hen Benjamin Libet and colleagues asked people to flex a wrist whenever they felt like it, and to note on a fast clock when they first felt the urge. Brain activity preparing the movement began before the reported urge. Few experiments in neuroscience have been argued over more.

the document

Libet, Gleason, Wright & Pearl, "Time of conscious intention to act in relation to onset of cerebral activity", Brain. No free copy that we could verify; the DOI goes to the publisher.

what it says

Participants watched a spot circling a clock face and reported where it was when they first became aware of wanting to move (W). Scalp electrodes recorded the readiness potential, a slow negative drift first described by Kornhuber & Deecke (1965). Averaged over many trials, the readiness potential began several hundred milliseconds before the reported W time, which itself came a fraction of a second before the movement [paraphrase, not a quotation]. Libet concluded that the brain initiates voluntary acts unconsciously, and later proposed that consciousness could still veto an act in the last moments (Libet, 1985).

what's contested

What is the readiness potential? Schurger, Sitt & Dehaene (2012) proposed an accumulator model: if a decision to move is triggered when spontaneous neural noise crosses a threshold, then averaging the activity before each movement produces a slow ramp, even though nothing was decided until the crossing. On this account the readiness potential is an artefact of averaging, not a sign of an earlier decision [paraphrase, not a quotation]. Schurger et al. (2021) review the evidence. Haggard & Eimer (1999) found that W time covaried with the lateralised readiness potential, not the general one, and Trevena & Miller (2010) found similar pre-movement activity when people decided not to move.

The veto. Brass & Haggard (2007) identified brain activity associated with intentionally withholding an action. Schultze-Kraft et al. (2016) had participants play against a computer that read their readiness potential in real time, and found a "point of no return" late in preparation, after which a movement could no longer be cancelled.

Bigger claims. Soon et al. (2008) decoded which of two buttons people would press from brain activity seconds before the reported decision, though only somewhat above chance; Fried et al. (2011) predicted movement timing from single neurons. Maoz et al. (2019) found a readiness potential before arbitrary choices but not before deliberate ones that mattered to the participant, which limits what the paradigm can say about meaningful decisions [paraphrase, not a quotation].

methodological critiques

The W report is the weak point. Banks & Isham (2009) delayed an auditory signal after the movement and found reported W times shifted later, so the "moment of decision" is partly inferred after the fact. Dennett & Kinsbourne (1992) argued that asking when, precisely, a person became conscious of something presupposes a single finishing line in the brain that may not exist. Guggisberg & Mottaz (2013) review evidence that awareness of a decision is a process extended in time. And the readiness potential is a trial average; it does not show what happens on any one trial.

later reception

The experiment became a standard exhibit in arguments about free will, often far beyond what it shows. Among researchers the consensus has moved toward reading it as a study of the timing of spontaneous, arbitrary movements and of introspective timing reports, both interesting, neither decisive about freedom.

References

1. Libet et al. (1983). Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential): the unconscious initiation of a freely voluntary act. Brain. doi:10.1093/brain/106.3.623
2. Kornhuber & Deecke (1965). Hirnpotentialänderungen bei Willkürbewegungen und passiven Bewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale. Pflügers Archiv für die gesamte Physiologie des Menschen und der Tiere. doi:10.1007/BF00412364
3. Libet (1985). Unconscious cerebral initiative and the role of conscious will in voluntary action. Behavioral and Brain Sciences. doi:10.1017/S0140525X00044903
4. Schurger, Sitt & Dehaene (2012). An accumulator model for spontaneous neural activity prior to self-initiated movement. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1210467109
5. Schurger et al. (2021). What Is the Readiness Potential?. Trends in Cognitive Sciences. doi:10.1016/j.tics.2021.04.001
6. Haggard & Eimer (1999). On the relation between brain potentials and the awareness of voluntary movements. Experimental Brain Research. doi:10.1007/s002210050722
7. Trevena & Miller (2010). Brain preparation before a voluntary action: Evidence against unconscious movement initiation. Consciousness and Cognition. doi:10.1016/j.concog.2009.08.006
8. Brass & Haggard (2007). To Do or Not to Do: The Neural Signature of Self-Control. The Journal of Neuroscience. doi:10.1523/jneurosci.0924-07.2007
9. Schultze-Kraft et al. (2016). The point of no return in vetoing self-initiated movements. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.1513569112
10. Soon et al. (2008). Unconscious determinants of free decisions in the human brain. Nature Neuroscience. doi:10.1038/nn.2112
11. Fried, Mukamel & Kreiman (2011). Internally Generated Preactivation of Single Neurons in Human Medial Frontal Cortex Predicts Volition. Neuron. doi:10.1016/j.neuron.2010.11.045
12. Maoz et al. (2019). Neural precursors of decisions that matter—an ERP study of deliberate and arbitrary choice. eLife. doi:10.7554/eLife.39787
13. Banks & Isham (2009). We Infer Rather Than Perceive the Moment We Decided to Act. Psychological Science. doi:10.1111/j.1467-9280.2008.02254.x
14. Dennett & Kinsbourne (1992). Time and the observer: The where and when of consciousness in the brain. Behavioral and Brain Sciences. doi:10.1017/S0140525X00068229
15. Guggisberg & Mottaz (2013). Timing and awareness of movement decisions: does consciousness really come too late?. Frontiers in Human Neuroscience. doi:10.3389/fnhum.2013.00385

see also