Benjamin Libet (
April 12, 1916- July 23, 2007) was a researcher in the physiologydepartment of the University of California, San Francisco, and a pioneering scientist in the field of human consciousness. In 2003, he was the first recipient of the "Virtual Nobel Prize in Psychology" from the University of Klagenfurt, "for his pioneering achievements in the experimental investigation of consciousness, initiation of action, and free will". [ [http://www.uni-klu.ac.at/psy/cognition/index_e.html Virtual Nobel Prize web site.] This prize has no relation to the Nobel Prizeof the Swedish Nobel Foundation.] In his [http://www.uni-klu.ac.at/psy/cognition/fr_nobel_E2003.html acceptance speech] , Libet summarized his life's research and highlighted his work on conscious volitional acts and the Subjective backward referral or "antedating" of sensory experience">antedating of sensory awareness.
In the 1970s, Libet was involved in research into
neuralactivity and sensation thresholds. His initial investigations involved determining how much activation at specific sites in the brain was required to trigger artificial somaticsensations, relying on routine psychophysicalprocedures. This work soon crossed into an investigation into human consciousness; his most famous and controversial experiment demonstrates that the unconscious electrical processes in the brain called Bereitschaftspotential(or readiness potential) discovered by Lüder Deeckeand Hans Helmut Kornhuberin 1968 precede conscious decisions to perform volitional, spontaneous acts, implying that unconscious neuronal processes precede and potentially cause volitional acts which are retrospectively felt to be consciously motivated by the subject.
Volitional acts and readiness potential
In order to gauge the relationship between unconscious readiness potential and subjective feelings of volition and action, Libet required an objective method of marking the subject's conscious experience of the will to perform an action in time, and afterward comparing this information with data recording the brain's electrical activity during the same interval. [Libet, B., Gleason, C. A., Wright, E. W., and Pearl, D. K. (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", 106:623-642.] [ Libet, B. (1985). Unconscious cerebral initiative and the role of conscious will in voluntary action. "Behavioral and Brain Sciences", 8:529-566.] For this, Libet required specialized pieces of equipment.
The first of these was the
cathode ray oscilloscope, an instrument typically used to graph the amplitudeand frequencyof sound waves. With a few adjustments, however, the oscilloscope could be made to act as a timer: instead of displaying a series of waves, the output was a single dot that could be made to travel in a circular motion, similar to the movements of a second hand around a clock face. This timer was set so that the time it took for the dot to travel between intervals marked on the oscilloscope was approximately forty-three milliseconds. As the angular velocity of the dot remained constant, any change in distance could easily be converted into the time it took to travel that distance.
To monitor brain activity during the same period, Libet used an
electroencephalogram, or EEG. The EEG uses small electrodesplaced at various points on the scalp that measure neuronal activity in the cortex, the outermost portion of the brain, which is associated with higher cognition. The transmission of electrical signals across regions of the cortex causes differences in measured voltage across EEG electrodes. These differences in voltage reflect changes in neuronal activity in specific areas of the cortex.
To measure the actual time of the voluntary motor act, an electromyograph (EMG) recorded the muscle movement using electrodes on the skin over the activated muscle of the forearm. The EMG time was taken as the zero time relative to which all other times were calculated.
Researchers carrying out Libet’s procedure would ask each participant to sit at a desk in front of the oscilloscope timer. They would affix the EEG electrodes to the participant’s scalp, and would then instruct the subject to carry out some small, simple motor activity, such as pressing a button, or flexing a finger or wrist, within a certain time frame. No limits were placed on the number of times the subject could perform the action within this period.
