Libet experiment

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In the Libet experiment , it was shown that the motor center of the brain began preparing for a movement before one became aware that one had decided to perform that movement immediately. The time interval is around 0.35 s, the real movement then takes place around 0.2 s later. The physiologist Benjamin Libet carried out the series of experiments in 1979. Its importance for the philosophy of mind has been the subject of lively discussion. Even today, the experiment is often cited in the debate over the concept of human free will .

Experimental setup and implementation

The starting point was experiments by William Gray Walter and Hans Helmut Kornhuber & Lüder Deecke , in which it had been shown that with a simple hand movement, about one second elapsed between a certain initiating nerve activity in the motor cortex of the brain and the actual execution of the movement. On the other hand, it was Libet's everyday experience that the time span between conscious sensation of intention and actual execution of the action was much shorter.

Libet's experiment: (0) rest until (1) the readiness potential is measured, (2) the test subject becomes aware of his decision and notes the position of the red dot and (3) takes action.

The aim of his experiment was therefore to determine, in addition to the times at which brain activity and muscle activity began, also the time when the test subject consciously makes the decision to act now. The action should consist of a hand movement carried out at an arbitrary point in time. The muscle activity was measured by an electromyogram (EMG), the brain activity by means of the standby potential in the motor cortex by an EEG .

Libet had to develop a new method for the precise determination of the moment of the appearance of the conscious will to act. Because if the test person had to give a signal, that would have meant that he was performing another action, the timing of which would inevitably have been too imprecise due to the inevitable and relatively variable reaction time. Instead, Libet let his test subjects look at a fast running clock, which was realized by a circling point of light on an oscilloscope . One revolution takes 2.56 seconds, so that with a reading accuracy of 6 ° (corresponding to 1 second for a normal second hand), an accuracy of around 40 ms is achieved. The test persons should note the position of the clock at the point in time at which they felt the conscious “urge” (“urge”) to move their hand, and then communicate the noted position.

To check the accuracy of this procedure, a part of the skin of the test subjects was electrically stimulated in a preliminary experiment. You should then use the oscilloscope clock to indicate the time of stimulation. With a mean deviation of ~ 50 ms compared to the real time of the stimulus, the measurement method was sufficiently accurate.

Before the actual experiment, the test subjects were asked to choose any point in time to move their right hand (“at any time they felt the urge or wish to do so”) and to note the status of the watch at that point in time . This was repeated 40 times with each subject. In one series of experiments you should comply with an occurring intention to act as quickly as possible, in a second you should allow up to a second to elapse between the decision to act and the voluntary (to be specified) execution, i.e. plan the movement in advance.

Result

When evaluating the measurement results, the zero point on the time scale was always placed on the start of muscle activation, which could be determined beyond doubt using the EMG. The time intervals of 40 EEG recordings of a test person were averaged relative to this reference point. Such averaging is usually necessary in order to be able to reliably evaluate such data.

Relative to the defined zero point of the start of muscle activity, i.e. the actual movement, the average times measured were as follows:

  • If the subject had planned the movement in advance, the readiness potential set in 1050 ms beforehand in the brain;
  • If the action was to be carried out spontaneously, the ready potential set in 550 ms beforehand in the brain;
  • The time at which the voluntary decision to act became conscious was then, in both cases 200 ms before the start of muscle activity.

The remarkable thing about this result was that the point in time at which the voluntary decision was made clearly after the point in time at which an introductory nerve activity characteristic of the movement had already started in the motor cortex. Since the preliminary experiment had ensured that the inaccuracies in the time given by the test subjects were considerably smaller than the relevant time delay of the perceived volitional decision, it followed that the latter could not causally cause the activation of the motor cortex.

Trial by Haggard and Bucket

Libet's results sparked a controversial discussion on the subject of free will. Among other things, it was criticized that the test subjects had no opportunity to make a real decision, but only to determine the time of a movement that had already been decided before the clock was started.

The neurophysiologist Patrick Haggard and the psychologist Martin Eimer therefore repeated the experiment in a modified form in 1999. They introduced an alternative course of action. The test person should only decide during the experiment whether he wanted to move his right or left index finger. The specific readiness potential for the selected movement was measured in the left or right hemisphere of the brain. In this experiment, too, the activation of the motor cortex was on average before the reported point in time of the conscious decision to act.

Nevertheless, methodological objections continued to be raised: the mean point in time of the reported decision to act in Libet's experiments and various follow-up experiments varied greatly; Even within the respective experiments, the differences between the test subjects were considerable. The reported voluntary decision in Haggard and Eimer was two out of eight people before the lateralized readiness potential.

These deviations were explained on the one hand by the fact that a voluntary decision was not an event that could be described and dated with sufficient accuracy to be reported uniformly by the test persons. On the other hand, it is known that the dating of impressions of different sensory modalities is dependent on attention: stimuli on which attention is focused are predated relative to other stimuli. Such effects could also have impaired the reliability of the test subjects' readings from the watch.

Interpretation of Libets

Libet himself first concluded from his results that the decision to act is made by unconscious brain processes before it penetrates consciousness as an intention; the conscious decision is therefore not the cause of the action. He saw the freedom of will and responsibility of man in question.

