|Born||(1916-04-12)April 12, 1916|
|Died||July 23, 2007(2007-07-23) (aged 91)|
|Alma mater||University of Chicago|
|Institutions||University of California, San Francisco|
|Doctoral advisor||Ralph W. Gerard|
Benjamin Libet (; April 12, 1916, Chicago, Illinois – July 23, 2007, Davis, California) was a pioneering scientist in the field of human consciousness. Libet was a researcher in the physiology department of the University of California, San Francisco. 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".
Libet graduated from the University of Chicago, where he studied with Ralph Gerard.
In the 1970s, Libet was involved in research into neural activity and sensation thresholds. His initial investigations involved determining how much activation at specific sites in the brain was required to trigger artificial somatic sensations, relying on routine psychophysical procedures. This work soon crossed into an investigation into human consciousness; his most famous experiment was meant to demonstrate that the unconscious electrical processes in the brain called Bereitschaftspotential (or readiness potential) discovered by Lüder Deecke and Hans Helmut Kornhuber in 1964 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. The experiment has caused controversy not only because it challenges the belief in free will, but also due to a criticism of its implicit assumptions. It has also inspired further study of the neuroscience of free will.
Volitional acts and readiness potential
To gauge the relation 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. For this, Libet required specialized pieces of equipment.
The first of these was the cathode rayoscilloscope, an instrument typically used to graph the amplitude and frequency of electrical signals. 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 (EEG). The EEG uses small electrodes placed 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 error of 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.
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 activity within the brain - the change in EEG signals reflecting this buildup came to be called Bereitschaftspotential or readiness potential. 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 7 seconds before the subject was aware of their decision.
Implications of Libet's experiments
Libet's experiments suggest to some that unconscious processes in the brain are the true initiator of volitional acts, and free will therefore plays no part in their initiation. If unconscious brain processes have already taken steps to initiate an action before consciousness is aware of any desire to perform it, the causal role of consciousness in volition is all but eliminated, according to this interpretation. For instance, Susan Blackmore's interpretation is "that conscious experience takes some time to build up and is much too slow to be responsible for making things happen."
Libet finds that conscious volition is exercised in the form of 'the power of veto' (sometimes called "free won't"); the idea that conscious acquiescence is required to allow the unconscious buildup of the readiness potential to be actualized as a movement. While consciousness plays no part in the instigation of volitional acts, Libet suggested that it may still have a part to play in suppressing or withholding 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 20 milliseconds prior to an act are occupied by the activation of the spinalmotor neurones by the primary motor cortex, and the margin of error indicated by tests utilizing the oscillator must also be considered).
Libet's experiments have received support from other research related to the Neuroscience of free will.
Reactions by dualist philosophers
It has been suggested that consciousness is merely a side-effect of neuronal functions, an epiphenomenon of 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 dualist 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.
A more general criticism from a dualist-interactionist perspective has been raised by Alexander Batthyany who points out that Libet asked his subjects to merely "let the urge [to move] appear on its own at any time without any pre-planning or concentration on when to act". According to Batthyany, neither reductionist nor non-reductionist agency theories claim that urges which appear on their own are suitable examples of (allegedly) consciously caused events because one cannot passively wait for an urge to occur while at the same time being the one who is consciously bringing it about. Libet's results thus cannot be interpreted to provide empirical evidence in favour of agency reductionism, since non-reductionist theories, even including dualist interactionism, would predict the very same experimental results.
Daniel Dennett argues that no clear conclusion about volition can be derived from Libet's experiment because of ambiguities in the timings of the different events involved. Libet tells when the readiness potential occurs objectively, using electrodes, but relies on the subject reporting the position of the hand of a clock to determine when the conscious decision was made. As Dennett points out, this is only a report of where it seems to the subject that various things come together, not of the objective time at which they actually occur.
Suppose Libet knows that your readiness potential peaked at millisecond 6,810 of the experimental trial, and the clock dot was straight down (which is what you reported you saw) at millisecond 7,005. How many milliseconds should he have to add to this number to get the time you were conscious of it? The light gets from your clock face to your eyeball almost instantaneously, but the path of the signals from retina through lateral geniculate nucleus to striate cortex takes 5 to 10 milliseconds — a paltry fraction of the 300 milliseconds offset, but how much longer does it take them to get to you. (Or are you located in the striate cortex?) The visual signals have to be processed before they arrive at wherever they need to arrive for you to make a conscious decision of simultaneity. Libet's method presupposes, in short, that we can locate the intersection of two trajectories:
• the rising-to-consciousness of signals representing the decision to flick
• the rising to consciousness of signals representing successive clock-face orientations
so that these events occur side-by-side as it were in place where their simultaneity can be noted.
Subjective backward referral or "antedating" of sensory experience
Libet's early theory, resting on study of stimuli and sensation, was found bizarre by some commentators, including Patricia Churchland, due to the apparent idea of backward causation. Libet 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 presented Libet's work as suggesting a backward step in time made by a non-physical mind. Edoardo Bisiach (1988) 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 concluded 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.
Conscious mental field theory
In the later part of his career, Libet proposed a theory of the conscious mental field (CMF) 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. This proposal is related to electromagnetic theories of consciousness.
