Above:  FIGURE

Most people think that their beliefs and desires and thoughts and intentions can cause them to do things, to move when they want to move, to speak when they have something to say, to make promises they intend to keep, to make choices after deliberating and weighing the pros and cons, to climb mountains because they are there, or to act in a particular, freely chosen way because of loyalty or love or hatred or anger.  Furthermore, most people believe they are morally responsible for such actions.  This way of conceiving the sources of human action is sometimes called common sense or belief/desire psychology,¹ but it has also been called “folk psychology,” a somewhat disparaging and pejorative term that not so subtly implies that only “simple folk” think this way and that it will eventually be replaced by modern scientific advances, particularly in neuroscience.  And when that day arrives, folk psychology’s beliefs, desires, and intentions will either remain as “shorthand descriptions for more fundamental neurophysiological properties of the underlying brains and bodies” ² or, more radically, completely replaced by the eliminative materialism of “a matured neuroscience” ³ that no longer has any room at all for talk of beliefs, desires, thoughts, hopes, fears, or dreams.  Consider how the neurophilosopher Patricia Churchland, on arriving home after a frustrating faculty meeting, greets her husband using the language of eliminative materialism: “Paul, don’t speak to me, my serotonin levels have hit bottom, my brain is awash in glucocorticoids, my blood vessels are full of adrenaline, and if it weren’t for my endogenous opiates I’d have driven the car into a tree on the way home. My dopamine levels need lifting.  Pour me a Chardonnay, and I’ll be down in a minute.” ⁴

One of the important events that has contributed to the attack on folk psychology’s beliefs and desires and intentions was the publication over 40 years ago of Benjamin Libet’s famous paper that concluded “that cerebral initiation even of a spontaneous voluntary act … can and usually does begin unconsciously.” ⁵ Libet based his conclusion that the brain decides before you decide on his data that showed that brain activity related to a simple, spontaneous voluntary hand movement began almost 0.5 seconds before the person reported consciously experiencing the will or urge or intention to move.  It is impossible to overestimate the impact of this paper on how we think about the efficacy of conscious intention, one of the basic elements of folk psychology.

Libet’s experiment

Libet wanted to know the relation between the pre-movement brain activity known as the Readiness Potential⁶ and the pre-movement conscious intention to move.  Replicating Kornhuber and Deeke’s 1965 protocol for the Readiness Potential, he instructed his subjects to make a series of simple, spontaneous hand movements.  The movements should be spaced apart by at least 2.56 seconds and should be spontaneous, without any planning and without any regular interval between them.  EMG (electromyogram) activity was also recorded from the skin over the forearm muscles to allow the exact onset time of the movement to be determined.

Brain activity was measured by recording the electroencephalogram (EEG) from the scalp at the vertex at the top of the head in the midline.  An example of a Readiness Potential from one of Libet’s subjects can be seen in the Figure (at the top of this article) as the light blue tracing labeled “Libet RP 6a” that begins about 1 second before the movement, which occurred at time 0 on the x axis.  Each Readiness Potential represented the average of the EEG activity before 40 movements.

To determine the time of the conscious intention to move Libet asked the subjects to watch a rotating “dot clock” while they were performing the task.  This clock is illustrated in the Figure as a small circle with numbers (5, 10, 15, …) outside the circle and a red dot inside the circle. The dot rotated around the circle continuously and took 2.56 seconds to complete each rotation.  Subjects were told not to try to follow the dot around, but after making each movement they were asked to report the dot’s position at “the time of appearance of his conscious awareness of ’wanting’ to perform a given self-initiated movement.” ⁷ Libet termed this time the subject’s “W” time (W for willing or wanting). Libet further stated that “This experience was also described as ’urge’ or ’intention’ or ’decision’ to move, though subjects usually settled for the words ’wanting’ or ’urge.”’ ⁸

The results of Libet’s experiment are illustrated in the Figure.  The average W times of each of the five subjects are plotted as filled red dots, and the average onset times of each subject’s Readiness Potential are plotted as filled blue dots. A black line connects the red and blue dots of each individual subject.  Note that for all subjects the onset time of the Readiness Potential is earlier than the W time.  The average of all five Readiness Potential onset times is -0.75 seconds before the movement, indicated by the solid blue line labeled as B_Libet (B for Brain).  The average of all five W times is -0.30 seconds before the movement, indicated by the solid red line labeled as W_Libet. So, on average the Readiness Potential began 0.45 seconds before the W’s “conscious awareness of urge” time.  This difference led Libet to conclude, as mentioned earlier, “that cerebral initiation even of a spontaneous voluntary act . . . can and usually does begin unconsciously.” ⁹ In other words, the brain decides before you decide.

