CHAMPIONS of free will take heart. A landmark 1980s experiment that purported to show free will doesn鈥檛 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 鈥渞eadiness potential鈥�. 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).
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Since then, others have quoted the experiment as evidence that free will is an illusion 鈥� a conclusion that was always controversial, particularly as there is no proof the RP represents a decision to move.
Long sceptical of Libet鈥檚 interpretation, 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鈥檚 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, ).
鈥淭he 鈥榬eadiness potential鈥� doesn鈥檛 indicate a decision has been made before the person was aware of it鈥�
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鈥檚 results to reinterpret Libet鈥檚 experiment, in which volunteers were not prompted to make a decision. The audio tone 鈥渃hanges the paradigm鈥�, so the two can鈥檛 be compared, he says. What鈥檚 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, ).
But , a psychologist at Swarthmore College in Pennsylvania, says that Brass鈥檚 results do 鈥渟eem to undermine Libet鈥檚 preferred interpretation鈥�, though they don鈥檛 contradict it outright.