To test the proposed causal ability of the CMF to affect or alter neuronal functions, Libet proposed an experimental design, 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. Detailed description of the proposed experimental test is as follows:
A small slab of sensory cortex (subserving any modality) is neuronally isolated but kept viable by making all the cortical cuts subpially. This allows the blood vessels in the pia to project into the isolated slab and provide blood flow from the arterial branches that dip vertically into the cortex. The prediction is that electrical stimulation of the sensory slab will produce a subjective response reportable by the subject. That is, activity in the isolated slab can contribute by producing its own portion of the CMF.
Libet further elaborated on CMF:
The CMF is not a Cartesian dualistic phenomenon; it is not separable from the brain. Rather, it is proposed to be a localizable system property produced by appropriate neuronal activities, and it cannot exist without them. Again, it is not a ‘‘ghost’’ in the machine. But, as a system produced by billions of nerve cell actions, it can have properties not directly predictable from these neuronal activities. It is a non-physical phenomenon, like the subjective experience that it represents. The process by which the CMF arises from its contributing elements is not describable. It must simply be regarded as a new fundamental ‘‘given’’ phenomenon in nature, which is different from other fundamental ‘‘givens,’’ like gravity or electromagnetism.
Dr. Robert W. Doty, professor of Neurobiology and Anatomy at the University of Rochester:
|“||Benjamin Libet's discoveries are of extraordinary interest. His is almost the only approach yet to yield any credible evidence of how conscious awareness is produced by the brain. Libet's work is unique, and speaks to questions asked by all humankind.||”|
Dr. Susan J. Blackmore, visiting lecturer at the University of the West of England, Bristol:
|“||Many philosophers and scientists have argued that free will is an illusion. Unlike all of them, Benjamin Libet found a way to test it.||”|
- ^"2003 Virtual Nobel Prize Acceptance Speech". cognition.uni-klu.ac.at. Archived from the original on 2011-07-06. Retrieved 2010-04-09.
- ^Virtual Nobel Prize web site.[dead link] This prize has no relation to the Nobel Prize of the Swedish Nobel Foundation.
- ^Squire, Larry R. (ed.). "The History of Neuroscience in Autobiography".
- ^Kornhuber H.H.; Deecke L. (1965). "Hirnpotentialänderungen bei Willkürbewegungen und passiven Bewegungen des Menschen: Bereitschaftspotential und reafferente Potentiale". Pflügers Arch. 284: 1–17. doi:10.1007/BF00412364.
- ^Libet, Benjamin; Gleason, Curtis A.; Wright, Elwood W.; Pearl, Dennis 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. doi:10.1093/brain/106.3.623. PMID 6640273.
- ^Libet, Benjamin (1985). "Unconscious Cerebral Initiative and the Role of Conscious Will in Voluntary Action". The Behavioral and Brain Sciences. 8: 529–566. doi:10.1017/s0140525x00044903.
- ^Keim, Brandon (April 13, 2008). "Brain Scanners Can See Your Decisions Before You Make Them". Wired News. CondéNet. Retrieved 2008-04-13. and Chun Siong Soon; Marcel Brass; Hans-Jochen Heinze; John-Dylan Haynes (April 13, 2008). "Unconscious determinants of free decisions in the human brain (Abstract)". Nature Neuroscience. Nature Publishing Group. 11 (5): 543–5. doi:10.1038/nn.2112. PMID 18408715. Retrieved 2008-04-13.
- ^Wegner D., 2002. The Illusion of Conscious Will. Cambridge, MA: MIT Press.
- ^"Archived copy". Archived from the original on 2016-03-21. Retrieved 2009-12-20.
- ^V. S. Ramachandran, New Scientist, 5 Sep 1998, p. 35
- ^"Archived copy". Archived from the original on 2012-11-08. Retrieved 2011-03-01. Interview with V. S. Ramachandran, The Hindu
- ^Velmans, Max (2000). Understanding Consciousness. London: Routledge. pp. 35–37. ISBN 0-415-22492-6.
- ^Batthyany, Alexander: Mental Causation and Free Will after Libet and Soon: Reclaiming Conscious Agency. In Batthyany und Avshalom Elitzur. Irreducibly Conscious. Selected Papers on Consciousness, Universitätsverlag Winter Heidelberg 2009, p.135ff.
- ^Libet B.; Wright E.; W., Gleason (1983). "Readiness potentials preceding unrestricted spontaneous pre-planned voluntary acts". Electroencephalographic and Clinical Neurophysiology. 54: 322–325. doi:10.1016/0013-4694(82)90181-x.
- ^"Freedom Evolves" by Daniel Dennett, p. 231
- ^Dennett, D. The Self as Responding and Responsible ArtefactArchived July 1, 2016, at the Wayback Machine.
- ^ abcdLibet, Benjamin; Wright Jr., Elwood W.; Feinstein, Bertram; Pearl, Dennis K. (1979). "Subjective Referral of the Timing for a Conscious Sensory Experience - A Functional Role for the Somatosensory Specific Projection System in Man". Brain. 102: 193–224. doi:10.1093/brain/102.1.193.
- ^Churchland, Patricia Smith (Jun 1981). "On the Alleged Backwards Referral of Experiences and its Relevance to the Mind-Body Problem". Philosophy of Science. 48 (2): 165–181. doi:10.1086/288989.
- ^Libet, Benjamin (1981). "The Experimental Evidence for Subjective Referral of a Sensory Experience Backwards in Time: Reply to P.S. Churchland". Philosophy of Science. 48: 182–197. doi:10.1086/288990.
