Benjamin Libet

Libet was the son of Ukrainian Jewish immigrants. Gamer Libitsky, his paternal grandfather, came to America in 1865 from a town called Brusyliv in Ukraine. His mother, Anna Charovsky, emigrated from Kyiv in 1913. His parents met in Chicago and married in 1915 and had Libet the next year. He had a brother, Meyer, and a sister, Dorothy. Libet attended a public elementary school and John Marshall High School. In 1939, Libet graduated from the University of Chicago, where he studied with Ralph W. 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 1965 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.

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 ray oscilloscope, 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. …

There is no majority agreement about the interpretation or the significance of Libet's experiments. However, 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".

Such a conclusion would be overdrawn as in a subsequent run of experiments, Libet found that even after the awareness of the decision to push the button had happened, people still had the capability to veto the decision and not to push the button. So they still had the capability to refrain from the decision that had earlier been made. Some therefore take this brain impulse to push the button to suggest just a readiness potential which the subject may either then go along with or may veto. So the person still has power over his or her decision.

For this reason, Libet himself regards his experimental results to be entirely compatible with the notion of free will. He 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 spinal motor neurons by the primary motor cortex, and the margin of error indicated by tests utilizing the oscillator must also be considered). …

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. …

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.

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.