Adaptation: Why your brain loves to tune out
The human eye is capable of processing visual information far more quickly than any computer.
The
constant whir of a fan. The sensation of the clothes against your skin.
The chair pressing against your legs. Chances are that you were not
acutely aware of these until I pointed them out. The reason you had
somehow forgotten about their existence? A fundamental brain process
that we call adaptation.
Our brains are remarkably good at
cancelling out all sorts of constants in our everyday lives. The brain
is interested in changes that it needs to react or respond to, and so
brain cells are charged with looking for any of these differences, no
matter how minute. This makes it a waste of time registering things that
are not changing, like the sensation of clothes or a chair against your
body, so the brain uses adaptation to tune this background out,
allowing you to focus on what is new.
If you don’t believe me, try
this simple, but startling demonstration. First, hold your eyeball
perfectly still. You could use calipers to do this, or a
drug that paralyses the eye muscles,
but my favourite method is to use my thumb and index finger. Using the
sides of your thumb and finger, press on the bone of the eye socket,
through your upper and lower eyelids. Do this gently. Try it with one
eye first, closing the other eye or covering it with your hand.
With
your eye fixed in position, keep your head still and soon you will
experience the strangest thing. (You will have to stop reading at this
point. I don't mind. We will pick up when you have finished). After a
few seconds the world in front of you will fade away. As long as you are
holding your eyeball perfectly still, you will very quickly discover
that you can see nothing at all. Blink, or move your head, let go of
your eye and the world will come back. What's going on?!
Now you see it…
For
all of our senses, when a certain input is constant we gradually get
used to it. As you are holding your eye still, exactly the same pattern
of light is falling on each brain cell that makes up the receptors in
the back of your eye. Adaptation cancels out this constant stimulation,
fading out the visual world. The receptors in your eye are still
processing information. They have not gone to sleep. They simply stop
firing as much, reducing the messages they pass on about incoming
sensations – in effect the message passed on to the rest of the brain is
"nothing new... nothing new... nothing new…". You can make your brain
cells spring into action by moving your eye, or by waving your hand in
front of your face. Your hand, or anything moving in the visual world,
is enough of a change to counteract the adaptation.
This sounds
like it could go badly wrong. What if I am watching something, or
someone, I am thinking hard about it, and I forget to move my eyes for a
few seconds. Will adaptation mean that thing disappears? Well, yes, it
could in principle. But the reason it does not happen in practice is due
to an ingenious work-around that the evolution has built into the
design of the eyes – they constantly jiggle in their sockets. As well as
the large rapid eye movements we make several times a second, there is
also a constant, almost unnoticeable twitching of the eye muscles that
means that your eyes are never absolutely still, even when you are
fixing your gaze on one point. This prevents any fading out due to
adaptation.
You can see this twitching when you look at a single point of light
against a dark background (such as a single star in the sky, or a
glowing cigarette end in a totally dark room). Without a frame of
reference your brain will be unable to infer a stable position of the
point of light. Every twitch of your eye muscles will seem like a
movement of the point of light (a phenomenon called
the autokinetic effect).
Adaptation
is so useful for the brain's processing of information that it has been
kept by evolution, even in basic visual processing, and this extra
muscle twitching has been added in to prevent too much adaptation
causing problems for us. But the basic mechanism is still there, as my
eye experiment revealed.
Once you understand adaptation, you
discover that it is all around us. It is the reason people shout when
they come out of nightclubs (they have got used to the constant high
volume, so it does not seem as loud to them as it does to the people
they wake up on the way home). It is why a smell that might have hit you
as overpowering when you first enter a room can actually be ignored
after you've got used to it. And it is related to the phenomenon of
word alienation,
whereby you repeat a word so often it loses its meaning. But most of
the time it operates quietly, in the background, helping to filtering
out the things that do not change, so that we can concentrate on the
more important tasks of those that do.
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