- ^Eccles J.C. (1985). "Mental summation: The timing of voluntary intentions by cortical activity". Behavioral and Brain Sciences. 8: 542–543. doi:10.1017/s0140525x00044952.
- ^Bisiach, E. (1988). The (haunted) brain and consciousness. In (A. Marcel and E. Bisiach, eds) Consciousness in Contemporary Science. Oxford University Press, ISBN 0-19-852237-1.
- ^Libet, Benjamin (2004). Mind Time - The Temporal Factor in Consciousness. Harvard University Press. ISBN 0-674-01320-4.
- ^Libet, B. (2004). op. cit. pp. 86-87.
- ^Libet, B. (2004). op. cit., pp. 157-184.
- ^Libet, B. (2004). op. cit., pp. 172-179.
- ^Libet, Benjamin (2006). "Reflections on the Interaction of the Mind and Brain"(PDF). Progress in Neurobiology. 78: 322–326. doi:10.1016/j.pneurobio.2006.02.003. Archived from the original(PDF) on 2011-07-18.
- ^ abLibet, B. (2006). op. cit., p. 324. PDFArchived July 18, 2011, at the Wayback Machine.
- ^Perlman, D. (2007). "Benjamin Libet - neurophysiologist studied the nature of free will", The San Francisco Chronicle, August 18, 2007. ObituaryArchived November 16, 2011, at the Wayback Machine.
- ^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. CommentaryArchived May 18, 2008, at the Wayback Machine..
- 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 0-14-028389-7.
- Pauen Michael (2004). "Does Free Will Arise Freely?". Scientific American Mind. 14: 1.
- In his Virtual Nobel acceptance speech, Libet summarized his life's research and highlighted his work on conscious volitional acts and the antedating of sensory awareness.
1 (-500 ms) EEG measures Readiness potential
2 (-200 ms) Person notes the position of the dot when decides
3 ( 0 ms) Act
by Ted Honderich
What are the differences between the philosophy of consciousness and the science of it? That general question as well as the particular one about mind and brain is raised is by the neurophysiological research of Benjamin Libet et al. This reseach has lately been given a kind of attention by Daniel Dennett and Marcel Kinsbourne in The Nature of Consciousness: Philosophical Debates (1997) edited by Ned Block, Owen Flanagan and Guven Guzeldere. It has also surfaced in The Volitional Brain: Towards a Neuroscience of Free Will (1999), edited by Libet, Anthony Freeman & Keith Sutherland, and John Searle's Royal Institute of Philosophy Annual Lecture. The research finding is to the effect that a conscious event happens before the relevant brain event. Thus It is used to refute many theories that identify the two events, and also a common theory of psychoneural lawlike connection that supposes the events are simultaneous. Karl Popper and John Eccles, in The Self and Its Brain, depended on the research to argue for a certain dualistic picture, in fact a "trialistic" picture, since it includes the brain, and mental or conscious events, and something certainly distinct from them, the Self-Conscious Mind. The research, naturally enough, also helps out with determinism and freedom. It serves to save our free will or power of origination. Very useful research then -- if OK. The following paper from The Journal of Theoretical Biology, first published undered the title 'The Time of a Conscious Sensory Experience and Mind-Brain Theories', looks into the the question. It was attended to confidently by Professor Libet -- Subjective Antedating of a Sensory Experience and Mind-Brain Theories: Reply to Honderich. The reply in turn got a response -- Honderich's 'Is The Mind Ahead of the Brain -- Rejoinder to Benjamin Libet'.
Abstract: Libet et al., like Popper & Eccles, have the view that some single hypothesis about the time of occurrence of a conscious sensory experience has certain consequences for various mind-brain theories. The view involves a fundamental inconsistency, which may cast doubt on certain experimental findings. It involves two hypotheses rather than one. The preferable hypothesis, which itself is doubtful, has been thought to have mind-brain consequences principally because it has not been distinguished from the other and different hypothesis. The different hypothesis, which in fact does have the given consequences, is entirely unacceptable.
Libet (1978, 1981) and Libet et al. (1979) claim to advance a single hypothesis about the time of a conscious sensory experience in relation to the time when there occurs a certain physical condition, "neuronal adequacy" for the experience. This can be and has been taken to throw doubt on theories that identify mind with brain (Feigl, 1960; Armstrong, 1968; Davidson, 1980) and on a dualistic theory of psychophysical lawlike correlation (Honderich, 198l a,b).
Popper & Eccles (1977) use the contentions of Libet et al. as evidence for a greatly different dualism, one which denies psychophysical lawlike correlation and asserts a kind of surveillance and control of the brain by "the self-conscious mind", despite some acting of the brain on "the self-conscious mind".
The contentions derive in part from many previous findings (Libet, 1965, 1966, 1973; Libet et al., 1964, 1967, 1972, 1975).
The aims of the present paper are first, fully to demonstrate an inconsistency involving two hypotheses, and to note a conclusion about experimental findings that follows from this; second, to raise a considerable doubt about the preferable hypothesis; third, to show why it has been supposed that the hypothesis is evidence against an identity theory of mind and brain and the theory of psychophysical lawlike correlation, and evidence for "the self-conscious mind"; fourth, to show why the hypothesis, if true, provides no evidence in either case, and fifth, to show that a different hypothesis, which if tenable would provide the evidence, is wholly untenable.
