There is an idea in literature called "harmful sensation".
The idea that a sight, a pernicious idea, a sound, a song, can get into your head and
stay there for a very long time and actually do you harm.
Pernicious earworms aside, there is actually a real thing that can get inside your head
and affect the way your brain works just by looking at it.
It's called the McCollough effect, and just a warning: I realise that telling people on
the internet not to do something is like telling a 5-year-old not to stick beans up their nose,
they are just going to stick beans up their nose, but seriously: if you work with text
or video, don't do this.
The McCollough effect revolves around these images. If you stare at them for long enough
-- we're talking anything up from 15 seconds to 15 minutes -- you'll get, as you would
with, say, blank colours, the opposite appear in your brain.
If you stare at a green light for long enough, you'll see red for a little while afterwards.
But this doesn't work at the retinal level: the McCollough Effect, for some reason, works
in the level of the brain.
You induct these for long enough - 15 minutes - you're going to see the inverse colours
on those gratings any time you look at something like, say, these railings, all the time, for
anything up to three months.
The McCollough effect is a very rare thing, it's quite scary, it's a real image that can
lock into your brain, change the way it works, and stick around for a good while.
It is a real harmful sensation, and that is something quite scary that you might not have known.
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