THE ROUGH GUIDE TO PSYCHOLOGYoften occurs when we walk onto a broken-down escalator of the kind that
used to seem irritatingly common on the London Underground. The
brain learns through experience that it needs to perform certain adjust-
ments to compensate for the movement of the escalator, to stop you
plunging head over heels. For many people, these adjustments become
so automatic that they occur inappropriately even when the escalator is
broken. So when you first step onto the defunct escalator, you’ll prob-
ably find you experience an involuntary body-wobble, creating an odd
sensation.
WHO’S IN CHARGE?
Never mind brief moments on autopilot, a famous study by Benjamin
Libet in the early 1980s challenged the idea that “you” are ever really
in control of your movements. Libet exploited the fact that whenever
we make a voluntary movement our brains exhibit a spike of prepara-
tory electrical activity that can be recorded by electrodes placed on the
scalp. Libet asked participants to move one of their fingers and to also
watch the second-hand (actually a rotating dot) on a clock, and to note
Rubber-arm illusion
For this illusion, you need one of those rubber arms that you can
buy from a joke shop – alternatively a stuffed rubber glove might do
the trick. Put the fake arm on a table in front of you, in a plausible
position that could correspond to one of your real arms. Next, place
your corresponding real arm under the table, out of view. Now you
need a friend to stroke your real arm with a feather or pen, and you
must watch as they simultaneously stroke the fake arm in perfect
synchrony with your real arm. Hopefully, you’ll soon experience the
strange sensation that you can feel the rubber arm being stroked
as if it were your own! This happens because your brain integrates
the sight of the rubber arm being stroked with the feeling of your
real arm being stroked, thus remapping where it thinks your arm
is located in space. Attempts have recently been made to put this
illusion to practical use. In 2009, a team of researchers in Sweden
reported that they were able to use the illusion to help amputees
experience a feeling of touch in a prosthetic arm. Henrik Ehrsson
and colleagues stroked the stump of an amputee and stroked their
prosthesis in synchrony. Many amputees struggle to develop any
sense of ownership of a prosthetic limb, and the researchers hope
that the illusion could be used to help counter this.