During the experiment, the subject would be asked to note the position of the dot on the oscilloscope timer when "he/she was first aware of the wish or urge to act" (control tests with Libet's equipment demonstrated a comfortable
margin of errorof only -50 milliseconds). Pressing the button also recorded the position of the dot on the oscillator, this time electronically. By comparing the marked time of the button's pushing and the subject's conscious decision to act, researchers were able to calculate the total time of the trial from the subject's initial volition through to the resultant action. On average, approximately two hundred milliseconds elapsed between the first appearance of conscious will to press the button and the act of pressing it. As of 2008, the upcoming outcome of a decision could be found in study of the brain activity in the prefrontal and parietal cortex up to 10 seconds before the subject was aware of their decision. [cite news|author=Keim, Brandon|title=Brain Scanners Can See Your Decisions Before You Make Them|url=http://www.wired.com/science/discoveries/news/2008/04/mind_decision|date= April 13, 2008|work=Wired News|publisher=CondéNet|accessdate=2008-04-13 and cite journal|author=Chun Siong Soon, Marcel Brass, Hans-Jochen Heinze, John-Dylan Haynes|title=Unconscious determinants of free decisions in the human brain (Abstract)|url=http://www.nature.com/neuro/journal/vaop/ncurrent/abs/nn.2112.html|date= April 13, 2008|doi=10.1038/nn.2112|journal=Nature Neuroscience|publisher=Nature Publishing Group|accessdate=2008-04-13|volume=11|pages=543]
Researchers also analyzed EEG recordings for each trial with respect to the timing of the action. It was noted that brain activity involved in the initiation of the action, primarily centered in the secondary
motor cortex, occurred, on average, approximately five hundred milliseconds "before" the trial ended with the pushing of the button. That is to say, researchers recorded mounting brain activity related to the resultant action as many as three hundred milliseconds "before" subjects reported the first awareness of conscious will to act. In other words, apparently conscious decisions to act were "preceded" by an unconscious buildup of electrical charge within the brain - this buildup came to be called Bereitschaftspotentialor readiness potential.
Implications of Libet's experiments
Libet's experiments suggest unconscious processes in the brain are the true initiator of volitional acts, therefore, little room remains for the operations of
free will. If the brain has already taken steps to initiate an action before we are aware of any desire to perform it, the causal role of consciousness in volition is all but eliminated.
Libet finds room for free will in the interpretation of his results only in the form of 'the power of veto'; conscious acquiescence is required to allow the unconscious buildup of the
readiness potentialto be actualized as a movement. While consciousness plays no part in the "instigation" of volitional acts, it retains a part to play in the form of suppressing or withholding from certain acts instigated by the unconscious. Libet noted that everyone has experienced the withholding from performing an unconscious urge. Since the subjective experience of the conscious will to act preceded the action by only 200 milliseconds, this leaves consciousness only 100-150 milliseconds to veto an action (this is because the final 50 milliseconds prior to an act are occupied by the activation of the spinal motor neuronesby the primary motor cortex, and the margin of error indicated by tests utilizing the oscillator must also be considered).
It has been suggested that consciousness is merely a side-effect of neuronal functions, an
epiphenomenonof brain states. Libet's experiments are proffered in support of this theory; our reports of conscious instigation of our own acts are, in this view, a mistake of retrospection. However, some philosophers have disputed this conclusion:
:"in short, the [neuronal] causes and correlates of conscious experience should not be confused with their "
ontology" [...] the "only" evidence about what conscious experiences are like comes from first-person sources, which consistently suggest consciousness to be something other than or additional to neuronal activity." [cite book | last = Velmans | first = Max | authorlink = | coauthors | title = Understanding Consciousness | publisher = Routledge| date =2000 | location =London | pages =pp 35-37 | url = | doi = | isbn =0-41522-492-6 ]
It is also possible to dispute the meaning of the experiments with regard to free will on a more simple methodological question: Libet asked his subjects to note the position of the dot the moment at which they became aware of making a decision. The assumption contained in the interpretation of the results, is that it took no time to note the position of the dot. Another account would be that in fact, given their instructions, subjects had to make a decision to note the position of the dot, but that this itself would take some time, and would interfere with the decision to move the wrist.
