Champions of free will, take heart. A landmark 1980s experiment that purported to show free will doesn't exist is being challenged.
By Anil Ananthaswamy / Source: New Scientist
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.
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.
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.
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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