Benjamin Libet (; April 12, 1916 – July 23, 2007) was an American neuroscientist who was a pioneer in the field of human consciousness. Libet was a researcher in the physiology department of the University of California, San Francisco.
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.
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 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. 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. Since then, even the readiness potential argument has been refuted.
Implications of Libet's experiments
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). However, Max Velmans has argued: "Libet