What is maintained by the authors derives from two sets of findings (1.1 to 1.3 and 2.1 to 2.4 below) pertaining to neuronal activity and to the temporal order of a subject’s pairs of sensory experiences, and also from (3) findings having to do with a "primary" evoked potential in somatosensory cortex. Very roughly, findings 1.1 to 1.3 are to the effect that there is a certain delay in sensory experience, but finding 2.1 apparently conflicts with this. Finding 3, with the help of 2.2 to 2.4, is used to give an explanation of the apparent conflict.
(1.1) Experiments on human subjects, with their agreement, and in conjunction with surgical treatment, are said to show that after trains of stimulation are applied directly by inserted electrodes to postcentral cortex there is a considerable delay, up to about 0.5 sec after the beginning of the train, before cortical activity reaches "neuronal adequacy" for eliciting a conscious sensory experience. What is said to be delayed, to repeat, is precisely the physical condition of "neuronal adequacy", as distinct from the experience itself, whatever is to be said of its time.
(1.2) There is said to be a very similar delay with direct subcortical stimulation trains.
(1.3) There is also said to be a very similar delay with single pulses of peripheral stimulation, say a single pulse to the skin of the hand.
(2.1) If a single-pulse stimulus to skin at just above threshold level is applied after (say 0.2 or 0.3 sec) the beginning of a stimulus train direct to somatosensory cortex, it would be expected that subjects would report that conscious experience for the skin stimulus began after conscious experience for the cortical stimulus. This would be expected on the basis of 1.1 and 1.3. Reports of tests, however, were predominantly of experience for the skin stimulus beginning before experience for the cortical stimulus.
(2.2) There is no such surprising order of conscious experiences reported with subcortical stimulation. If a skin stimulus is applied after the beginning of a subcortical stimulus train, subjects report that sensory experience owed to the skin stimulus began after sensory experience owed to the subcortical stimulation.
(2.3) Similarly, if the beginning of subcortical stimulation is simultaneous with a skin stimulus, subjects report simultaneous sensory experiences.
(2.4) Similarly again, if a skin stimulus is applied before the beginning of subcortical stimulation, subjects report that experience of the skin stimulus came before experience owed to the subcortical stimulation.
(3) Peripheral stimuli and subcortical stimuli quickly elicit a relatively localized "primary" evoked potential in somatosensory cortex, owed importantly to the specific (lemniscal) projection system. The onset of this potential, the arrival of a fast projection volley, is only about 15 msec after a stimulus to the hand. However, a stimulus train applied direct to somatosensory cortex does not elicit a similar type of response.
The first set of findings (1.1 to 1.3) are to the effect that "neuronal adequacy" for any sensory experience is achieved only after a certain delay. Finding 2.1, which is crucial, appears to conflict with this. That is, the ordering by subjects of their experiences (experience-owed-to-skin-stimulus before experience-owed-to-cortical-stimulation) suggests that the experience of the peripheral stimulus occurs considerably sooner than about 0.5 sec later. There is thus a contrast with cortical stimulation. However, there is no such contrast evident with subcortical stimulation, as indicated by 2.2, 2.3 and 2.4.
The two sets of findings, together with (3) the finding of a "primary" evoked potential, are said to issue in an hypothesis about the timing of conscious sensory experiences owed to peripheral and subcortical stimulation, and also an explanation of what is postulated in the hypothesis. The explanation of what is postulated in the hypothesis has to do in part with the specific (lemniscal) projection system and the "primary" evoked potential.
1. What Is The Hypothesis?
Some statements made by the authors are of one hypothesis, or suggest it. Other statements are of, or suggest, a different hypothesis. It will be necessary, partly for fairness, to quote extensively. Unless otherwise indicated, all page references in what follows are to Libet et al. (1979).
The following statements are somehow to the effect that an experience occurs at a certain time but is "antedated" to the time of certain earlier events, the "primary" evoked potential.
[a] "[The hypothesis] postulates (a) the existence of a subjective referral of the timing for a sensory experience, and (b) a role for the specific (lemniscal) projection system in mediating such a subjective referral of timing." (p. 193)
[b] "(1) Some neuronal process associated with the early or primary evoked response, of SI (somatosensory) cortex to a skin stimulus, is postulated to serve as a ‘time-marker’. (2) There is an automatic subjective referral of the conscious experience backwards in time to this time-marker, after the delayed neuronal adequacy at cerebral levels has been achieved (see Fig. 2). The sensory experience would be ‘antedated’ from the actual delayed time at which the neuronal state became adequate to elicit it; and the experience would appear subjectively to occur with no significant delay from the arrival of the fast projection volley." (pp. 20 1-2. Cf. Libet, 1978, p. 75)
[c] "The results obtained in these experiments provide specific support for our present proposal, that is, for the existence of a subjective temporal referral of a sensory experience by which the subjective timing is retroactively antedated to the time of the primary cortical response (elicited by the lemniscal input)." (p. 217)
[d] "The specific projection system is already regarded as the provider of localized cerebral signals that function in fine spatial discrimination, including the subjective referral of sensory experience in space. Our present hypothesis expands the role for this system to include a function in the temporal dimension. The same cortical responses to specific fast projection inputs would also provide timing signals. They would subserve subjective referral in such a way as to help ‘correct’ the subjective timing (relative to the sensory stimulus), in spite of actual substantial delays in the time to achieve neuronal adequacy for the 'production’ of the conscious sensory experience." (pp. 220-1)
[e] "… for a peripheral sensory input, (a) the primary evoked response of sensory cortex to the specific projection (lemniscal) input is associated with a process that can serve as a ‘time-marker’; and (b) after delayed neuronal adequacy is achieved, there is a subjective referral of the sensory experience backwards in time so as to coincide with this initial ‘time-marker’." (p. 222)
These statements are certainly not all clear, but their burden is the following hypothesis. Such a conscious experience as that of a skin stimulus occurs only when "neuronal adequacy" is achieved, but it is somehow "antedated" of "referred" to an earlier time. That is, despite what is said of the experience’s involving "subjective referral backwards in time", the experience itself occurs only about 0.5 sec after the beginning of stimulation. The experience does not occur at the time to which it is "referred". We can call this the "delay-and-antedating" hypothesis. It is also what is conveyed by the authors, as will be worth noting, in one of their diagrams and several other passages.
