Your brain decides your actions seconds before you become aware of them
In 1983, neurophysiologist Benjamin Libet recorded electrical activity in the brain while volunteers chose when to flick their wrists. Electrodes detected a 'readiness potential' rise in the motor cortex roughly 350 to 500 milliseconds before participants consciously registered the intent to act. Modern fMRI studies have extended this gap, decoding predictable motor choices in frontal and parietal regions up to several seconds prior to conscious subjective awareness.
The Oscilloscope Clock and the Wrist Flick
In the mid-1960s, German neurophysiologists Hans Helmut Kornhuber and Lüder Deecke discovered that voluntary hand and foot movements are preceded by a distinct electrical shift in the brain. Recorded using electroencephalography (EEG), this slow electrical buildup was named the Bereitschaftspotential, commonly translated as the readiness potential. The signal reliably appeared hundreds of milliseconds before a person physically carried out a simple movement. While this demonstrated that the motor cortex and supplementary motor areas prepare for physical execution well in advance, it left a central question unresolved: at what point in this physiological sequence does conscious awareness of the urge to act actually emerge?
In 1983, American neurophysiologist Benjamin Libet and his colleagues devised an experiment to measure that exact subjective timing. Libet had participants sit facing a specialized cathode-ray oscilloscope screen displaying a dot that rotated in a rapid circle, functioning as a high-speed clock where each cycle took just under three seconds. Volunteers were told to rest their hands and, whenever they experienced the spontaneous urge to do so, carry out a sudden, simple movement such as flexing their wrist or tapping a finger.
Crucially, participants were instructed not to plan the movement in advance, but to act completely on impulse. Afterward, they were asked to report the precise position of the revolving spot on the clock face at the exact instant they first became aware of their intention or urge to move. By synchronizing this reported subjective moment with electromyogram recordings of muscle contraction and continuous EEG recordings of the readiness potential, Libet could compare the timing of unconscious brain activity, subjective conscious intent, and physical action.