The conclusions about Libet´s experiments have to be discussed, because for the movements of hand or fingers, only the
procedural memoryis involved. Whereas for a discussion about the free will of a person, we should be aware, that our individual knowledge/experiences is stored in the episodic memory. To make a good decision we have to weigh up different arguments - but the movement of a finger needs no moralized decision.
ubjective backward referral or "antedating" of sensory experience
It is possible that Libet's theory has been misunderstood to some extent, and that his results have been used by materialist polemicists, since the question of the nature of the mind is a highly political one. It is worth noting that his early theory, resting on study of stimuli and sensation,Libet, B., Wright, E. W., Feinstein, B., and Pearl, D. (1979). Subjective referral of the timing for a conscious sensory experience: A functional role for the somatosensory specific projection system in man. "Brain", 102(1):193-224.] was found bizarre by some commentators, including
Patricia Churchland, [Churchland, P. S. (1981). On the alleged backwards referral of experiences and its relevance to the mind-body problem. "Philosophy of Science", 48:165-181.] due to the apparent idea of backward causation. Libet [Libet, B. (1981). The experimental evidence for subjective referral of a sensory experience backwards in time: Reply to P. S. Churchland. "Philosophy of Science", 48:181-197.] argued that data suggested that we retrospectively "antedate" the beginning of a sensation to the moment of the primary neuronal response. People interpreted Libet's work on stimulus and sensation in a number of different ways. John Eccles [Eccles, J.C. (1985). Mental summation: The timing of voluntary intentions by cortical activity. "Behavioral and Brain Sciences", 8:542-543.] presented Libet's work as suggesting a backward step in time made by a non-physical mind. Edoardo Bisiach (1988) [Bisiach, E. (1988). The (haunted) brain and consciousness. In (A. Marcel and E. Bisiach, eds) "Consciousness in Contemporary Science". Oxford University Press, ISBN 0-19852-237-1.] described Eccles as tendentious, but commented:
"This is indeed the conclusion that the authors (Libet, et al.) themselves seem to be willing to force upon the reader. [...] They dispute an alternative explanation, suggested by Mackay in a discussion with Libet (1979, p. 219) to the effect that 'the subjective referral backwards in time may be due to an illusory judgment made by the subject when he reports the timings', and more significant, Libet, et al. (1979, p. 220) hint at 'serious though not insurmountable difficulties' for the identity theory (of mind and matter) caused by their data."
Libet later concludedLibet, B. (2004). "Mind time: The temporal factor in consciousness", Cambridge, MA: Harvard University Press. ISBN 0-67401-320-4.] that there appeared to be no "neural mechanism" that could be viewed as directly mediating or accounting for the subjective sensory referrals backward in time [emphasis Libet's] . Libet postulated that the primary evoked potential (EP) serves as a "time marker". The EP is a sharp positive potential appearing in the appropriate sensory region of the brain about 25 milliseconds after a skin stimulus. Libet's experiments demonstrated that there is an automatic subjective referral of the conscious experience backwards in time to this time marker. The skin sensation does not enter our conscious awareness until about 500 milliseconds after the skin stimulus, but we subjectively feel that the sensation occurred at the time of the stimulus.
For Libet, these subjective referrals would appear to be purely a mental function with no corresponding neural basis in the brain. Indeed this suggestion can be more broadly generalized:
"The transformation from neuronal patterns to a subjective representation would appear to develop in a mental sphere that has emerged from that neuronal pattern. [...] My view of mental subjective function is that it is an emergent property of appropriate brain functions. The conscious mental cannot exist without the brain processes that give rise to it. However, having emerged from brain activities as a unique 'property' of that physical system, the mental can exhibit phenomena not evident in the neural brain that produced it." [Libet, B. (2004). op. cit. pp. 86-87.]
Conscious Mental Field Theory
In the later part of his career, Libet proposed a theory of the "conscious mental field" (CMF) [Libet, B. (2004). op. cit., pp. 157-184.] to explain how the mental arises from the physical brain. The two main motivations prompting this proposal were: (1) the phenomenon of the unity of subjective conscious experience and (2) the phenomenon that conscious mental function appears to influence nerve cell activity.