FIG. 1. Figure 2 from Libet et al. (1979) p. 201. Reproduced with permission.
[f] Their Fig. 2 (p. 201), reproduced here as Fig. 1, has to do in the main with the "primary" evoked potential. It is said of the figure in part: "Diagram representing the ‘average evoked response’ (AER) recordable on the surface of human primary somatosensory cortex (SI) in relation to the.., hypothesis on timing of the sensory experience. Below the AER, the first line shows the approximate delay in achieving the state of ‘neuronal adequacy’ that appears (on the basis of other evidence) to be necessary for eliciting the sensory experience. The second line shows the postulated retroactive referral of the subjective timing of the experience, from the time of neuronal adequacy’ backwards to some time associated with the primary surface-positive component of the evoked potential." (p.201)
Presumably "neuronal adequacy" is not taken as necessary for eliciting what has already happened, earlier in time. The experience, at the later time, is merely "referred" to the earlier.
(g) It is stated (p. 221) that the hypothesis in question "deals with the problem of a substantial neuronal time delay apparently required for the ‘encoding’ of a conscious sensory experience, by introducing the concept of a subjective referral of sensory experience in the temporal dimension."
[h] Finally, there is what is said of a quite different idea, owed to Donald MacKay, and more particularly of an amendment of it. MacKay’s idea
"accepts our proposal that there is substantial delay in achieving neuronal adequacy with all inputs, peripheral or central; but it would argue that, in those cases where there is apparent antedating of the subjective timings of the sensory experience, the subjective referral backwards in time may be due to an illusory judgement made by the subject when he reports the timings…For example, it could be argued that during the recall process, cerebral mechanisms might ‘read back’ via some memory device to the primary evoked response and now construe the timing of the experience to have occurred earlier than it in fact did occur." (pp. 219-20)
An amendment of this idea is contemplated, one that is said in fact to turn it into the hypothesis of Libet et al. "…if any ‘read back’ to the primary timing signal does occur, it would seem simpler to assume that this takes place at the time when neuronal adequacy for the experience is first achieved, when the ‘memory’ of the timing signal would be fresher; such a process would then produce the retroactive subjective referral we have postulated." (p. 220)
The burden of all that has been reported here so far, then, is that such a conscious experience as that of a skin stimulus occurs only at the time when "neuronal adequacy" has been achieved, about 0.5 sec after the beginning of stimulation, despite what is said of "antedating".
(I) On the other hand the authors’ Fig. 1 (p. 199), reproduced here as Fig. 2, pertaining to the crucial finding 2.1, about subjects’ surprising ordering of experiences, could hardly be clearer in its quite different import.
FIG. 2. Figure I from Libel et al. (1979) p. 199. Reproduced with permission.
The S-experience (experience of a skin stimulus) is specified as "actually before C-experience" (experience owed to cortical stimulation). It is shown as occurring only a few msec after the stimulus-pulse (S-pulse). In the note to the diagram it is said: "If S were followed by a... delay of 500 msec of cortical activity before neuronal adequacy’ is achieved, initiation of S-experience might.., have been expected to be delayed until 700 msec of C [stimulus train to somatosensory cortex] had elapsed. In fact, S-experience was reported to appear subjectively before C-experience..." The note, although perhaps less definite in its intention, thus accords with the diagram.
(II) What the crucial finding 2.1 established, it is said and implied, is that conscious experience of certain stimulation did not occur at the time of "neuronal adequacy". "If the subjective experience were to occur at the same time as the achievement of neuronal adequacy in the case of either stimulus, one would expect the subject to report that the conscious sensory experience for the C stimulus began before that for the threshold S pulse (Fig. 1) ….However, the pooled reports were predominantly those of sensory experience for the C (cortical) stimulus beginning after, not before, that for a delayed threshold S pulse.. (pp. 199-200)
(III) It is stated, of the crucial finding 2.1, that "the subjective experience of the skin stimulus occurs relatively quickly after the delivery of the S pulse, rather than after the expected delay of up to about 500 msec for development of neuronal adequacy following the S input." (p. 200) That is, the skin-stimulus experience itself occurs earlier, rather than after the expected delay.