And then all hell broke loose.

Libet’s impact

Libet’s findings were replicated by many other labs¹⁰ and, not surprisingly, his conclusions have been frequently cited in philosophical discussions about human free will and human moral responsibility.  If my brain decides long before I decide, does this mean, as Daniel Wegner wrote, that my free will is an illusion and my consciousness is an “epiphenomenon” that has as much to do with human action as the “turn signals are to the movements of motor vehicles,” ¹¹ a mere “feeling that occurs to a person”? ¹²  Although Wegner did not base his conclusions on Libet’s experiment, he cited Libet’s work as consistent with his conclusions, as did Sam Harris, who obviously had Libet’s work in mind when he wrote in Free Will that “One fact now seems indisputable: Some moments before you are aware of what you will do next— a time in which you subjectively appear to have complete freedom to behave however you please—your brain has already determined what you will do.” ¹³  And, more recently, Robert Sapolsky refers to Libet on 28 of the 512 pages (5.4%) of his 2023 book Determined: A Science of Life Without Free Will.¹⁴  Libet himself did not endorse such interpretations of his experiment, arguing in his original 1983 paper that “There could be a conscious ‘veto’ that aborts the performance even of the type of ‘spontaneous’ self-initiated act under study here,” ¹⁵ and in 1999 he wrote that “we must recognize that the almost universal experience that we can act with a free, independent choice provides a kind of prima facie evidence that conscious mental processes can causatively control some brain processes.” ¹⁶  However, the genie was out of the bottle, and Libet’s results and conclusions had a life of their own and became key elements of what might be called “folk neuroscience” whose influence extended far beyond neuroscience and into philosophy,¹⁷ law,¹⁸ and popular culture.¹⁹  For example, a Google search using the terms “libet” and “free will” yields 77,100 hits, and a YouTube search using “libet experiment” yields 1,258 hits. And, using Wikipedia’s August 2024 article on the “Neuroscience of Free Will” as another index of impact on popular culture, Libet’s experiment is the first experiment mentioned and is discussed in detail.  So Libet’s experiment reporting “unconscious initiation of a freely voluntary act” raised a huge problem for the folk psychology of intentional acts and free will.  How can the will be free if the brain has already decided what to do before conscious willing or intention occurs?  In recent years, however, two serious challenges to Libet’s conclusion have arisen, one concerning the Readiness Potential and the other with Libet’s dot clock, as described in detail in my recent review article.²⁰

Is the Readiness Potential a valid measure of brain activity leading to movement?

The first challenge came from a number of studies that seriously questioned whether the Readiness Potential actually measured brain activity preparing for movement.  For example, Pockett and Purdy²¹ found that a significant number of finger movements in their experiment were not preceded by a Readiness Potential, weakening the causal link between the Readiness Potential and movement.  And the laboratories of Aaron Schurger and Stefan Schmidt have both found that the Readiness Potential itself may actually be an artifact of averaging spontaneous changes in the EEG that are not related to the movement at all.²² See Aaron Schurger’s guest article “What does neuroscience have to say about free will” on this website for a full discussion of the problems with the Readiness Potential.²³

The uncertain status of the Readiness Potential raises the question of whether there is another measure of brain activity before movement that could be used instead.  In 2011, the neurosurgeon Itzhak Fried and his coworkers recorded neuronal activity from depth electrodes implanted into the medial frontal lobe during performance of the Libet clock task in “12 subjects with pharmacologically intractable epilepsy to localize the focus of seizure onset.” ²⁴  Each depth electrode included nine microwires capable of recording single and multiunit neuronal activity, and 760 units (264 single units and 496 multiunits) were recorded in the Supplementary Motor Area (SMA), pre-SMA, and Anterior Cingulate Cortex of the 12 patients.  As seen in the Figure they found “progressive neuronal recruitment over ∼1500 ms [1.5 seconds] before subjects report making the decision to move . . . [with a] progressive increase [thin blue line in figure going towards upper right] or decrease [thick blue line in figure going towards lower right] in neuronal firing rate, particularly in the supplementary motor area (SMA), as the reported time of decision was approached.” ²⁵  Small histograms of representative single neurons increasing and decreasing before movement touch the two blue lines; the black vertical line in each small histogram indicates the onset of movement.

