Eli5: Why do things moving quickly in our peripheral vision startle us so much? Especially in a normally safe place like home
In: Biology
The cells in the eye (called “rods”) that are responsible for peripheral vision (as opposed to “cones” which give detail to the tiny area you are focused on at any given time) evolved to detect faint movements and changes quickly (though with little detail or even color), as an evolutionary defense mechanism against predators, against which you need to be able to react quickly and thoughtlessly as they approach.
Because throughout pretty much all of human evolution, and up until relatively very recently, reacting quickly to things in your peripheral vision was very important for survival. As part of that, it’s safer to assume something is a threat (and be startled) than to assume it’s friendly and potentially lose that precious reaction time.
Basically, even though you know you are in a safe place, your subconscious mind still has hundreds of thousands of years of instincts tell you that any movement in your peripherals is a potential threat and to act accordingly.
think about life as an ancient human. fast animals running at you can be a serious, life-ending threat. so being able to tell when an animal is running at you is very important.
if an animal is directly in front of you, it’s much easier to assess the threat level. if a lion is laying down 100 feet away and starts running at you, you instantly know you should run the other way. but it’s not really that surprising – you saw it the whole time.
however, peripheral vision isn’t good enough to tell if the thing coming at you is a lion…or a small bird.
the early humans that noticed quick things in their peripheral vision had an advantage compared to those who didn’t – if an animal might be running at you, every second counts, and if you notice it sooner you can run sooner.
overtime it got to the point that as soon as you see quick movement in your peripheral vision, your body gets alerted and ready to move. and this instinct is super useful because there’s not really a big downside to being alerted by that in terms of survival. and so this instinct got passed down through humanity (and many prey animals), because not getting eaten is very important
so today, you will get startled even in a safe environment, because being startled for a second is a much better alternative than being eaten. better safe than sorry!
Humans used to be big cat prey. Leopards and lions stalk from where they think the prey isn’t looking, and humans have exceptional forward vision due to how our eyes are set and our brain’s ability to remember. If a human wants to, they cannot be fooled by “this wasn’t here thirty seconds ago but it’s very still and natural.”
So attacking us from the front does not work. Or it at least has a very high casualty rate for the cat, which is the same thing in evolution.
Reacting quickly to something in peripheral vision saved your ancestors and their kids.
You get startled in your own home because the movement is out of the ordinary for your environment. If you’re out shopping, you know people are moving all around you so your brain just notes it and moves on. At home alone? Could be a random dustbunny kicked up by the AC coming on, could be an axe murderer! Similarly, sudden noise at night when it’s usually quiet of course wakes people up, but if you are used to sleeping somewhere with lots of traffic or forest noises and they all suddenly go silent, it’ll also wake you because the little part of your brain that’s still paying attention goes “*wait that’s not right!*”
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