(IV) It is flatly stated that "subjective experience of a peripherally-induced sensation is found to appear without the substantial delay found for the experience of a cortically induced sensation." (p. 222)
(V) Very importantly, it is noted that findings 1.1, 1.2 and 1.3 above, about delay in achieving "neuronal adequacy", are not to be taken in a natural way, as asserting or implying that the experiences in question are subject to the given delay. That is left open. "The two timings, for subjective experience vs. neuronal adequacy, might not necessarily be identical." (p. 200) (Cf. Libet, 1978, p. 75)
(VI) It is stated that the hypothesis in question introduces "an asynchrony or discrepancy between the timing of a subjective experience and the time when the state of ‘neuronal adequacy’ associated with the experience is achieved." (p. 221)
(VII) It is stated that there is "a dissociation between the timings of the corresponding ‘mental’ and ‘physical’ events." (p. 222)
The burden of these statements (I to VII), although some phrases might be taken as ambiguous, is that a conscious experience occurs earlier rather than later, i.e. before about 0.5 sec after the beginning of stimulation rather than about 0.5 sec after the beginning of stimulation. We can call this the "no-delay hypothesis".
As remarked, Eccles uses the hypothesis of Libet et al., whatever it is, to argue for the existence of "the self-conscious mind". Eccles states the hypothesis a number of times. Compare (i) and (ii) with (iii) and (iv).
(i) "The experiments of Libet on the human brain.., show that direct stimulation of the somaesthetic cortex results in a conscious sensory experience after a delay as long as 0.5 sec ... although there is this delay in experiencing the peripheral stimulus, it is actually judged to be much earlier, at about the time of cortical arrival of the afferent input.... This antedating process does not seem to be explicable by any neurophysiological process. Presumably it is a strategy that has been learnt by the self-conscious mind.., the antedating of the sensory experience is attributable to the ability of the self-conscious mind to make slight temporal adjustments, i.e. to play tricks with time..." (Popper & Eccles, 1977, p. 364)
(ii) "...Libet developed a most interesting hypothesis, namely that, though a weak single (SS) just threshold single skin stimulus requires up to 0.5 sec of cortical activity before it can be experienced, in the experiencing process it is antedated by being referred in time to the initial evoked response of the cortex (Popper & Eccles, 1977, p. 259)
(iii) "The ... experimental design tested the supposition that a just-threshold single skin stimulus (SS) was effective in producing a conscious sensation after the same incubation period. . . as a just-threshold train of cortical stimulation (CS), which is as long as 0.5 sec. If that were so, when the SS was applied during the minimal CS train, the SS should be experienced after the CS; but it was usually experienced before!" (Popper & Eccles, 1977, p. 257)
(iv) "There can be a temporal discrepancy between neural events and the experiences of the self-conscious mind." (Popper & Eccles, 1977, p. 362)
Passages i and ii are to the effect that the experience is later (i.e. about 0.5 sec after the beginning of stimulation) and is somehow antedated.
Passages iii and iv say otherwise: the experience is earlier (i.e. not nearly
so much as about 0.5 sec after the beginning of stimulation).
There is the same inconsistency suggested by two parts of a diagram (Popper & Eccles, 1977, Fig. E2-3, p. 258) reproduced here as Fig. 3.
In part D, which has to do with tests whose details are not all relevant to the present point, a just-threshold single skin stimulus (SS2) is shown as giving rise to an experience whose "actual time" is about 0.6 sec later. There is "antedating", somehow involving "ER [evoked response] time", which is not further explained. However, in part B of the diagram, which pertains to the crucial finding (2.1) above, the SS experience is shown as actually occurring earlier rather than later. (CS is cortical stimulation.) The diagram specifies "SS experience before CS [experience]".
The inconsistency in the work of Libet et al. and Eccles, and in other reports of the work of Libet et al. (Cotman & McGaugh, 1980, pp. 806-7), is worth noting for itself. Not both the hypothesis that there is delay in experience but "antedating", the "delay-and-antedating hypothesis", and the hypothesis that there is no such delay, the "no-delay hypothesis", can be true.
That conclusion carries corollaries, of course. One is that if the delay-and-antedating hypothesis is true, then if there are putative findings which entail the no-delay hypothesis, those findings are false. The like corollary has to do with taking the no-delay hypothesis to be true. Any putative findings which entail the delay-and-antedating hypothesis are then false. It is not within my competence, however, to examine these findings in detail, notably the crucial finding 2.1.
If the inconsistency itself and hence the fact that there are two hypotheses in question are important, partly for a further conclusion of mine, it can confidently be said that it is the delay-and-antedating hypothesis to which the authors, at bottom, are committed. It is the preferable hypothesis. In my view, the authors’ failure to notice, despite their commitment, that there are two inconsistent hypotheses in question has had an effect on their understanding of the consequences of the delay-and-antedating hypothesis, consequences for the mind-brain issue. It is my suggestion that failing to notice the inconsistency, and hence that there are two hypotheses on hand, has been significant in enabling Libet et al. and Eccles and Popper (Popper & Eccles, 1977, pp. 531, 565), to come to certain conclusions about the mind-brain identity theory, the dualistic theory of psychophysical lawlike correlation, and "the self-conscious mind".
2. Is The "Delay-and-Antedating Hypothesis" Acceptable?
To consider another matter first, however, how are we to understand the preferable hypothesis? It is in part that an experience is somehow "antedated" or "retroactively antedated" or "referred back" to a "time-marker". What does that come to? Very little account of the supposed phenomenon itself, as distinct from what is supposed to explain it, is given.
