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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 and controversial experiment demonstrates that unconscious electrical processes in the brain (called 'readiness potential') 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
Equipment and methodology
In order to gauge the relationship between unconscious readiness potential ('RP') 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 ray oscilloscope, an instrument typically used to graph the amplitude and frequency of 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. Developed in 1936 by British scientist William Grey Walter and inspired by work performed in 1929 by German psychologist Hans Berger, 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.
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 charge within the brain - this buildup came to be called Bereitschaftspotential or readiness potential.
The implications of Libet's experiments
If unconscious processes in the brain are the true initiator of volitional acts, as Libet's experiments suggest, then 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 himself finds room for free will in the interpretation of his results, but in a massively reduced role to that which we are used to. This is in the form of 'the power of veto', in which conscious acquiescence is required to allow the unconscious buildup of RP to be actualised as a movement. Thus, 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. Certainly, as Libet notes, we have all experienced the withholding from performing an unconscious urge. If it is recalled that 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 neurones by the primary motor cortex, plus the margin of error indicated by tests utilising the oscillator must also be considered). That issue hasn't been solved, however, and studies must continue. For further discussions of Libet's findings with regard to free will, see Michael Pauen, "Does Free Will Arise Freely?" in Scientific American Mind, Volume 14, Number 1: 2004.
It has been suggested that consciousness is merely a side-effect of neuronal functions, an epiphenomenon of brain states. On the face of it, Libet's experiments offer support to this theory; our reports of conscious instigation of our own acts is, arguably, a mistake of retrospection. However, for consciousness to be reduced to brain states, Leibniz's law would have to be observed; that is, for A to be the same as B, the properties of A must be the same as the properties of B.
Phenomenally speaking, the properties of consciousness are entirely unlike the properties of either its neural causes or correlates. Reducing consciousness to being simply a brain state is fraught with philosophical difficulty:
- "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" (Max Velmans, Understanding Consciousness, Routledge, London, 2000: 35-37; emphases author's own).
Furthermore: Does the possibility of those conscious act to be a mistake of retrospection necessarily means that we aren't able to change them? (having in mind the possibility of a hypothetical subjective perception power to alter that act- even with a mistaken view); Also, as stated previously in this text, for Libet, consciousness instigation doesn't eliminate the veto of that instigation, at least in some scenarios.
Thus the ramifications of Libet's experiments remain a point of philosophical contention.
- Benjamin Libet, Anthony Freeman, & 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
- Max Velmans, Understanding Consciousness, Routledge, London, 2000, ISBN 0-415-18655-2
- Daniel C. Dennett, Consciousness Explained, Little Brown, 1991, ISBN 0-316-18065-3
- Virtual Nobel for psychology Including video of Libet's speech.
- Short account of Libet's experiments and theory
de:Benjamin Libet es:Benjamín Libet
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