I used breakpoint analysis²⁶ to analyze the time series of the averaged normalized firing rates for neurons increasing and neurons decreasing before movement to determine the time points when each time series changed its course, thus yielding the onset times of changes in neuronal activity related to the movement.  As seen in the Figure the onset time for averaged neurons increasing was -1.66 seconds before movement, while the onset time for averaged neurons decreasing was slightly earlier, at -1.96 seconds before movement.  This time point is indicated with a vertical thick blue line labeled B_Fried (B for Brain). Finally, note that Fried’s pre-movement neuronal activity began long before his average W time of -0.18 second he measured using a version of Libet’s clock; Fried’s W time is indicated in the Figure by vertical red line labeled W_Fried.  Thus, even though Libet’s Readiness Potential data can be questioned, Fried’s neuronal activity results confirmed Libet’s basic finding that brain activity related to movement generation begins long before the conscious decision or intention to move as given by the W time.  However, there was another problem.

Does Libet’s dot clock and its W time really detect the onset of intention?

In 2008, Matsuhashi and Hallet at the NIH were the first to replace Libet’s dot clock with “tone probes” to measure conscious intention in subjects performing the Libet task’s spontaneous movements.²⁷ The subjects were instructed to “veto” the next movement if, when they heard a brief tone probe beep, they were conscious of any thought or intention to move.  If, on the other hand, they were not aware of any intention to move, subjects were to ignore the tone.²⁸ A typical result from one of their subjects is shown in the histogram with green bars in the upper left of the Figure.  Each bar shows the number of tones (y axis) in 50 msec bins that were followed by movements in the 5 seconds before the movement at time 0.  There is an obvious decrease, beginning about 2 seconds before the movement, in the number of tones with movements, which is explained by the fact that most tones occurring in this interval caused the subject to veto the next movement, thus creating a gap.  This gap, therefore, corresponds to the presence of intention!  The mean onset time of intention of their 15 subjects was at -1.2 seconds before movement, with the earliest subject having an onset of intention at ‑2.5 seconds before the movement.  This is radically different from the average Libet dot clock onset time for conscious intention of -0.3 second before the movement.  And in 5 out of 15 subjects the onset of intention began before the onset of the subject’s Readiness Potential.

In 2016 and 2019, Verbaarschot and her coworkers also used tone probes instead of the dot clock in their “Advance to Next Slide When You Want” (2016) or “Object Game” (2019) versions of the Libet task.²⁹  In their 12 subjects in the “Advance to the Next Slide When You Want” Libet task, the mean onset time of intention measured by tone probes was -2.17 seconds before the movement, with the earliest subject having a onset time of -3.249 seconds before the movement.  Almost identical results were seen in the 16 subjects in the 2019 “Object Game” study, where the mean onset time of intention measured by tone probes was -2.5 seconds before the movement, with the earliest subject having an onset time of -3.25 seconds before movement.  The graph labeled “B” from Fig. 8 of their 2019 study is seen in the Figure just below the Matsuhashi and Hallett histogram.  In this graph they plotted the percentage of (tones ignored)/(tones scheduled) for every 0.1 second interval in the five seconds before the movements, allowing them to display the time course of the development of intention.  The part of their graph outlined in the red box (from -3 to 0 seconds) is replotted as the thick red line in the large plot below their graph.  Using time series breakpoint analysis,³⁰ the onset of intention is at -2.2 seconds.  This time point is indicated with a vertical thick red line labeled T_Verbaarschot (T for Tone).  And note that the peak of intention is seen at -0.4 seconds before the movement, suggesting that Libet’s dot clock W time measures only the peak of the intentional process rather than its much earlier onset and progression.

Putting it all together

When the time course of intention before movement as measured by tone probes (thick red line)³¹ is compared to the time course of human neuronal firing rate decreases before movement (thick blue line),³² there is no difference between the onset times.  Both intention and neuronal activity related to movement begin about 2 sec before movement.  Thus, “No difference” is the correct answer to Libet’s original question about the relation between pre-movement brain activity and pre-movement conscious intention to move.  This means that Libet’s 1983 conclusion that there was “unconscious initiation of a freely voluntary act” was wrong.  The fundamental cause of his error was the assumption that the conscious intention or urge to move was a moment or instant or “single point in time” that could be captured by a dot clock rather than “a multistage process developing over time.” ³³ It took the tone probe experiments of Matsuhashi and Hallett and Verbaarschot and coworkers to provide the experimental evidence that this was so.

It is interesting to note that in 1985 two of the commentaries on Libet’s article in Behavioral and Brain Sciences³⁴ suggested that intention might be a process rather than a moment.  James Ringo³⁵ wrote that in Libet’s experiment “subjects may be reporting the ’peak of an urge’ that actually has an extent in time.  That is, perhaps we should not imagine the production of an instantaneous urge . . . ; instead the urge may have a start, a rise, and a peak.”  And Lawrence Marks³⁶ anticipated the development of tone probes when he wrote that he “would like to be able to query the subject” asking “Do you think you are likely to want to move your wrist within the next few tenths of a second?”  And Marks continued, “I strongly suspect that subjects would be much more likely to acknowledge an intent to act at ’test moments’— that is, during the supposed unconscious interval between the occurrence of the readiness potential and the first awareness of an intent to act—than at ’controls’.”