Shortly thereafter, Libet went on to the thesis that there is a time window of around 100 ms within which the conscious will can still prevent an action that has already been initiated ( veto or control function of the will). In this sense, consciousness can “select results determined by the will and bring them under its control”. He substantiated this position with further experiments which showed that a readiness potential does not necessarily lead to an action, but can be canceled up to about 50 ms before muscle activation. He calculated the cited 100 ms from the 200 ms from the conscious decision to muscle activation, minus the 50 ms within which the movement can no longer be stopped, as well as corrected by the 50 ms that had resulted in the pre-experiment as a systematic reading error of the clock .

Libet further speculated that the veto itself was not initiated unconsciously, but took place directly on a conscious level. However, he did not base this assumption on experimental results. Instead, he argued that alternative assumptions would lead him to unsatisfactory conclusions about free will. With reference to the prohibitive formulation of many social rules ("You should n't ..."), he saw the moral responsibility of man restored on the basis of his assumptions.

Experiments on the awareness of voluntary decisions by Kühn and Brass from 2009 indicate that veto decisions are also made unconsciously and are only felt as free decisions afterwards. Libet's original and most extensive interpretation of his results would have been subsequently confirmed after decades.

Trial by John-Dylan Haynes

In 2015, the experiments were further checked by a team led by brain researcher John-Dylan Haynes at the Bernstein Center for Computational Neuroscience at the Charité in Berlin. The results indicate that the test persons can veto the beginning of the movement even after the willingness potential to move has started. Aborting the start of a movement was found to be possible if stop signals occurred earlier than 200 ms before the start of the movement. The time span of 200 ms is therefore a point of no return , from which the start of the movement can no longer be aborted. But even after the movement has started it is possible to cancel or change the movement itself while it is taking place.

According to Haynes, the freedom of human volition is far less restricted than assumed. The Libet experiments are out of date. There is no evidence that humans get their decisions dictated by the brain.

criticism

From the perspective of the philosopher Peter Rohs, the Libet experiment does not refute the assumption of freedom. What people know from self-experience as free decisions is not described with the Libet experiment. Against the background of the results of John-Dylan Haynes from 2015, Rohs also points out that a refutation of the assumption of freedom by brain research is not fundamentally unthinkable, even if predictions over fractions of a second as in the Libet experiment due to temporality mental processes are not to be regarded as relevant. But forecasts would have to be long-term if they were to refute freedom.

See also

swell

  1. ^ WG Walter, R. Cooper, VJ Aldridge, WC McCallum, AL Winter: Contingent negative variation: An electrical sign of sensorimotor association and expectancy in the human brain. In: Nature. Volume 203, July 1964, ISSN  0028-0836 , pp. 380-384, PMID 14197376 .
  2. ↑ Hans H. Kornhuber and Lüder Deecke: Changes in brain potential during voluntary movements and passive movements of humans: readiness potential and reacting potentials. In: Pflügers Arch Physiol (1965), 281, pp. 1-17. doi : 10.1007 / BF00412364 PDF
  3. ^ Benjamin Libet: Do we have a free will? In: Journal of Consciousness Studies , 5, 1999, p. 49.
  4. ↑ Patrick Haggard and Martin Eimer: On the Relation between Brain Potentials and the Awareness of Voluntary Movements . Experimental Brain Research 126: 128-133, 1999.
  5. ^ Benjamin Libet: Unconscious cerebral initiative and the role of conscious will in voluntary action. In: The Behavioral and Brain Sciences , 8, 1985, pp. 529-566.
  6. ↑ Benjamin Libet : Do we have free will? In: Christian Geyer (ed.): Brain research and free will. To interpret the latest experiments. Suhrkamp, ​​2004, p. 268ff. ISBN 3-518-12387-4
  7. ^ Kühn, Simone, and Brass, Marcel: "Retrospective construction of the judgment of free choice". Consciousness and Cognition 18 (1), 2009, pp. 12-21. PMID 18952468
  8. ↑ a b Joachim Müller-Jung : Liberated at last! In: Frankfurter Allgemeine Zeitung . January 30, 2016, accessed September 9, 2019 .
  9. ↑ How free is the will really? Berlin scientists examine basic patterns of decisions. In: Charité / press releases. December 17, 2015, accessed September 14, 2019 .
  10. ↑ Matthias Schultze-Kraft, Daniel Birman, Marco Rusconi, Carsten Allefeld, Kai Görgen, Sven Dähne, Benjamin Blankertz, John-Dylan Haynes: Point of no return in vetoing movements . In: William T. Newsome (Ed.): Proceedings of the National Academy of Sciences of the United States of America . tape 113 , no. 4 , 2016, p. 1080-1085 , doi : 10.1073 / pnas.1513569112 (English).
  11. ↑ Peter Rohs : Spirit and Presence . Draft of an analytical transcendental philosophy . mentis, Münster 2016, ISBN 978-3-95743-071-7 , pp. 159–160 ( limited preview in Google Book Search).

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