Regarding the unity of conscious experience, it was increasingly evident to Libet that many functions of the cortex are localized, even to a microscopic level in a region of the brain, and yet the conscious experiences related to these areas are integrated and unified. We do not experience an infinite array of individual events but rather a unitary integrated consciousness, for example, with no gaps in spatial and colored images. For Libet, some unifying process or phenomenon likely mediates the transformation of localized, particularized neuronal representations into our unified conscious experience. This process seemed to be best accountable in a "mental sphere" that appears to emerge from the neural events, namely, the "conscious mental field".
The CMF is the mediator between the physical activities of nerve cells and the emergence of subjective experience. Thus the CMF is the entity in which unified subjective experience is present and provides the causal ability to affect or alter some neuronal functions. Libet proposed the CMF as a "property" of an emergent phenomenon of the brain; it does not exist without the brain but emerges from the appropriate system of neural activity.
To test the proposed causal ability of the CMF to affect or alter neuronal functions, Libet proposed an experimental design, [Libet, B. (2004). op. cit., pp. 172-179.] which would surgically isolate a slab of cerebral cortex (in a patient for whom such a procedure was therapeutically required). If electrical stimulation of the isolated cortex can elicit an introspective report by the subject, the CMF must be able to activate appropriate cerebral areas in order to produce the verbal report. This result would demonstrate directly that a conscious mental field could affect neuronal functions in a way that would account for the activity of the conscious will.
Dr. Robert W. Doty, professor of Neurobiology and Anatomy at the
University of Rochester: [Perlman, D. (2007). "Benjamin Libet - neurophysiologist studied the nature of free will", "The San Francisco Chronicle", August 18, 2007. [http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/08/18/BANFRK1TG.DTL Obituary] ]
Dr. Susan J. Blackmore, visiting lecturer at the
University of the West of England, Bristol: [Blackmore, S. (2007). "Mind over matter? Many philosophers and scientists have argued that free will is an illusion. Unlike all of them, Benjamin Libet found a way to test it.", commentary at "Guardian Unlimited", August 28, 2007. [http://commentisfree.guardian.co.uk/sue_blackmore/2007/08/mind_over_matter.html Commentary] .] cquote|Many philosophers and scientists have argued that free will is an illusion. Unlike all of them, Benjamin Libet found a way to test it.
*Benjamin Libet, Anthony Freeman, and J. K. B. Sutherland, Editors, "The volitional brain: Towards a neuroscience of free will".
Imprint Academic, 1999. ISBN 0-907845-50-9.
*Benjamin Libet, "Mind time: The temporal factor in consciousness", Perspectives in Cognitive Neuroscience.
Harvard University Press, 2004. ISBN 0-674-01320-4.
Daniel C. Dennett, " Freedom Evolves". Allen Lane, 2003. ISBN 0-14028-389-7.
*Michael Pauen (2004). Does Free Will Arise Freely?, "Scientific American Mind", 14(1).
* [http://www.latimes.com/news/printedition/asection/la-me-libet27aug27,1,3692178.story?coll=la-news-a_section&ctrack=1&cset=true Obituary] from "The Los Angeles Times", August 27, 2007.
* [http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2007/08/18/BANFRK1TG.DTL Obituary] from "The San Francisco Chronicle", August 18, 2007.
* [http://www.davisenterprise.com/articles/2007/07/30/obituaries/160obit0.txt Obituary] from "The Davis Enterprise", July 27, 2007.
* [http://www.uni-klu.ac.at/psy/cognition/fr_nobel_E2003.html?http://www.uni-klu.ac.at/psy/cognition/nobelpreis_2003E.html Virtual Nobel Prize in Psychology] Including video of Libet's acceptance speech.
* [http://www.consciousentities.com/libet.htm Short account of Libet's experiments and theory]
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