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Language: en-GB

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There is an idea in literature called "harmful
sensation".

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The idea that a sight, a pernicious idea,
a sound, a song, can get into your head and

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stay there for a very long time and actually
do you harm.

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Pernicious earworms aside, there is actually
a real thing that can get inside your head

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and affect the way your brain works just by
looking at it.

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It's called the McCollough effect, and just
a warning: I realise that telling people on

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the internet not to do something is like telling
a 5-year-old not to stick beans up their nose,

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they are just going to stick beans up their
nose, but seriously: if you work with text

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or video, don't do this.

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The McCollough effect revolves around these
images. If you stare at them for long enough

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-- we're talking anything up from 15 seconds
to 15 minutes -- you'll get, as you would

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with, say, blank colours, the opposite appear
in your brain.

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If you stare at a green light for long enough,
you'll see red for a little while afterwards.

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But this doesn't work at the retinal level:
the McCollough Effect, for some reason, works

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in the level of the brain.

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You induct these for long enough - 15 minutes
- you're going to see the inverse colours

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on those gratings any time you look at something
like, say, these railings, all the time, for

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anything up to three months.

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The McCollough effect is a very rare thing,
it's quite scary, it's a real image that can

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lock into your brain, change the way it works,
and stick around for a good while.

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It is a real harmful sensation, and that is
something quite scary that you might not have known.

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