The central idea appears to be that a subject has a conscious sensory experience including or involving or accompanied by the belief or impression that it is not then happening. He may have a conscious sensory experience, reportable as "sensation in right hand", which conscious sensory experience occurs about 0.5 sec after stimulation, and after another sensory experience. His experience of the right-hand sensation includes or involves or is accompanied by the belief or impression that the sensation occurred significantly earlier than about 0.5 sec after stimulation, before the other sensory experience. There seem to be very great difficulties in this idea - which, it must be remembered, is crucially different from Mackay’s simpler idea.
It is true, surely, that the actual having of any conscious sensory experience includes or involves or is accompanied by the belief or impression that the experience is present. That is, it is happening now. Further, it might be said that there is the belief or impression that the experience is after another experience, immediately prior in time. The having of any experience, that is, somehow brings in a belief or impression as to a temporal property (presentness) and perhaps also a temporal relation (after another experience) (Honderich, 1977).
But then the supposed phenomenon of a conscious sensory experience we have been considering, the phenomenon of "antedating" or "referring back’’ , involves imputing something very like certain self-contradictory beliefs to subjects. It involves, more precisely, imputing something like simultaneously-held, fully explicit self-contradictory beliefs to subjects, as distinct from the common sort of self-contradiction where the conflicting beliefs are not brought together. The supposed phenomenon, by way of a kind of summary description, is the phenomenon of a conscious experience which a subject might describe in the words "present-sensation past" or "now-sensation then", or perhaps "later-sensation earlier".
Libet et al. say that the processes in referral or antedating are to be regarded as "unconscious and ‘automatic’ in nature and… not distinguishable by the subject" (p. 220). What processes are there in question is not entirely clear. However, we cannot choose to regard a conscious sensory experience as something of which the subject is unaware. It seems, to repeat, that a conception of presentness and perhaps of a temporal relation enters into the having of any sensation. Are there really certain experiences such that the involved belief or impression as to presentness is, so to speak, simultaneously denied? On the assumption that a belief or impression of a temporal relation enters into the having of any sensation, can it be that there are certain experiences such that the belief or impression is simultaneously denied?
As illustrated above in [d], it is said that the supposed "antedating" phenomenon is related to something else also owed to the specific (lemniscal) projection system. "…the concept of subjective referral in the spatial dimension, and the discrepancy between subjective and neuronal spatial configurations, has long been recognized and accepted; that is, the spatial form of a subject sensory experience need not be identical with the spatial pattern of the activated cerebral neuronal system that gives rise to this experience." (p. 221) "The newly proposed functional role for the specific projection system would be additional to its known role in spatial referral and discrimination." (p. 222)
In fact, however, there is no relevant analogy whatever between "temporal referral", of the kind with which we have been concerned, and the given discrepancy between (a) spatial experience and (b) neuronal spatial configurations. The latter discrepancy obviously involves no kind of self-contradictory experience, the simultaneous occurrence of contradictory beliefs or impressions.
Libet remarks that hypotheses are the weaker for involving ad hoc assumptions (Libet, 1978, p. 74). It is also said that hypotheses are the weaker for involving "added assumptions" (p. 220). It is my own second and tentative conclusion here that the delay-and-antedating hypothesis is open to objection along these lines. However, it is not within my competence to judge the findings which are put in question, or whose interpretation is put in question, if the delay-and-antedating hypothesis is rejected.
3. Explanation of Mind-Brain Conclusions
To turn now to mind-brain theories, it is said (Libet, 1978, p. 80) that "on the face of it, an apparent lack of synchrony between the ‘mental’ and the ‘physical’ would appear to provide an experimentally-based argument against ‘identity theory’, as the latter is formulated by Feigl, Popper, etc."
However, it is allowed that a certain reply to the argument is possible. The reply is not specified. It is then said that "a temporal dissociation between the mental and the physical events would further strain the concept of psychophysiological parallelism or, if one prefers, of co-occurrence of corresponding mental and neuronal states. It could thus have an impact on the philosophical interpretation of such parallelism or co-occurrence when formulating alternative theories of the mind-brain relationship."
In the later (1979) article it is said, differently, that "dissociation between timings of the corresponding ‘mental’ and ‘physical’ events" raises serious but not insurmountable difficulties for identity theories, but that the dissociation does not contradict "the theory of psychophysical parallelism or correspondence" (pp. 221-2). The seeming change of mind about dualistic theories is not explained.
In a still later article it is said that "the temporal discrepancy creates relative difficulties for identity theory, but.. . these are not insurmountable" (1981, p. 196). It is said, further, that the data are "compatible with the theory of ‘mental’ and ‘physical’ correspondence" (1981, p. 182), and that the data do "introduce a novel experimentally-based feature into our views of psychophysiological correspondence, with some interesting philosophical implications that merit analysis" (1981, p. 183). Further, it is said, without explanation: "What we are discussing is not any denial of correspondence between mental and physical events, but rather the way in which the correspondence is actually manifested" (1981, p. 195).
I am uncertain what to make of what is said, so vaguely, of psychophysical "correspondence". Can such "correspondence" hold between events at different times? Does such "correspondence" require only such a loose connection between the brain and the "self-conscious mind" as posited by Eccles and Popper?
