What is a readiness potential in Benjamin Libet's free-will experiments?

In the context of Benjamin Libet’s experiments on free will, the readiness potential is an electrical change in the brain detected via EEG.1

Libet’s team observed that this potential occurs between 350 and 850 milliseconds before a participant becomes consciously aware of their desire to perform a voluntary movement. These findings were interpreted by some as evidence that voluntary actions originate unconsciously, suggesting that the brain initiates movement before the conscious mind makes a decision.1

While these results have been used to argue against the existence of free will and moral responsibility, the interpretation remains highly controversial. Critics argue that the experimental design—which requires subjects to monitor their own internal states during spontaneous, arbitrary movements—may lead to inaccurate reporting or fail to capture the complex, deliberative processes typically associated with human agency. Libet himself maintained that free will could still function through a "veto" power, allowing individuals to consciously override these unconsciously generated impulses.1