Want to learn something weird about how your brain works?
There's a physical problem with our eyes - we move them in short fast bursts called
saccades. These are very quick, synchronized movements, which allow the eye to scan across your field of vision for what it thinks are the critical points of reference.
By "quick", think in the order of 900°/s angular movement speed - the eyeball could rotate fully nearly three times a second, if that was possible. The problem is that leads to blurred vision during motion. Having your vision turn into a blurry mess every time you move your eyes is obviously not a good idea, so our brains hide it from us.
Imagine you're an engineer and you have this problem. What are some solutions to this issue?
1. Make vision go black during movement. (Some VR games actually use this method.)
2. Just keep showing the last thing we saw prior to movement, until a blur-free image is again available.
Both are possible options that work, although with different downsides, but your brain uses neither of these options.
When a saccade occurs, your brain puts your visual system on "pause". You're not seeing blackness, you're just not seeing anything - full stop. You literally have no vision while this is occurring. (Yes, this is happening to you right now. Cool, hey?) When you finish your saccade, you see what you now see at the new eye orientation. But what happened in the meantime? What's freaky is that your brain now pretends it can time travel. It doesn't just show you the image at the current time, but it
time-shifts it backwards so that you
think you were seeing it the whole time your eyes were moving.
This works because your brain has no independent clocking reference, so if it feels like fooling itself by distorting linear time - it can. You can see this effect happen for yourself if you watch an analog clock with a ticking second hand. (Here's one I prepared earlier:
https://www.youtube.com/watch?v=G735Q4amKZc). Watch the second hand, then look away (move just your eyes, not your head), then look back to the second hand. It will seem to take longer than a second to initially move, then it resumes moving as normal. That's your brain and visual system lying to you about linear time to cover up for the physical limitations of your eyes. This effect has been known for over a century, it's called "saccadic masking" or chronostasis. Your visual system can distort time by up to half a second to mask the effect of saccade blurring.
An engineer would also wonder why a vision-pausing system wouldn't cause all sorts of weird effects with moving objects. Wouldn't they appear to stutter when they move? No, because your brain has a second perception hack to avoid you perceiving this. You can again see this happening by looking at a clock with a smoothly moving (as opposed to ticking) second hand like this:
https://www.youtube.com/watch?v=iAr3e7VoKv8. Try the same experiment as before with looking at the second hand, looking away, and back again. In this instance, the second hand doesn't exhibit the initial pause like the ticking hand, because your brain recognizes it's moving and adjusts what you see to make sure it sees the "right" thing. The perception hack is only really obvious with periodically moving things like a clock hand, because it's not moving constantly (so not triggering the movement-during-chronostatis hack) but it moves
at a set rate, so you can still perceive that that rate appears to change.
Because we all think we've got a fair sort of handle on how the world works, it's tempting to think of your eyes and visual system as a camera dumping a video feed into your conscious brain where you sort out what's happening for yourself, but that's very, very much not the case. What you
think you see and what your eyes
actually see are two completely different things.
The first and biggest is the blind spot. Vertebrate eyes are wired backwards so we've got a blind spot in each eye were the nerves enter into the back of the eye. About 6 degrees of your vision in each eye is just not there, as there's no light sensitive cells present in that part of the eye. But you don't see a blind spot, assuming your eyes are undamaged. Does the other eye perhaps fill in the blank spot for you? Try it, close one eye - there's now no way for the other eye to fill in the gap. You still won't see a blind spot... your visual system is again lying to you, and
making up content it thinks is there. You literally cannot see what you
think you see.
The second is colour vision. You can see in colour, right?
All of your vision is colour, edge to edge? What's interesting is that most of your cone cell light receptors (which are the ones sensitive to colour) are in the fovea, a little spot in the center of your vision:
Outside of that center-of-vision spot, you have very little color perception. There's some but it's very limited compared to your main colour vision. But if you shift your attention to your peripheral vision right now, it's in colour - how? Again, your vision system is lying to you. It's remembering what colours things are and guessing and filling in the gaps. It's basically doing a colorisation process on your non-central vision.
Then there are other interesting effects like "action-specific perception". If you get a number of white balls of various sizes and toss them at someone, then ask them to estimate the size of the balls thrown at them, they'll have a certain size estimate, right?
Now repeat the experiment but ask them to try to hit the balls back with a bat, and suddenly all the estimates shift larger. They actually see the ball as bigger
because they need to hit it. Their vision system exaggerates object size to make it easier to see, based on need.
As mentioned above, your vision is not a camera. Perfect accuracy is not one of its goals, and objective reality is not very important. What is important to the evolution of the visual system is any trick that
helps you survive, no matter how strange or weird it is. So if you want an accurate visual representation of what things look like - use a camera. Not your eyes.
Now you know about your eyes and visual system being poor cameras that lie to you, you might still think that at least they're consistent, time-wise. They don't screw with your sense of time passing, just to make up for visual defects, right?
Nope - if you can't get it done in time, just turn back the clock and
pretend you did. That's a perfectly good solution to your visual system.
Want another example of saccadic masking? Go and look into a mirror. No matter how close you bring it to your eyes, and how much you look around, you will never see your eyes move. How is that the case? Simple,
you're blind during those moments. But you still
think you are seeing.
More proof? Try the same thing again, but using your phone's selfie camera. It's not a mirror, there's a slight delay between the camera "seeing" the image and it being displayed on the screen - so now you
can see your eyes performing saccades.
Look at wikipedia's example of the blindspot below. Stare at L with only your left eye, adjust the distance, and the R will disappear. You don't see "nothing" or "black", you see the background, because you expect to, so your brain fills in the space with what you expect to see.
This is why laser damage your retina can be so dangerous. Your visual system already hides "holes" in your vision, what's one more to hide? You can damage a small spot of your retina and your visual system covers it up. The problem is you don't realise the damage has been done, until eventually you accumulate so much damage that your visual system simply cannot hide it all and your vision rapidly degrades.
The other reason many lasers are so dangerous is that they don't trigger the same responses as regular incoherent light. Your pupil reflex is only triggered by some special cells in the center of your eye, so an off-center laser might not trigger an aversion reflex and cause your iris to contract. Infrared laser light outside your visual range of perception is just as dangerous as visible laser light, but can't trigger your blink reflex, so damage is still done to your light perception system.
Cephalopods actually have their eyes structured differently to vertebrates; their nerve fibres run behind the retina, so they have no blind spot.
Want another examples of how crazy your vision system is?
There was an experiment back in 1890 where someone wore glasses made with internal mirrors to flip their vision laterally. After about 8 days, they could see just fine with them on. Their vision system had started "flipping" the image to compensate. It only took them a few hours to get back to normal after taking these glasses off, though.
This is strange enough, but what is really freaky is that -
your eyes already do this. Based on how our vision is wired, we actually see everything upside down. Your visual system flips it vertically for you, so what you "see" aligns with the world as you interact with it via your other senses like touch.
So if you every wonder if you really saw something, it's possible you didn't - you just think you did!
