September 4th, 2020
Imagine that you’ve been completely blind (not to be confused with ‘legally blind‘) for as long as you’ve existed. One day you receive the world’s first eye transplant and you are assured it was completely successful. The doctors don’t want to overwhelm you, so they place a simple still image in front of you and then remove the bandages. This is what you might expect to see: 1
Unlike in the movies, you won’t see the world perfectly (and as you do there’s a good chance you won’t recognize what you’re seeing).
Like a camera, the lens of the eye refracts to the retina, turning everything upside down. Within the first week or two of our lives, or in this case seeing for the fist time, our brains figure things out and turn things right-side up.
This process can be reversed. Experiments have been done with special goggles that turn everything ‘upside down’. Within 10 days the brain compensates and makes everything look ‘normal’ (presumably it takes another 10 days to rewire the brain once the goggles are removed). In the meantime, you might miss noticing a famous person
Hocus Focus
Also during this time we learn to focus our eyes, but that takes a little longer, around eight weeks. After a month the same picture might appear something like this:
Blindspotting
By now you’ll have noticed the two black circles on the first two pictures.
Images coming into your eyes are refracted onto the retina. The retina has to communicate to the brain. This is done through the optic nerve. Unfortunately, the part of the retina where the optic nerve is ‘plugged in’ is useless for ‘seeing’ resulting in blind spots in both eyes about 15 degrees away from the center.
The blind spots take up a significant portion of the field of vision (you can test your blind spot here). Fortunately most of us have a second eye to cover for the blind spot in the first eye (and back-up cameras when we can’t use the other eye).
The Third Dimension
Finally, for the first few months everything will appear two-dimensional to us, becuase our individual eyes only see in two dimensions. Fortunately, we have the second eye to help us out there as well. The brain uses a few different methods to help us gauge distance and put things in perspective.
Being blind in one eye makes life difficult, especially for athletes (and Cyclopes). I know of only one pro athlete who overcame this hurdle.
Scanners (don’t) Live in Vain
Our area of focus at any particular microsecond is very small relative to our field of view, like looking through a cardboard tube. When we are actively using our eyes, usually when looking at something unfamiliar, we scan all these areas rapidly and stitch them together into a complete picture in our mind2. Humans can process 10-12 images per second.
Look at this picture for a few moments:
Notice your eyes moving about the picture trying to take in all the action. You can’t take in the whole picture without moving your eyes…and this is a very small portion of your field of vision.
Now, without moving your head, notice what else is around you, below, above, and to either side of the computer screen. While you were studying the picture you were most likely ‘blind’ to those areas…you weren’t scanning them.
But you had a ‘back-up image’ stored in your mind so that if someone had come up behind you and put their hands over your eyes, you could have described pretty well the area around your screen looked like.
…and you weren’t completely blind. Some part of your vision was keeping tabs on those areas so that if any unexpected movement had occurred within your field of vision you’d be alerted immediately and focused on that.
I am a (movie) Camera…
Movies create the illusion of movement by showing us 24 still pictures (Frames) every second. As stated above, Humans can only process around 12 images a second. When we are presented with a greater number, the brain interprets it as movement. This is true of live action, cartoons, claymation, or any other type of film. You can create your own ‘Flipbook movie’ with a small pad of paper and a little patience.
This limit on frame rate, both on film and in real life, is the cause of the ‘Wagon-wheel effect’. A more complete discussion of frame rates is available here.
The Eyes Don’t Have It?
There are most likely other examples of ‘defects’ in our vision. But hopefully I’ve presented enough here to convince you that “Seeing is not believing” if you didn’t feel that way already.
It’s just been in the last decade that we’ve begun to realize that in most cases Humans are terrible eyewitnesses (which has had some terrible consequences for some innocent people over the years – but that’s a topic for a different type of blog).
If you’d like more evidence, there are hundreds of other optical illusions out there, like the Wagon-wheel effect, that take advantage of the disconnect between what your eyes actually see and how your mind thinks it sees that may help convince you.
Our Lyin’ Eyes / Our Amazin’ Bayesian Brain
Hopefully I’ve disillusioned you from the natural inclination to feel like our eyes are these flawless windows providing perfect input to our brains on everything within our field of vision instantly.
But why do we feel this way? Because of our remarkable minds.
Bayes Theorem
[This brief explanation is cribbed from my second favorite blog, SlateStarCodex – much more on Bayes in another post]
“Bayes Theorem is a mathematical framework for integrating new evidence with prior beliefs”. In other words, you have many preconceived notions of what you expect to see when your ‘field of vision’ suddenly changes (such as going through a door to a new room) or waking up and opening your eyes.
Here’s a quick example…
…even though the middle letters of both words are exactly the same, you not only read this as “THE CAT” but most likely you didn’t even give much thought to the middle letters until it was pointed out to you. That’s because reading the middle ‘letters’ exactly how they are printed results in nonsense, so your mind seamlessly ‘corrects’ the gibberish letters, first into an H and then an A.
While newborns don’t really have any of these preconceptions because everything is new to them, that’s not the case for the person who started this whole post out.
If you’ve just gained use of your eyes after being totally blind your whole life, your frame of reference will be based on all your experiences up to then. Your mind will start working immediately to reconcile what you’re ‘seeing’ with what it expects. The images will be flipped ‘right-side-up’, the blurriness will go away, the blind spots will be filled in, things will clear up and your mind will paint an apparently perfect beautiful picture…
…and just like the rest of us, you will believe you have ‘perfect vision’.
Thanks for reading! I plan to update this post every so often. Constructive comments are most welcome, especially pointing out spelling/grammatical errors, readability issues, broken/missing links, etc.