NS 2727: Free will not an illusion after all http://www.newscientist.com/...: "
NS 2727: Free will not an illusion after
all
http://www.newscientist.com/
article/mg20327274.400-
free-will-not-an-illusion-after-all.html
et seq. * 23 September 2009 by Anil
Ananthaswamy
CHAMPIONS of free will take heart. A
landmark 1980s experiment that purported to show free will doesn't exist
is being challenged.
In 1983, neuroscientist Benjamin Libet
asked volunteers wearing scalp electrodes to flex a finger or wrist.
When they did, the movements were preceded by a dip in the signals being
recorded, called the "readiness potential". Libet interpreted this RP as
the brain preparing for movement.
Crucially, the RP came a few tenths of a
second before the volunteers said they had decided to move. Libet
concluded that unconscious neural processes determine our actions before
we are ever aware of making a decision (Brain, vol 106, p 623).
Since then, others have quoted the
experiment as evidence that free will is an illusion--a conclusion that
was always controversial, particularly as there is no proof the RP
represents a decision to move.
Long sceptical of Libet's interpretation,
Jeff Miller and Judy Trevena of the University of Otago in Dunedin, New
Zealand, attempted to tease apart what prompts the RP using a similar
experiment, with a key twist.
They also used scalp electrodes, but
instead of letting their volunteers decide when to move, Miller and
Trevena asked them to wait for an audio tone before deciding whether to
tap a key. If Libet's interpretation were correct, Miller reasoned, the
RP should be greater after the tone when a person chose to tap the key.
While there was an RP before volunteers
made their decision to move, the signal was the same whether or not they
elected to tap. Miller concludes that the RP may merely be a sign that
the brain is paying attention and does not indicate that a decision has
been made (Consciousness and Cognition, DOI:
10.1016/j.concog.2009.08.006).
Miller and Trevena also failed to find
evidence of subconscious decision-making in a second experiment. This
time they asked volunteers to press a key after the tone, but to decide
on the spot whether to use their left or right hand. As movement in the
right limbs is related to the brain signals in the left hemisphere and
vice versa, they reasoned that if an unconscious process is driving this
decision, where it occurs in the brain should depend on which hand is
chosen. But they found no such correlation.
Marcel Brass of Ghent University in Belgium
says it is wrong to use Miller and Trevena's results to reinterpret
Libet's experiment, in which volunteers were not prompted to make a
decision. The audio tone "changes the paradigm", so the two can't be
compared, he says. What's more, in 2008, he and his colleagues detected
patterns in brain activity that predicted better than chance whether or
not a subject would press a key, before they were aware of making a
decision (Nature Neuroscience, DOI: 10.1038/nn.2112).
But Frank Durgin, a psychologist at
Swarthmore College in Pennsylvania, says that Brass's results do "seem
to undermine Libet's preferred interpretation", though they don't
contradict it outright.
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