But what about the Haynes lab’s 2008 fMRI study³⁷ of subjects performing the Libet task that reported fMRI changes in brain blood flow in frontopolar cortex and precuneus/posterior cingulate cortex as early as 10 seconds before the subjects reported their conscious intention to make L/R choice voluntary hand movements?  This study used a “stream of letters” variant of the Libet dot clock to determine the time of conscious intent, which was reported as 1 second before the movement.  As discussed extensively above, Libet-type clocks are completely unable to determine the onset of intention, so the authors of this study actually have no idea when intention began.  In addition, because of limitations imposed by the fMRI method, they had to use extremely long intervals (mean of 21.6 seconds) between movement choices.  Given that tone probe intention onsets were 2-3 seconds before movements when movement intervals were about 5 seconds, it would not be surprising if tone probes found the onset of intention as early as 10-12 seconds in experiments with movement intervals of 21.5 seconds.

Conclusion

Libet’s conclusion of that there is “unconscious initiation of a freely voluntary act” is wrong.  You and your brain decide together.  And so folk psychology survives the Libet experiment, and we have reason to believe the legendary words of Jerry Fodor that “my wanting is causally responsible for my reaching, and my itching is causally responsible for my scratching, and my believing is causally responsible for my saying,” ³⁸ thereby making us authentic and responsible human agents.

References

1. J.A. Fodor, Psychosemantics: The Problem of Meaning in the Philosophy of Mind (Cambridge MA, MIT Press, 1987).

2. C. List, Why Free Will is Real (Cambridge MA,  Harvard University Press, 2019).

3. P.M. Churchland, Matter and Consciousness (Cambridge MA, MIT Press, 1988); P.M. Churchland, Neurophilosophy:Toward a Unified Science of the Mind/Brain (Cambridge MA, MIT Press, 1989).

4. L. MacFarquhar, “Two Heads”, The New Yorker, February 5, 2007.

5. B. Libet, C.A. Gleason, E.W. Wright, and D.K. Pearl, “Time of conscious intention to act in relation to onset of cerebral activity (readiness-potential). The unconscious initiation of a freely voluntary act,” Brain 106, 623–664, 1983.

6. H.H. Kornhuber and L. Deecke, “Hirnpotentialänderungen bei willkürbewegungen und passiven bewegungen des menschen: Bereitschaftspotentialund reafferente potentiale”. Pflügers Archiv für die gesamte Physiologie des Menschen und der Tiere 284, 1–17 (1965). [English translation: H.H. Kornhuber, L. Deecke, “Brain potential changes in voluntary and passive movements in humans: readiness potential and reafferent potentials,”  Pflügers Archiv – European Journal of Physiology 468, 1115–1124, (1965).] URL: https://doi.org/10.1007/s00424-016-1852-3.

The Readiness Potential was described as a surface-negative potential (voltage) of 10-15 µV that was largest over the precentral region contralateral to the limb that moved; it was discovered in 1965 by Kornhuber and Deeke by averaging EEG (electroencephalogram) activity before a series of simple, spontaneous, voluntary hand or foot movements. The Readiness Potential was not seen before single movements but required averaging of at least 20 and often 100 movements to be clearly visible. The subjects were told not to execute the movements rhythmically but at irregular intervals of at least 15 seconds.

7. Libet et al., ibid.

8. Libet et al., ibid.

9. Libet et al., ibid.

10. P. Haggard and M. Eimer, “On the relation between brain potentials and the awareness of voluntary movements,” Experimental Brain Research 126:128–133 (1999); I. Keller and H. Heckhausen,  “Readiness potentials preceding spontaneous motor acts: voluntary vs. involuntary control,” Electroencephalography and clinical neurophysiology 76: 351–361 (1990); J. Trevena and J. Miller,  “Brain preparation before a voluntary action: Evidence against unconscious movement initiation,” Consciousness and Cognition 19:447–456 (2010); M.N. Braun, J. Wessler, and M. Friese, “A meta-analysis of Libet-style experiments,” Neurosci Biobehav Rev 128: 182–198 (2021).

11. D.M. Wegner, The Illusion of Conscious Will (Cambridge MA, MIT Press, 2002).

12.. Wegner, ibid.

