don't pay attention to (peripheral vision is a huge one for this) because it gets it right 99% of the time. The times it doesn't tend to produce things like shadow people or things that make you double-take and focus on an object because the fix the brain did was off enough for you to recognize it but focusing gave it more data to do it right.

tl;dr your brain is fuckin' weird and makes shit up constantly and when it doesn't have to do that it's a lazy fuck that sits on the couch and demands sugar, fat, and caffeine.

Hello, guy with lifelong vision impairment due to one of my eyes not being favoured by my brain since childhood, here. It sucks that I'll probably never know how normal people see the world.

You just learned me that, during my childhood i saw some childrens with a patch on one eye, it all makes sense now ! It's very facinating to see how the brain work

This happened to me! I can close my left eye and nothing in my vision changes, but if I keep the left one open and close the right one, everything jumps to the side. It's funny to keep winking with my right side and watch everything jump back and forth. Eye doctors did try to correct it when I was younger with eye patches, but no luck. Though I think my right eye overcompensated and got worse as a result, as it now has a prescription literally double the strength of the left eye

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This is the correct answer. To expand on it: You don't need a fancy machine. Lenses, including our corneas, work both ways. Therefore you can just shine a narrow beam of light into the eye, moving it across the eye serveral times (Skiascopy). If the beam moves across the retina in the opposite direction of the movement of the light, you know the focal point lies in front of the retina. If it moves across in the same direction as the beam, the focal point lies behind. Now you can just hold up test lenses and see with which lens the reflection of the beam of light stops moving, meaning the retina is in focus. This is basically the old school method of doing it, but is much easier explained by drawings or a video.

An MRI would never be used to measure focusing properties of the eye in a clinical setting. It would be a very expensive, time consuming and “invasive” (probably not the right word) way to measure the length of the eye but I doubt it could measure the focusing power of the cornea or lens.

Autorefraction or retinoscopy would measure the focusing power of the whole eye. You can derive the power of the lens from biometry (measures the length of the eye and focusing power of the cornea).

>An MRI would never be used to measure focusing properties of the eye in a clinical setting.

Not clinically, no. But for research purposes a group could absolutely write a grant proposal to study development of the eye in early childhood.

Doubtful you could get grant approval when there are cheaper, faster and more accurate methods of measurement available. Especially when it has been studied quite extensively already.

https://www.surveyophthalmol.com/article/S0039-6257(21)00134-X/fulltext

Oh hey, I knew about the bad eye thing lol my right eye doesn't turn to the right. My peripheral is pretty strong but my body favors my left eye entirely

Your tl;dr is hilarious but true!

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I’m not 100% sure what you’re asking but I’ll try.

If you are asking in gereral; Most babies can focus quite well (like the lens in a camera) on an object but they see it in low resolution. Like they’re recording in 144p instead of HD. Their brains aren’t well developed but the resolution improves as they develop.

They can measure this resolution in a variety of ways. One way is an optokenetic drum, once the baby is a few months old. The eyes will follow a stripey pattern travelling in one direction. If the baby can’t see or resolve the pattern their eyes will not follow it. You can adjust the pattern (bigger or smaller stripes) until the baby can/can’t follow it. With toddlers you can show them two cards, one with a faint picture and one without a picture. Watch where their eyes go and go to the next set of cards.

If you asking how we know a specific baby can’t see, we measure the focusing ability of the babies eye with a special light and lenses. And how well the baby can see with and without glasses with the methods above. If the baby falls significantly outside normal values, we give the glasses and/or eyepatches. If they are slightly outside normal values we might re-examine them in a few months.

my eye doctor told me a story about an older dude in england. lived alone. his children came round now and then to make sure he was looking after hisself and his cupboards had food in them. for a decade or so this was the routine. one night he had a bad fall and during the course of his recovery and assessment if he could still live alone he went the doctors, they sent him to the eye doctors, the eye doctors concluded he was fully blind and had been for a long time

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There was a bit of investigation into neglect from the children had they just let him bumble around this house fully blind for a few years but ultimately they concluded his brain had built an image of his house, the furniture, everything within the property from when he could still see stuff, so though he could not see, he could still see how it all was in his brain so he never seemed like he was blind to his children

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I'd love to know if there's a link to that. It's a story that always stuck with me and made me wonder.