It is clear how the findings as to the timing of conscious experiences may be taken to threaten an identity theory, or the theory of psychophysical lawlike connection, or parallelist theories. Evidently a conscious occurrent or process or experience cannot be identical with a physical event or process if the mental and physical items have different temporal locations or durations. Consider the claim that a conscious experience was identical with the physical process which constituted "neuronal adequacy" for it. The claim must be false if the experience and the processes occurred at different times.
Again, the theory of psychophysical lawlike correlation naturally takes the correlated mental and physical items to be simultaneous. That is part of the theory. Evidently, if the studies of Libet et al. did establish of certain mental and physical items that they were not simultaneous, this would indeed raise a difficulty for the given theory of psychophysical lawlike correlation. The same is true, evidently, of a parallelist theory denying psychophysical lawlike correlation but involving psychophysical simultaneity, the parallelism taken as simply inexplicable or somehow owed to ongoing divine intervention. It is a theory, perhaps, which has no contemporary defenders.
Finally, if there were mental events separate in time from their physical bases, that might be taken as going some way towards supporting the theory of the self-conscious mind. Given what seems to me the obscurity of that theory, I shall not attempt to say more about why that might be true. Eccles’ remark in passage (i) above carries the idea that something plays tricks with time, which thing is the self-conscious mind. (There, admittedly, he is to be taken as speaking of the "trick" of "antedating".)
It is clear, tosum up, that the findings as to the timings of conscious experiences may be taken to have these various consequences if the findings are taken as issuing in the no-delay hypothesis. That is one fact, about which I shall say a word more in a moment. Another fact, to repeat, is that clearly it is the delay-and-antedating hypothesis that is favoured by the authors, despite their inconsistency.
It is my conjecture that the authors, not having clearly distinguished the two quite different hypotheses about timing, have supposed that the studies in question have somehow established the no-delay hypothesis, to the effect that certain mental and physical items are not simultaneous. That, fundamentally, is why they draw their conclusions about the mind-brain relation. However, they must choose one or the other of the two hypotheses, and the one they favour, the delay-and-antedating hypothesis, is not at all to the effect that certain mental and physical items lack simultaneity.
4. Failure of Mind-Brain Conclusions
The experience of the skin stimulus, on the delay-and-antedating hypothesis, is simultaneous with the physical process which is taken to constitute "neuronal adequacy" for it. The experience may indeed be of a strange self-contradictory kind. This in itself, so long as psychophysical simultaneity is preserved, is no problem whatever for either an identity theory, or the theory of psychophysical lawlike correlation, or a theory of psychophysical parallelism. Nor, evidently, is a problem raised by the fact that the experience in one part somehow has reference to an earlier time. That in itself no more makes the experience non-simultaneous with its "neuronally adequate" physical process than does the very different referring feature of an ordinary memory or recall experience make that experience non-simultaneous with its "neuronally adequate" physical process-or time-distortion in an hallucination make that experience non-simultaneous with the related physical process.
5. Failure of "No-Delay Hypothesis"
It may be supposed, I have said, thatboth an identity theory and the theory of psychophysical lawlike correlation would be affected by the no-delay hypothesis, the hypothesis to which the authors do not incline, but which appears to have influenced their thinking. Still, that is not all that is to be said of the hypothesis. It would not only put two things at different times, thereby threatening certain mind-brain theories. The no-delay hypothesis is as follows: "neuronal adequacy" for a certain experience is achieved only about 0.5sec after the beginning of stimulation, but the experience occurs before then.
What is "neuronal adequacy"’? It appears to be a kind of neuronal condition which is a sufficient condition for the emergence of the experience. There are certain quite general philosophical problems here (Honderich, 1982, p. 311f) but it must appear that the no-delay hypothesis is in fact false because self-contradictory: it asserts that something cannot occur before a certain time - before a sufficient condition occurs - and that it does. It is true, as remarked above, that if the studies of Libet et al. did establish of certain mental and physical items that they were not simultaneous, this would raise a difficulty for, say, the theory of psychophysical lawlike correlation. Still, it must be false that a mental item occurs before the physical item on whose later existence it depends.
In fact, in my view, identity theories are open to a conclusive objection of a philosophical kind (Honderich, 1981a, pp. 294-8; 1981b, pp. 430-4) while the dualist theory of psychophysical lawlike correlation is open to no such objection, and is increasingly confirmed by neuroscience. The theory of the self-conscious mind, in my view, faces insurmountable objections of philosophical and neuroscientific kinds. These propositions, about the three mind-brain theories, cannot be defended here.
Professor Libet made a reply, in the next issue of The Journal of Theoretical Biology. My rejoinder to his reply is ''Is the Mind Ahead of the Brain -- Rejoinder to Benjamin Libet'.
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I am most grateful for comments on earlier drafts of this paper by Professors R. Audley, G. Burnstock, D. Colquhoun and P. D. Wall, Drs M. Budd, G. Falk, A. R. Gardner-Medwin and V. Glover, Mr R. E. Rawles, an anonymous referee for The Journal of Theoretical Biology, and colleagues who attended a seminar at the Centre for Neuroscience, University College London. I also thank Professor Libet and Sir John Eccles. Their comments have improved the paper but not changed my mind. Above all I am grateful to Professor J. Z. Young.
Originally published as 'The Time of a Conscious Sensory Experience and Mind-Brain Theories', Journal of Theoretical Biology (1984)110, pp. 115-129
For my view of some philosophy that has something to do with Libet's work, see Mind the Guff -- An Examination of John Searle's Thinking on Consciousness and Freedom.
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