13. S. Harris, Free Will (New York, Free Press, 2012).

14.. R. Sapolsky, Determined: A Science of Life Without Free Will (New York, Penguin Press, 2023).

15. Libet et al., ibid.

16. B. Libet, “Do we have free will?” Journal of Consciousness Studies 6(8–9): 47–57 (1999).

17. A search on the PhilPapers website https://philpapers.org for all matches of libet yielded 245 hits.

18. If my brain made me do it, why are you putting me in jail? See  S. Cave, “There’s no such thing as free will but we’re better off believing in it anyway,” The Atlantic (June 2016). https://www.theatlantic.com/ magazine/archive/2016/06/theres-no-such-thing-as-free-will/480750/;  R. Sapolsky, Determined: A Science of Life Without Free Will (New York, Penguin Press, 2023); J. Greene J. and J.  Cohen, “For the law, neuroscience changes nothing and everything,” Philos Trans R Soc Lond B Biol Sci, 359(1451) 2004, pp. 1775–1785;  W. Sinnott-Armstrong and L. Nadel, Conscious Will and Responsibility: A Tribute to Benjamin Libet (New York, Oxford University Press, 2011).

19. E. Racine, V. Nguyen, V. Saigle, et al., “Media Portrayal of a Landmark Neuroscience Experiment on Free Will,” Sci Eng Ethics 23: 989–1007 (2017).

20. E.J. Neafsey, “Conscious Intention and Human Action: Review of the Rise and Fall of the Readiness Potential and Libet’s Clock,” Consciousness and Cognition 94:103171 (2021).

21. S. Pockett and S. Purdy, “Are Voluntary Movements Initiated Preconsciously? The Relationships Between Readiness Potentials, Urges and Decisions,” in Conscious Will and Responsibility. A tribute to Benjamin Libet, L. Nadel and W. Sinnott-Armstrong (Eds.) (New York, Oxford University Press, 2010), pp. 34-46.

22. A. Schurger, J.D. Sitt, and S. Dehaene, “An accumulator model for spontaneous neural activity prior to self-initiated movement,” PNAS 109, E2904–E2913 (2012); S. Schmidt, H.G. Jo, M. Wittmann, and T. Hinterberger,  “‘Catching the waves’—slow cortical potentials as moderator of voluntary action,”  Neuroscience and biobehavioral reviews 68, 639–650 (2016).

23. https://catholicscientists.org/articles/what-does-neuroscience-have-to-say-about-free-will/

24. I. Fried, R. Mukamel, and G. Kreiman, “Internally Generated Preactivation of Single Neurons in Human Medial Frontal Cortex Predicts Volition,” Neuron 69:548-562 (2011).

25. Fried, 2011, ibid.

26. A. Zeileis, F. Leisch, K. Hornik, and C. Kleiber, “Strucchange: An R package for testing for structural change in linear regression models,” Technische Universität Dortmund, Sonderforschungsbereich 475: Komplexitätsreduktion in multivariaten Datenstrukturen #26 (2001).

27. M. Matsuhashi and M. Hallett, “The timing of the conscious intention to move,” The European Journal of Neuroscience 28, 2344–2351 (2008).

28. The tone probes method was developed by Matsuhashi and Hallett based on studies of “mind wandering” in which it was found that more mind wandering detected with a “probe-caught” methodology than with a “self-caught” methodology. See: J. Smallwood and J.W. Schooler,  “The restless mind,” Psychol Bull 132:946–958 (2006).

29. C. Verbaarschot, P. Haselager, and J. Farquhar, “Detecting traces of consciousness in the process of intending to act,” Experimental Brain Research 234:1945–1956 (2016); C. Verbaarschot, J. Farquhar, and P. Haselager, “Free Wally: Where motor intentions meet reason and consequence. Neuropsychologia 133:107156 (2019).

30. Zeileis et al., 2001, ibid.

31. Verbaarschot et al., 2019, ibid.

32. Fried et al., 2011, ibid.

33. Verbaarschot et al., 2016, ibid.

34. B. Libet, “Unconscious cerebral initiative and the role of conscious will in voluntary action,” Behavioral and Brain Sciences 8:529–566 (1985).

35. J.L. Ringo, “Timing Volition: Questions of What and When About W,” Behavioral and Brain Sciences 8: 550–551 (1985).

36. L.E. Marks, “Toward a psychophysics of intention,” Behavioral and Brain Sciences 8: 547 (1985).

37.. C.S. Soon, M. Brass, H.-J. Heinze, and J.-D. Haynes, “Unconscious determinants of free decisions in the human brain,” Nature Neuroscience 11: 543–545 (2008).

38. Fodor, 1987, ibid.