The Neuroscience-y Reason You Can't Spot Your Own Typos
This is why it's so hard to spot your own typos, like the ones in the last sentence.
University of Sheffield psychologist Tom Stafford says as much to Wired:
"When you're writing, you're trying to convey meaning. It's a very high level task," he said.
As with all high level tasks, your brain generalizes simple, component parts (like turning letters into words and words into sentences) so it can focus on more complex tasks (like combining sentences into complex ideas). "We don't catch every detail, we're not like computers or NSA databases," said Stafford. "Rather, we take in sensory information and combine it with what we expect, and we extract meaning."
This is how it shakes out experientially: When you're reading your own stuff, your eyes might be dutifully scanning over your sentences, but all you're really conscious of is the meaning you're trying to get across, rather than these words you're using to convey it.
Psychologists call this process generalization, which is one of your mind's shortcuts for storing information. If you've ever driven to work when you were trying to go to your friend's house, you're familiar with it: Instead of reasoning out where you're going, you glide away on autopilot. Same thing with editing - you're just easily rolling to the assumed destination.
What's more terrifying for the would-be author is that the generalization process is fast and easy, and since it feels easy, it's hard to be vigilant. Similarly, highlighting feels like a good way to study - after all, it feels so easy to run your marker over the page - but it doesn't help you remember information better. On the other hand, blurry fonts lead to better recall.
The trick with editing, then, is to de-familiarize yourself with your words as much as possible. Stafford says that you should change the font, switch out background colors, or print the thing out and carve it up with a pen - all these things help communicate to your brain that it needs to pay attention.
Otherwise you could be coasting into errors, Stafford says. "Once you've learned something in a particular way, it's hard to see the details without changing the visual form."