mind & identity · new to this edition
half a second early
you decide to move your wrist. an electrode on your scalp saw it coming before you did. benjamin libet asked who made the decision.
Most arguments about free will take place in armchairs. This one began in a laboratory. In 1965 Hans Kornhuber and Lüder Deecke found a slow build of electrical activity over the scalp in the moments before a voluntary movement, which they called the Bereitschaftspotential, or readiness potential. In the early 1980s the neurophysiologist Benjamin Libet asked a simple question about it: does the build start before or after you consciously decide to move? His answer has been quoted ever since as proof that free will is an illusion, and contested ever since as proof of nothing of the kind. Answer each step before reading on. The later steps test whether your earlier answers still hold.
your brain began preparing before you felt the urge. who decided to move?
You are in Libet’s laboratory with electrodes on your scalp. In front of you is a clock face with a spot of light revolving round it. Your task is to flex your wrist whenever you feel like it: no signal, no plan, no reason. Each time, you note where the spot was at the moment you first felt the urge or intention to move, and you report it afterwards.
You do this many times. Then the trials are lined up by the instant your wrist moved, and averaged. Libet reported that for spontaneous movements like yours, the readiness potential began about 550 milliseconds before the movement. Your reported moment of wanting to move came at about 200 milliseconds before it. The brain’s build was already under way for roughly a third of a second before you were aware of any intention at all.
Who decided to move your wrist?
further reading
- time of conscious intention to act in relation to onset of cerebral activity (readiness-potential) original paper
- unconscious cerebral initiative and the role of conscious will in voluntary action original paper
- an accumulator model for spontaneous neural activity prior to self-initiated movement original paper
- neural precursors of decisions that matter: an erp study of deliberate and arbitrary choice original paper
- neuroscience of free will wikipedia